Chemical Hazards in the Workplace - American Chemical Society

12 Solid Sorbents for Workplace Sampling

Downloaded by UNIV OF ARIZONA on January 7, 2013 | http://pubs.acs.org Publication Date: April 2, 1981 | doi: 10.1021/bk-1981-0149.ch012

ELLEN C. GUNDERSON and ELLEN L. FERNANDEZ SRI International, Menlo Park, CA 94025

Worker exposure to toxic vapors is monitored by collecting samples in the breathing zone of the worker. These samples are usually returned to a laboratory for analysis. A suitable sampling pump and collection device are required. Lightweight, battery-operated pumps are available in a variety of flowrates. Collection devices are usually sorbent tubes or bubblers or impingers, but bags and evacuated containers have also been used. This discussion will focus on solid sorbents as collection media. The examples presented in this paper are based on results of our laboratory method development and validation studies. These studies, performed at both SRI International and Arthur D. Little, Inc., were supported by the National Institute for Occupational Safety and Health (NIOSH) from 1974 to 1979. In an effort to provide validated sampling and analytical methods for determining worker exposure to toxic substances, we validated existing methods when possible and developed and validated new procedures when no methods were available. Evaluation and testing of solid sorbents played a major role throughout this work (1). Typical of solid sorbent tubes used to sample a workplace environment are the commercially available charcoal tubes. These glass tubes are about 5-6 cm in length (6-mm O.D. by 4-mm I.D.), containing approximately 100 mg of sorbent in a front section and 50 mg of sorbent in a back section separated by a polyurethane plug. Other sorbent tubes vary depending on their application, the most obvious difference being the sorbent contained in the tube. The amount of sorbent may be increased to increase the capacity, or larger I.D. tubing may be used to reduce the pressure drop and allow higher sampling rates. The backup sorbent section may be contained in a separate tube, for instance, i f thermal desorption is used or i f the analyte has been shown to migrate during storage. The sorbent tube is placed near the worker's breathing zone, and the outlet of the tube is attached to a calibrated personal sampling pump. A known volume of air is drawn through the tube. A l ternatively, several passive charcoal badges are currently commercially available. No sampling pump is required for these devices, 0097-6156/81/0149-0179$05.00/0 © 1981 American Chemical Society

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

C H E M I C A L HAZARDS IN T H E W O R K P L A C E

180

and the d i f f u s i o n r a t e o f the substance i n t o the c h a r c o a l d e t e r mines the amount c o l l e c t e d . P a s s i v e badges may soon be a v a i l a b l e w i t h sorbents other than c h a r c o a l . The same f a c t o r s may be r e l e vant i n s e l e c t i n g a sorbent f o r a s p e c i f i c compound u s i n g these pass i v e sampling methods. F a c t o r s A f f e c t i n g Sorbent S e l e c t i o n S o l i d sorbents are m a t e r i a l s w i t h a microporous s t r u c t u r e , whose i n t e r n a l pores and o u t e r s u r f a c e s a r e a c c e s s i b l e f o r s o r p t i o n . T y p i c a l sorbents used f o r the c o l l e c t i o n o f a i r samples have nomina l s i z e of 20/40 mesh, w i t h pore diameters l e s s than 50 Â, g i v i n g r i s e to s u r f a c e areas up to 1000 m /g. Most s o l i d sorbents r e l y on vapors being sorbed by a p h y s i c a l a d s o r p t i o n mechanism: the substance enters the i n t e r n a l pores o f the sorbent and i s h e l d there by a t t r a c t i v e f o r c e s c o n s i d e r a b l y weaker and l e s s s p e c i f i c than those o f chemical bonds. These weakl y a t t r a c t i v e f o r c e s f a c i l i t a t e d e s o r p t i o n f o r subsequent a n a l y s i s . The mechanisms f o r p h y s i c a l a d s o r p t i o n have been s t u d i e d e x t e n s i v e l y and are d e s c r i b e d mathematically by equations such as the Langmuir isotherm. Other sorbent tubes are designed t o t r a p vapors by a chemical mechanism. Chemical s o r p t i o n i s almost always accomplished by coati n g a s o l i d sorbent o r support w i t h the d e s i r e d r e a c t a n t . I n t h i s way, the substance o f i n t e r e s t i s not only removed from the gas stream b e i n g sampled, b u t i s a l t e r e d c h e m i c a l l y . T h i s type o f sorpt i o n has the advantages o f b e i n g more s e l e c t i v e and rendering react i v e substances s t a b l e . Many v a r i a b l e s may a f f e c t the a b i l i t y o f a sorbent to c o l l e c t an a n a l y t e . Some o f the r e l e v a n t f a c t o r s are discussed below.


