Analytical Techniques in Occupational Health ... - ACS Publications

3 Occupational Health Analytical Quantitation

Chemistry

Using X - R a y Powder

Diffraction

DONALD D. DOLLBERG, MARTIN T. ABELL, and BRUCE A. L A N G E

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Downloaded by UNIV OF ARIZONA on January 13, 2013 | http://pubs.acs.org Publication Date: April 22, 1980 | doi: 10.1021/bk-1980-0120.ch003

National Institute for Occupational Safety and Health, 4676 Columbia Parkway, Cincinnati, O H 45226

Particulate contaminants are of extreme importance in occupational health because of their effect on the respiratory system. Such contaminants may be grouped into several physiological categories depending on their overall effect (1). Three categories are of importance to this discussion: (1) inert, (2) minimal pulmonary fibrosis producing, and (3) extensive pulmonary fibrosis producing. Dust aerosols which produce no known injuries when inhaled but may cause discomfort and minor irritation to the lung, are classed as nuisance and/or inert. Examples of such dusts include particulate clay, limestone, gypsum or aluminum oxide. I f a dust contaminant i s known to produce nodulation (discrete deposits of particulate) and diffuse fibrosis (growth of nonelastic tissue) in the lung, the material i s classed as minimal pulmonary fibrosis producing. Dusts i n this category include barium sulfate, iron oxide and t i n oxide. The third category, extensive pulmonary fibrosis producing, includes dusts such as silica and asbestos. These dusts are known to produce a significant degree of nodulation and fibrosis in the lung. Exposure to inorganic chemicals in the workplace has been traditionally evaluated using elemental analysis. However, in recent years some attention has been given to the toxic effects of specific compounds rather than elements, e.g., chromic acid (2), nickel subsulfide (3), zinc oxide (4), and sodium hydroxide (5). I t i s therefore important that the occupational health chemist develop the capability to identify and quantitate chemical compounds. To this end, X-ray powder diffraction (XRD) i s a unique tool for 1Current Address: Construction Products Div., W.R. Grace, Inc., 62 Whittemore Ave, Cambridge, Mass., 02140. This chapter not subject to U.S. copyright. Published 1980 American Chemical Society In Analytical Techniques in Occupational Health Chemistry; Dollberg, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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crystalline p a r t i c u l a t e a n a l y s i s . Every species has a unique powder d i f f r a c t i o n p a t t e r n ; thus, i t i s p o s s i b l e to i d e n t i f y each s p e c i e s i n a sample by e i t h e r a manual or computerized search (&) o f c r y s t a l l i n e standards* Upon i d e n t i f i c a t i o n o f t h e compounds present i n t h e s a m p l e , e a c h may b e q u a n t i t a t e d since the diffracted intensity f o r a given profile o r peak of the diffraction pattern i s p r o p o r t i o n a l t o t h e amount present i n t h e sample, Furthermore, t h e nond e s t r u c t i v e n a t u r e o f XRD p e r m i t s additional analyses by o t h e r t e c h n i q u e s s h o u l d t h i s b e n e c e s s a r y . Recognizing the applicability of XRD to o c c u p a t i o n a l h e a l t h chemistry, Lennox and Leroux (1) suggested a number o f c h e m i c a l s p e c i e s w h i c h w o u l d be s u i t a b l e f o r XRD a n a l y s i s : a r s e n i c t r i o x i d e , b e r y l l i u m oxide, mica, vanadium oxides, calcium fluoride i n ceramic m a t e r i a l s , a s w e l l a s a number o f o r g a n i c s such as DDT, l i n d a n e and chlordane. Unfortunately, the g e n e r a l a p p l i c a t i o n o f XRD t o t h e q u a n t i t a t i o n o f industrial hygiene samples h a s n o t been r e a l i z e d a n d the m a j o r i t y o f these analyses a r er e s t r i c t e d t o free s i l i c a and t o a l e s s e r extent asbestos and t a l c . Table I lists several XRD a n a l y t i c a l methods r e c e n t l y d e v e l o p e d i n t h e NIOSH l a b o r a t o r i e s . F o r each analyte, the analytical range, detection limit and analytical precision arelisted. The method numbers refer t o t h e NIOSH Manual o f A n a l y t i c a l Methods ( 8 ) , As i n d i c a t e d i n t h e t a b l e , t h e r e are several NIOSH methods a v a i l a b l e f o r f r e e s i l i c a a n a l y s i s . Method No, P&CAM 1 0 9 i n c o r p o r a t e s t h e i n t e r n a l s t a n d a r d approach as developed by Bumsted (£), T h e o t h e r t w o m e t h o d s S315 a n d P&CAM 2 5 9 a r e b a s e d o n t h e s u b s t r a t e standard method, The major d i f f e r e n c e between t h etwo i s t h e direct sampling on s i l v e r membrane filters (S-315), This paper will address the various methods o f quantitation, sample collection and procedures f o r matrix absorption c o r r e c t i o n s t h a t have been used i n this laboratory f o r the analysis of crystalline p a r t i c u l a t e contaminants i nt h e workplace. Methods o f Q u a n t i t a t i o n Because X-ray powder d i f f r a c t i o n d e a l s w i t h s o l i d samples, t h ea n a l y t i c a l v a r i a b l e s a r e d i f f e r e n t from those associated with the analysis of liquid or solution samples, P r i n c i p l e among t h e s e are particle s i z e e f f e c t s , uniform sample surface, c r y s t a l l i n i t y and X-ray absorption. Although p a r t i c l e s i z e and a nonuniform sample surface a r es e r i o u s problems, t h e i r

In Analytical Techniques in Occupational Health Chemistry; Dollberg, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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TABLE I Permissible Exposure Levels And XRD A n a l y t i c a l M e t h o d s


Analyte

Standard mg/M^

Analytical ~ R a n g e , mg/M^

Relative Std,Dev,

NIOSH Method

Zinc oxide

5-0 ( a )

0,03-2,4

0,10

P&CAM 222

Zirconium oxide

5,0 (£)

0 , 0 3 - 1 -5

0,08

P&CAM 250

Chrysotile

0,15(ç)

0,025-0,25

0,07

P&CAM 309

0,012-0,25

0,14

Fibrous tremolite Quartz

0,05 ( a )

