J. Aerosol Sci.. Vol. 9 pp. 261 to 264. Pergamon Press Ltd. 1978. Printed m Grlmt Britain.
PREPARATION OF MONODISPERSE TAGGED POLYSTYRENE AEROSOL USING AN AEROSOL CENTRIFUGE D. P. BHANTI, S~ K. DUA, P. KOTRAPPA and N. S. PIMPALE Health Physics Division, Bhabha Atomic Research Centre, Bombay 400 085, India (First received 5 October 1977 and in revised form 14 December 1977) Abstract--A method of preparing and aerosolizing monodisperse particles of polystyrene tagged with s ICr is discussed. The procedure consisted of four steps: generating polydisperse polystyrene aerosol by nebulizing polystyrene solution in xylene containing SlCr in the form of chromium acetyl acetonate, separating and collecting this polydisperse aerosol in monodisperse size groups on a polythene collection foil of Lovelace Aerosol Particle Separator (LAPS), resuspending the monodisperse particles in water and the subsequent aerosolization. Because of the presence of xylene vapour in the aerosol stream, a new sampling procedure was needed for the second step. The stream air needed for the LAPS was taken from the aerosol chamber itself through a series of filters in order to match the vapour concentrations between the stream air and aerosol air, an essential step to achieve separation in the LAPS. It was possible to obtain tagged aerosols in the size range of 0.7 ~m to 5/~m with a standard deviation of less than 10 per cent.
INTRODUCTION Radioactively tagged aerosols are needed both in aerosol research and inhalation studies. A four step method for the preparation of monodisperse aerosols for inhalation experiments had been described earlier, Kotrappa and Moss (1971), Kotrappa et al. (1972b). The four steps adopted in the technique are : (1) preparation of polydisperse aerosol of the material by nebulizing its solution, (2) separation and collection of the polydisperse aerosol in monodisperse aerodynamic size groups using an aerosol centrifuge, (3) resuspension of monodisperse particles of various size groups in a suitable liquid medium and (4) the aerosolization of the suspension to obtain monodisperse aerosol. The technique was applied for the preparation of monodisperse polystyrene aerosol tagged with StCr.
P R O D U C T I O N OF P O L Y D I S P E R S E AEROSOL The solution of polystyrene in xylene containing desired quantity of 5tCr was made as follows (Rundo et al. (1968), Sharma et al. (1975)): s 1Cr isotope in the form of CrC1 a was mixed with inactive CrCl 3 to make up a volume of 25 ml aqueous solution containing 250/zg of Cr. This solution along with 3 ml of N H , O H was transferred to 500 ml reflux flask with a reflux condenser. About 15 ml of freshly distilled acetyl acetone was added along with a few ceramic beads. Refluxing was done for 2-3 hr. After cooling the solution was transferred to a separating funnel containing 20 ml of chloroform. The chromium acetyl acetonate extracted into organic phase was separated and washed with 6 NH2SO , to remove the impurities and finally washed with distilled water. The solution was dried under an infra-red lamp and dissolved into 10 ml of xylene. Care should be taken not to dry the complex to complete dryness, otherwise it will not dissolve in xylene easity. This was the stock solution of organic complexehromium acetyl acetonate soluble in xylene. A suitable amount of stock solution was added to 2% solution of polystyrene in xylene to obtain the desired concentration of 5tCr activity. This solution was taken in all metal nebuliser (Kotrappa et al. (1976)) and nebulised using compressed air at 20 p.s.i. The nebuliser output was mixed with diluting air in the proportion of 1:2 and led into an aerosol chamber. This chamber provides dry polystyrene aerosol needed for subsequent separation. 261
D . P . BHANTI, S. K. DUA, P. KOTRAPPA and N. S. PIMPALE
TEST S E P A R A T I O N
BY A E R O S O L C E N T R I F U G E
The aerosol centrifuge used in the present study was the Lovelace Aerosol Particle Separator (LAPS), described elsewhere (Kotrappa and Light (1972a)). It was operated at 3000 r.p.m, with 5.2 l./min of total flow and aerosol sampling rate of 0.4 l./min. The sample was taken into LAPS from the aerosol chamber.The deposit on the collection foil, when examined by a collimated beam of white light, did not show the Tyndall spectra indicating no separation of particles. Further, most of the deposit was confined to the lower half of the collection foil. Deposit up to a distance of 12 cm showed some diffused colours (Tyndall spectra) indicating some separation. Kops et al. (1974) have also reported a similar behaviour of polystyrene aerosol in their aerosol centrifuge. They recommended that the concentration of