Determination of heavy metals in soil, mushroom and plant samples by atomic absorption spectrometry

Determination of heavy metals in soil, mushroom and plant samples by atomic absorption spectrometry

Microchemical Journal 74 (2003) 289–297 Determination of heavy metals in soil, mushroom and plant samples by atomic absorption spectrometry ¨ Mustafa...

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Microchemical Journal 74 (2003) 289–297

Determination of heavy metals in soil, mushroom and plant samples by atomic absorption spectrometry ¨ Mustafa Tuzen* ¸ University, 60250 Tokat, Turkey Faculty of Science and Arts, Chemistry Department, Gaziosmanpasa Received 5 December 2002; received in revised form 3 February 2003; accepted 8 February 2003

Abstract The concentrations of heavy metals in the soil, mushroom and plant samples collected from Tokat, Turkey have been determined by flame and graphite furnace atomic absorption spectrometry after dry ashing, wet ashing and microwave digestion. The study of sample preparation procedures showed that the microwave digestion method was the best. Good accuracy was assured by the analysis of standard reference materials. The relative standard deviations for all measured metal concentrations were lower than 10%. In all cases, quantitative analytical recoveries ranging from 95 to 103% were obtained. Metal accumulation factors were calculated for mushroom and plant samples. High ratio of plants to soil cadmium, zinc and copper concentrations indicate that these elements are accumulated by mushrooms. Results obtained are in agreement with data reported in the literature. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Atomic absorption spectrometry; Digestion; Heavy metals; Soil; Mushroom; Plant

1. Introduction Heavy metals are considered to be one of the main sources of pollution in the environment, since they have a significant effect on its ecological quality w1x. Human activity leads to increasing levels of heavy metal contamination in the environment. Heavy metals owing to atmospheric and industrial pollution accumulate in the soil and influence the ecosystem nearby w2x. The determination of heavy metal in soil samples is very important in monitoring environmental pollution w3x. Lead, cadmium, iron, copper, manganese, zinc, etc., were chosen as representative trace metals *Tel.: q90-356-2521582; fax: q90-356-2521585. ¨ E-mail address: [email protected] (M. Tuzen).

whose levels in the environment represent a reliable index of environmental pollution. Metals like iron, copper, zinc and manganese are essential metals since they play an important role in biological systems, whereas lead and cadmium are nonessential metals as they are toxic even in traces w4x. The essential metals can also produce toxic effects when the metal intake is excessively elevated. Recently, both international and Turkish studies have drawn attention to the metal pollution of soil w5,6x and plant samples w7–11x. But, such a study has not been yet carried out in Tokat, Turkey. Tokat is a developing industrial–agricultural city in the central Anatolia-Turkey and has a population of 114 000. It is famous for industrial plants (textile, nutrition, sugar, etc.).

0026-265X/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0026-265X(03)00035-3

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¨ M. Tuzen / Microchemical Journal 74 (2003) 289–297

Mushroom has been used as a bioindicator by various researchers to determine the heavy metal pollutions w12–15x. Compared to green plants, mushroom can build up large concentrations of some heavy metals such as Pb, Cd, Hg, and a great effort has been made to evaluate the possible danger to human health from the ingestion of mushrooms w16x. Determination of total heavy metal concentration in soils requires matrix destruction. The reliability of heavy metal determination in its complex matrices mainly depends on the dissolution process used. Both the wet and dry ashing procedures are slow and time consuming. In recent years, closed vessel microwave digestions have been developed as a rapid and reproducible sample preparation method for a great variety of complex matrices w17–21x. Decomposition of solid samples is an important step in combined analytical methods. In most cases, when using highly sensitive measuring methods, such as flame atomic absorption spectrometry (FAAS), graphite furnace AAS, ICPAES, ICP-MS, the sample is measured in an aqueous solution w22x. Combined analytical methods are favoured for multi element analysis of environmental and biological samples at very high speed. Sequential and simultaneous determinations of the elements can be made using the above analytical techniques w23x. In this study, the levels of heavy metals in soil, mushroom and plant samples collected from Tokat, Turkey were determined by flame and graphite furnace AAS after various digestion methods.