2

Surface Area and Mesh S i z e . 20/40 mesh sorbent i s g e n e r a l l y used to minimize the pressure drop across a sorbent tube. Some s o r bents, such as Tenax-GC, are n o t a v a i l a b l e i n these mesh s i z e s , b u t can s t i l l be used by i n c r e a s i n g the c r o s s - s e c t i o n a l area o f the samp l i n g tube to lower the o v e r a l l pressure drop f o r these s m a l l e r s o r bent p a r t i c l e s . Surface areas vary from very low i n Tenax-GC t o over 1000 m /g i n a c t i v a t e d c h a r c o a l . I n some cases, the g r e a t e r surface area may i n c r e a s e the c a p a c i t y o f the sorbent. 2

Pore S i z e and D i s t r i b u t i o n . The pore diameter must be s u f f i c i e n t t o a l l o w the substances o f i n t e r e s t to migrate i n t o the pores to the adsorbing s u r f a c e . Sorbents w i t h very s m a l l pores, such as the carbon molecular s i e v e s , are used t o c o l l e c t s m a l l molecules l i k e permanent gases (e.g., methyl formate on Carbosieve B ) . Surface Groups. Another f a c t o r a f f e c t i n g s o r p t i o n c h a r a c t e r i s t i c s i s the chemical nature o f the adsorbing s u r f a c e s . S o l i d s o r bent s u r f a c e s may c o n t a i n h y d r o x y l o r phenyl groups, a c i d o r base groups, o r even r e a c t i v e groups such as o l e f i n s . P o l a r i t i e s a l s o


12.

GUNDERSON A N D F E R N A N D E Z

vary greatly enhance

Solid Sorbent

among s o r b e n t s .

the s e l e c t i v i t y

Some s o r b e n t s

for specific

With the p o s s i b l e exception

Sampling

181

are s p e c i a l l y coated

to

substances.

o f pore s i z e

and d i s t r i b u t i o n ,

these c h a r a c t e r i s t i c s c a n a f f e c t b o t h p h y s i c a l and c h e m i c a l s o r p t i o n mechanisms. In chemical s o r p t i o n , the o v e r r i d i n g f a c t o r i s the s u r f a c e groups on the s o r b e n t . However, the s o r b e n t substrate i t s e l f can a f f e c t the a b i l i t y o f the coated sorbent to c o l l e c t a substance, rapidly.

e s p e c i a l l y i f the d e s i r e d r e a c t i o n

does n o t o c c u r


T a b l e I shows some o f t h e c h a r a c t e r i s t i c s o f t h e commonly u s e d sorbents. T h e c h a r c o a l s a r e b y f a r t h e most f r e q u e n t l y u s e d s o l i d sorbent f o r organic vapors. O v e r 130 methods h a v e b e e n v a l i d a t e d i n our f i v e - y e a r study u s i n g coconut, p e t r o l e u m , and s y n t h e t i c c h a r coal. The o t h e r s o r b e n t s i n c l u d e s i l i c a g e l , used p r i m a r i l y f o r a m i n e s , and porous p o l y m e r s , u s e d f o r s u b s t a n c e s n o t amenable t o c o l l e c t i o n on c h a r c o a l o r s i l i c a g e l . O t h e r r e s e a r c h e r s have used o t h e r s o r b e n t s , i n c l u d i n g F l o r i s i l , a l u m i n a , and m o l e c u l a r s i e v e s . The c h a r a c t e r i s t i c s o f t h e s u b s t a n c e t o b e c o l l e c t e d a l s o be c o n s i d e r e d , i n c l u d i n g t h e v a r i a b l e s o f p h y s i c a l p r o p e r t i e s , i c a l p r o p e r t i e s , r e a c t i v i t y , and i n t e r f e r e n c e s .

