Recent advances in sample preparation methods for elemental and isotopic analysis of geological samples

Recent advances in sample preparation methods for elemental and isotopic analysis of geological samples

Journal Pre-proof Recent advances in sample preparation methods for elemental and isotopic analysis of geological samples Wen Zhang, Zhaochu Hu PII: ...

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Journal Pre-proof Recent advances in sample preparation methods for elemental and isotopic analysis of geological samples

Wen Zhang, Zhaochu Hu PII:

S0584-8547(19)30154-5

DOI:

https://doi.org/10.1016/j.sab.2019.105690

Reference:

SAB 105690

To appear in:

Spectrochimica Acta Part B: Atomic Spectroscopy

Received date:

25 March 2019

Revised date:

10 July 2019

Accepted date:

27 August 2019

Please cite this article as: W. Zhang and Z. Hu, Recent advances in sample preparation methods for elemental and isotopic analysis of geological samples, Spectrochimica Acta Part B: Atomic Spectroscopy(2018), https://doi.org/10.1016/j.sab.2019.105690

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© 2018 Published by Elsevier.

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Recent advances in sample preparation methods for elemental and isotopic analysis of geological samples

Wen Zhang, Zhaochu Hu*

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State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences,

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China University of Geosciences, Wuhan 430074, China

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Corresponding author: Zhaochu Hu, State Key Laboratory of Geological Processes and Mineral

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Resources, China University of Geosciences, Wuhan 430074, China. ([email protected] Tel.: +86 27 61055600, Fax: +86 27 67885096)

Submitted to Spectrochimica Acta Part B

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Abstract Knowledge of the elemental compositions and isotopic ratios of geological samples is a prerequisite for geochemical research. For whole-rock elemental composition and isotopic analyses, sample digestion is a crucial step and often the limiting factor for achieving reproducible and accurate analytical results. This review focuses on the recent advances in sample digestion methods, especially on open-vessel acid digestion, high-pressure acid digestion, microwave digestion, alkali fusion, high-pressure asher, and sample preparation methods for laser ablation-inductively coupled

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plasma-mass spectrometry (LA-ICP-MS) analysis. This review addresses concerns and discusses the attributes and limitations of these sample digestion methods along with examples of their

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applications in geochemical analysis. Particular attention is placed on sample digestion with

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ammonium fluoride (NH4F) and ammonium bifluoride (ammonium hydrogen fluoride, NH 4HF2), which are recent new research hotspots. Whole-rock analysis by LA-ICP-MS is becoming a

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promising technique because of rapid developments in sample preparation. It is evident that only the appropriate sample digestion methods can provide us with high-quality analytical results and

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guarantee the success of further investigations.

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Keywords: sample digestion, geological sample, ICP-MS, LA-ICP-MS, ammonium bifluoride, ammonium fluoride

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Journal Pre-proof 1. Introduction 2. Acid digestion 2.1 Open-vessel acid digestion 2.2 High-pressure acid digestion 2.3 Microwave digestion 2.4 NH4F and NH4HF2 digestion 3. Alkali fusion 4. High-Pressure Asher

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5. Preparation of geological samples for LA-ICP-MS 5.1 Pressed powder pellets

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5.2 Fusion glasses

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6. Conclusion

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1. Introduction

Knowledge of the elemental compositions and isotope ratios in geological samples is of high

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significance to research in earth science and other disciplines. Even with modern instruments, geological sample digestion is the basis for accurate analysis of elemental and isotopic compositions

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[1-3]. Mass spectrometry has been a dominant technique to determine a comprehensive range of

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trace elements and isotopic ratio analysis of geochemical interest [4]. The major purpose of sample digestion is to convert a sample into a form that is suitable for accurate and stable chemical analysis in an analytical instrument (e.g., mass spectrometer). In many cases, solid samples are converted to the solution form because of adequate homogeneity of a solution. Modern instrumental analysis poses many requirements with regard to the properties of the final sample solution. Taking inductively coupled plasma-mass spectrometry (ICP-MS) as an example, which is by far the major technique of choice for elemental analysis at the trace level, the total dissolved solids (TDS) in the final solution is required to be low to avoid serious matrix effects, clogging of cones, and signal instability. Potential sources of contamination should be carefully controlled, including reagent purity, cleanliness of test vessels and environment, and cross-contamination during operational processing. Moreover, complete sample digestion is an important prerequisite for achieving accurate multielement analytical results. This is particularly true for geological samples, which usually 3

Journal Pre-proof contain refractory minerals. The loss of elements in the form of precipitation or volatilization should also be avoided. To summarize, sample digestion is a fundamental and critical stage in the process of sample analysis, which directly affects the accuracy and precision of the analysis. In addition, sample digestion is often tedious and time-consuming, especially for geological samples. It is thus the limiting factor for sample throughput [1, 3, 5-7]. Natural geological samples are polyphase systems consisting of heterogeneous admixtures of minerals of contrasting chemical compositions and physical properties. Specifically, some resistant accessory minerals (e.g., zircon and garnet) generally control the whole-rock budget for some of the

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key trace elements (e.g., Nb, Ta, Zr, Hf, rare earth elements (REEs), U, and Th), which are important geological process indicators. In addition, because of the differences in the occurring state of

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minerals and physicochemical properties such as melting point, boiling point, volatility, chemical

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valence, and ionic radius, elements have different behaviors during the sample digestion procedure. As a result, there is still no single, universal sample preparation method that can provide quantitative

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recovery of all elements in all types of geological sample [3]. The development of multiple independent sample digestion methods is an inevitable requirement in the field of sample digestion,

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and it is also essential to evaluate and compare data with each other. Commonly used sample

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digestion methods can be divided into two classes, acid digestion and fusion. These two classes also can be subdivided into a number of different categories, depending on the combinations of reagents,

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digestion parameters, and digestion devices. In addition, techniques such as Carius tube and highpressure asher (HPA) system (HPA-S) are proposed separately, although they also use acid reagents for digestion. For the basic knowledge of these methods, other publications [1, 3, 8, 9] are recommended.

Compared to the rapid development of instrument hardware and mass spectrometric analytical technology, the progress in geological sample digestion method is overall very slow [10]. Fortunately, many conventional sample digestion methods have had new technological innovations during the past few years. In addition to ensuring complete recovery of elements and good analytical reproducibility, new sample digestion methods focus on multielement capability at ultra-low content. Many aspects have also been actively investigated, including high sample throughput, reduced labor and reagent consumption, simple digestion processes, and operator safety. A significant 4

Journal Pre-proof advancement is the use of two digestion reagents: ammonium fluoride (NH4F) and ammonium bifluoride (ammonium hydrogen fluoride, NH 4HF2). These reagents can replace hydrofluoric acid in acid digestion [11, 12]. On the other hand, there has been increasing interest in the quantitative analysis of major and trace elements by laser ablation-ICP-MS (LA-ICP-MS) bulk analysis, which provides several advantages over solution ICP-MS, including minimal sample preparation, low blanks, high sample throughput, and green chemistry concept [13-16]. This review focuses on the recent advances in sample digestion for geological samples. The authors do not attempt to discuss all dissolution methods; rather, common (or typical) methods that

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are routinely used in the laboratory are presented. Detailed introductions about the NH 4F and NH4HF2 digestion methods and the sample preparation of LA-ICP-MS analysis are summarized.

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Additionally, research progress on conventional acid digestion, fusion method, and HPA-S is also

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presented.

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2. Acid digestion 2.1 Open-vessel acid digestion

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Open-vessel acid digestion has long been a popular and simple method for the digestion of

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inorganic and organic materials in chemical laboratories. Open-vessel acid digestions refer to acid attack in open containers or screw-top vials (lowly pressurized) placed on a hot plate [3]. Strong

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mineral acids (HNO3, HCl, and HClO4) and the weak mineral acid HF are generally used for digestion of geological samples. Open-vessel acid digestion is successful when applied to most finegrained mafic rocks or geological samples without refractory minerals. Ma et al. [17] investigated the reproducibility of elemental analysis of basaltic samples by open-vessel acid digestion with a HNO3+HF mixture at 120 oC for >12 hours. For more than forty elements, the analytical results obtained from open-vessel acid digestion were consistent with those obtained from high-pressure acid digestion, indicating the reliability of open-vessel digestion. The complete dissolution of FeMn nodules with open-vessel acid digestion was reported by Berezhnaya and Dubinin [18] for the determination of platinum group elements (PGEs, Ru, Pd, Ir, and Pt) and Au. Good agreement was achieved between open-vessel acid digestion and microwave digestion operated under elevated temperature and pressure. In addition, open-vessel acid digestion is widely used in isotopic analysis 5

Journal Pre-proof of geological samples such as Li [19], Mg [20], Ca [21], Cu-Fe-Zn [22], Ba [23], Ag [24], and others, which are considered to be absent or present only in trace quantities in refractory minerals. However, the maximum digestion temperature for open-vessel acid digestion is limited by the boiling point of the corresponding acid or acid mixture (e.g., boiling points of 38.3% HF = 112 °C, 68% HNO3 = 122 °C, and 20.24% HCl = 110 °C) under ambient pressure, which is a major limitation [3]. It is thus difficult to apply open-vessel acid digestion to felsic rocks because of the incomplete dissolution of refractory minerals such as zircon [4, 25-27], which requires more vigorous digestion conditions. Generally, high-pressure acid digestion (bomb digestion) or alkali

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fusion is recommended for the digestion of granites. 2.2 High-pressure acid digestion

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Modern high-pressure acid digestions (bomb digestion) consist of a polytetrafluoroethylene

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(PTFE) beaker with a lid, which fits tightly into an outer, stainless-steel pressure jacket [3]. These bombs withstand very high pressures (7–12 MPa) when the sample and acids are subjected to high

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temperatures (110–250 oC) [28, 29]. This method enables effective digestion of refractory phases, which are not decomposed or only partially decomposed in the open vessel and microwave digestion.