200-mesh sieve and transferred to polyethylene bottles until analysis. Mushroom and plant samples were washed with distilled water and dried at 105 8C for 24 h. The dried samples were ground, then homogenized using an agate pestle and stored in polyethylene bottles until analysis. 2.2. Reagents All reagents were of analytical reagent grade unless otherwise stated. Double deionized water (Milli-Q Millipore 18.2 MVycm resistivity) was used for all dilutions. HNO3, H2SO4, H2O2, HF, HClO4 and HCl were of suprapur quality (E. Merck). All the plastic and glassware were cleaned by soaking in dilute HNO3 (1q9) and were rinsed with distilled water prior to use. The element standard solutions used for calibration were prepared by diluting a stock solution of 1000 mgyl (Pb, Cd, Co, Cr, Mn, Ni) supplied by Sigma and (Cu, Zn, Fe) by Aldrich. 2.3. Apparatus A Perkin Elmer AAnalyst 700 atomic absorption spectrometer with deuterium background corrector was used in this study. Pb, Cd, Co, Cr, Mn and Ni in plant samples were determined by HGA graphite furnace using argon as inert gas. Other measurements were carried out in an airyacetylene flame. The instrumental parameters and operating conditions are given in Table 1. 2.4. Digestion procedures

2.1. Sampling

Three types of digestion procedures were applied. Optimum digestion conditions were given below.

Soil (control) samples were taken at random at the location of mushroom and plant sampling from uncontaminated agricultural lands. The other soil samples were taken at measurement points located near high-density traffic roads and near the textile plants in Tokat, Turkey. The sampling area is shown in Fig. 1. Soil samples were taken in a depth of approximately 0–15 cm. The samples were dried at 110 8C and ground to pass through

2.4.1. Dry ashing One gram of sample (mushroom and plant) was placed into a high form porcelain crucible. The furnace temperature was slowly increased from room temperature to 450 8C in 1 h. The samples were ashed for approximately 4 h until a white or grey ash residue was obtained. The residue was dissolved in 5 ml of HNO3 (25% vyv) and the mixture, when necessary, was heated slowly to

2. Material and methods

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291

Fig. 1. The map showing the sampling area of mushroom, plant and soil samples.

dissolve the residue. The solution was transferred to 25-ml volumetric flask and made up to volume. The sample solutions were not clear. A blank digest was carried out in the same way. 2.4.2. Wet ashing Digestion of mushroom samples was performed using an oxi-acidic mixture of HNO3:H2SO4:H2O2 (4:1:1) (12 ml for a 1-g sample). This mixture was heated up to 150 8C for 4 h and brought to a volume of 25 ml with deionized water. A blank digest was carried out in the same way. For the digestion of plants, 1.00 g of sample was placed into an erlenmeyer and 3 ml of concentrated HNO3 and 1 ml of concentrated HCl (3q1, (vyv)) were added. This mixture was heated for 3 h at 85 8C until the solubilization of the sample was complete and then diluted to 25-ml volume with deionized water. A blank digest was carried out in the same way.

For the digestion of soil, 0.5 g of sample was placed into a beaker and digested for 6 h at 90 8C with concentrated HCl:HNO3 (3:1) mixture (8 ml) and concentrated HClO4 (3 ml). The residue was filtered and diluted to 25 ml with deionized water. The sample solutions were not clear. A blank digest was carried out in the same way. 2.4.3. Microwave digestion Milestone Ethos D closed vessel microwave system (maximum pressure 1450 psi, maximum temperature 300 8C) was used in this study. Soil sample (0.25 g) was placed in a Teflon vessel (100 ml capacity) and digested with 6 ml of HCl (30%), 2 ml of HNO3 (65%) and 2 ml of HF (40%) in microwave digestion system for 26 min and diluted to 25 ml with deionized water. A blank digest was carried out in the same way. For the mushroom and plants, 0.25 g of sample was digested with 6 ml of HNO3 (65%) and 1 ml of H2O2 (30%) in microwave digestion system for

292

Table 1 Instrumental analytical conditions of element analyses FAAS Acetylene (lymin)

Air (lymin)

Wavelength (nm)

Slit width (nm)

Lamp current (mA)

Fe Cu Mn Zn Ni Cr Pb Co Cd

2.0 2.0 2.0 2.0 2.0 2.5 2.0 2.0 2.0

17.0 17.0 17.0 17.0 17.0 17.0 17.0 17.0 17.0

248.3 324.8 279.5 213.9 232.0 357.9 283.3 240.7 228.8

0.2 0.7 0.2 0.7 0.2 0.7 0.7 0.2 0.7

30 15 20 15 25 25 30 30 4

Instrumental conditions

Pb

Cd

Ni

Cr

Mn

Co

Argon flow (mlymin) Sample volume (ml) Modifier (ml)

