14
Atomic Absorption Spectroscopy in the Occupational
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Health Laboratory
ALLEN W. VERSTUYFT Chevron Research Company, 576 Standard Avenue, Richmond, CA 94802
Metals and metallic compounds are among the toxic substances most often found in workplace environments (1,2). Industrial hygienists and hygiene chemists must accurately determine the presence and amount of toxic metals and their compounds in the industrial environment. Accurate methods for the quantification of metals in biological and atmospheric samples are required for the industrial hygienist to properly evaluate the environment. Atomic absorption spectroscopy (AAS) has been the primary method of analysis for toxic metals because AAS is sensitive, specific, and rapid especially compared to colorimetric analysis. When the Occupational Safety and Health Act (PL 91-596) was enacted in 1970, colorimetric analyses were among the primary methods for quantifying metallic contaminants. Although some colorimetric techniques are still routinely used in industrial hygiene laboratories, the methods development programs of NIOSH, OSHA, research institutes, industrial laboratories, and universities have developed and improved the routine use of AAS for industrial hygiene laboratories. Rapid improvements in AAS instrumentation have allowed determination of microgram quantities of toxic metals collected on filters or in impingers and found in bodily fluids and tissues. Atomic absorption has become the primary method for determining metal concentrations in industrial hygiene samples. The types of samples that can be analyzed in AAS will be discussed along with acid digestion methods and AAS atomization techniques. No attempt will be made to thoroughly review the theory
0-8412-0539-6/80/47-120-241$06.25/0 © 1980 American Chemical Society
Dollberg and Verstuyft; Analytical Techniques in Occupational Health Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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OCCUPATIONAL HEALTH CHEMISTRY
o f AAS, s i n c e e x c e l l e n t reviews and t e x t s are a v a i l a b l e . B r i e f l y , when ground s t a t e atoms of an element are vaporized by a conventional flame or e l e c t r o t h e r m a l source, absorption of a resonance l i n e emitted from a reference lamp, hollow cathode or e l e c t r o d e l e s s discharge, occurs decreasing the transmitted r a d i a t i o n . The change i n the u l t r a v i o l e t or v i s i b l e l i g h t i s detected and q u a n t i f i e d by a photodetector. The concentraation of the element i s measured by the percentage of absorbance of l i g h t by the ground s t a t e of the s p e c i f i c element. The reader u n f a m i l i a r with AAS i s r e f e r r e d to S l a v i n (3) and other e x c e l l e n t t e x t s f o r a thorough explanation of AAS. Flame methods are the conventional atomization sources used i n AAS f o r i n d u s t r i a l hygiene (Table I ) . A i r / a c e t y l e n e at 2150-2400°C i s used f o r the e a s i l y atomized elements l i k e l e a d , cadmium, and z i n c . R e f r a c t o r y metals such as tungsten or vanad ium r e q u i r e h o t t e r n i t r o u s oxide/acetylene atomization a t 2600-2800°C. The need f o r greater s e n s i t i v i t y and multielement a n a l y s i s from a s i n g l e f i l t e r has increased the use of e l e c t r o thermal atomization f o r t i n , vanadium, n i c k e l , and other d i f f i c u l t elements. Formation of hydrides combined with flame atomi z a t i o n has been used i n some cases to increase s e n s i t i v i t y . These atomization techniques are used i n NIOSH, AIHA, and APHA approved (recommended) methods (5-7). Although the purpose of t h i s d i s c u s s i o n i s AAS techniques, one must b r i e f l y consider c o l l e c t i o n of samples. The primary method of c o l l e c t i n g metal dust samples and fume samples are 0.8 urn and 0.45 urn mixed c e l lulose ester f i l t e r s . The f i l t e r i s d i s s o l v e d i n a c i d or leached with d i l u t e base, a c i d , or d i s t i l l e d water to give an analyte f o r AAS a n a l y s i s . A general procedure Ρ & CAM 173 (_5), was developed by NIOSH f o r the a n a l y s i s of metals. This method provides a s t a r t i n g point and standard of comparison f o r the a n a l y s i s of metals.