polystyrene aerosol should be extremely low to obtain any meaningful separation. Such a procedure cannot be adopted in our case because of our interest in preparing significant quantities of monodisperse polystyrene particles in the various size groups. A detailed study done in our laboratory (Bhanti et al. (1977)) indicated that the LAPS will not separate the particles if the vapour concentrations between aerosol air and stream air do not match. It was also noted that the performance of LAPS did not improve in unmatched condition in spite of neutralising the aerosol by electrostatic charge neutraliser, indicating that the phenomenon is not due to electrostatic charge on the aerosols. In the experiment described above the aerosol air contained xylene vapour whereas the stream air had none. Aerosol air containing xylene is heavier than the stream air. This difference in density can cause convective mixing in the centrifugal field. The simplest way to attain the matched condition was to use the air originating from the aerosol chamber itself as stream air after passing it through a series of filters and a pressure vacuum pump. Since the filters might absorb some vapour it was necessary to run the air through them for some time before sampling in the LAPS. Figure 1 shows the schematics of the arrangement found suitable for separation. Figure 2 gives electron micrographs of aerosol particles from a given location on a foil under matched and unmatched conditions. The standard deviation at any given location for the matched condition was 5-6,°~o. In the unmatched condition the standard deviation was in the range of 40-509/0, indicating almost no separation. MONODISPERSE
A polythene sheet of dimensions 46.5 x 3.2 x 0.01 cm was used as the collection foil and a 4 hr sample was taken in the LAPS in the matched conditions. The foil was cut into strips of suitable lengths to provide a spread of 30o on either side of the desired particle size (Kotrappa
@____ B __J
--0 --C SF
Fig. 1. Experimental arrangement : A, compressed a~r : B, pressure regulator valve : C. matertal for nebulisation: D, nebuliser: E, diluting air; F, glass fibre paruculate filter: G, aerosol chamber, capacity 55 I. ; H, aerosol vent ; I, LAPS assembly ; J, aerosol inlet for LAPS ; K, stream air inlet for LAPS; M, critical orifice; P, vacuum pump; PV, pressure vacuum pump: SF, series of filters in line, glove box filter, respirator filter, glass fibre filter: MF. mdlipore filter.
Monodisperse tagged polystyreneaerosol
w._.,. . . . . . . . . . . . . . ~ .................... O
tt ml O
Fig. 2. Electron micrographs of polystyreneparticles taken from grids placed at 36 cm (from the laminator end) on the collection foil. (a) is for unmatched conditions and (b} is for the mateh~ conditions. et al. (1972b)). The deposit could be easily resuspended into water containing a little
surfactant (Tween-80) by ultrasonic agitation. The suspension of a desired size was nebulised to get monodisperse aerosols. The particle concentration was adjusted so as to provide 95~o singlets on subsequent nebulisation (Raabe (1968)). ASSESSMENT OF M O N O D I S P E R S E
St6ber and Fiachsbart (1971) and Prodi and Spumy (1976) have measured the size distribution of polystyrene latex spheres and carnauba wax aerosols, respectively, using an aerosol centrifuge. They state that the result obtained with the centrifuge are more reliable than their electron microscope data. We have also used the LAPS for the assessment of monodispersity of the polystyrene aerosol. Monodisperse suspensions of polystyrene were nebulised and sampled on clean polythene foil in the LAPS. The sample was analysed under an optical microscope. The position of the deposit agreed with the location from where the suspension was prepared. The standard deviation was less than 10~o in all the cases. Similar observations were also made when the strip was analysed for 5~Cr by gamma counting. DISCUSSION This method of preparation of S~Cr-tagged polystyrene particles h a s s o m e limitations. Starting from about 9 mCi one gets the separated particles of only about 30/~Ci. In order to obtain usable quantities of the separated monodisperse size groups, one may have to start with significant amounts ofradioactivity. The separation efficiency can be improved by using a number of LAPS units in parallel (Raabe et al. (1975)}. The advantages of the method are: • (1) various size groups are prepared in a single production run, (2) activity tagging is proportional to the volume of aerosol particle because of the very nature of generation of feed aerosol prior to separation, (3) activity does not leach out of the t)articles (Sharma et al. (1975)).