250 20 5

250 20 10

250 20 5

250 20 5

250 20 5

250 20 5

Heating program temperature, 8C (ramp time (s), hold time (s)) Drying 1 Drying 2 Ashing Atomization Cleaning

100 (5, 20) 140 (15, 15) 700 (10, 20) 1800 (0, 5) 2600 (1, 3)

100 (5, 20) 140 (15, 15) 850 (10, 20) 1650 (0, 5) 2600 (1, 3)

100 (5, 20) 140 (15, 15) 1300 (10, 20) 2500 (0, 5) 2600 (1, 3)

100 (5, 20) 140 (15, 15) 1600 (10, 20) 2500 (0, 5) 2600 (1, 3)

100 (5, 20) 140 (15, 15) 1200 (10, 20) 2300 (0, 5) 2600 (1, 3)

100 (5, 20) 140 (15, 15) 1300 (10, 20) 2400 (0, 5) 2600 (1, 3)

GFAAS

¨ M. Tuzen / Microchemical Journal 74 (2003) 289–297

Element

¨ M. Tuzen / Microchemical Journal 74 (2003) 289–297 Table 2 Operating conditions for samples in microwave digestion system Steps

Time (min)

Power (W)

Soil 1 2 3 4 Vent: 8 min

6 6 8 6

250 400 550 250

Mushroom and Plant 1 2 3 4 5 Vent: 8 min

2 2 6 5 8

250 0 250 400 550

23 min and diluted to 25 ml with deionized water. A blank digest was carried out in the same way. All sample solutions were clear. Digestion conditions for microwave system are given in Table 2. In order to validate the method for accuracy and precision, certified reference materials (National Institute for Environmental Science—NIES, No. 3 Chlorella) and the polluted farmland soil standard reference material (SRM, GBW 08303) were analysed for corresponding elements. The results are shown in Tables 3 and 4.

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2.5. Analytical procedure Detection limit is defined as the concentration corresponding to three times the standard deviation of ten blanks. Detection limit values of elements as microgram per liter in flame AAS were found to be 0.02 for Cd, 0.12 for Co, 0.08 for Cr, 0.07 for Cu, 0.11 for Fe, 0.05 for Mn, 0.14 for Ni, 0.45 for Pb and 0.02 for Zn. The concentrations of Cu, Zn and Fe were determined in plant samples using FAAS. The other elements (Cd, Pb, Co, Cr, Mn and Ni) were below detection limit of FAAS. These elements in plant samples were determined using graphite furnace AAS by autosampler. During analyses, internal argon flow rate through the graphite tube was 250 mlymin; gas flow was interrupted during atomization. Sample volume, ramp and hold times for the drying, ashing, atomization and cleaning temperatures were optimized before analysis to obtain maximum absorbance and minimum background. Matrix modifiers were added 200-mg NH4H2PO4 for Pb, 15-mg Pdq10mg Mg(NO3)2 for Cd, 50-mg Mg(NO3)2 for Co, Mn, Ni and Cr. Most of the matrix was removed before the atomization step and less interference occurred during atomization. Each graphite furnace atomic absorption spectroscopic analysis calls for 20 ml of solution and 5–10 ml of the matrix modifier was added if necessary. Characteristic mass for 0.0044 absorbance was found to be 0.5

Table 3 Observed and certified values (mgyg)a of elemental concentrations in SRM (GBW 08303 polluted farmland soil) as average"S.D. Element

Cd Co Cr Cu Fe Mn Ni Pb Zn

Certified value

1.20 13.0 112 120 2.97 519 40 73 260

Observed value Wet digestion

Recovery (%)

Microwave digestion

Recovery (%)

1.14"0.05 12.5"0.10 106"7.3 118"5.7 2.88"0.10 493"22 38"1.9 70"4.2 252"10

95 96 95 98 97 95 95 96 97

1.18"0.03 12.8"0.05 110"4.1 124"3.5 2.99"0.12 515"13 39"1.1 73"2.1 264"12

98 98 98 103 101 99 98 100 102

Each value is the average of five separate digestions. a Fe (%).