must chem-

Physical Properties. The s i z e , m o l e c u l a r w e i g h t , and v a p o r p r e s s u r e determine the pore s i z e necessary to t r a p and r e l e a s e a substance e f f i c i e n t l y . L a r g e m o l e c u l e s such as p e s t i c i d e s cannot be c o l l e c t e d and r e c o v e r e d on m o l e c u l a r s i e v e s ; n o r w i l l s m a l l m o l e c u l e s l i k e gaseous h y d r o c a r b o n s be r e t a i n e d w e l l b y porous p o l y m e r s . Chemical P r o p e r t i e s . I f a s u b s t a n c e i s h i g h l y p o l a r , i t may b e r e a d i l y c o l l e c t e d b y c h a r c o a l , b u t i t may b e d i f f i c u l t o r e v e n impossible to recover i t . A l s o , some s u b s t a n c e s may b e r e a d i l y h y d r o l y z e d a n d i t may b e b e s t t o c o l l e c t t h e s e o n h y d r o p h o b i c s o r bents

like

a porous polymer.

Reactivity. Some s u b s t a n c e s may decompose o r b e a l t e r e d i n s t r u c t u r e a f t e r c o l l e c t i o n , shipment, and s t o r a g e o f samples on some s o r b e n t s . O f t e n d e c o m p o s i t i o n can be minimized by s e l e c t i n g sorbents that are i n e r t o r w i l l s t a b i l i z e the substances o f i n t e r est. An example i s the c o l l e c t i o n o f amines o n a c i d - c o a t e d s i l i c a gel. T h e a c i d f o r m s a s a l t and p r e v e n t s t h e o x i d a t i o n o f t h e amine groups. Interferences. an

I n t e r f e r i n g substances

i n d u s t r i a l environment.

affect

not only

sorbent,

Their

identities

may a l s o

the s t a b i l i t y of the substance

but also

the capacity

be p r e s e n t

in

a n d c o n c e n t r a t i o n s may of interest

on the

of the sorbent.

A substance c o l l e c t e d on a s o l i d sorbent i s u s u a l l y desorbed with a solvent. D i f f e r e n t s o l v e n t s g i v e r i s e to d i f f e r e n t r e c o v eries. We h a v e f r e q u e n t l y f o u n d i t n e c e s s a r y t o t e s t s e v e r a l s o r b e n t / s o l v e n t c o m b i n a t i o n s t o d e t e r m i n e which b e s t c o l l e c t s and




182

T A B L E I.

PROPERTIES O F SOLID SORBENTS

SURFACE AREA, SORBENT Charcoals

COMPOSITION

m /gm 2

PORE SIZE J

Carbon

Coconut based

800-1000

20

Petroleum based

800-1000

18-22

Synthetic (Carbosieve B) Silica gel

SiO,

1000

10-12

300-800

20-40

Porous polymers Tenax GC

2,6-Diphenyl-p-phenyleneoxide

19

Chromosorb 101

Styrene/divinylbenzene

50

Chromosorb 102/X A D-2

Styrene/divinylbenzene

300-400

85

Chromosorb 103

Cross-linked polystyrene

15-25

3000-4000

Chromosorb 104

Acrylonitrile/divinylbenzene

100-200

600-800

Chromosorb 106


700-800

50

Chromosorb 108


100-200

235

Porapak Q

Ethylvinyl/divinylbenzene

3000-4000


75

12.