D.P. BHANTI,S. K. DUA, P. KOTRAPPAand N. S. PIMPALE
It is possible to o b t a i n a better m o n o d i s p e r s e a e r o s o l with less t h a n 10°o s t a n d a r d d e v i a t i o n by m a k i n g a p r o d u c t i o n run at a s a m p l i n g rate o f 0. I 1./min i n s t e a d of at 0.4 I./min. This is b e c a u s e o f the expected i m p r o v e d s e p a r a t i o n at the lower s a m p l i n g rate. H o w e v e r , s a m p l i n g at 0.4 I./min is c o n s i d e r e d a d e q u a t e for the p r o d u c t i o n of a m o n o d i s p e r s e a e r o s o l n e e d e d for i n h a l a t i o n studies. T h e s a m p l i n g p r o c e d u r e d e v e l o p e d in the c u r r e n t w o r k solves the p r o b l e m of m a t c h i n g the v a p o u r c o n c e n t r a t i o n s b e t w e e n a e r o s o l a n d s t e a m air. Such p r o c e d u r e allows the use of the L A P S as a n a e r o d y n a m i c size a n a l y s e r for an a e r o s o l c o n t a i n i n g a variety of v a p o u r s . E x a m p l e s o f such a e r o s o l s a r e cigarette s m o k e , boiler exhaust, a u t o m o b i l e exhaust, a e r o s o l s g e n e r a t e d by c o n d e n s a t i o n m e t h o d s , etc.
Acknowledgements--This work was carried out under partial support from Internauonal Atomic Energy Research
Contract No. 1449/RB. The authors wish to thank Mr. S. D. Soman for his encouragement. Mr. V. B. Menon for his assistance and Dr. R. Krishnan for electron microscopy. REFERENCES Bhanti, D. P., Dua, S. K. and Kotrappa, P. (1977) To be published. Kotrappa, P. and Moss, O. R. (1971) HIth Phys. 11(4), 531. Kotrappa, P. and Light, M. E. (1972a) Rev. scient. Instrum. 43(8), 1106. Kotrappa, P., Wilkinson, C. J. and Boyd, H. A. (1972b) HIth Phys. 22(6), 837. Kotrappa, P., Mayya, Y. S., Raghunath, B., Menon, V. B. and Bhanti, D. P. (1976) Ann. occup. Hy~l. 19, 363. Kops, J., Hermans, L. and Van de Vate, J. F. (1974) J. Aerosol Sci. 5, 379. Prodi, V. and Spumy, K. R. (1976) J. Aerosol Sci. 7, 43. Raabe, O. G, (1968) Am. ind. Hyg. Ass. J. 29, 639. Raabe, O. G., Boyd, H. A., Kanapilly, G. M., Wilkinson, C. J. and Newton G. J. (1975) HIth Phys. 28, 655. Rundo, J., Taylor, B. T., Booker, D. V., Newton, D. and Scargill, D. (1968) Nature land. 217, 642. Sharma, R. C., Somasundaram, S., Kotrappa, P., Haridasan, T. K., Surendran, T,, Kapur, D. K., Bhanti, D. P. and Pimpale, N. S. (1975) Assessment of Chest Burdens of Plutonium. Proceedings of Diagnosis and Treatment of Incorporated Radionuclides IAEA-SR-6/50, 177. St6ber, W. and Flachsbart, H. (1971) J. Aerosol Sci. 2, 103.