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Other advantages of the high-pressure digestion include shortening the digestion time, reducing acid

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reagent consumption, avoiding the losses of volatile elements, and a lower procedure blank [3]. Therefore, high-pressure acid digestion is particularly suitable for trace and ultra-trace analysis.

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Note that high-pressure acid digestion does not include the closed-vessel microwave digestion method, which is discussed separately in Section 2.3. Effective digestion of refractory accessory minerals is an important advantage of the highpressure acid digestion, which has been shown constantly and consistently in recent reports. The results of the GeoPT proficiency testing programme provide an opportunity to recognize that inappropriate acid digestion schemes lead to unsuccessful multielement analysis in some special samples. Potts et al. [4] compared the analysis results of a basalt sample and those of a sediment sample in GeoPT. The former is considered a “well-behaved” rock, but the latter contains zircon or other refractory minerals. The ICP-MS and acid digestion test results of most elements in the basalt sample obtained from different laboratories were consistent with the results of other independent analyses. However, the variably low results for Y, heavy rare earth elements (HREEs), and Zr were 6

Journal Pre-proof observed for the sediment sample. A comparison of the reported procedures revealed that the poor recoveries were caused by the incomplete digestion of zircon when open-vessel digestions and microwave digestions were used. In contrast, when the samples were digested even under relatively mild acid conditions but under high pressure, such as 2 ml HF per 100 mg of sample, digestion temperature of 180 °C, and digestion time of 48 hours, complete recovery can be obtained. Potts et al. [4] thus recommended this condition as a reliable dissolution method for silicate rock that could be expected to contain zircon or other refractory minerals. In addition, a comparison between highpressure acid digestion and open-vessel acid digestion was studied for analyzing trace elements in

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the iron-formation reference materials [26]. Lower recoveries of HREEs were observed in the openvessel acid digestion due to incomplete digestion of the refractory phases. In contrast, quantitative

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recoveries of HREEs were achieved with high-pressure acid digestion.

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The dissolution process of geological materials in complex matrix is still not fully understood. Use of HF alone has been considered a more efficient way to digest geological materials than with

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HF-HNO3 mixture [30]. Recently, Okina et al. [31, 32] conducted a series of studies on the conventional high-pressure acid digestion to understand the effect of acid mixture during digestion

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and evaporation. The results indicated that the digestion of refractory accessory minerals is

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controlled by the concentration of HF in the acid mixture. Quantitative recoveries of Zr and Hf can be achieved only when the HF concentration in an acid mixture is more than 35%, with the digestion

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performed at >180 oC for >16 hours. The absolute amount of HF requirement for rock digestion is 5–10 times higher than the theoretical one based on stoichiometry. In contrast, the source of HF is not important [31]. Further investigations were focused on the effects of the residual fluoride ion in the final solution during evaporation [32]. The authors found that the critical concentration of fluoride ion is in the range of 20–80 μg ml−1, depending on the elemental compositions of the rock sample. When the residual fluoride ion was below the critical concentration, the recoveries of Ta and Nb were reported to be low. Moreover, the concentration of residual fluoride ion in the final solution prepared for the analysis mainly depends on the type of acid mixtures used during evaporation. Evaporation with HClO4 alone or an HClO4+H3BO3 mixture would significantly reduce the residual fluoride ion compared to evaporation with HNO 3 only, resulting in hydrolysis of high-field strength elements (HFSE) and low recoveries of Ta and Nb. However, to eliminate 7

Journal Pre-proof fluoride influence, evaporation with HClO4 alone or an HClO4+HNO3 mixture was proposed, but it should be finished shortly after the appearance of HClO4 vapor. The formation of insoluble fluorides that coprecipitate trace elements is a problem [10, 30]. To suppress the formation of insoluble fluorides, many methods have been studied, including evaporation with HClO4 [30, 33-35] or boric acid [36-38], adjustment of the [(Mg+Ca)/Al] ratio of the sample [10, 34, 39, 40], reduction of pressure in the digestion process [11, 12], decrease in the sample amount [30], and control of sample dryness during evaporation [5, 41]. However, a thorough understanding of the fluoride formation mechanism and complete inhibition of fluoride formation

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remains unclear. Chen et al. [42] thought that insoluble fluorides could exist in various forms of fluoro complexes including (Ca, Mg)F2, AlF3, Al(OH)3, or Al(OH, F)3. The authors suggested that

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a re-dissolution process, after evaporation with HNO3 at 190oC for 2 hours under high pressure,

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could dissolve these fluorides. In addition, the reagents of NH 4HF2 and NH4F have been recently reported to prevent the formation of fluorides in open-vessel acid digestion and microwave digestion

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[11, 12, 43].

The new sample preparation methods for some trace elements that are not commonly measured

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were developed by high-pressure acid digestion [44, 45]. He et al. [44] reported on the analysis of

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Br and I in soils and sediments by ICP-MS. Samples were digested with an HNO3+HF mixture under high pressure. A serious loss of Br and I was found at a high digestion temperature of 190 °C

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due to volatile forms of Br and I (e.g., as HBr, HI, and I2). Therefore, an optimized digestion procedure of heating at 140 °C for 15 min was recommended. A dilution with 5% ammonia solution was used to stabilize Br and I and eliminate their memory effects on the sample introduction system of ICP-MS.

2.3 Microwave digestion Microwave digestion is based on the ability of a certain substance (solvent and/or reagent) to absorb microwave radiation and transform its energy into heat energy [3, 46-51]. Compared with conventional heating, microwave heating is many times more efficient. The interaction of microwave radiation with sample and reagents causes both ionic migration and dipole rotation, resulting in rapid bulk heating of the reaction mixture with consequent decomposition of the sample. Microwave digestion procedures are classified according to their operational modes: open-vessel 8

Journal Pre-proof microwave-assisted digestion and closed (pressurized)-vessel procedures [3]. The latter have the advantages of reducing the digestion time, contamination, loss of volatile species, and reagent and sample consumption. Online microwave-assisted digestion of solid samples is also known [52]. Microwave digestions have been developed to include a variety of sample matrices such as basalts, soils, sediments, coals, airborne particulates, sludge, and organic environmental and biological samples [3, 5, 8]. However, several authors have reported that microwave digestion does not always allow for the accurate analyses of elements such as Cr, Zr, Hf, and HREEs because of the relatively short

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digestion time commonly used in a microwave system, especially when refractory minerals (e.g., zircon, chromite, rutile, corundum, and cassiterite) are present. Therefore, its applicability seems

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restricted to dissolve basalts [5] or to obtain part of trace elements from rocks [53], ores [54], and

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soils [55]. Fedyunina et al. [56] made efforts for the complete digestion of refractory minerals by microwave digestion. They found that the conventional microwave digestion with an HF+HNO3

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mixture at 210 oC for one hour could quantitatively digest basic rocks but not felsic rocks. The addition of HCl and H3BO3 in the digestion process significantly enhanced the digestion efficiency

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and quantitatively transformed andesites and some types of granites. However, some syenite,

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granodiorite, and granite, which are rich in albite (NaAlSi 3O8), are still nonquantitatively digested. The main reason was attributed to the incomplete digestion of the complex aluminum-silicates in

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the microwave field. Magaldi et al. [43] evaluated the NH4HF2 microwave digestion method for multielement analysis in geological reference materials from felsic to ultramafic composition. Low recoveries of Zr and Hf in granodiorite GSP-2 and granite G-2 were reported, indicating the incomplete digestion of zircon. Therefore, microwave digestion cannot be treated as the universal technique for all types of rocks [56]. 2.4 NH4F and NH4HF2 digestion In recent years, the traditional reagents NH4F and NH4HF2 have been utilized for new applications in geological sample digestion for trace element determination by ICP-MS. [11, 12, 40, 43, 57, 58]. These two reagents are very similar in their working principle and digestion process; therefore, they can be considered as one type of digestion method. However, some physicochemical differences between them, such as digestion temperature and digestion time, should be noted. Additionally, to 9

Journal Pre-proof the best of our knowledge, there is currently no comprehensive review on these two digestion reagents. Therefore, the content presented in this section is expanded slightly beyond the field of geological analysis sample preparation. In this section, we extensively discuss the applications of NH4F and NH4HF2 in radioanalytical chemistry, environmental science, agriculture science, and metallurgy for multielement analysis or elemental extraction and cover some previously published papers. NH4F and NH4HF2 are white, water-soluble crystalline salts, and soluble in water. The physical properties of NH4F and NH4HF2 are summarized in Table 1. Much of the commercial interest in

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NH4F and NH4HF2 stems from their chemical reactivity as less hazardous substitutes for hydrofluoric acid [59, 60]. More than 2072 papers on NH4F have been published (Fig. 1, data from

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Web of Science). These publications show that NH4F is mainly used as an electrolyte solution or

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fluorine precursor to synthesize high-purity materials or nanomaterials in the field of material science, such as TiO2 [61, 62], CeO2 [63], LaF3 [64], CaF2 [65], etc. In addition, certain physical

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properties of nanomaterials can be improved through their modification with NH4F. For example, fluorine was doped to enhance the photocatalytic activity of TiO 2 with NH4F treatment [66, 67].