Solid Sorbent

desorbs the substance o f i n t e r e s t . c r i t e r i a to be acceptable.

Sampling

183

A solvent must meet c e r t a i n


The analyte must be e f f i c i e n t l y recovered. The usual mechanism f o r s o l v e n t desorption i s s e l e c t i v e displacement o f the a n a l y t e . S e l e c t i v e displacement occurs as a more p o l a r s o l v e n t d i s p l a c e s a l e s s p o l a r one on c h a r c o a l , j u s t as a more a c t i v e i o n d i s p l a c e s a l e s s a c t i v e one on i o n exchange r e s i n s . CS2 i s f r e q u e n t l y used to recover substances from c h a r c o a l , but simple a l c o h o l s cannot be d i s placed from c h a r c o a l by CS2, and i t i s necessary to add l % - 5 % o f another a l c o h o l t o the CS2 t o f a c i l i t a t e d e s o r p t i o n . Frequently, low r e c o v e r i e s can be increased by i n c r e a s i n g the q u a n t i t y o f s o l vent, i f a n a l y t i c a l s e n s i t i v i t y permits. P r o s p e c t i v e s o l v e n t s may be chosen based on p o l a r i t y o r s o l u b i l i t y o f the a n a l y t e . I t must be compatible w i t h the a n a l y t i c a l method. The f r e quent use o f CS2 as a s o l v e n t i s favored because CS2 produces a low response when a n a l y s i s i s performed by gas chromatography w i t h flame i o n i z a t i o n d e t e c t i o n (GC/FID). L i k e w i s e , low UV-absorbing s o l v e n t s a r e f r e q u e n t l y used i n h i g h performance l i q u i d chromatography (HPLC) t o minimize s o l v e n t i n t e r f e r e n c e when using a UV detector. I t must not react w i t h the a n a l y t e . For example, CS2 cannot be used t o desorb amines because chemical r e a c t i o n s occur. I t must be compatible w i t h the sorbent. We have found that v a r i o u s s o l v e n t s , notably CS2 and CH2CI2, d i s s o l v e some porous p o l y mers and are u n s u i t a b l e f o r use. L i k e w i s e , aqueous bases produce a g e l - l i k e substance w i t h s i l i c a g e l and should be avoided. The s o l v e n t should be n o n t o x i c , where p o s s i b l e . E a r l y work o r sorbent/sorbent combinations f r e q u e n t l y used benzene o r CCli^ as desorbents. I f p o s s i b l e , these should be replaced by l e s s t o x i c solvents. Method V a l i d a t i o n Tests must be performed on a p r o s p e c t i v e sorbent t o ensure v a l i d i t y o f the sampling and a n a l y t i c a l methods. The c r i t e r i a e s t a b l i s h e d by NIOSH f o r t e s t i n g a sorbent are summarized below. Det a i l s o f these c r i t e r i a are given i n Reference 2_. Recovery. The recovery o f the substance from the sorbent must be a t l e a s t 75%; we have p r e f e r r e d recovery t o be g r e a t e r than 90% i n our s t u d i e s . For v a l i d a t e d methods, r e c o v e r i e s o r d e s o r p t i o n e f f i c i e n c i e s are given i n NIOSH Backup Data Reports ( 3 ) . Capacity. Breakthrough volume should be a t l e a s t 1.5 times the recommended sampling volume a t twice the OSHA standard a t 80%


184

C H E M I C A L HAZARDS IN

THE

WORKPLACE

r e l a t i v e humidity. This requirement i s to ensure that there i s s u f f i c i e n t c a p a c i t y of the sorbent tube, w i t h s u f f i c i e n t margin f o r i n t e r f e r i n g substances that may be present. The humidity i s a l s o important because h i g h humidity has been shown to reduce the capac i t y of c h a r c o a l and s i l i c a g e l .