0,030-1,3 0,025-0,4 0,025-2,4

0,07 0,10

P&CAM 109 S 315 P&CAM 259

( a ) NIOSH recommended s t a n d a r d (JD) A m e r i c a n C o n f e r e n c e o f G o v e r n m e n t a l I n d u s t r i a l Hygienists (&) W e s t German mass s t a n d a r i ; c , f , r e f , 3 0 , effects c a n be r e d u c e d o r e l i m in a t e d t h r ough c a r e f u l sample p r e p a r a t i o n , Field s a m p le c r y s t a ll i n i t y i s , p e r h a p s , t h e m o s t u n c o n t r o l l a b l e νa r i a b l e w h i c h e f f e c t s i s q u a n t i t a t i v e XRD a n a l y s i s , 0 f te n t h e an a l y t e generated i n t h e w o r k p l a c e u n d er a d v e r s e c o n d i t i o n s ; c o n d i t i o n s s i g n i f ic a n t l y d i f f e r e n t f r o m t h o se u s e d i n Thus, t h e l a b o r a t o r y f o r t h e p r e p a r a t i o n o f s t a n d ar d s . t o l e s s e n t h e e f f ec t o f t h e v a r ia t i o n i n d e g r e e o f crystallinity b et w e e n s t a n d a r d s a n d s am p l e s , t h e d i f f r a c t i o n p e a k ar e a r a t h e r t h a n peak h e i g ht m u s t b e measured, Both h i s t o r ic a l l y a n d c u r r e n t l y , X-ray a b s o r p t i o n b y t h e s a m p l e h a s h a d a m a j o r i m p a c t o n t h ed e v e l o p m e n t of analytical me t h o d o l o g y . W h i l e t h e t h e or y o f X - r a y a b s o r p t i o n i s compl e x , t h e o b s e r v e d e f f e c t is straight forward. A s X - ra y s p a s s t h r o u g h a m a t e r ia l t h e y a r e



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absorbed; the e x t e n t o f the a b s o r p t i o n depends upon the thickness and nature of the absorbing medium, D i a g r a m a t i c a l l y the s p e c i f i c f a c t o r s i n f l u e n c i n g X-ray absorption are pictured in Fig, I, The sample t h i c k n e s s ( t ) and the d i f f r a c t i o n a n g l e (Θ) determine the path length of the X-rays through the absorbing m a t e r i a l , while the chemical composition of the matrix determines the degree of absorption per u n i t l e n g t h . By k n o w i n g t h e t h i c k n e s s ( o r a r e a , m a s s a n d d e n s i t y ) o f the sample and i t s exact chemical composition i t i s p o s s i b l e t o c a l c u l a t e t h e a b s o r p t i o n e f f e c t and t h e r e b y make c o r r e c t i o n s . H o w e v e r , f o r an a c t u a l f i l t e r s a m p l e c o l l e c t e d i n the workplace environment these parameters are rarely known and often impossible to determine. Therefore, to account f o r the X-ray absorption effect on q u a n t i t a t i v e a n a l y s i s , two e x p e r i m e n t a l procedures have been u t i l i z e d i n the X-ray laboratory: the internal s t a n d a r d p r o c e d u r e and t h e s u b s t r a t e s t a n d a r d procedure, Internal Standard Procedure, The f i r s t o f t h e s e absorption correction methods was developed by Alexander and K l u g (10) and i n v o l v e s t h e a d d i t i o n o f a known amount o f an i n t e r n a l s t a n d a r d t o t h e sample. If the a n a l y t e and the i n t e r n a l s t a n d a r d have d i f f r a c t i o n profiles at approximately the same angle, their intensities will be e q u i v a l e n t l y i n f l u e n c e d b y m a t r i x absorption. Thus, the i n t e n s i t y r a t i o of the internal standard to that of the analyte can be u s e d a s a q u a n t i t a t i v e measure o f the amount of a n a l y t e present. To effectively use the i n t e r n a l s t a n d a r d procedure, a number of requirements m u s t be met. First, the standard should have a s t r o n g d i f f r a c t i o n peak near t h a t o f t h e a n a l y t e , but n o t so c l o s e as t o interfere. Secondly, the density, particle size and mass absorption coefficient of the a n a l y t e and standard should be s i m i l a r . T h i r d l y , the a n a l y t e , s t a n d a r d and any m a t r i x m u s t be c h e m i c a l l y i n e r t relative to each other and f i n a l l y , a homogeneous m i x t u r e o f s a m p l e and s t a n d a r d m u s t be g e n e r a t e d . The major drawbacks to t h i s method as a g e n e r a l quantitative procedure i n c l u d e the rather stringent requirements f o r the i n t e r n a l standard, the p r e p a r a t i o n of a homogeneous m i x t u r e of s t a n d a r d and sample, and the a d d i t i o n a l time r e q u i r e d f o r the measurement of the two phases. Furthermore, s i n c e i t i s not always p o s s i b l e t o know i n a d v a n c e w h e t h e r the m a t r i x c o n t a i n s strongly absorbing m a t e r i a l s , the internal standard m u s t be added to every sample as a precautionary measure,



DOLLBERG

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Ag

Figure 1.

X-Ray Powder

Diffraction

FILTER

Geometric factors that affect the degree of x-ray absorption

American Chemical

Society Library

1155 16th St. N. w.

In Analytical Techniques in Occupational Health Chemistry; Dollberg, D., et al.; Washington, D. C. Society: 2003$ ACS Symposium Series; American Chemical Washington, DC, 1980.