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NH4F is also a traditional surface treatment agent for surface morphology modification, substrate

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activation, and silica-based substance etching. NH4F has attracted much recent research attention in the search for high-performance materials for electrochemical energy storage [68-71] and is also

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widely applied in semiconductor technology to etch or clean the surface of silica-based substances [72-76]. Similarly, NH4HF2 is also applied to areas of new material synthesis [77-80], formation of electroless coatings [81-83], modification of surface morphology [84, 85], and etching of glasses and other silicon materials [86-88]. However, its focus in research is not as prevalent as NH4F (Fig. 1, only 178 publications). We found that only a small fraction of NH4F (4%) and NH4HF2 (8%) reported applications are related to analytical chemistry (Fig. 1). NH4F tends to release ammonia gas on heating and is converted to the more stable form NH4HF2 (Equation 1) [60]. NH4HF2 can release ammonia gas and hydrogen fluoride gas on heating (Equation 2).

Equation 1: 2NH4 F

→ NH4 HF2 + NH3 10

Journal Pre-proof Equation 2: NH4 HF2 → NH3 + 2HF

Therefore, NH4F and NH4HF2 can replace HF and take part in reactions with many substances (such as oxides, hydroxides, and salts) at room temperature or on heating to form ammonium fluorometallates or oxofluorometallates. They are considered promising reagents for fluorination of minerals, metal oxides, ores, and geological samples. Analysts in different fields have established sample pretreatment methods based on NH4F and NH4HF2 for subsequent chemical analysis. Alternatively, these reagents can effectively extract elements from concentrates, ores, or rocks in

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the metallurgical industry. Geochemistry analysis

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NH4F and NH4HF2 reagents can be used as promising sample digestion reagents in lieu of HF

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acid to break the Si-O bond in minerals. The fluorination reactions of silicon-containing minerals of different compositions and structures with NH 4HF2 have been summarized by Laptash and

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Maslennikova [89]. In the mid-1980s to early 1990s, Kolikova et al. [90, 91] already reported the interactions between NH4F/NH4HF2 and silicate minerals by stirring solid NH4F/NH4HF2 with

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powdered minerals at ambient temperatures, resulting in partial decomposition of the silicates.

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However, these reagents have long been ignored in the field of trace element analysis of geological samples. In the last decade, Mariet et al. [92] suggested the use of HNO3+H2O2+NH4F mixture

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instead of HNO3+HF+HClO4 in conventional open acid digestion for lichen, basalt, and soil. In their study, 50 mg (100 mg for lichen) of reference materials was weighed into Teflon flasks, and the flasks were placed on a sand bath at 240 oC for 10 hours with an HNO3-NH4F mixture containing 1 ml NH4F (47 %). Thirteen elements in the basalt reference material were determined with an estimated bias of <10%. Subsequently, Hu et al. [57] reported the application of NH4F to highpressure bomb digestion of geological samples for multi-element analysis by ICP-MS. Thirty-three trace elements present in a wide variety of geological samples were completely recovered by digestion lasting for 24 hours with a NH4F+HNO3 mixture at 190 oC. These two pioneering studies showed the capacity of the NH4F reagent to release trace elements from geological samples, ranging from mafic rocks to felsic rocks. However, the processes are time consuming. In the study reported by Hu et al. [57], a sample amount of more than 150 mg under the high-pressure bomb digestion 11

Journal Pre-proof condition yielded white precipitates in the final solution, resulting in significant losses of some trace elements such as Sc, Y, Sr, REEs, and Th. Further investigations on the use of NH4F or NH4HF2 reagents for geological sample digestion were carried out by Hu’s group. Use of NH4F or NH4HF2 along with open-vessel digestion (Savillex screw-top Teflon vial) was developed to digest various rock reference materials for multielement analysis [11, 12]. The high boiling points of NH4F and NH4HF2 allow the use of an elevated digestion temperature, which enables rapid decomposition of refractory phases in open vessels. Thirty-seven elements at trace levels were completely recovered. From the results reported in these

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two representative papers, the advantages of NH4F and NH4HF2 digestion technologies are summarized as follows:

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(1) Efficient and complete digestion of geological samples in the open-vessel technique

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The open-vessel acid digestion has long been a popular and simple method for the digestion of inorganic and organic materials in chemical laboratories. It is a low-pressure (technically speaking,

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lowly pressurized) digestion conducted using open vials or screw-top Teflon vials. Therefore, the maximum digestion temperatures are limited by the boiling point of the corresponding acid or acid

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mixture under ambient pressure. The limited digestion temperature not only prolongs digestion time

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but also is often insufficient for the dissolution of refractory minerals such as zircon. The high boiling points of NH4F and NH4HF2 (260 oC and 239.5 oC) allow elevated digestion temperatures

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in open vessels. Consequently, the rock samples can be completely digested by NH4F at 250 oC in 1.5–2 hours or by NH4HF2 at 230 oC in 2–3 hours. The applicable sample types include felsic igneous rocks, which contain refractory minerals such as zircon. It is worth noting that mixing of NH4F or NH4HF2 reagents with water, in particular with HNO3 and HCl, moderates their digestion ability. We speculate that the added HNO3 or HCl reacts with NH4F or NH4HF2 and releases HF rapidly. The released HF is rapidly lost through evaporation at elevated temperature, resulting in a decrease in F- ion concentration in the residual reagent, leading to a reduced digestion capability in open vessels. (2) NH4F/NH4HF2 open-vessel acid digestion methods are not hampered by the formation of insoluble fluorides due to the low-pressure digestion environment There is consensus that use of HF to digest silicate samples in a high-pressure acid digestion 12

Journal Pre-proof environment could cause formation of insoluble fluorides, which subsequently incorporate large proportions of trace elements [10, 30, 34, 35]. Previous studies reported that factors influencing the formation of insoluble fluorides are the relative proportions of Al:Mg:Ca in the sample, digestion pressure, and test portion mass of the sample powder [10, 30, 34]. However, the procedures for NH4F or NH4HF2 open-vessel digestion methods are performed in a low-pressure environment. Zhang et al. [11] reported that no insoluble fluorides were observed when using NH4HF2 in the open-vessel technique for granodiorite GSP-2, even when the test portion size of GSP-2 was increased to 200 mg. Hu et al. [12] performed digestion of six geological reference materials, with

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amounts ranging from 50 mg to 200 mg, using NH4F by open-vessel acid digestion, and they also did not observe the formation of insoluble fluorides with the naked eye. In addition, excellent

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agreement between the measured and the reference values were achieved in both studies. The

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absence of insoluble fluorides was attributed to the low-pressure digestion environment. This advantage guarantees the accurate analysis of some important and fluoride precipitate-sensitive

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elements (e.g., Rb, Sr, REEs, and Th) in geological samples. (3) Commercial NH4F and NH4HF2 reagents can be effectively purified by the sub-boiling

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distillation purification system, similar to HF, HNO3, and HCl

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Modern ICP-MS instrumentation enables the measurement of extremely low concentrations of elements in geological samples. The influence of reagent blank, which, in many cases, determines

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the lower limit of quantitation in the analytical results, becomes increasingly important with decreasing elemental concentrations. For unpurified NH 4F and NH4HF2, significant contamination levels for some transition metal elements such as V, Cr, Co, Ni, Cu, and Zn, have been reported. After purification treatment with sub-boiling distillation, the purified NH4F and NH4HF2 provide satisfactory reagent background similar to that obtained with the conventional HF-HNO3 acid digestion method [12, 13, 58]. Ultrapure NH4F and NH4HF2 reagents are beneficial for ultra-trace multi-element determination. In addition to the use of sub-boiling distillation to obtain ultrapure NH4F and NH4HF2, commercial high-purity reagents are currently available, for example, from Sigma-Aldrich as trace-metal grade NH4F (99.99%) and NH4HF2 (99.999%). The latter has been used for trace element analysis by O’Hara et al. [58]. (4) NH4F or NH4HF2 digestion methods are comparatively safer to handle and economical 13

Journal Pre-proof Compared to the corrosive and toxic HF, which is in the solution form, NH4F and NH4HF2 are solids and thus are comparatively safe for the operator during handling. The NH4F or NH4HF2 digestion method does not require the use of large amounts of acid reagents as in conventional open digestion methods and avoids the expensive high-pressure digestion bomb. The relatively economical price of NH4F and NH4HF2 compared to HF renders another advantage in reducing costs. (5) There is largely reduced risk in introducing high total dissolved solids (TDS) and complex polyatomic ion interference

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Excessive NH4F and NH4HF2 reagents can be effectively removed during the evaporation process, and thus, they do not remain in the final solution. For digestion of silicate minerals, NH4F and

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NH4HF2 reagents release hydrofluoric acid and break the strong Si-O bonds, ultimately forming

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(NH4)2SiF6. Subsequent thermal treatment in the presence of HNO 3 will result in the formation of SiF4, thus removing this major elemental component from the silicate matrix, which further reduces

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TDS in the final solution. The constituents of NH 4F and NH4HF2 reagents (H, N, and F) are consistent with those of the HF and HNO3 mixture. Additional polyatomic ion interferences are not

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analysis.