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Table 4 Observed and certified values (mgyg)a of elemental concentrations in SRM (NIES, No. 3 Chlorella) as average"S.D. Element

Cu Fe Zn Co Mn Pb Cd

Certified value

3.5"0.3 0.185"0.01 20.5"1.0 0.87"0.05 69"5 0.60"0.04 0.026"0.005

Observed value Dry ashing

Recovery (%)

Wet ashing

Recovery (%)

Microwave digestion

Recovery (%)

3.4"0.7 0.180"0.06 19.7"1.5 0.83"0.09 67"8 0.57"0.08 0.024"0.009

97 97 96 95 97 95 92

3.3"0.6 0.182"0.05 19.9"1.8 0.84"0.08 68"7 0.58"0.07 0.025"0.010

95 98 97 97 99 97 96

3.6"0.5 0.187"0.04 20.3"1.4 0.85"0.09 69"6 0.59"0.06 0.026"0.008

103 101 99 98 100 98 100

Each value is the average of five separate digestions. a Fe (%).

pg for Cd, 6.0 pg for Co, 3.0 pg for Cr, 13 pg for Ni, 2.0 pg for Mn and 10 pg for Pb. 3. Results and discussion It is desirable to use a higher ashing temperature in graphite furnace in order to remove the matrix efficiently for many analytes in food, biological and environmental samples w23x. The ashing and atomization temperatures of heavy metals were increased using different chemical modifiers. The maximum ashing and atomization temperatures obtained were 700 and 1800 8C for Pb, 850 and 1650 8C for Cd, 1300 and 2500 8C for Ni, 1600 and 2500 8C for Cr, 1200 and 2300 8C for Mn and 1300 and 2400 8C for Co. T-test was used in this study. The comparison of dry, wet and microwave digestion methods showed no statistically significant differences in results. Therefore, the microwave digestion procedure was preferred because this procedure is more proper with respect to both time and recovery than dry and wet digestion. The disadvantage of this method is expensive and needed of some experience. The standard deviations of the dry and wet digestion methods are higher than those of the microwave digestion method. The accuracy of the method was evaluated by means of heavy metals determination in SRM. The achieved results were in good agreement with certified values. The results from the analysis of SRM were all within the 95% confidence limit.

Heavy metal levels in the analysed samples are given in Table 5. All metal concentrations were determined on a dry weight basis. Textile productions may be containing high cadmium. So, cadmium concentrations in the soil samples taken from near the textile plants were found to be three times higher than those of roadside soil. In general, heavy metal levels in the soil samples taken from roadside were found to be higher than those of soil samples near the plants. Control samples were taken from remote areas which are not influenced by the traffic and textile plants. Minimum values are consisted of control station. The lead concentration in the samples increased with increasing traffic volume. The number of vehicles on roads is approximately 800 vehiclesyh in Tokat w24x. Generally, the mean levels of heavy metals in soil samples taken from roadside and near the plants in Tokat, Turkey are lower than those reported in Refs. w3,8,11,25x. Mushroom and plant samples were collected from uncontaminated agricultural lands. The highest metal concentrations were found in Amanita soliteria (poisonous) and Lentinus tigrinus (inedible) mushroom species. Heavy metal levels in poisonous and inedible mushrooms were found to be higher than those of edible mushrooms. Metal accumulation factors in mushrooms were calculated as 17.4, 0.16, 0.16, 0.13, 6.8, 0.06, 0.32, 0.09 and 2.8 for Cd, Pb, Co, Cr, Cu, Fe, Mn, Ni and Zn, respectively. The results are shown in Table 6. Metal bio-accumulation factors (smetal concen-

Sample no.

Samplea

Pbb

Cd

Co

Cr

Mn

1 2 3

Soil Control Near roadside Near textile plants

15.3"1.2 45.1"3.6 25.4"1.6

0.25"0.01 1.63"0.12 4.88"0.27

4.20"0.32 12.4"1.0 5.60"0.41

10.9"1.1 31.7"2.6 19.7"0.1

139"12 320"31 258"23

32"3 54"3.7 41"4

7.50"0.27 38.3"1.7 24.2"0.2

38"2 60"5 74"6

7368"135 10344"400 9574"304

4 5 6 7 8

Mushroom Lentinus tigrinus (inedible) Amanita soliteria (poisonous) Morchella esculenta (edible) Lactarius deliceus (edible) Russula delica (edible)