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Eumstead (£) c h o s e n a t i v e f l u o r i t e a s a n i n t e r n a l standard f o r the analysis of quartz i n coal dust. His procedure consisted of mixing 0 , 2 0 mg f l u o r i t e i n t o each water suspension o f standard and sample and d e p o s i t i n g on s i l v e r membrane f i l t e r s f o r a c a l i b r a t i o n c u r v e ( f l u o r i t e / q u a r t z i n t e n s i t y r a t i o v s , mg quartz). The a p p l i c a t i o n o f t h i s method t o c o a l dust samples containing less than 1% q u a r t z produced acceptable results; r e l a t i v e standard d e v i a t i o n (RSD) was 18,2? Substrate Standard Procedure, The second method, a l s o known a s t h e e x t e r n a l s t a n d a r d p r o c e d u r e , depends on the measurement o f a d i f f r a c t i o n peak from t h e s u b s t r a t e s u p p o r t i n g t h e sample. This method i s based on t h e work o f two i n d e p e n d e n t g r o u p s , Leroux, Lennox and Kay ( J M ) d e v e l o p e d a p r o c e d u r e f o r c a l c u l a t i n g a n absorption c o r r e c t i o n based on t h e d i f f r a c t e d and transmitted i n t e n s i t i e s from a bulk sample, Williams (12) s i m p l i f i e d t h i s p r o c e s s by m o u n t i n g t h e s a m p l e on a f i n e grained copper metal foil and measuring the intensity of t h e d i f f r a c t e d r a d i a t i o n from t h i s f o i l with and without the sample i n place, A correction factor can then be c a l c u l a t e d f r o m the observed a t t e n u a t i o n o f t h e copper d i f f r a c t i o n peak, Leroux and co-workers (13>1U) e x t e n d e d t h e W i l l i a m s t e c h n i q u e b y r e p l a c i n g t h e c o p p e r f o i l w i t h a s i l v e r membrane f i l t e r and a p p l i e d t h e method t o t h e a n a l y s i s o f q u a r t z . To calculate the absorption correction,Γ, t h e f o l l o w i n g equation i s employed: r =

-R

Τ

In

where: R = s i nθ substrate T

I

A

W

1

/ s i nθ

analyte

^

= d i f f r a c t e d intensity f o rthe substrate with sample d e p o s i t i o n diffracted intensity f o rthe s u b s t r a t e without sample d e p o s i t i o n

The observed m u l t i p l i e d by weight, The

i n t e n s i t y or weight o f analyte i s then r to give the corrected intensity or substrate standard method r e q u i r e s that


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t h e s u b s t r a t e be c r y s t a l l i n e ( s u c h a s a s i l v e r membrane f i l t e r ) a n d assumes a uniform sample d i s t r i b u t i o n , The major drawback t o t h i s approach i s the possibility of overlap of the analyte profile with that of the substrate profile,


Sample C o l l e c t i o n and P r e p a r a t i o n For occupational health investigations, personal sampling i s done i nthe b r e a t h i n g zone o f t h e exposed worker. T o a c c o m p l i s h t h i s , a p o r t a b l e s a m p l i n g pump must be a t t a c h e d t o the worker a n d a c o l l e c t i o n device p o s i t i o n e d near the breathing zone, u s u a l l y attached t o the s h i r t c o l l a r . This device consists o f a plastic holder (filter cassette) containing t h e membrane filter. T y p i c a l f i l t e r s used f o r the collection of p a r t i c u l a t e s i n c l u d e M i n e S a f e t y A p p l i a n c e ' s (MSA) FWSB (polyvinylchloride) a n d Gelman Sciences DM-5000 (polyacrylonitrile-polyvinylchloride), Flow r a t e s f o r p e r s o n a l s a m p l i n g u s u a l l y r a n g e f r o m 1,0 - 2,0 L / m i n w i t h 1,7 L / m i n b e i n g t h e r e c o m m e n d e d s a m p l i n g r a t e . When t h e w o r k i n g environment i s suspected of containing dusts formed by comminution (quartz, asbestos, talc), a size selecting device (cyclone) i s added t o the sampling t r a i n t o i n s u r e the c o l l e c t i o n o f only respirable p a r t i c l e s . The a d d i t i o n o f the cyclone i s a d i s t i n c t a d v a n t a g e f o r XRD a n a l y s i s s i n c e this p r o d u c e s a sample i n w h i c h the p a r t i c l e s i z e i s known and a s t a n d a r d c a n be s e l e c t e d t o m a t c h , A sampling setup with a cyclone anda closed face f i l t e r cassette i s shown i n F i g u r e 2, Dust l a d e n a i r e n t e r i n g the s i d e of t h e c y c l o n e m u s t make a n a b r u p t c h a n g e i n d i r e c t i o n to be p u l l e d t h r o u g h the filter. Larger p a r t i c l e s impact o n the s i d e s o f the c y c l o n e a n d do not reach t h e filter. Sampling procedures a r e often dependent on t h e method o f sample p r e p a r a t i o n a s w e l l a s t h e p h y s i c a l and chemical properties o f the analyte. For most a n a l y t e s t h a t are c o l l e c t e d by t h e above method t h e usual procedure i s t o a s hthe f i l t e r i ne i t h e r al o w temperature plasma asher o r a muffle furnace, disperse the residue i n a s u i t a b l e l i q u i d such a s i s o p r o p a n o l using ultrasonic agitation, andf i l t e r t h e suspension through a s i l v e r membrane f i l t e r . I n a d d i t i o n , i f the i n t e r n a l standard method i s used, t h e chosen standard must be added t o t h e residue suspension priorto filtration. Regardless o f the a n a l y t i c a l procedure chosen f o r the a n a l y s i s , s e v e r a l advantages are d e r i v e d from t h e ashing/re-deposition process. These advantages i n c l u d e



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Figure 2. Personal sampling device consisting of a filter cassette and a 10-mm nylon cyclone

Figure 3. Illustration of a dust sample collected with a closed face cassette showing dust deposition in the center of the filter