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present, making digestion methods with NH4F and NH4HF2 suitable and compatible for ICP-MS

The processes of NH4F/NH4HF2 open-vessel acid digestion methods described by Hu et al. [12]

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and Zhang et al. [11] are listed in Fig. 2. NH4F has a higher boiling point than NH 4HF2, which allows the use of a higher digestion temperature in open vessels. Therefore, digestion with NH4F can be reduced to 2 hours at 250 °C compared to that by the NH4HF2 method, which requires 3 hours at 230 °C. However, according to our understanding, the NH 4HF2 reagent is receiving more attention and wider application, possibly because high-purity NH4HF2 is more readily available than NH4F. Gaschnig et al. [93, 94] employed the NH4HF2 open-vessel method to digest their subset of glacial diamictite samples for trace element analysis. O’Neil et al. [95] found that the simple HNO3+HF digestion in closed Savillex Teflon beakers cannot effectively dissolve amphibolite samples, resulting in Zr and Hf contents. Then, a second dissolution with NH4HF2 in screw-top Teflon vials was performed following the procedure described by Zhang et al. [11]. The measured Zr and Hf concentrations in the NH4HF2 open-vessel acid digestion method were reported to be 14

Journal Pre-proof comparable to previously determined results. Furthermore, bauxite was digested with NH4F or NH4HF2 in open vessel for trace element analysis [40]. Bauxite is composed principally of aluminum hydroxides and different minerals including refractory ores (zircon, tourmaline, and rutile). The conventional bomb digestion method with HF/HNO3 results in a significant loss of REEs due to the formation of insoluble AlF 3 precipitates. The NH4F or NH4HF2 open-vessel digestion method was reported to be able to completely digest resistant minerals in bauxite samples in a short period of time (5 hours). White precipitates and semi-transparent gels were obtained in both digestion methods, but these

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precipitates and gels can be dissolved by HClO4. O‘Hara et al. [58] systematically evaluated the efficiency of using NH4HF2 to digest a range of inorganic sample types. Their study focused on the

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effects of NH4HF2-to-sample ratios and the NH4HF2 digestion time. The results showed that the

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required NH4HF2-to-sample ratio depends on the specific material being decomposed. For glass and quartz, the required ratios are consistent with predictions from stoichiometry, whereas with soil and

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low-grade U ore standards, mass ratios in excess of stoichiometric estimations are more effective. The advantages of the NH4F and NH4HF2 reagents described above attract analysts to use them

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in other sample preparation techniques (Table 2). Husáková et al. [96] reported an NH4F microwave

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digestion method to determine silicon and boron in fertilizers, sludge, plants, and fly ash. Fifty milligrams of the sample was digested with 5 mL of 100 g L-1 NH4F for 15 min at 180 °C. This

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method avoids the risk of analyte loss owing to the volatile SiF4 and BF3 formed in the conventional HF digestion process. The fluorination products of Si and B formed with the use of NH4F, such as (NH4)2SiF6, NH4BF4, mixtures of (NH4)2BOF3, (NH4)3B3O3F6, and other ammonium oxofluoroborates and hydroxofluoroborates, are thermally stable. NH4HF2 was allowed to react with pyrolyzed coal residue combined with microwave digestion for developing a simple and reliable analytical method for coal samples. The quantitative recoveries of thirty-four elements in coal samples demonstrated that the NH4HF2 reagent can be used as a replacement to the highly toxic HF solution [97]. Magaldi et al. [43] evaluated the NH4HF2 microwave digestion method for multielement analysis of geological reference materials from felsic to ultramafic composition. Samples (100 mg) were digested with a mixture of 2 ml saturated solution of NH4HF2 and 2 ml of ultrapure water at ~200 oC for 30 min. During the digestion procedure, no fluoride precipitations 15

Journal Pre-proof were observed in nine felsic and intermediate rock matrices. Quantitative recoveries of forty-one trace elements in twenty-one mafic and igneous rocks were achieved, including shale. The recoveries of Cr in two ultramafic rocks (DTS-1 and JP-1) were 90–110% but only 85% in the ultramafic rock OKUM, which was attributed to the incomplete digestion of the more resistant cores of the chromite grains. Low recoveries of Zr and Hf in granodiorites GSP-2 and granite G-2 were observed, but the digestion was complete for the other four felsic rock reference materials. The authors speculated that the difference in zircon crystallinity, composition of the original zircon, or metamict degree could lead to incomplete digestion. He et al. [98] developed a new analytical

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method for the simultaneous determination of Cl, Br, and I in geological materials based on NH 4HF2 open-vessel digestion. The authors found that Cl, Br, and I are not lost during NH4HF2 digestion at

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temperatures of 200−240 °C for 0.5−12 hours. They speculated that the alkaline atmospheric

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environment caused by NH3 during NH4HF2 digestion can suppress the volatilization loss of Cl, Br, and I.

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NH4HF2 open-vessel digestion has also been introduced as a pretreatment technique for LA-ICPMS analysis of whole-rock sample [16, 99]. Zhang et al. [16] described an NH4HF2 digestion

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method as sample preparation for rapid determination of major and trace elements in silicate rocks

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using LA-ICP-MS. Powered rock after digestion with NH4HF2 becomes ultrafine powder, which offers excellent cohesion and homogeneity suitable for laser ablation without the addition of a

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binder. Because of the complete destruction of silicate minerals by NH 4HF2, the digested products presented similar structure, morphology, and particle size, achieving consistency in physical properties in pressed powder pellets and matrix-matched ablation. Internal standard can be spiked into reference materials and samples. With this method, ten major elements and thirty-five trace elements were simultaneously analyzed by LA-ICP-MS. The analytical results of six reference materials were in general agreement with the recommended values, with discrepancies of less than 10% for most elements. The analytical precision is within 5% for most major elements and within 10% for most trace elements. Grate et al. [99] reported the use of NH4HF2 for the digestion of uranium ore for subsequent LA-ICP-MS analysis. Spikes, in the form of single element standards of In, Tb, and Bi, and isotopic 235U were added into the uranium ore sample. LA-ICP-MS analysis of the fluorinated products showed that the reproducibility of the isotope ratio of 238U/235U can reach 16

Journal Pre-proof 0.1%, better than that of the elemental ratios of 238U/115In, 238U/159Tb, and 238U/209Bi (8–12%). These results provided a new concept of incorporating internal standards into a solid for analysis by LAICP-MS to improve calibration and quantification in solid-based analysis. In contrast to the preparation scheme presented by Zhang et al. [16], who heated the fluorinated products to 400 oC to remove the residual NH4HF2, Grate et al. [99] directly analyzed the fluorinated products and emphasized that a common matrix can be obtained from samples and reference materials, which is similar to matrix-assisted laser desorption ionization (MALDI) and solution analysis; the initial sample is only a small fraction in the measured material, so that the laser sampling is defined by a

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common matrix and not by the sample itself. Radioanalytical Chemistry

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In recent years, the NH4HF2 digestion method has attracted more attention in nuclear forensics

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and emergency responses. Development of in-field and rapid dissolution techniques is important for decreasing response time following a nuclear event, for example, nuclear detonation, nuclear

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accident, monitoring of nuclear material, or technology for nuclear nonproliferation [100, 101]. Conventional bulk analytical methods (i.e., acid digestion methods and fusion methods) are difficult

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to reach the requirements of in-field and rapid analyses. Acid digestion methods are generally time-

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consuming and require the use of HF solution for silicates and other refractory minerals that are expected to occur in nuclear debris [101]. HF solution, which is corrosive and toxic, is difficult for

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in-field use because of operator safety considerations and their highly regulated transportation [100]. For conventional fusion methods, high temperature (>600 oC) and, thus, a furnace is needed. It should be noted that in situ research usually does not allow the use of these experimental conditions. For example, Wang et al. [102] reported that analyzing Pu in the Fukushima Daiichi Nuclear Power Plant accident was strictly limited to a specific small area where the use of high-temperature oven operation was restricted. In addition, high temperatures for the melt could result in loss of Cs and other semi-volatile fission products [38, 103]. NH4HF2 is considered as a replacement for HF to establish the low temperature fusion method for in-field and rapid trace element and isotope analysis. The dissolution method similar to that reported by Zhang et al. [11] was adapted for the digestion of nuclear debris (trinitite), geological reference materials, and urban canyon matrix for trace element analyses [38, 101]. The results 17

Journal Pre-proof showed that most of the elements (with the exception of Zr) and the radiotracer can be completely recovered with a fusion time of 10 min at a digestion temperature of 230 oC. In the optimized procedure, the total dissolution time was shortened to <3 hours. The new NH 4HF2 dissolution method with indirect sonication at a low temperature of 55 oC was attempted by the same group for the digestion of refractory nuclear debris [37]. Boric acid was added to form monofluoroboric acid, which limits the formation of insoluble fluorides. Quantitative recovery for most elements in United States Geological Survey (USGS) and National Institute of Standards and Technology (NIST) reference materials can be obtained with 30 minutes of sonication, including DNC-1a (Dolerite),

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SDC-1 (Mica Schist), QLO-1a (Quartz Latite), and BHVO-2 (Basalt) but not for Zr in the DNC-1a and SDC-1. However, low recoveries of trace elements in AGV-2 (andesite), NIST 278 (obsidian),

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and NIST 1413 (high-alumina sand) were observed. The authors suspected that the low

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concentration of HF (releasing from NH4HF2) and low digestion temperature in the sonication technique caused difficulty to dissolve TiO2 and Al2O3 completely, resulting in low recoveries of

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many elements [37]. Hubley et al. [104] reported the use of the NH4HF2 sonication method for the digestion of the surrogate post-detonation urban debris, which is a vitrified material and contains

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high levels of Fe and Ca. Compared to the conventional NH 4HF2 digestion method and microwave

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digestion method, the NH4HF2 sonication method resulted in Fe-bearing black particles and lower determined concentrations for all elements.