Storage S t a b i l i t y * I t must be shown that no g r e a t e r than 10% l o s s occurs i n samples s t o r e d f o r seven days under ambient c o n d i t i o n s . This w i l l ensure ample time to s h i p the samples from the f i e l d to the l a b o r a t o r y . P r e l i m i n a r y t e s t s may be performed on spiked samples, but the f i n a l t e s t i n g must be done w i t h samples c o l l e c t e d from t e s t a i r , s i n c e d i s t r i b u t i o n o f the a n a l y t e on the s o r bent i s d i f f e r e n t f o r spiked samples. P r e c i s i o n and Accuracy. S t a t i s t i c a l requirements f o r p r e c i s i o n and accuracy have been e s t a b l i s h e d to ensure t h a t the method i s r e p r o d u c i b l e and f r e e from b i a s . Commonly Used Sorbents The sorbents most commonly used i n i n d u s t r i a l hygiene sampling are c h a r c o a l , s i l i c a g e l , and the porous polymers. In a d d i t i o n , a number o f methods have been developed u s i n g coated sorbents. Each sorbent i s discussed b r i e f l y i n t h i s s e c t i o n . Charcoal. A c t i v a t e d coconut c h a r c o a l has gained the s t a t u s as the almost u n i v e r s a l s o l i d sorbent. Petroleum-based c h a r c o a l i s l e s s a c t i v e , but i s a l s o w i d e l y used. Charcoal i s a very e f f e c t i v e sorbent and i s g e n e r a l l y used f o r c o l l e c t i o n of nonpolar o r g a n i c s o l v e n t vapors. I t a l s o c o l l e c t s p o l a r o r g a n i c s , but they f r e q u e n t l y cannot be recovered. However, many organic substances that are r e a c t i v e , p o l a r , or oxygenated (e.g., chloroprene, a c e t i c a c i d , and acetone) have been s u c c e s s f u l l y c o l l e c t e d and recovered from charc o a l . Substances f o r which c h a r c o a l tube methods have been v a l i dated are l i s t e d i n Table I I . Many of the c h a r c o a l tube methods are based on NIOSH Method P&CAM 127 (4) f o r o r g a n i c s o l v e n t s . I n t h i s method, a known volume of a i r i s drawn through a c h a r c o a l tube to trap o r g a n i c vapors, the c h a r c o a l i s t r a n s f e r r e d to a v i a l , and the sample i s desorbed w i t h carbon d i s u l f i d e . The sample i s analyzed by gas chromatography (GC) w i t h flame i o n i z a t i o n d e t e c t i o n (FID). Most methods use CS2 as the d e s o r p t i o n s o l v e n t because i t y i e l d s good r e c o v e r i e s from c h a r c o a l and produces a very low flame response. Charcoal does have l i m i t a t i o n s that can a f f e c t i t s performance. Humidity may reduce the c a p a c i t y of the c h a r c o a l tube, or i n t e r f e r i n g substances may d i s p l a c e the substance of i n t e r e s t . Frequent periods of h i g h l e v e l exposure may exceed the breakthrough volume f o r very v o l a t i l e m a t e r i a l s . The s y n t h e t i c charcoals o r g r a p h i t i z e d carbons, such as Carbos i e v e B, are s i m i l a r i n p h y s i c a l c h a r a c t e r i s t i c s to the n a t u r a l



T A B L E II.