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the e l i m i n a t i o n o f most o r g a n i c interférants, t h e formation o f a uniform dust d i s t r i b u t i o n on the f i l t e r , the low flat X-ray background o f the silver f i l t e r (SMS.) and t h e potential f o r making absorption corrections. However, i fthe analyte i s sensitivet o t h i s preparative technique, e,g,, chemically reactive during ashing, then a n a l t e r n a t e sample c o l l e c t i o n method must be employed. Since a u n i f o r m i l y t h i c k dust l a y e r m u s t b e p r e s e n t e d t o t h e X - r a y beam, t h e s t a n d a r d closed face f i l t e r cassette andcyclone cannot be used because o f t h e tendency t o accumulate a greater q u a n t i t y o f dust i n t h e c e n t e r than a t t h e edge o f t h e filter. Figure 3 i s a good example o f t h i s problem. Although experimental evidence i s l a c k i n g , t h e open face cassette tends t o collect the required uniform dust l a y e r . However, t h i s c o l l e c t i o n device i s more prone t o abuse by t h e worker, i s d i f f i c u l t t o t r a n s p o r t back t o t h e l a b o r a t o r y without d i s t u r b i n g t h e uniform dust l a y e r , a n d does not collect the respirable fraction. When s a m p l e s a r e c o l l e c t e d w i t h a n open face f i l t e r cassette and analyzed directly, the internal standard method i s n o t amenable to the analysis, Henslee and Guerra (l£) developed a procedure f o r direct analysis o f quartz on p o l y v i n y l c h l o r i d e f i l t e r s ; however, t h e procedure d i d not allow f o r absorption corrections, A l l e n , Samimi, Ziskindand W e i l (11) a l s o a n a l y z e d f o r q u a r t z d i r e c t l y o n o r g a n i c membrane f i l t e r s . T h e i r method o f determining matrix absorption c o r r e c t i o n s i s based on the percent attentuation o f t h e sample background compared t ot h e background o f t h e standards, Altree-Williams (18) modified t h e s u b s t r a t e standard method t o accommodate samples c o l l e c t e d d i r e c t l y on Nuclepore filters. M a t r i x a b s o r p t i o n was accounted f o r by mounting a c l e a n s i l v e r membrane f i l t e r b e n e a t h t h e Nuclepore filter. The absorption c o r r e c t i o n was then determined as described previously. I n agreement with AltreeWilliams, preliminary experiments i n this laboratory suggested that this procedure i s viable f o rthe measurement o f samples which cannot be t r e a t e d by t h e standard preparative techniques, e,g,, organic solids. Standards

Preparation

There a r e two basic l a b o r a t o r y methods f o r t h e preparation o f f i l t e r standards--dust generation and the l i q u i d suspension technique. Dust generation has a d i s t i n c t advantage because o f the a b i l i t y t o produce atmospheres s i m i l a r t o t h e w o r k p l a c e w h i c h c a n t h e n be



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sampled u s i n g standard, t e c h n i q u e s . Equipment f o r dust g e n e r a t i o n c a n be a s s i m p l e a s t h e a p p a r a t u s described by Leroux (JLSL) o r a s s o p h i s t i c a t e d a s t h a t shown i n F i g u r e 4, T h i s a p p a r a t u s c o n s i s t s o f a Wright Dust Feeder (£Q), a c h a r g e n e u t r a l i z e r and a d u s t chamber f i t t e d w i t h an e x h a u s t manifold, The a t t a c h m e n t of sampling devices to the manifold permits the c o l l e c t i o n o f t h e g e n e r a t e d d u s t o n t h e membrane f i l t e r s , A set of filter standards are prepared by v a r y i n g t h e s a m p l i n g t i m e s and t h e c o n c e n t r a t i o n o f t h e dust i n t h e chamber. Although dust generation may appear relatively straight forward, the technique requires considerable e x p e r t i s e and does n o t e x a c t l y match t h e treatment o f f i e l d samples; hence, this procedure is not generally applicable i n a routine analytical laboratory. For the l i q u i d suspension t e c h n i q u e , a weighed quantity of the standard (particle s i z e < 10 ym) i s dispersed i n a suitable l i q u i d such as i s o p r o p a n o l . F o r most a p p l i c a t i o n s , s u s p e n s i o n s i n t h e range 10-50 mg/L a r e adequate. F i l t e r c a l i b r a t i o n standards are e a s i l y p r e p a r e d by p i p e t t i n g a l i q u o t s o f t h e s u s p e n s i o n and f i l t e r i n g t h r o u g h a 0,45 ym s i l v e r membrane f i l t e r ; however, classical analytical techniques must be modified when dealing with suspensions. During the p i p e t t i n g o p e r a t i o n , t h e a l i q u o t must be b r o u g h t up t o the mark, I f t h e s u s p e n s i o n i s t r e a t e d as a s o l u t i o n , i t i s p o s s i b l e , due t o s e t t l i n g within the p i p e t t e , that a h i g h l y c o n c e n t r a t e d d r o p o f s u s p e n s i o n w i l l be l o s t when t h e p i p e t t e i s a d j u s t e d t o t h e mark from above. To f a c i l i t a t e t h e p r e p a r a t i o n o f a u n i f o r m d u s t layer, a s m a l l volume of the suspending liquid (isopropanol) i s added t o the f i l t r a t i o n apparatus p r i o r to the a d d i t i o n of the a l i q u o t , Vacuum i s not applied until the aliquot h a s been added and a l l washings of the f i l t r a t i o n apparatus have been accomplished, In order to obtain a uniform suspension, the s t a n d a r d must be g r o u n d t o a p a r t i c l e s i z e < 10 ym. This c a n be a c h i e v e d u s i n g a l i q u i d n i t r o g e n " f r e e z e r " m i l l and s i e v i n g t h e ground m a t e r i a l u s i n g either dry or wet sieving. For compounds which tend to agglomerate during dry sieving, the wet sieving technique o f Kupel (2J_) i s p r e f e r a b l e . To i n s u r e complete dispersal of the solid throughout the suspending liquid, an u l t r a s o n i c bath o r probe i s essential, There has been c o n s i d e r a b l e concern among XRD researchers on t h e e r r o r s which may result from p i p e t i n g a l i q u o t s from a s u s p e n s i o n , e r r o r s which c o u l d


DOLLBERG ET A L .

X-Ray Powder


WRIGHT DUST FEEDER

Diffraction

AIR TO DUST FEEDER DILUTION AIR

CHARGE NEUTRALIZER

EXHAUST (TO HOOD)

Figure 4.

Dust generation system: the Wright dust feeder introduces dust into the air stream at a constant rate to produce test atmospheres.