2.78"0.31 4.17"0.35 1.43"0.11 1.96"0.15 2.30"0.12

5.45"0.66 7.50"0.56 1.08"0.10 2.84"0.22 4.27"0.32

0.85"0.10 1.32"0.11 0.28"0.03 0.46"0.02 0.54"0.04

1.53"0.12 2.66"0.18 1.05"0.10 0.87"0.05 1.28"0.08

63.2"5.3 74.3"4.8 25.4"2.3 36.5"2.3 21.7"2.0

5.14"0.42 4.64"2.3 1.18"0.10 1.73"0.14 2.69"0.14

60.8"5.7 96.2"5.1 42.9"2.6 31.4"2.8 24.7"1.5

145"13 188"15 45"4 82"7 78"5

371"25 835"54 146"12 288"21 485"33

9 10 11

Plant Tomato Pepper Corn

32"3.0 40"3.7 57"5.3

12"1.1 18"1.5 29"2.2

19"1.2 24"2.1 21"1.9

37"2.5 48"3.7 89"7.4

76"3.8 93"0.4 63"5.5

60"5.2 43"2.9 89"6.8

8.0"0.6 7.6"0.3 9.6"0.7

3.6"0.2 6.4"0.4 7.7"0.5

4.5"0.2 8.1"0.8 9.7"0.6

a b

ns5. Concentrations of Pb, Cd, Co, Cr, Mn and Ni (mgykg) in plant samples, all others (mgykg).

Ni

Cu

Zn

Fe

¨ M. Tuzen / Microchemical Journal 74 (2003) 289–297

Table 5 Concentration of heavy metals in soil, mushroom and plant samples collected from Tokat, Turkey

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Table 6 Bio-accumulation factors of heavy metals from soil to mushrooms and plants, ns5 Sample

Pb

Cd

Co

Cr

Mn

Ni

Cu

Zn

Fe

Mushroom Lentinus tigrinus Amanita soliteria Morchella esculenta Lactarius deliceus Russula delica Mean

0.18 0.27 0.09 0.13 0.15 0.16

13.8 30.0 4.3 21.8 17.1 17.4

0.20 0.31 0.07 0.11 0.13 0.16

0.14 0.24 0.09 0.08 0.12 0.13

0.45 0.53 0.18 0.26 0.16 0.32

0.16 0.15 0.04 0.05 0.08 0.09

8.1 12.8 5.7 4.2 3.3 6.8

3.8 4.9 1.2 2.2 2.1 2.8

0.05 0.11 0.02 0.04 0.07 0.06

Plant Tomato Pepper Corn Mean

0.002 0.003 0.004 0.003

0.005 0.006 0.005 0.005

0.003 0.004 0.008 0.005

0.0005 0.0007 0.0005 0.0006

0.002 0.001 0.003 0.002

1.1 1.0 1.3 1.1

0.09 0.17 0.19 0.15

0.0006 0.001 0.001 0.0009

0.048 0.072 0.116 0.079

tration in plantymetal concentration in soils) in this study are lower than those reported in Refs. w26,27x. While Cd, Cu and Zn were being accumulated at a high ratio by mushrooms, the other elements were being accumulated at a low ratio. The mean concentrations of metals in mushroom samples were generally similar to those found in other studies w14,16,28x. The fact that toxic metals are present in high concentrations in the fruiting bodies of fungi, from an area greatly favoured by mushroom pickers, is of particular importance in relation to the FAOy WHO w29x Standards for Pb and Cd as toxic metals. The maximum permissible dose for an adult is 3 mg Pb and 0.5 mg Cd per week, but the recommended doses are only one-fifth of those quantities. The amount of heavy metal contents are related to species of mushroom, collecting site of the sample, age of fruiting bodies and mycelium, and distance from the source of pollution w30x. According to Stijve and Besson w31x, the mechanism by which some heavy metals are accumulated is somewhat obscure although it seems to be associated with a chelation reaction with the sulfhydryl groups of protein and especially with methionine. 4. Conclusions The dry and wet digestion methods are more time-consuming and complicated than microwave digestion method without any advantage in terms