Acetic acid Acetone Acetonitrile Acrylonitrile Aliyl alcohol Ally 1 chloride n-Amyl acetate sec-Amyl acetate Arsine Benzene Benzyl chloride Bromoform Butadiene 2-Butanone 2-Butoxy ethanol sec-Butyl acetate t-Butyl acetate η-Butyl acetate sec-Butyl alcohol t-Butyl alcohol η-Butyl alcohol η-Butyl glycidyl ether p-tert-Butyl toluene Camphor Carbon disulfide Carbon tetrachloride Chlorobenzene Chlorobromomethane Chloroform Chloroprene Cumene Cyclohexane Cyclohexanol Cyclohexanone Cyclohexene Diacetone alcohol Dichlorodifluoromethane Dichloromonofluoromethane Oichlorotetrafluoroethane 1,1 -Dichloro-1 -nitroethane o-Dichlorobenzene p-Dichlorobenzene 1,1-Dichloroethane Dichloroethyl ether 1,2-Dichloroethylene Difluorodibromomethane Diisobutyl ketone Dimethylamine Dioxane Dipropylene glycol methyl ether

Solid Sorbent

Sampling

V A L I D A T E D C H A R C O A L METHODS

2-Ethoxyethanol Ethyl chloride Epichlorhydrin 2-Ethoxyethylacetate Ethyl acetate Ethyl acrylate Ethyl alcohol Ethyl benzene Ethyl bromide Ethyl chloride Ethyl butyl ketone Ethyl ether Ethyl formate Ethylene chlorhydrin Ethylene dibromide Ethylene dichloride Ethylene oxide Fluorotrichloromethane Glycidol Heptane Hexachloroethane Hexane 2-Henanone Hexone Isoamyl acetate Isoamyl alcohol 1 so butyl acetate Isobutyl alcohol Isophorone Isopropyl acetate Isopropyl alcohol Isopropyl glycidyl ether Isopropyl ether Mesityl oxide Methyl (n-amyl)ketone (2-heptanone) Methyl chloride Methylcyclohexanol 5-Methyl-3-heptanone Methyl acetate Methyl acrylate Methyl bromide Methyl cellosolve Methyl cellosolve acetate Methyl chloroform Methyl iodide Methyl isoamyl acetate Methyl isobutyl carbinol

α-methyl styrene Methylal Methylcyclohexane Methylene chloride Naphthalene Naphtha Octane Pentane 2-Pentanone Petroleum distillates Phenyl ether vapor Phenyl glycidyl ether η-Propyl acetate Propyl alcohol η-Propyl nitrate Propylene dichloride Propylene oxide Pyridine Stoddard solvent Stryene Tellurium hexafluoride 1,1,2,2-Tetrachloro-1,2difluoroethane 1,1,1,2-Tetrachloro-2,2difluoroethane 1,1,2,2-Tetrachloroethane Tetrachloroethylene Tetrahydrofuran Tetramethyl succinonitrile Toluene 1,1,2-Trichloro-1 2,2-trifluoroethane 1,1,2-Trichloroethane Trichloroethylene 1,2,3-Trichloropropane Trifluorobromomethane Turpentine f

Vinyl toluene Xylene

Synthetic Charcoal Carbosi Alkyl mercury compounds Methyl formate



186

THE

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c h a r c o a l s . However, the s u r f a c e i s not as c h e m i c a l l y a c t i v e perhaps due to i m p u r i t i e s i n n a t u r a l c h a r c o a l s . In some cases t h i s may be used to advantage. For i n s t a n c e , methyl formate c o u l d not be r e covered from coconut o r petroleum c h a r c o a l , but s a t i s f a c t o r y r e s u l t s were obtained w i t h Carbosieve B.


S i l i c a G e l . S i l i c a g e l i s a p o l a r adsorbent, i t s s u r f a c e cont a i n i n g h y d r o x y l groups. Thus p o l a r substances have a s t r o n g a t t r a c t i o n toward s i l i c a g e l . Since water i s h i g h l y p o l a r , s i l i c a gel r e t a i n s atmospheric m o i s t u r e , sometimes p r e f e r e n t i a l l y over other substances. This a f f i n i t y f o r moisture l i m i t s the use of s i l i c a g e l , although i n a dry environment i t would be an e x c e l l e n t sorbent. The compounds f o r which our l a b o r a t o r i e s have v a l i d a t e d methods u s i n g s i l i c a g e l as a c o l l e c t i o n medium are l i s t e d below: Acetylene tetrabromide Aniline Chloroacetaldehyde Cresol Di e thylaminoe thano1 Diethylamine Dimethylacetamide D ime thy 1 f o rmami de Dime thy lamine Ethylamine n-Ethylmorpholine