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OCCUPATIONAL H E A L T H CHEMISTRY

s i g n i f i c a n t l y a f f e c t the o v e r a l l p r e c i s i o n and a c c u r a c y of t h e method, H e n s l e e and G u e r r a (16) weighed their filters before and a f t e r d e p o s i t i o n of the standard, O f 30 f i l t e r s , 8MÏ a g r e e d w i t h d i l u t i o n d a t a t o w i t h i n ± 12 y g . Furthermore, calibration c u r v e s b a s e d on weight data d i d not show any better precision than those based on l i q u i d s u s p e n s i o n d a t a . These a u t h o r s c o n c l u d e d t h a t i t w o u l d be a d v a n t a g e o u s t o i n v e s t time in m u l t i p l e d e t e r m i n a t i o n s b a s e d s o l e l y on t h e l i q u i d suspension data, A similar comparison has been conducted i n t h i s laboratory. We h a v e b e e n s p e c i f i c a l l y concerned that the capillary t i p of the p i p e t t e might generate poor p r e c i s i o n by p r o m o t i n g t h e f o r m a t i o n o f agglomerates. To test for effects on the a c c u r a c y and p r e c i s i o n , t h r e e d i f f e r e n t t e c h n i q u e s were used f o r p l a c i n g pure talc on three sets of s i l v e r f i l t e r s . In the f i r s t technique samples of t a l c were weighed, suspended in i s o p r o p a n o l , and t h e e n t i r e s u s p e n s i o n f i l t e r e d t h r o u g h the silver filters. In the second and third techniques, a t r a n s f e r p i p e t t e and a t r a n s f e r p i p e t t e w i t h the capillary t i p removed were used to draw aliquots from a suspension c o n t a i n i n g 1000 yg/mL o f talc. The a v e r a g e t a l c p e a k i n t e n s i t i e s a n d a s s o c i a t e d s t a n d a r d d e v i a t i o n s were used t o a s s e s s and compare the precision and accuracy of the three methods. Statistical tests showed that the precision and accuracy a t t a i n e d w i t h e i t h e r p i p e t t e i s the same as t h a t found w i t h the weighed samples, demonstrating t h a t pipettes can be used to remove aliquots from suspensions w i t h c o n c e n t r a t i o n s as h i g h as 1000 ug/mL (22),

Haartz, Bolyard and Abell (£3.) used atomic a b s o r p t i o n spectrophotometry to check f i l t e r standards prepared by t h e s u s p e n s i o n t e c h n i q u e . Z i n c o x i d e was " s p i k e d " on D M - 8 0 0 f i l t e r s and q u a n t i t a t e d by AAS and XRD, Results from these measurements are shown i n T a b l e I I , An average d e v i a t i o n of 2 , λ% w a s found between the two techniques. Although t h i s data does g i v e a c o n s i d e r a b l e degree of c o n f i d e n c e t o the use of the suspension procedure for standards preparation, additional data f o r other a n a l y t e s i s needed for conclusive proof. Measurement and

Data A n a l y s i s

T h e r e a r e two m e t h o d s the d i f f r a c t e d i n t e n s i t y : Klug and Alexander (2JL) c a n o n l y be u s e d w h e n t h e y

f o r experimentally measuring peak h e i g h t and peak area, have noted t h a t peak h e i g h t s a r e known t o


3.

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Powder

Diffraction

55

TABLE I I C o m p a r i s o n o f XRD a n d A A S R e s u l t s For Zinc Oxide on F i l t e r s Nominal

Value

AAS

Results (â)


Pg

250 400 475 700 925

250,7 383-0 473-3 689-0 928,0

(a) Values r e p o r t e d three samples.

XRD

Results Pg ( â )

Percent Diff ,

249-3 367-7 467-7 683-3 691-3

0,5 4,0 1-3 0,8 4,0

are the average o f

be proportional to t h e corresponding integrated intensities. Furthermore, peak width a n d consequently peak h e i g h t a r e a function o f c r y s t a l l i t e size and lattice imperfection. Thus, f o r samples generated i n the workplace where t h e s e e f f e c t s c o u l d be s i g n i f i c a n t , it i s essential that integrated intensities be measured, Regardless o f which q u a n t i t a t i v e procedure i s c h o s e n , i«e,, i n t e r n a l s t a n d a r d o r s u b s t r a t e standard, the measurement o f the analyte profile i s identical* T y p i c a l l y , f i l t e r samples contain l i g h t dust loadings, generally a m a x i m u m o f t w o mg t o t a l d u s t , a n d t h e e x p e c t e d a n a l y t e w e i g h t may t y p i c a l l y b e l e s s t h a n 5 0 0 \igé To p r e c i s e l y measure t h e i n t e g r a t e d i n t e n s i t ya t this level, considerably longer counting times a r e required than might ordinarily be employed i n XRD analysis. Experimentally, i n t e n s i t y measurements a r e usually made i n one o f two ways. The usual approach u t i l i z e s a sealer/timer t o accumulate thediffracted radiation as t h e profile i s continuously scanned. Alternatively, using a computer controlled diffractometer, integrated intensities are easily obtained by step scanning through the analyte profile, A c o u n t i n g r a t e o f 10 s e c p e r 0 , 0 2 2 0 s t e p ( 0 , 1 2 / m i n ) is typical. With the exception o f c h r y s o t i l e , which has a very broad primary peak, t h e t y p i c a l a n a l y t g r e q u i r e s s t e p s c a n n i n g t h r o u g h a 20 r a n g e o f 1,0-1,5 which r e q u i r e s approximately 15-20 minutes o f a n a l y s i s time. F o r c h r y s o t i l e , which h a s a r a t h e r broad primary profile (£2), t h e analysis time