of digestion efficiency. The use of microwave digestion system in soil, mushroom and plant samples provides a better, safer and cleaner method of sample preparation. The accuracy of the method was checked and confirmed by SRMs. It can be concluded that the 3:1 of HCl:HNO3 in 8-ml acid mixture is best for digesting soil samples. The use of the HFyHClyHNO3 mixture allows the determination of the total content of the elements analysed in soil samples. HF was not used for soil in wet digestion because it is dangerous in open system. Recovery studies show that the digestion methods used in this study are satisfactory and reproducible for analysis of soil, mushroom and plant samples. While mushrooms accumulate Cd, Zn and Cu at high ratio, plants do not accumulate them. References w1x J. Sastre, A. Sahuquillo, M. Vidal, G. Rauret, Anal. Chim. Acta 462 (2002) 59–72. w2x A.S. Al-Radady, B.E. Davies, M.J. French, Sci. Total Environ. 145 (1994) 143–156. w3x C.Y. Zhou, M.K. Wong, L.L. Koh, Y.C. Wee, Environ. Monit. Assess. 44 (1997) 605–615. w4x H.A. Schroeder, The Trace Elements and Nutrition, Faber and Faber, London, 1973. w5x M. Soylak, O. Turkoglu, J. Trace Microprobe Tech. 17 (1999) 209–217. w6x J.N. Beck, J. Sneddon, Anal. Lett. 33 (10) (2000) 1913–1959. w7x S. Yoshida, Y. Muramatsu, Int. J. Environ. Anal. Chem. 67 (1997) 49–58.

¨ M. Tuzen / Microchemical Journal 74 (2003) 289–297 w8x N. Singh, V. Pandey, J. Mısra, M. Yunus, K.J. Ahmad, Environ. Monit. Assess. 45 (1997) 9–19. w9x M. Yaman, Y. Dilgin, S. ¸ Gucer, ¨ ¸ Anal. Chim. Acta 410 (2000) 119–125. w10x M. Demir, S. ¸ Gucer, ¨ ¸ T. Esen, J. Agric. Food Chem. 38 (1990) 726–728. w11x S. Shallari, C. Schwartz, A. Hasko, J.L. Morel, Sci. Total Environ. 209 (1998) 133–142. w12x M. Tuzen, ¨ ¸ Food Chem. 63 M. Ozdemir, A. Demirbas, (2) (1998) 247–251. w13x M. Tuzen, ¨ ¸ Z. Lebensm. M. Ozdemir, A. Demirbas, Unters. Forsch. A 206 (1998) 417–419. w14x E. Sesli, M. Tuzen, ¨ Food Chem. 65 (1999) 453–460. w15x M.A. Garcia, J. Alonso, M.I. Fernandez, M.J. Melgar, Arch. Environ. Contam. Toxicol. 34 (1998) 330–335. w16x C.H. Gast, E. Jansen, J. Bierling, L. Haanstra, Chemosphere 17 (4) (1988) 789–799. w17x D. McGrath, Talanta 46 (1998) 439–448. w18x M. Tuzen, ¨ Anal. Lett. 35 (10) (2002) 1667–1676. w19x Z. Mester, M. Angelone, C. Brunori, C. Cremisini, H. Muntau, R. Morabito, Anal. Chim. Acta 395 (1999) 157–163.

297

w20x Ju. Inanova, R. Djingova, S. Korhammer, B. Markert, Talanta 54 (2001) 567–574. w21x A.C. Szolnoki, M. Bathori, G. Blunden, Microchem. J. 67 (2000) 39–42. w22x G. Knapp, Microchim. Acta 2 (1991) 445–455. w23x M. Tuzen, ¨ Food Chem. 80 (2003) 119–123. w24x M. Tuzen, ¨ J. Trace Microprobe Tech., (2003). w25x M. Linde, H. Bengtsson, I. Oborn, Water, Air, Soil Pollut. 1 (2001) 83–101. w26x M. Blanusa, A. Kucak, V.M. Varnai, M.M. Saric, J. AOAC Int. 84 (6) (2001) 1964–1971. w27x L. Racz, V. Oldal, Microchem. J. 67 (2000) 115–118. w28x M. Isıloglu, ¸ ˘ F. Yılmaz, M. Merdivan, Food Chem. 73 (2001) 169–175. w29x FAOyWHO, List of Maximum Levels Recommended for Contaminants by the Joint FAOyWHO Codex. Alimentarius Commission, Second Series, CACyFAL, Rome, 3 (1976) 1–8. w30x P. Kalac, J. Burda, I. Staskova, Sci. Total Environ. 105 (1991) 109–119. w31x T. Stijve, R. Besson, Chemosphere 2 (1976) 151–158.