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In another study, a 1% NH4HF2 solution (w/v) was used to digest uranium oxides (UO2 and U3O8) from simulated filter swipe media with sample heating to 80 oC for 50–60 min [100]. These conditions yield average uranium recoveries of 100% for U3O8 and 90% for UO2. A digestion time of one hour was considered suitable and timely for in-field monitoring of nuclear material. For measurements of Pu concentration and associated isotope ratios in soils and sediments, Wang et al. [102] reported an NH4HF2 fusion method to extract Pu from the sample with a hot plate with 15 min heating-up time and 15 min fusion at 250 oC. The recovery of Pu was more than 90% for a 0.5–1 g sample. The whole analytical procedure took merely 8 hours, which is of benefit for rapid Pu determination in environmental samples for nuclear emergency response or environmental studies. Representative NH4HF2 digestion methods used in radioanalytical chemistry are listed in Table 2. Environmental Science 18

Journal Pre-proof In the field of industrial hygiene monitoring, occupational exposure or skin contact to beryllium (Be) can cause Be sensitization and chronic Be disease [105, 106]. Therefore, fast and simple analytical methods have been developed for Be analysis in the field. A new analytical method that uses a dilute aqueous solution of NH4HF2 for extraction of Be, followed by fluorescence measurement for the determination of trace amounts of Be in airborne and surface samples, was developed [100]. Dilute NH4HF2 (1–3%) solution has been demonstrated to be effective in the digestion of Be metal and BeO [106, 107], soil and sediment [108], BeO in polyvinyl alcohol wipes [105], or air filters [109].

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Agriculture Science

In agricultural soil research, the relationship between crop yields and concentrations of

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phosphorus (P) in soils (P fertilizer) has been paid attention for a long time. Phosphorus in soil can

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be extracted with a mixture of NH4F (0.03 mol/L) and H2SO4 (0.03 mol/L) [110, 111]. The modified extractant with 0.025 mol/L HOAc, 0.25 mol/L NH4OAc, and 0.015 mol/L NH4F was effective

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under a wide range of soil conditions for simultaneous extraction of available phosphorus and potassium [112]. To know the status of various P forms in calcareous soils, a fractionation scheme

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(Al-P) [113-115].

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of inorganic phosphorus was suggested. NH4F was used to extract the P from Al oxyhydroxides

In addition, Prietzel and Hirsh [116] reported the use of a 0.5 M NH4F solution at a soil:solution

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ratio of 1:5 for the extraction of inorganic sulfur in acid forest soils, followed by subsequent determination of the SO42- concentration in the extract. The proposed method is more efficient than conventional procedures in which inorganic S is extracted with phosphate or bicarbonate solution because of its ability to extract both SO42- that are absorbed on the mineral surfaces as well as precipitated as [Alx(OH)y(SO4)z] [116, 117]. Metallurgy In metallurgical industry, NH4HF2 has been used to extract elements from concentrates, ores, or rocks. The fluoride processing of nonbauxite aluminum ores with NH 4HF2 was reported for the extraction of alumina, aluminum fluoride, amorphous silica, and other useful components [118]. The research groups of Medkov et al. [119] and Khanchuk et al. [120] employed NH4HF2 for the extraction of noble metals and lanthanides from carbon-bearing rocks. New separation processes 19

Journal Pre-proof for niobium, tantalum, and scandium from tantalite or ferrocolumbite were developed, with the use of NH4HF2 as an alternative to hydrofluoric acid [121-123]. To extract zirconium from the refractory mineral of zircon, microwave-assisted digestion with ammonium acid fluoride (NH4F·1.5HF) was investigated, and complete digestion was achieved by successive microwave digestion steps combined with an aqueous wash procedure [124, 125]. Other hydrometallurgical processes with the use of NH4HF2 include extraction of REEs from monazite concentrate [126], high-purity ammonium hexafluorosilicate and amorphous silica from quartz sands [127], and titanium from titaniferous concentrates [128]. In these applications, NH4HF2 is considered easier and safer to handle than

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aqueous HF [122, 123]. Another advantage of NH4HF2 is its lower cost because NH4HF2 (including NH4F) is a by-product of the production of superphosphates [129] and local plants producingNF3,

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for example [123].

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Reported problems

Although many of the publications described above have reported successful digestion of samples

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and quantitative recovery of multiple elements with NH4F or NH4HF2 reagents, some contrasting results have been reported recently. Chiweshe et al. [130] attempted to digest chromitite

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concentrates at 250 oC using NH4HF2 in a ratio of 1:20 for 1 hour and then determined Ru, Os, and

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Pt by inductively coupled plasma optical emission spectrometry (ICP-OES). Low recoveries of Ru, Os, and Pt were observed and compared with the results obtained with the use of ammonia and

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sodium phosphate fluxes at 800 oC. There are some problems in Chiweshe’s digestion procedure, for example, the use of open platinum crucibles and too high a digestion temperature at 250 oC, which exceeds the decomposition temperature of NH4HF2. According to our experience, NH4HF2 decomposes and evaporates at 250 oC from an open crucible, resulting in a decrease in digestion efficiency. O’Hara et al. [58] used the NH4HF2 open-vessel digestion method to digest 50 mg pure zircon powders (size of 90% of particles is ≤ 23 μm) at 230 oC. Approximately 90% of the zircon can be decomposed when the digestion time exceeds 12 hours and the reagent amount (300 mg) is 6 times the sample amount (50 mg). The main reason for the slow and incomplete digestion of pure zircon powders was reported to be the highly coordinated crystal structure and the formation of a diffusional fluoro-silicate-zirconate boundary that retards the rate of decomposition. However, the 20

Journal Pre-proof authors also pointed out that a small component of zircon in natural samples (e.g., natural levels in soils, sediments, and rocks) could be successively decomposed, given adequate stoichiometric excess of the NH4HF2 reagent. Zhang et al. [11] likewise demonstrated successful zircon decomposition in milled granodiorite certified reference material (GSP-2). Research from Magaldi et al. [43] showed low recoveries of Zr and Hf in granodiorite GSP-2 and granite G-2 (mean recoveries of ca. 70%) when digested with NH4HF2 in a microwave oven even when providing enough amount of HF. The reason for the low recoveries is not clear, and more

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investigations are needed in the future.

3. Alkali fusion

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Alkali fusion is a rapid and effective technique for the decomposition of rocks that contain

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refractory minerals [3]. Fusion of rock samples with appropriate flux results in the formation of glasses that are readily soluble in acids or water and can be subsequently introduced into an ICP-

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MS through conventional means. Decomposition efficiency by fusion is greater than that by acid digestion mainly because of the high fusion temperature (480–1200 oC). Common fluxes are alkali

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salts including LiBO2, Li2B4O7, K2B4O7, KOH, K2CO3, NaOH, Na2O2, and Na2CO3 [3, 131]. Alkali

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fusion requires a high flux-to-sample ratio for complete decomposition of the sample. This resulted in high blank levels, TDS content, and matrix effects. To reduce the TDS and matrix effect, the final

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sample should be diluted by large amounts of acids, which consequently increases the detection limit for trace element analysis. Moreover, some elements cannot be analyzed in fusion samples due to volatilization (Pb, Sb, Sn, Zn, and Cs) or contamination (Li and B from fluxes). Polyatomic interferences are generated from the fluxes of LiBO2/Li2B4O7 [10]. Alkali fusion is thus traditionally used for the analysis of major elements, and only part of trace elements in geological samples is determined by X-ray fluorescence (XRF), ICP-MS, and ICP-OES. As alkali fusion always leads to substantial contamination of the analyzed material by introduction of salts, many efforts have been made to minimize the amount of the flux. Park et al. [27] reported a revised alkali fusion method using a mixed flux of LiBO 2+Li2B4O7 for the determination of trace elements in geological samples. The low ratio of flux to sample (2:1) was applied to fuse rock powders at 1020 oC for 10 min. The fused products (glass beads) were grounded 21

Journal Pre-proof and digested with HF+HNO3+HClO4 in PTFE vessels. The limit of quantification (LOQ) for thirty trace elements was comparable with that of conventional open-vessel acid digestion due to the limited amount of high-purity flux. Sintering or fusion with sodium peroxide (Na2O2) was considered as a promising tool for sample preparation, as it offers several advantages over fusion with lithium borate compounds [132]. Na2O2 is a strongly oxidizing alkali flux. Most minerals (zircon, tourmaline, cassiterites, or chromites) found in silicate rocks can be rapidly decomposed by sintering with Na2O2 at a temperature of 480±20 oC. Low-temperature sintering with Na2O2 causes slight risk of analyte loss. The sintered

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cake can be easily dissolved in water. Hydrolysis of Fe and Ti precipitate and entrain lanthanides and other trace elements, especially the high-field strength elements [132]. This process results in a

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separation between some trace elements and matrix elements (such as Na and Si). The final solution

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then has a low salt content. With this procedure, Cotta and Enzweiler [10] accurately determined a large number of trace elements in the precipitates, including REEs, Ni, Sc, Co, Sr, Y, Ba, Zr, Nb,

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Cd, Sn, Sb, Hf, Pb, and Th. However, quantitative recoveries of Ta, Rb, and U were limited due to the variable co-precipitation efficiency. Bokhari and Mersel [132] proposed an optimized Na2O2

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sintering method for geological samples. A long digestion time of 120 min was required if refractory

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minerals (such as zircon and chromite) were present; otherwise, samples without refractory minerals could be digested in 30–45 min. Fifty elements could then be determined through the combination

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of the supernatant and the precipitates including Si, Ti, Al, Fe, Mg, Ca, K, and P. Silicate remains in the alkali fusion method. When the melt is poured into an acidic media, soluble silicic acid monomers (Si(OH)4) are formed, and they continually condense to form silica gels [133]. Silica gel is a colloidal solution of silicic acid and always used as an adsorbent [134, 135]. The presence of silica gel could carry down some elements, and this leads to loss of these elements. In addition, the condensation products of silica gel will reduce flow in the following separationpurification processing [133]. To overcome this problem, Michaud and Larivière [133] performed flocculation with polyethylene glycol polymer to remove silica before the pre-concentration processing of both actinides and strontium in the environmental solid samples. The high-molecularweight polymer can interact with colloidal and subcolloidal silica in the acidic aqueous solution (through hydrogen bonding) and form a precipitate, called a floc. Using the optimized procedure, 22