OCCUPATIONAL H E A L T H


56

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increases to approximately one hour. While i ti s p o s s i b l e t o reduce t h e a n a l y s i s time somewhat, there w i l l be a c o n s e q u e n t l o s s i n s e n s i t i v i t y . To account f o r a b s o r p t i o n e f f e c t s , an a d d i t i o n a l measurement i s r e q u i r e d . With the internal standard method, t h e p r o f i l e f o rt h e i n t e r n a l s t a n d a r d must be measured f o r both c a l i b r a t i o n standards and samples. The rather small quantity o f analyte present i n the sample r e q u i r e s t h e a d d i t i o n o f a s i m i l a r quantity of internal standard ( * 200 yg) necessitating the measurement o f t h e standards p r o f i l e under essentially the same conditions as that of the analyte. Consequently, t h e i n t e r n a l standard method r e q u i r e s an a d d i t i o n a l measurement time approximately equal t o t h a t of t h e a n a l y t e . Under c o n d i t i o n s noted above, this would amount t o a p p r o x i m a t e l y thirty minutes total a n a l y s i s time p e r sample. In t h e s u b s t r a t e s t a n d a r d method t h e a b s o r p t i o n effect i s determined using t h e transmittance r a t i o (T =I / I ), The d e t e r m i n a t i o n of the transmittance involves a d d i t i o n a l measurements, i , e , , steg scanning over the s i l v e r p r o f i l e , t o obtain I . and IA„- Ι i s m e a s u r e d f o r e a c h f i l t e r s a m p l e a n d i f c a n 61 o b t a i n e d several ways a s d i s c u s s e d below, These additional m e a s u r e m e n t s c a n be p e r f o r m e d v e r y q u i c k l y c o m p a r e d t o the more l e n g t h y measurements o f t h e i n t e r n a l s t a n d a r d because t h e s i l v e r peak i s q u i t e i n t e n s e , A step scan of the s i l v e r diffraction profile plus background c o u n t i n g t i m e c a n be a c c o m p l i s h e d i n a b o u t t w o minutes w i t h b e t t e r t h a n 1% p r e c i s i o n , Abell, et,al, (££) c o n s i d e r e d s e v e r a l p r o c e d u r e s for determining I « The s i m p l e s t , an averaging method, i n v o l v e s one a d d i t i o n a l measurement f o r each sample and standard. Samples and standards are prepared using filters from t h e same b o x ; I . i s determined f o r each f i l t e r by s t e p scanning througn the silver 100 l i n e after the diffraction profile ofthe analyte i s measured, Standards having l e s s than 200 yg of material are essentially "clean" s i l v e r f i l t e r s f o r absorption purposes; therefore, the diffracted intensities determined f o r these standards are e s s e n t i a l l y 1 ° v a l u e s . The average o f t h e i n d i v i d u a l I values i s an estimate of I f o r an i n d i v i d u a l f i l t e r w i t h i n a g i v e n b o x . A n R S D o r a p p r o x i m a t e l y 45& was f o u n d when t h i s p r o c e d u r e was used t o determine Τ A

A

Α β :

s

A g :

s

g

A

A

There are two "single filter" methods f o r d e t e r m i n i n g T, O n e i n v o l v e s m e a s u r i n g I . o n a c l e a n filter before d e p o s i t i n g t h e sample and measuring I. and analyte intensities. The other involves



3.

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measuring t h eanalyte ands i l v e r i n t e n s i t i e s on a dust laden f i l t e r and then measuring^I? on t h e r e v e r s e side. The l a t t e r approach was f i r s t proposed by Leroux (JJt), Thatcher (26) found a l a r g e d i f f e r e n c e i n s i l v e r intensity from t h e two sides o f clean f i l t e r s and adopted t h e former method, A l t r e e - W i l l i a m s (£2) also adopted t h e former method a f t e r f i n d i n g a 7% R S D i n intensities from different filters. The large differences found by these r e s e a r c h e r s d i s a g r e e w i t h t h a t f o u n d f o r t h e s m a l l e s t p o r e s i z e ( 0 , 4 5 ym) s i l v e r filters b y A b e l l g t . a l * (£5.), They a l s o found t h e f r o n t r e v e r s e method t o have s l i g h t l y b e t t e r p r e c i s i o n than t h e a v e r a g i n g method. The following example illustrates a typical application o f an absorption correction calculation, A dust sample c o n t a i n i n g 135 yg c h r y s o t i l e asbestos a n d 4365 yg t a l c was d e p o s i t e d o n a s i l v e r membrane f i l t e r . The net diffracted intensity f o r chrysotile when compared t o an e s t a b l i s h e d c a l i b r a t i o n curve indicated only 88 y g p r e s e n t . Comparison o f t h e s a m p l e s s i l v e r diffracted intensity ( I .= 424S4 c o u n t s ) to the established average vafue ( I ? = 59379 counts) indicated that an absorption effect was present. From this intensity d a t a t h e t r a n s m i t t a n c e , (T : L / I ? ) , was d e t e r m i n e d t o be 0,713Τ c a n t h e n be d e t e r m i n e d either by s o l v i n g Eq, 1 o r from a t a b l e o f c o r r e c t i o n factors which c a n be p r e p a r e d b e f o r e hand either manually o r by computer. I n t h i s case, Τ was 1,6, Thus, t h e c o r r e c t e d w e i g h t o f c h r y s o t i l e was 1 3 8 yg i n excellent agreement w i t h t h e amount " s p i k e d " on t h e filter. Proponents o f t h e i n t e r n a l s t a n d a r d procedure have questioned the validity o f t h e substrate standard method t o adequately correct f o rm a t r i x a b s o r p t i o n , Leroux and coworkers (11,13.,lit) h a v e presented data which support t h e method; i n addition, several measurements were performed i n this laboratory t o verify the validity o f t h e method (£2), M i x t u r e s o f c h r y s o t i l e i nt a l c (1-7?) were prepared and various quantities "spiked" on s i l v e r filters. Table I I I illustrates theresults obtained after correcting f o r matrix a b s o r p t i o n a s compared with theuncorrected data, Overall, there i s excellent agreement between the c o r r e c t e d w e i g h t a n d t h e " s p i k e d " w e i g h t . As i s t h e c a s e f o r any quantitative analysis scheme, t h e p r e c i s i o n and accuracy o f t h e results depends significantly on t h e c a l i b r a t i o n standards. Because t h e q u a n t i t i e s o f dust t o be measured correspond with t h e lower end o f t h e instrumental range, long counting times, a s p r e v i o u s l y noted, a r e f

g


OCCUPATIONAL

H E A L T H CHEMISTRY

TAELE I I I QUANTITATIVE MEASUREMENT OF USING CORRECTED & UNCORRECTED


Percent Chrysotile in Talc

7 7 7 5 7 7 7 7 5

Uncorrected Weight

CHRYSOTILE INTENSITIES

Actual Weight

Corrected Weight

151 118 93 93 67 60 46 28 93

175 140 105 125 70 63 49 28 125

188 139 106 119 74 62 49 28 119

75 60 35 33 26

100 75 50 40 30

99 77 44 39 31

3 3

113 88 63 56 45 37 31

180 135 90 72 54 45 36

191 138 87 71 55 44 36

1 1 1 1 1 7

61 49 42 34 23 93

150 100 70 50 30 105

168 102 78 52 27 106

5 5

5 5 3 3 3 3 ?



3.