Methylamine Methyl a l c o h o l Morpholine Nitrobenzene p-Nitrochlorobenzene Nitrotoluene Phenyl e s t e r - b i p h e n y l vapor mix o-Toluidine Xylidine

This l i s t i n c l u d e s s e v e r a l a l i p h a t i c and aromatic amines that are s o l u b l e i n water and t h e r e f o r e are not s i g n i f i c a n t l y a f f e c t e d by r e t e n t i o n of moisture on the s i l i c a g e l sorbent tube. In some cases, such as w i t h chloroacetaldehyde, c o l l e c t i o n of moisture may even improve the c o l l e c t o r . Chloroacetaldehyde forms a very s t a b l e hydrate when c o l l e c t e d from humid atmospheres w i t h s i l i c a g e l , bes i d e s being e f f i c i e n t l y c o l l e c t e d from a dry environment. Porous Polymers. When c h a r c o a l and s i l i c a g e l are not accepta b l e — b e c a u s e of poor recovery, low breakthrough volume, o r poor storage s t a b i l i t y — p o r o u s polymers should be considered. These polymers i n c l u d e Tenax-GC and the XAD, Chromosorb, and Porapak s e r ies. They are r e l a t i v e l y i n e r t , hydrophobic, and u s u a l l y of h i g h s u r f a c e area. Most porous polymers do not r e t a i n v o l a t i l e s , i n c l u d ing water and s o l v e n t s , but t h i s may be an advantage when sampling an atmosphere c o n t a i n i n g h i g h concentrations o f water or s o l v e n t vapors. L i s t e d below are the compounds f o r which methods have been v a l i d a t e d i n our s t u d i e s u s i n g porous polymers. These methods have been s t u d i e d more thoroughly t h a t others because they presented more problems than those where c h a r c o a l o r s i l i c a g e l could be used. This i n f o r m a t i o n i s contained i n the Backup Data Reports f o r each method.


12.



Tenax-GC A l l y l g l y c i d y l ether 2-Aminopyridine Diphenyl EGDN and/or n i t r o g l y c e r i n Phosphorus y e l l o w XAD-2 A n i s i d i n e (o- and/or p-) Ethyl s i l i c a t e Methyl methacrylate Nicotine Nitroethane Quinone Tetraethyl lead Tetramethyl lead

Solid Sorbent

Sampling

187

Chromosorb 102 Phosdrin Heptachlor Chromosorb 104 η-Butyl mercaptan Chromosorb 108 1-Chloro-l-nitropropane Chromosorb 103 Formic a c i d Porapak Q Furfural alcohol Methylchohexanone Chromosorb 106 Nitromethane