Journal Pre-proof most of silica gel can be removed (97–99%), depending on the analyst matrix and the vessel used for flocculation. A flux-free fusion technique was an alternative method to eliminate the problems caused by the fluxes. The rock samples are melted directly at 1300 oC to 1800 oC, and glasses are formed by rapid quenching. Shimizu et al. [136] used a SiC electric furnace to fuse powdered sample at 1500 oC or 1600 oC with regulated oxygen fugacity. The formed glasses were digested with a HF+HClO4 mixture, followed by ICP-MS analysis. Twenty-four trace elements can be quantitatively recovered, including Zr and Hf in the zircon-bearing granitic samples. However, the determination of highly

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volatile elements such as Pb and Tl was not feasible. Bao et al. [137] reported a new flux-free fusion method using a closed boron nitride vessel at 1400 oC or 1600 oC for 1 min for intrusive rocks or

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extrusive rocks, respectively. The high temperature and minimum duration of heating are sufficient

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to melt the rock samples and suppress evaporation of the volatile elements. The fusion glass fragments then were digested by a three-step acid treatment in closed screw-top Savillex vials on a

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hot plate, including HF+HNO3+HClO4, HF+HNO3 mixtures, and HNO3 alone. Although undissolved small zircon grains were still found in the granodiorite after high-temperature fusion at

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1600 oC, the high recovery for Zr and Hf can be achieved due to the effective acid digestion

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processes in 6 hours at 150 oC. Low procedural blanks were obtained. Thirty-six trace elements were completely recovered in different rock reference materials, including the highly volatile Rb,

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Cs, and Pb. Moreover, Bao et al. [138] used this preparation method to accurately determine the HfSr-Nd isotopic compositions in a series of geological rock reference materials.

4. High-Pressure Asher

In the 1980s, Knapp designed a new sample digestion system to solve the problems encountered in the traditional polytetrafluoroethylene (PTFE) bomb digestion method, such as low decomposition temperature and leak-related element losses or contamination [139, 140]. He used a pure quartz vessel instead of a PTFE bomb and placed the quartz vessel inside an autoclave that could be filled with nitrogen [139, 140]. The quartz vessel can be operated at a pressure up to 100 bar and at the maximum temperature of 320 oC. This digestion system was called high-pressure asher by Knapp. Its initial main purpose is for the digestion of various organic materials. After years 23

Journal Pre-proof of development, the HPA technology has also been combined with microwave technology for the digestion of the biological materials [141] or used to directly digest various geological samples [142-152]. Currently, commercial high-pressure digestion systems, for example, the HPA and Multiwave 7000 (HPA+Microwave) (Anton Paar GmbH, Austria), are available. The new HPA systems (HPA-S) withstand a high pressure of approximately 130 bar and a digestion temperature of 300–320 oC. HPA-S has been widely used in the determination of highly siderophile elements (HSE: Re and platinum group elements (PGE)) [142-147], strongly chalcophile elements (S, Se, and Te) [146-150], Os isotope ratios [151], and Se isotopes [150, 152] in ultrabasic or basic rocks.

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The advantages of HPA-S are the low-level blanks, low detection limits, and hence better precision in low-concentration samples [145]. Generally, inverse aqua regia is used to digest the samples

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because most siderophile and chalcophile elements are considered to be mainly hosted by base metal

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sulfides or oxides in mantle rocks [150]. More rigorous digestion parameters are needed for highly siderophile elements, for example, digestion temperatures from 280 oC to 320 oC and digestion

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pressures from 125 bar to 130 bar. However, a lower digestion temperature of 220 oC is often used for S-Se and Te extraction together with a lower digestion pressure of 100 bar. The digestion time

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for element extraction in siderophile and chalcophile is variable and ranges from 2 to 16 hours

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depending on the sample types. Recently, studies have indicated that some elements (e.g., Re, Pt, Pd, Ru, Se, and Te) could be hosted inside the silicate minerals or glassy matrices of rock samples,

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especially for basaltic rocks [150, 153, 154]. To improve the digestion of the silicate minerals, an additional hot plate HF-desilicification step was suggested [143, 147, 148, 150, 153, 154]. In modern HPA-S, the quartz pressure vessel is replaced by a glassy carbon tube so that HF can be used. Cotta and Enzweiler [10] reported the complete digestion of rock samples containing refractory phases using HPA-S with HF at 260 oC for 3 hours, shortening considerably the digestion time compared with that of conventional high-pressure acid digestion. Wang et al. [155] used highpressure acid digestion and HPA-S digestion with HF-HNO3 to determine S, Cu, Se, Mo, Ag, Cd, In, Te, Ba, Sm, W, Tl, and Bi in geological reference materials and carbonaceous chondrites. Both methods as described in Wang et al. [155] yielded precise results, but HPA-S digestion provided higher digestion efficiency. However, both studies reported encountering the formation of insoluble fluoride precipitate during the HPA-S digestion process. A combination of addition of Mg and 24

Journal Pre-proof evaporation with HClO4 was suggested to eliminate the formation of fluoride precipitates [10]. Moreover, high blanks from glassy carbon vessels exist for some trace elements, for example, Mo [155]. Furthermore, glassy carbon vessels need to be carefully handled, as they are fragile and expensive [155].

5. Preparation of geological samples for LA-ICP-MS LA-ICP-MS is a powerful in situ micro-analytical technique for the determination of major, minor, and trace elements as well as isotope ratios in a wide variety of applications [2, 156-158].

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LA-ICP-MS provides several advantages, including the ability for multi-elemental analysis, simple sample preparation, low blanks, and high-throughput sample [2, 159]. LA-ICP-MS not only permits

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the spatial characterization of heterogeneities in solid materials with a high spatial resolution [159-

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161] but also provides a means of whole-rock geochemical analysis (e.g., rocks, sediments, soils, and ores) [14, 16, 162-164]. LA-ICP-MS is a micro-sampling technique. Production of a stable,

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homogeneous, and mechanically resistant target is a prerequisite for bulk geological sample analysis by LA-ICP-MS. The pros and cons of LA-ICP-MS are beyond the scope of this review but can be

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5.1 Pressed powder pellets

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found elsewhere [2, 156-160].

The pressed powder pellet technique is a common method of sample preparation for both XRF

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analysis and LA-ICP-MS. The pressed powder pellet technique has several advantages including its simple and rapid operation, flexible addition of internal standard elements or isotopic spikes, and lack of volatile element loss. In recent years, an important development of pressed powder pellet is the concept of ultrafine powder. Summarizing the conclusion from previous studies, particle size of the sample is the single most influential factor in controlling the subsequent powder tableting, laser ablation behavior, sampling representativeness, and homogeneity of element or isotopic distribution. Mukherjee et al. [14] concluded that small particle size, that is, <10 μm, is a prerequisite for homogeneity and hence acceptable data quality for micro-scale analysis. Moreover, granitic rocks or samples containing refractory minerals such as zircon require extra grinding to achieve grain sizes below 5 μm for better precision. Mechanical grinding is still the most common approach to reduce particle size, although there is a risk of contamination of the sample [162]. Dry milling can 25

Journal Pre-proof reduce particle sizes to <1 μm (20 min) but may result in loss of volatile elements (e.g., Pb and Sn) due to overheating of the container during grinding [165]. Wet milling has been used to avoid the heating problem. Elaborated operating protocols of wet milling have been reported, including ballto-powder ratio, water-to-powder ratio, milling time, and others. Research results indicated that for most rock samples, 30–45 min grinding with the planetary ball mill or the vibratory mill is sufficient to reduce the particle size to below 10 μm [14, 162-164]. The mineral combination in the sample significantly affects the particle grinding efficiency. It is more difficult to grind hard crystals (e.g., zircons) in a soft matrix (e.g., feldspars or clay minerals) than grinding mineral assemblages with a

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similar hardness [162]. After wet grinding of granitoids, it was found that a small amount of large particles remained [162, 164].

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The grounded powder is typically pressed to tablets with or without binder. The addition of binder

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is considered to improve mechanical resistance and cohesion of pellets. The cohesion of the test sample played an important role in precision because the more compact the pellet, the more

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reproducible was the laser–solid interaction [165]. Different substances have been investigated as binders including polyvinyl alcohol (PVA) [166, 167], boric acid [168, 169], Ag powder [170], and

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vanillic acid [171]. However, sample dilution and contamination are always concerning. Peters and

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Pettke [172] compared the effects of six binders in the preparation of pressed powder tablets. Pellets bound with graphite and PVA are very brittle. Poor repeatability was observed when collagen

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hydrolysate was used. Microcrystalline cellulose (MCC)- and vanillic acid-bound pellets had the smoothest ablation patterns, but MCC was purer than vanillic acid and thus was chosen as the optimal binder. The addition of 20% MCC did not significantly dilute the sample and contaminate the element. Hennekam et al. [173] and Shaheen et al. [174] applied high-purity epoxy resin to soak sediment powders or basaltic rock powders for the preparation of rigid disks for LA-ICP-MS analysis. To completely infiltrate the resin into the powders, a vacuum impregnation device was applied to remove the interstitial air bubbles. This method provided low-contamination disk with appropriate hardness for LA-ICP-MS analysis. Rock samples and external standards can be prepared simultaneously with the same process. However, the introduction of resin leads to isobaric interferences with polyatomic ions of carbon, for example, 12

C18O1H on 31P, and 23Na12C40Ar on 75As [174]. 26

40

Ar12C on

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Cr,

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C12C on

24

Mg,

Journal Pre-proof Because ultrafine powder offers good cohesive force, the nonbinder pelleting method has become popular in recent years. The elimination of binders simplifies sample preparation and avoids the potential risk of elemental contamination. In some reports, ultrafine powder (<10 μm) pellets were reported to exhibit a very stable ablation signal that is comparable to that of glasses [16, 162]. However, Mukherjee et al. [14] reported that the real-time signal fluctuation of a pressed pellet of basalt powder standard (BHVO-2) was more severe than that of NIST SRM 612 glass. Wu et al. [164] also observed less stable transient signal in pellets during laser ablation compared with fusion glasses. The less stable signal of pellets can be attributed to the heterogeneous distribution of the

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polyphase minerals.