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necessary to compensate t o some e x t e n t f o r the increased degree o f i m p r e c i s i o n . Thus, several filter c a l i b r a t i o n s t a n d a r d s a t each l e v e l must be prepared t o obtain the best c a l i b r a t i o n curve, A linear regression analysis o f t h e n e t i n t e n s i t y y_s w e i g h t o f s t a n d a r d data helps t o insure the best p r e c i s i o n , Of the several parameters associated with an a n a l y t i c a l method, d e t e c t i o n l i m i t i s a very important but often a little understood parameter. I n X-ray powder d i f f r a c t i o n the d e t e c t i o n l i m i t i s defined as the amount o f m a t e r i a l which will produce a net intensity that i s equal t o three times the standard deviation o f t h e background a s measured over a time period equal to that used i n measuring the corresponding peak (28.), I t cannot be emphasized too s t r o n g l y that the d e t e c t i o n l i m i t i s useful only i n ascertaining an analyte's presence with a reasonable degree o f confidence. Quantitation i s generally impossible at this level, I n a d d i t i o n , the range o f the c a l i b r a t i o n curve between the d e t e c t i o n l i m i t and the lowest measured standard i s a "gray area". M e a s u r e m e n t s made i n t h i s r e g i o n a r e b a s e d e s s e n t i a l l y on extrapolation o f t h e c a l i b r a t i o n curve s o that r e s u l t s must be c o n s i d e r e d a s u n r e l i a b l e . Matrix

Interference??

Quantitative a n a l y s i s o f f i e l d samples must, o f course, be p e r f o r m e d under identical conditions t o those used i n analyzing the standards, Prior to the a c t u a l a n a l y s i s , a powder p a t t e r n o f a bulk sample such as r a f t e r dust, an area sample, o r a h e a v i l y loaded f i l t e r i s obtained t o determine information about t h e matrix. This information may necessitate some modification to the analytical method, such as quantitatively m e a s u r i n g the s e c o n d a r y peak because o f a matrix i n t e r f e r e n c e with the primary peak. Such a modification usually r e s u l t s i n poorer sensitivity because o f the reduced i n t e n s i t y o f the secondary peak. The number o f c o m p o u n d s w i t h p r o f i l e s t h a t o v e r l a p the primary analyte p r o f i l e are p o t e n t i a l l y very l a r g e , but t h e number o f compounds which are potential i n t e r f e r e n c e s i no c c u p a t i o n a l h e a l t h u s u a l l y depends on the industry i n which t h e analyte i s collected, Altree-Williams (18.) c o l l e c t e d s a m p l e s at several i n d u s t r i e s where q u a r t z a n d k a o l i n i t e o r f e l d s p a r / m i c a


60

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TABLE I V Matrix

Interferences


Analyte

Interference

Quartz

Mica, b i o t i t e , potash, feldspars, siliraanite, zircon, graphite, iron carbide, lead sulfate

Zinc Oxide

α-iron o x i d e , (NHi ) Zn(SO ) f

Chrysotile

2

l f

(NH ) ZnCl , 0 , Zn k

2

, 6 H

3

5

2

Chlorite, antigorite, lizardite, anthophyllite

were present, He d i d n o t f i n d t h e s e s u b s t a n c e s t o p r e s e n t a ni n t e r f e r e n c e t o t h eq u a r t z XRD a n a l y s i s , Freedman, e t , a l , (£2.) a n a l y z e d coal dusts which contained clay minerals using infrared absorption and XRD, They noted good agreement between t h e two t e c h n i q u e s p a r t i c u l a r l y when t h e s a m p l e s were ashed prior t o analysis. Some p o t e n t i a l i n t e r f e r e n c e s a r e l i s t e d i n Table I V f o rt h e quartz, c h r y s o t i l e and z i n c oxide primary peaks, I f organic interferencesare present, they can u s u a l l y bee l i m i n a t e d i n t h e ashing step. F o r t h e case where t h e r e i s s i g n i f i c a n t o v e r l a p , l i t t l e c a n be done u n l e s s t h e a n a l y t e has a secondary line o f sufficient intensity f o rquantitation. F o r e x a m p l e , 2 0 yg o f q u a r t z c a n b e q u a n t i t a t e d u s i n g t h e primary peak; w i t h t h e secondary peak, o n l y 100 yg c a n be q u a n t i t a t e d , If t h esecondary o r other l e s s intense peaks a r e not usable and t h e degree o f o v e r l a p i s n o t t o o severe, two other approaches a r ep o s s i b l e . The simpler approach i s t o use longer wavelength radiation. Changing from t h e w i d e l y used copper r a d i a t i o n (1,54A) to chromium radiation (2,29A) allows a slight improvement i n resolution. Figure 5 illustrates the improved r e s o l u t i o n o f z i r c o n i n t e r f e r e n c e with q u a r t z . This approach would only bep r a c t i c a l f o r t h e s i t u a t i o n where samples with a similar interference were regularly received. The second, a n d more c o m p l e x , approach t o the problem i s t h e use o f deconvolution o r peak stripping techniques. This approach i s being a c t i v e l y pursued b ya number o f r e s e a r c h e r s a n d shows promise a s a v i a b l e s o l u t i o n t o t h ei n t e r f e r e n c e



DOLLBERG E T AL.

X-Ray Powder

Diffraction

Figure 5. Effect of radiation wavelength on quartz and zircon profiles. The two profiles are better resolved with a chromium target tube (λ = 0.229 nm) than with a copper tube (λ = 0.154 nm).



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problem. Thus, a combination of r a d i a t i o n type, a deconvolution computer p r o g r a m and a j u d i c i o u s c h o i c e o f X - r a y o p t i c s may solve many o f the interference problems.


Conclusion The trend in industrial hygiene work is to i d e n t i f y the particular species responsible for an o c c u p a t i o n a l h e a l t h problem, although assessment of exposures to i n o r g a n i c m a t e r i a l s p r e v i o u s l y has most often b e e n b a s e d on e l e m e n t a l a n a l y s i s . When a s o l i d i n o r g a n i c c o m p o u n d i s t o be i d e n t i f i e d a n d quantified, X-ray diffraction should be among t h e approaches considered. T h i s paper has o u t l i n e d the use of X-ray powder d i f f r a c t i o n as a t o o l f o r the i d e n t i f i c a t i o n and q u a n t i t a t i o n o f c r y s t a l l i n e p a r t i c u l a t e s . I t has been shown that the substrate standard method is the preferred q u a n t i t a t i v e procedure f o r several reasons: (1) easy a d a p t a b i l i t y t o most analytes; (2) fast a n a l y s i s time (as compared to the internal standard procedure); and (3) accurate determination of matrix absorption effects. While there are a number of reasons why a given c o m p o u n d may n o t b e a m e n a b l e t o t h i s technique, i t i s l i k e l y that the l i s t of analytes w i l l be a d d e d t o i n t h e f u t u r e . Acknowledgement The a u t h o r s wish t o thank Dr, Janet Mr, J o h n L, H o l t z f o r t h e i r review and c r i t i c i s m of the manuscript.