General Flame - AAS A n a l y s i s The general methods, Ρ & CAM 173, uses mixed c e l l u l o s e e s t e r f i l t e r s f o r c o l l e c t i o n of the metal sample, a n i t r i c a c i d wet ashing, and flame AAS a n a l y s i s . N i t r i c a c i d wet ashing i s the main wet ashing technique used i n i n d u s t r i a l hygiene analy sis. I t i s s u f f i c i e n t l y vigorous f o r many of the common metals i n c l u d i n g l e a d , cadmium, and z i n c . These three metals are rou t i n e l y analyzed i n the P r o f i c i e n c y Aptitude T e s t i n g (PAT) pro gram operated by NIOSH (8) f o r q u a l i t y assurance i n i n d u s t r i a l hygiene l a b o r a t o r i e s a c c r e d i t e d by the American I n d u s t r i a l Hygiene A s s o c i a t i o n (AIHA) ( 9 ) . Other wet a c i d d i g e s t i o n s such as n i t r i c / s u l f u r i c a c i d s f o r antimony and t i n , or n i t r i c / p e r c h l o r i c a c i d s f o r n i c k e l , have r e s u l t s i n separate AAS methods that are adaptations of Ρ & CAM 173. The wet a c i d d i g e s t i o n i s p r e f e r r e d over dry ashing or low temperature ashing
Dollberg and Verstuyft; Analytical Techniques in Occupational Health Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Dollberg and Verstuyft; Analytical Techniques in Occupational Health Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
2
HN0 /HC1
S-2
Wavelength 217.6 nm
3
4
H S0 /HN0
173
3
0.04
Air
2
Flame/Air-C H (0) 2
0.04
Air
2
ug/ml
0.1 ug/ml
565 Matrix
1.0 ug
Detection L i m i t
565 nm
Wavelength
Flame/Air-C H (0) 2
Atomization
Atomic Absorption
Ashing
U r i n e / A i r , Urine
Apha 802, Ρ & C 107
Method No.
Pink
Reaction
Apha 301 Rhodamine-B
Colorimetric
ANTIMONY OSHA STANDARD 0.5 MG/CU M
TABLE I
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OCCUPATIONAL H E A L T H CHEMISTRY
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where v o l a t i l e species may be l o s t . The merits and l i m i t a t i o n of wet ashing techniques have been adequately d e t a i l e d e l s e where (10)» h e r e i n , wet a c i d d i g e s t i o n e f f e c t s with s p e c i f i c elements w i l l be d i s c u s s e d . The elements, a b s o r p t i o n l i n e s , flame types, and i n t e r f e r ences a s s o c i a t e d with the a n a l y s i s by Ρ & CAM 173 w i l l be d i s cussed and contrasted to other methods f o r these elements. Thirty-one metals are analyzed by Ρ & CAM 173; f i v e metals, A l , Be, Co, Cr, and V are l i s t e d as p o t e n t i a l l y only p a r t i a l l y digested by n i t r i c a c i d ; and N i , Sb, and Sn may need more v i g o r ous o x i d a t i o n than j u s t n i t r i c a c i d . Antimony. Antimony dust has been analyzed c o l o r i m e t r i c a l l y by formation of a Rhodamine-B complex, 565 nm (Table I ). Both Ρ & CAM 173 (5) and S-2 (11) provide AAS methods f o r antimony. Whereas the former method recommends 5:1 n i t r i c : s u l f u r i c a c i d d i g e s t i o n , the l a t t e r uses o n l y 2-ml n i t r i c a c i d at 140°C f o l lowed by 2 ml of 6 Ν h y d r o c h l o r i c a c i d . E i t h e r the n i t r i c / s u l f u r i c a c i d s (10% and 5% v/v) or a 10% t a r t a r i c a c i d (11) matrix may be used f o r the analyte s o l u t i o n . Although the normal a n a l y t i c a l wavelength i s 217.6 nm, when 10,000 ppm Pb or 1000 ppm Ca are present i n the f i n a l s o l u t i o n , then the 231.2 nm a n a l t y i c a l l i n e should be used. A new method f o r a n t i mony has been developed by NIOSH and w i l l be published i n volume four of the methods manual. Aluminum. Aluminum, as AloO^, i s a nuisance dust. There may be instances where elemental composition of the nuisance d u s t i s d e s i r e d ; t h e r e f o r e , A l i s i n c l u d e d i n Ρ & CAM 173. Aluminum i s d i f f i c u l t to d i s s o l v e i n n i t r i c a c i d and should be t r e a t e d as a r e f r a c t o r y metal. Since the n i t r o u s oxide/acetylene flame i s subject to many i n t e r f e r e n c e s , both 1000 ppm Cs and 1000 ppm La, a r e l e a s i n g agent, should be added to the f i n a l s o l u t i o n . A r s e n i c . A r s i n e generation or graphite furnace atomization are p r e f e r r e d to the conventional flame f o r the s e n s i t i v i t y and p r e c i s i o n required i n a n a l y z i n g t h i s element. The west ashing used i n S-309 (12) r e q u i r e s 10 ml n i t r i c a c i d and 1 ml of 60% p e r c h l o r i c a c i d with heating on a 400°c hot p l a t e . The sample i s analyzed at 193.7 nm with deuterium or hydrogen arc back ground c o r r e c t i o n . The a r s e n i c e l e c t r o d e l e s s discharge (EDL) source i s s u p e r i o r to the hollow cathode (HCL) lamp f o r s e n s i t i v i t y , n o i s e , and long-term s t a b i l i t y . The o l d e r c o l o r i m e t r i c a n a l y s i s for a r s e n i c used impinger c o l l e c t i o n of the analyte i n a p y r i d i n e s o l u t i o n of s i l v e r d i e t h y l d i t h i o c a r b a m a t e Ç7) that was q u a n t i t a t e d f o r the orange-yellow 540 nm complex (5,7). T h i s method i s s t i l l used f o r the r a p i d determination of a r s e n i c i n the f i e l d (Table I I ) .
Dollberg and Verstuyft; Analytical Techniques in Occupational Health Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Dollberg and Verstuyft; Analytical Techniques in Occupational Health Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980. 4
Wavelength 193.7 nm
Furnace
Furnace
Nitric
4
S-229
3
2
HN0 /HCL0
4
S-309
2
Hydr i d e/ Argo n-H
3
HN0 /H S0 /HCL0
2
139
2
Flame/Air-C H (0)
Nitric
173
4
Ashing
Atomization
2
2 4
Method No.