In summary, the pressed pellet prepared from ultrafine powder is applicable for the analyses of

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30–45 elements [162, 164], independent of whether a binder is used, in various geological samples

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(basic rocks, intermediate-acidic rocks, granitoids, and sediments). The precision or reproducibility (expressed in % relative standard deviation, RSD) of repeated measurements is better than 5% for

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most major elements and 10% for most trace elements with concentrations higher than 1 μg g -1. The analytical results of most elements were in general agreement with the recommended values, with

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discrepancies typically less than 10%. Moreover, some volatile elements (Hg) or trace elements not

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commonly measured (flux elements like Li, B; chalcophile elements like As, Sb, Tl, In, and Bi) also could be quantitatively analyzed [163, 172].

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However, the pressed powder pellet method has several shortcomings. First, the process of preparing a fine powder suffers the risk of contamination, for example, from grinding reagents (water or ethyl alcohol), abrasion of milling equipment (the milling ball and the milling vial), pellet tableting procedure, or binder, if used [164]. Elements reported to have susceptibility to contamination include B, Si, Cu, Zn, Mo, W, and Pb [162, 164, 172]. Second, there is a nugget effect from zircon and sulfides, even after the rock powder particle size is grounded to <10 μm or even to the nanometer level. Garbe-Schӧnberg and Müller et al. [162] observed unstable Zr and Hf signals in the pellet of granodiorite DR-N, which has a particle size of d50<1.5 μm. The precisions of Zr and Hf (14% and 11%) were significantly degraded than those of other elements (<2–5% RSD), indicating the heterogeneous distribution of zircon in the pellets. Mukherjee et al. [14] found that results of ultrafine powder pellets in granitic samples exhibited the poorest precision for Zr, Hf, 27

Journal Pre-proof and some HFSEs, which are generally associated with accessory zircon. Wu et al. [164] also reported large RSDs for Zr, Hf, Th, and U in powder pellets from granitoid rocks. The nugget effect of sulfides, which cause large discrepancies in some of the chalcophile elements, was shown by Peters and Pettke [172]. Therefore, further studies are needed to improve the pressed powder pellet method. Here, we once again propose the NH4HF2 digestion as a good sample preparation method for the pressed powder pellet method. Zhang et al. [16] found that the digestion products after NH4HF2 treatment were in the form of ultrafine powder (d80< 8.5 μm), allowing us to synthesize pressed

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powder pellets without any binder (Fig. 3). In contrast to mechanical grinding, NH4HF2 digestion is a new strategy for pulverizing and homogenizing geological samples by chemical means. Because

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of the complete destruction of physical structures of silicate minerals by NH 4HF2 digestion, the

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nugget effect can be eliminated naturally. The low process blank also can be achieved due to the

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purified NH4HF2 reagent.

5.2 Fusion glasses

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Fusion glasses can be produced by alkali fusion or flux-free fusion. The fusion product can be

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directly analyzed by XRF for major elements and some trace elements and can also be measured using solution ICP-MS after dissolution in dilute nitric or hydrochloric acids or with LA-ICP-MS.

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Alkali fusion is a conventional and efficient method to decompose geologic samples that contain refractory minerals. Recently, Monsels et al. [175] reported using lithium borate to fuse bauxites for the determination of trace elements by LA-ICP-MS (sample:flux = 1:4). The results showed that thirty trace elements were typically measured to have within 20% of their reference values with an “external” precision of <20% (RSD), which confirmed the applicability of LA-ICP-MS after lithium borate fusion for bauxite analysis. However, the introduction of the flux agents dilutes the trace element contents in samples, leading to increased limits of detection, and causes inevitable contamination (e.g., Li and B) to the samples and the ICP-MS instrument [156]. Park et al. [176] attempted to control the amount of flux used in the production of fusion glasses for LA-ICP-MS analysis. They compared normal (sample:flux = 1:5) and low-dilution (sample:flux = 1:2) fusion methods for three USGS reference materials (G-3 28

Journal Pre-proof granite, AGV-2 and BHVO-2). The fused glasses from the low-dilution fusion method yielded reproducible and accurate results for 24 trace elements, including Zr and Hf. Zhang et al. [177] also used the low-dilution fusion method to melt peridotites for trace element analysis using LA-ICPMS. In fact, it is difficult to directly fuse peridotites owing to their high liquidus temperatures (1725±30 °C for garnet-lherzolite and 1850 °C for dunite). Furthermore, peridotite melts cannot be quenched to homogeneous glass owing to high MgO concentrations, which lead to nucleation of olivine and other minerals. The authors chose synthetic albite (NaAlSi3O8) as the flux agent (melting point at 1450 oC) to reduce the melting point of peridotite. The homogeneous peridotite glasses were

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produced with albite heating at 1500 to 1550 °C for 10 to 15 min with a sample-to-flux ratio of 1:2 (low-dilution fusion). The albite flux did not introduce significant contamination except for Ga, Rb,

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Ba and La.

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The flux-free fusion method is another strategy for preparing fusion glass. The principle is simple: the sample is directly fused at 1300–1800 oC and then rapidly quenched to form a homogeneous

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glass. The development of this technology in recent years is focused mainly on the choice of heat sources. Early studies used a self-made heater with metal strips (W, Mo, and Ir) to rapidly melt the

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samples [178, 179]. A high-temperature furnace with a temperature control system is also frequently

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used as the heat source [164, 180]. Recently, Zhang et al. [15] reported a new approach using a high-energy infrared laser to melt the pressed pellet samples under optimized laser conditions (16

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ms pulse width and 5 pulse counts). In this study, fusion and instant cooling take only tens of milliseconds for each sample, bypassing the use of any chemical reagent or melting container (Fig. 4). Complete fusion of rock samples was achieved, including intermediate-acidic intrusive rocks. In summary, current technologies can simultaneously analyze more than forty major and trace elements in the flux-free fusion glasses by LA-ICP-MS for different geological samples [13, 15, 180]. The measured major elements matched the reference values to within 5–10%, and the measured trace element contents were generally consistent with the reference values within 5–15%. The routine precision of fusion glasses was generally better than 5–10% (RSD). How to suppress the loss of highly volatile elements (e.g., Zn and Pb) and completely melt the refractory minerals in coarse-grained rocks (e.g., granitoid) in the flux-free fusion methods are questions of interest. The loss of highly volatile elements (Zn, Pb, Sn, and Ge) during fusion at a 29

Journal Pre-proof high temperature was reported in cases where a metal strip heater or high-energy infrared laser was used as the heat source. Zhu et al. [13] suggested the use of a crucible of hexagonal boron nitride (h-BN) as a capsule for the preparation of glass samples. Element volatilization during hightemperature heating was effectively suppressed with the use of the h-BN crucible. Bao et al. [137] also used the BN vessel and a high-temperature furnace to prepare the fusion glasses for trace elements and Pb isotope ratio analysis. In addition, He et al. [180] employed a home-made Mo capsules with a graphite tube to load rock powders. The results showed that volatile elements (e.g., Zn, Ga, Cs and Pb) had a homogeneous distribution (<10–20%) and could be quantitated,

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confirming the effectiveness of the sealing assembly to suppress the loss of volatile elements. Wu et al. [164] selected a platinum crucible with a lid to eliminate the depletion of volatile elements.

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However, significant losses of Pb and Sn were observed when the melting time was >2 hours. In

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addition, it is difficult to completely melt the refractory minerals with the flux-free fusion method, especially for granites. He et al. [180] found the residual zircons in GPS-2 glasses after fusion at

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1600 oC for 35 min, resulting in heterogeneous distributions and the high RSD values (10–20 %) for Zr and Hf. To solve this problem, Wu et al. [164] designed a complex pretreatment scheme for

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the melting of granite. They first melted the rock powders in a high-temperature furnace at 1600 oC

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for 1 hour. Next, the silicate melt was quenched to glass and then ground in an automatic agate mortar for 15 min. The ground glass was melted again at 1600 oC for 1 hour. The grinding process

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significantly improved the homogeneity of the glass. The RSD values of Zr and Hf in five granite reference materials were better than 10%. However, one granite, JG-2, still had a relatively large RSD values for Zr and Hf even after a total of 4 hours of melting at 1600 °C and two grinding steps. Complete fusion of zircon was reported by Zhang et al. [15] with the use of a high-energy infrared laser to melt rock powders. The interactions between laser and rock powders produce a sufficiently high temperature to melt the refractory minerals. The recoveries of Zr and Hf in GPS-2 are high, indicating the complete fusion of zircons. However, the relatively large RSD values of Zr and Hf (~18%) confirmed slight heterogeneous distribution of these elements in the glass. However, high losses (20–70%) of highly volatile elements (Zn and Pb) are observed in the infrared laser fusion [15] (Fig. 4). The infrared laser fusion technique has high development potential because of its high efficiency and safe operation process, but the work to suppress the loss of volatile elements requires 30

Journal Pre-proof further research. In addition to the above-mentioned problems of volatile element losses and refractory mineral residues, some other problems have been observed in the current reported fusion glasses techniques. For example, partial loss of transition metals (V, Cr, Co, Ni, and Cu) was reported when a BN crucible was used as the container [13, 137]. Researchers thought that BN might reduce transition metal ions to their metallic states owing to its strong reducing power at high temperature. The metal segregation behavior could cause heterogeneity of some transition metals in the fused glasses. Contamination during the fusion process was also reported. The reactions between the melt and the

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fusion container could lead to contamination of some metal elements (Cr, Co, Cu, and Ni), especially for prolonged fusion process [164, 180]. For the alkali fusion methods, impure flux

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materials could be the main contamination source for some elements such as Cr, V, Cd, and La, in

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the lithium borate flux [175, 177].