C, H a a r t z a n d constructive

Disclaimer Mention of company names o r products does not c o n s t i t u t e e n d o r s e m e n t by t h e National Institute for O c c u p a t i o n a l S a f e t y and H e a l t h ,

Abstract X-Ray powder diffraction (XRD) has assumed increasing importance as an analytical technique for the identification and quantitation of contaminants in the workplace environment. Traditionally, the major use of this technique has been for the analysis of free silica, talc and asbestos; however, because of the increasing need for the quantitation of chemical



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compounds as opposed to elements, the importance of XRD in the analytical laboratory has grown considerably. This paper will reveiw the application of XRD to the analysis of dust contaminants collected on membrane filters. The advantages and disadvantages of the internal and substrate (external) standard procedures are also discussed. The latter method, currently i n use at NIOSH, is discussed in detail emphasizing the use of the silver membrane filter for re-deposition of the dust sample and for the correction of matrix absorption. This analytical procedure has been applied to the analysis of free silica, zinc oxide, zirconium oxide and chrysotile. Literature Cited 1. Pritchard, J.A., "A Guide to Industrial Respiratory Protection," DHEW Publication No. (NIOSH) 76-189, p. 201, National Instiute for Occupational Safety and Health, Cincinnati, Ohio, 45226, 1976. 2. "Criteria for a Recommended Standard...Occupational Exposure to Chromic Acid," DHEW Publication No. (HSM-11021), National Institue for Occupational Safety and Health, Cincinnati, Ohio, 45226, 1973. 3. "Criteria for a Recommended Standard...Occupational Exposure to Inorganic Nickel," DHEW Publication No. (NIOSH) 77-164, National Institute for Occupational Safety and Health, Cincinnati, Ohio, 45226, 1977. 4. "Criteria for a Recommened Standard...Occupational Exposure to Zinc oxide," DHEW Publication No. (NIOSH) 76-104, National Institute for Occuptional Safety and Health, Cincinnati, Ohio, 45226, 1976. 5. Criteria for a Recommended Standard...Occupational Exposure to Sodium Hydroxide," DHEW Publication No. (NIOSH) 76-105, National Institute for Occupational Safety and Health, Cincinnati, Ohio, 45226, 1976. 6. Johnson, G.G., Jr., Joint Committee on Powder Diffraction Standards. 7. Lennox, D. and Leroux, J., Ind. Hyg. and Occ. Med., (1953). 8. Taylor, D.G., "NIOSH Manual of Analytical Methods," 2nd ed., DHEW Publication No. (NIOSH) 77-157,



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National I n s i t u t e f o r Occupational Safety and Health, Cincinnati, Ohio, 45226, 1977. 9. Bumsted, H.E., Amer. Ind. Hyg. Assoc. J., (1973), 34, 150. 10. Alexander, L. a n d Klug H.P., Anal. Chem., (1948), 20, 886.


11. Leroux, J., Lennox, D.Η., and Kay, Κ., Anal. Chem. (1953), 25, 740.

12. Williams, P.O., Anal. Chem., (1959),31,1842. 13. Leroux, J. and Powers, C.A., Staub-Reinhalt Luft (Eng. ed.), (1969),29,26. 14. Leroux, J . , Davey, A.B.C. and Parlard, A., Amer. Ind. Hyg. Assoc. J. (1973), 34, 409. 15. Peters, Ε.Τ., "Evaluation of the National Institute f o r Occupational Safety and Health X-Ray Diffraction Method f o r the Determination of Free Silica in Respirable Dust," Final Report, Contract No. CDC 99-7451, May 1976. 16. Henslee, W.W. and Guerra, R.E., Advan. X-Ray Anal., (1977), 20, 139. 17.

Allen, G.C., Samini, R., Ziskind, M., amd Weil, H., Amer. Ind. Hyg. A s s o c . J., ( 1 9 7 4 ) , 35, 711.

18.

Altree-Williams, S.,Lee,J.,and M e z i n , N.V., Ann. Occup. Hyg. (1977), 20, 109.

19. Leroux, J . , Staub-Reinhalt, (1969),29,33. 20. Wright, Β.M., J. Sci. I n s t r . , (1950), 27, 12. 21. Kupel, R.E., Kinser, R.E., and Mauer, P.A., Amer. Ind. Hyg. Assoc. J., (1966), 29, 364. 22. Lange, B.A. and Haartz, J.C., Anal. Chem., (1979), 51, 520.

23. Haartz, J.C., Bolyard, M.L. and Abell, Μ.Τ., American Industrial Hygiene Assoc. Conf., Minneapolis, MN., (1975), Paper No. 145.



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24. Klug, H.P. and Alexander, L.E., "X-Ray Diffraction Procedures for Polycrystalline and Amorphous M a t e r i a l s , " John Wiley and Sons, New York, NY, 2nd ed., 1974,p.358. 25. Abell, Μ.Τ., Lange, B.A., Dollberg, D.D., and Hornung, R., To be presented at the 28th Annual Conference on A p p l i c a t i o n s of X-Ray Analysis, Denver, Colorado. 26. Thatcher, Mine Enforcement and Safety Administration Information Report No. 1021, (1975). 27. Altree-Williams, S., Anal. Chem. (1977), 49, 429. 28. Hertoys, Ρ and DeVries, J.L., Conference On X-Ray Spectroscopy, Swansea, 1966. 29. Freedman, R.W., Toma, S.Z. and Lang, H.W. Amer. Ind. Hyg. Assoc., J., (1974), 35, 411. 30. Schutz, A., a n d Wortowitz, H . J . , S t a u b Reinhalt Luft, (1973), 32, 445. RECEIVED October 31, 1979.


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