Atomic Absorption
Apha 302/803, Ρ & C 140
2
7
e
As - A g S C N ( C H )
6
Vasak - AGDDC
4
As + ( N H ) M o 0
Jacobs •- Molyblue
+
A s ( l I I ) + Zn(H )
Reaction
Gutzeit
Colorimetrie
2
4 2
H0
2
Ash-/Air
Air
Urine
Air
Matrix
535
540
830 nm
HgX
Wavelength
ARSENIC OSHA STANDARDS 0.5 AND 0.2 MG/CU M
TABLE I I
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0.002
0.002
0.001
0.2
ug/ml
0.1 ug
1 ug
1 ug
1 ug
Detection Limit
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OCCUPATIONAL H E A L T H CHEMISTRY
A r s e n i c can be determined i n u r i n e and a i r , Ρ & CAM 139, by f i l t e r c o l l e c t i o n and hydride flame AAS. Samples are analyzed i n 5 ml of 3:1:1 n i t r i c : s u l f u r i c : p e r c h l o r i c a c i d s on a hot p l a t e at 130-150°C. The sample may be wet ashed with 5 ml of 3:2 n i t r i c : s u l f u r i c acids when no p e r c h l o r i c a c i d hood i s a v a i l a b l e . The sodium borohydride i s added to the sample and a c i d i f i e d to generate a r s i n e . T h i s operation should be performed c a u t i o u s l y i n a w e l l - v e n t i l a t e d area because a r s i n e gas i s extremely t o x i c . A r s i n e has been determined c o l o r i m e t r i c a l l y or by c o l l e c t i o n on a c t i v a t e d charcoal and flameless AAS a n a l y s i s by S-229 (12). N i t r i c a c i d desorption of the charcoal o f f e r s a safe method of handling the a r s e n i c analyte than a r s i n e genera t i o n . This method does not s p e c i f y use of an EDL as does S-309, however, i t i s advisable i f an EDL source i s a v a i l a b l e . Barium. Before the r o u t i n e use of AAS, Ba was analyzed by emission spectrograph or a KMnO^ spot t e s t (13). A l k a l i and a l k a l i n e e a r t h metals are analyzed i n n i t r o u s oxide/acetylene flames with i o n i z a t i o n suppressants such as 1000 ppm Cs. For barium a n a l y s i s by Ρ & CAM 173, background c o r r e c t i o n must be used whenever greater than 1000 ppm calcium i s i n the analyte s o l u t i o n . There are strong Ca(0H) absorptions and emission at 553.6 nm, which i s the barium a n a l y t i c a l l i n e . Soluble barium may be determined by S-198 (12) using a hot water l e a c h , h y d r o c h l o r i c a c i d d i s s o l u t i o n , and n i t r o u s oxide/acetylene flame. In t h i s method, 1000-2000 ppm sodium as sodium c h l o r i d e i s used to minimize i o n i z a t i o n of barium i n the flame. Although background c o r r e c t i o n i s not mentioned i n t h i s method, i t i s s t r o n g l y recommended when calcium i s present. 2
B e r y l l i u m . The low exposure l i m i t f o r Be, 2 ug/cu m (TWA) o r 5 ug/cu m ( c e l i n g ) , make the d e t e c t i o n of t h i s analyte i n small samples very d i f f i c u l t . B e r y l l i u m has been analyzed by zenia complexation at 500 nm, and DC or spark spectrograph at 313.1 or 234.9 nm (6,7). Not a l l Be compounds are d i s s o l v e d by n i t r i c a c i d wet ashing described i n Ρ & CAM 173; t h e r e f o r e , more o x i d i z i n g 10:1:1 n i t r i c : s u l f u r i c : p e r c h l o r i c a c i d (12 ml t o t a l ) i s necessary. The flameless AAS, S-309, method suggests use of 10 ml of 3M HC1 f o r a f i n a l s o l u t i o n . Recent work shows a d d i t i o n of N a S 0 or (NH/) SO^ i s necessary i n standards to b a l ance s u l f a t e extracted from f i l t e r s . The new method on Be should be forthcoming i n Volume Four of the NIOSH manual. The background c o r r e c t o r i s necessary to minimize f a l s e p o s i t i v e s i g n a l from molecular s c a t t e r i n g at 234.9 nm. An o l d e r Be method, Ρ & CAM 121, d i s c u s s e s a i r , dust, ore, and swipe samples. More vigorous d i g e s t i o n procedures such as h y d r o f l u o r i c : n i t r i c a c i d f o r f i l t e r s or potassium fluoride:sodium p y r o s u l f a t e f u s i o n and n i t r i c a c i d d i g e s t i o n f o r ores. A n i t r o u s oxide/acetylene flame i s suggested f o r t h i s method r a r e l y used (Table I I I ) . 2
4
2
Dollberg and Verstuyft; Analytical Techniques in Occupational Health Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
14.
Atomic
VERSTUYFT
Absorption
247
Spectroscopy
ο t>0
3
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