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Conclusion

Sample decomposition is a fundamental and critical stage in the process of geochemical sample

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analysis. Although the speed of advancements in conventional sample digestion methods lags

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behind the development of modern instrumentation and analytical methods, sample preparation methods are still the main tools for the elements and isotopic ratio analysis in geological samples.

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The reagents NH4F and NH4HF2 as a substitute of hydrofluoric acid in acid digestion are important advancements in recent years. The NH4F/NH4HF2 open-vessel acid digestion methods have been established and resolved the fundamental problem of incomplete dissolution of refractory minerals in conventional open-vessel acid digestion. Other advantages such as easy purification, suppression of insoluble fluorides, simple and flexible operation, low equipment and labor cost, and relatively safe property make them suitable for geological sample preparation. LA-ICP-MS is a fascinating technology for whole-rock elements and isotope ratio analysis in geological samples. Sample preparation methods for LA-ICP-MS have received significant attention, such as ultrafine powder pellets and fusion glasses. Nevertheless, the accurate bulk determination of trace elements in some rock types (such as granite) was prevented due to the heterogeneous element distribution in the analyzed samples. Therefore, more works are needed to improve the sample preparation method to 31

Journal Pre-proof meet the analytical requirements of natural geological samples. We believe that the development and advancement of sample preparation methods are mainly dependent on three important aspects. The first is the innovation of modern instruments, which will change the design thought and specific process of sample preparation methods. For example, collision cell or high mass resolution technology in mass spectrometers can effectively remove polyatomic isobaric interference. This ability allows HCl, HClO4, H2SO4, or HPO3, which were previously used with caution, to be considered again as sample digestion reagents or solution media. If the problem of salt tolerance and matrix effect in ICP-MS is solved in the future, the sample

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solution does not need to be highly diluted (>1000 times). The amount of sample thus can be significantly reduced; of course, this must be accompanied by further reduction in the particle size

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of the sample powder to improve the representativeness. A smaller amount of sample and ultrafine

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powders will reduce the consumption of digestion reagents and shorten the digestion step and time, which is beneficial to the implementation of automated digestion process. The second aspect is new

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element or isotope analysis requirement. When new rock types or elements are found to offer new information or important research meaning in earth science, new sample preparation methods will

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also be rapidly developed to satisfy the needs of these specific sample or elemental analyses. In this

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case, the research work often does not focus on efficiency but the reliability of sample digestion to meet the requirements of high-precision and high-accuracy analysis. The third driving force for the

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development of sample preparation methods is the actual production requirements. A sample preparation method developed in a scientific research laboratory is not entirely suitable for highvolume production work, especially in the field of basic geological exploration and mineral resource exploration due to the large number of samples to be tested. The sample pretreatment method then is more concerned with the optimization of the digestion process. The main evaluation indicators are used to improve production efficiency, reduce labor intensity, reduce experimental cost, and eliminate environmental pollution as much as possible. In summary, the development of sample preparation methods should closely follow the progress of analytical instruments, changes in scientific research content, and the actual production needs. In addition, attention to new digestion reagents, new material-made digestion vessels, and new experimental equipment can facilitate advances in sample preparation technology. 32

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Acknowledgements We are very grateful to Dr. George Chan and the two anonymous reviewers for editorial work and constructive comments, which considerably improved this manuscript. This research is supported by the Chinese State Key Research and Development Program (2016YFE0203000), the National Nature Science Foundation of China (Grant No. 41603002, 41725013, and 41573015), the Science Fund for Distinguished Young Scholars of Hubei Province (2016CFA047), and the most special fund from the State Key Laboratories of Geological Processes and Mineral Resources, China

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University of Geosciences (MSFGPMR04 and MSFGPMR08).

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Journal Pre-proof Table captions Table 1 General physical properties of NH4F and NH4HF2.

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Table 2 Selection of typical sample treatment procedures for NH4F and NH4HF2 digestion methods.

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Journal Pre-proof Figure captions Fig. 1 Statistics of research based on NH4F and NH4HF2 collected from “Web of Science” and breakdown according to their application areas. Fig. 2 Flowchart showing details of NH4F/NH4HF2 open vessel acid digestion methods presented in Hu et al. [12] and Zhang et al. [11]. Fig. 3 SEM image of the NH4HF2 digestion products from BCR-2 (a); SEM image of surface and ablation crater on the pressed pellet of BCR-2 (b); laser ablation-generated transient signals from the pressed powder pellet of BCR-2 (c). These figures are cited from Zhang et al. [16].

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Fig. 4 Infrared laser fusion processes (a and b) and fused glasses of basalt reference materials (c and d).

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Journal Pre-proof Table 1. General physical properties of NH4F and NH4 HF2

Molar Mass Formula Ammonium fluoride

NH4F

Ammonium bifluoride

NH4HF2

Density

Phys. Form white crystal, hydroscopicity white crystal, hydroscopicity

Melting point

Boiling point

CAS

(C)

Reg. No.

3

(g/mol)

(g/cm )

(C)

37.037

1.015

238



12125-01-8

125

240 (decomposes)

1341-49-7

57.044

1.5

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Data taken from "CRC Handbook of Chemistry and Physics. 97st ed"

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Journal Pre-proof Table 2. Selection of typical sample treatment procedures for NH4F and NH4HF2 digestion Materials

Determined elements or isotope ratios

Reagent

Sample treatment Digestion Digestion time temperature

Geological materials Lichen, basalt, soil Basalt, andesite, granodiorite, granite

Sc, Cr, Co, Ni, Zn, As,Rb, Sr, Mp, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Gd, Tb, Yb, Hf, Ta, Pb, Th, U Li, Sc, V, Co, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, REEs, Ta, W, Tl, Pb, Th, U

HNO3+NH4F+H2O2

240oC in a sand bath

10 hours

NH4F+HNO3

190oC in an electric oven

24 hours

Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, REEs, Ta, Tl, Pb, Th, U

NH4HF2

230oC in an electric oven

3 hours

Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, REEs, Ta, W, Tl, Pb, Th, U

NH4F

250oC in an electric oven

2 hours

Solid

Si and B

NH4F

180°C in a microwave system

15 min

Coal

Be, V, Cr, Mn, Co, Ni, Cu, Zn, As, Rb, Sr, Mo, Cd, Sb, Cs, Ba, W, Tl, Pb, Th, U and REEs by ICP-MS Na, K, Ca, Mg, Fe, Al, Sc, Ti by ICP-OES

HNO3+NH4HF2+H2O2

From 150oC to 200oC in a microwave digestion system at 600 W

5-10 min

Bauxite

Li, Be, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Mo, Cs, Ba, REEs, Ta, Tl, Pb, Th, U

240oC in an electric oven

5 hours

Basalt, andesite, granodiorite, granite, microgabbro, rhyolite, nepheline syenite, diabase, dolerite, shale

Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Mo,Cd, Sn, Sb, Cs, Ba, REEs, W, Pb, Bi, Th, U

oo

Sr, Ba, Y, La, Ce, Nd, Ti, V, Mn, Fe, Cu, Zn, Mo, Zr, Ag, Hf, Pb, Th, U

NH4HF2 or NH4F+HClO4

o

NH4HF2

230 C in an electric oven

1-6 hours

NH4HF2+H2O+HNO3 for lowsilica rocks NH4HF2+H2O for felsic rocks

Approximately 200oC in a microwave digestion system at 1500 W

30 min

NH4HF2

230oC on a hot block

3 hours

U

1% NH4HF2 solution

80oC in an oven

1 hour

Dolerite, mica schist, andesite, quartz latite, basalt, obsidian, high alumina sand

Al, Si, Ti, Fe, Co, Cu, Zn, Rb, Sr, Zr, Nb, Sn, Cs, Ba, La, Ce, Nd, Sm, U

NH4HF2+HNO3

55oC from the indirect sonication horn

5, 15, 30, 60, and 120 min

Dolerite, mica schist, andesite, quartz latite, basalt, obsidian, high alumina sand

Al, Si, Fe, Co, Ni, Cu, Rb, Sr, Zr, Nb, Mo, Sn, Sb, Cs, Ba, La, Ce, Nd, Sm, U

NH4HF2

230oC on a hot block

10, 30, or 180 min

Al, Si, K, Ca, Fe, Mo, Cs, Ba, Sr, Nd, Ta, U, 235U/238U

NH4HF2

230oC on a hot plate

3 hours

Pu

NH4HF2

250oC on a hot plate

15 min

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Soil, U ore

f

Basalt, andesite, granodiorite, granite, rhyolite, sedimentary Basalt, andesite, granodiorite, granite, rhyolite, sedimentary, soil

Radioanalytical chemistry

Pr

Al, Fe, Co, Ni, Cu, Rb, Sr, Zr, Nb, Cs, Ba, La, Nd, Sm, Eu, Lu, Bi, Th, U, Pu

Trinitite

methods

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Soil and sediment

rn

Surrogate post-detonation urban debris

al

Solid uranium oxides from swipe media

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Journal Pre-proof Highlights

• An overview of the recent advances in sample digestion methods for geological samples is presented.

• Recently developed sample preparation methods for bulk silicate rock analysis by LA-ICP-MS are discussed.

• An in-depth review of the application of the new digestion reagents NH4F and

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NH4HF2 in different fields is provided.

49

Figure 1

Figure 2

Figure 3

Figure 4