Porous polymers are u s u a l l y most u s e f u l f o r the c o l l e c t i o n o f h i g h molecular weight and n o n v o l a t i l e substances such as p e s t i c i d e s . They may a l s o be s u i t a b l e f o r sampling many o f the compounds f o r which c h a r c o a l and s i l i c a g e l methods have been v a l i d a t e d . However, s i n c e c h a r c o a l and s i l i c a g e l tubes are l e s s expensive than porous polymer tubes, are commercially a v a i l a b l e , and are already w i d e l y used, they are the favored media. The most s e r i o u s l i m i t a t i o n t o the use o f porous polymers i s batch-to-batch v a r i a t i o n . We have found tremendous v a r i a t i o n i n recovery and c a p a c i t y t e s t s w i t h d i f f e r e n t manufacturer's l o t s o f the same sorbent. Often even an extensive clean-up procedure d i d not r e s u l t i n r e p r o d u c i b l e data. For t h i s reason, we recommend t e s t i n g s e v e r a l manufacturer's l o t s . This o f t e n r e q u i r e s more ex t e n s i v e method development than r e q u i r e d w i t h c h a r c o a l and s i l i c a gel methods. An example o f method development u s i n g porous polymers i s the work done f o r η-butyl mereaptan. η-Butyl mereaptan c o l l e c t e d on c h a r c o a l was found t o o x i d i z e r e a d i l y to the d i b u t y l d i s u l f i d e . I t was not f e a s i b l e t o analyze f o r the mercaptan as the d i s u l f i d e , be cause the d i s u l f i d e could a l s o be present i n the workplace. S i l i c a gel was an e x c e l l e n t c o l l e c t o r f o r the mercaptan i n a dry atmo sphere; however, a t 80% r e l a t i v e humidity the sorbent c o l l e c t e d moisture p r e f e r e n t i a l l y , and sorbent c a p a c i t y was s e v e r e l y reduced. Porous polymers such as XAD-2, Chromosorbs, and Porapacks were then i n v e s t i g a t e d . Most demonstrated s a t i s f a c t o r y c a p a c i t y and short term recovery, but long term s t o r a g e s t a b i l i t y on c e r t a i n s o r bents was a problem. Chromosorb 104, a porous polymer made from a c r y l o n i t r i l e - d i v i n y l b e n z e n e monomers r a t h e r than from s t y r e n e - d i vinylbenzene monomers as most others a r e , demonstrated s a t i s f a c t o r y r e c o v e r i e s a f t e r a 7-day storage p e r i o d . Since mercaptans a r e known t o r e a c t w i t h o l e f i n s and i n h i b i t p o l y m e r i z a t i o n s , i t was thought that the mercaptan might be i r r e v e r s i b l y r e a c t i n g w i t h unreacted monomers i n the polymer sorbent. Prewashing procedures d i d


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not s i g n i f i c a n t l y a l t e r the c h a r a c t e r i s t i c s of the sorbents. Chromosorb 104 appeared to have fewer r e a c t i v e s i t e s f o r the mercaptan.

The

Coated Sorbents. When c o l l e c t i o n and recovery o f a s p e c i f i c substance cannot be achieved u s i n g c h a r c o a l , s i l i c a g e l , or porous polymers, chemical s o r p t i o n w i t h a coated sorbent may be necessary. Compounds r e q u i r i n g t h i s method of c o l l e c t i o n are u s u a l l y too react i v e or unstable to be c o l l e c t e d and s t o r e d by other means. In t h i s case, a s p e c i f i c s t a b l e d e r i v a t i v e or unique product c h a r a c t e r i s t i c of the compound of i n t e r e s t i s d e s i r e d . Examples o f some of the products of chemical s o r p t i o n i n c l u d e (1) the formation of amine s a l t s when sampling f o r ammonia and o r ganic amines w i t h acid-coated s i l i c a g e l , (2) amalgamation o f s i l ver w i t h mercury when c o l l e c t i n g mercury vapor on s i l v e r - c o a t e d Chromosorb P, and (3) formation of the D i e l s - A l d e r adduct when c o l l e c t i n g cyclopentadiene on Chromosorb 104 coated w i t h maleic anhydride. V a l i d a t e d methods u s i n g coated sorbents are l i s t e d below. Sorbent Acid-coated s i l i c a g e l Ag-coated Chromosorb p TEA-coated molecular s i e v e HgCN-coated s i l i c a g e l HgCl2~coated s i l i c a g e l M a l e i c anhydride-coated Chromosorb 104

Analyte Ammonia n-Butylamine Mercury Nitrogen dioxide Phosphine Stibine Cyclopentadiene

Development and t e s t i n g of these s p e c i a l i z e d sorbents i s u s u a l l y a lengthy process. The r e a c t i o n must f i r s t be shown to proceed on the sorbent, i n a d d i t i o n to recovery, c a p a c i t y , and storage s t a b i l i ty t e s t s . The procedure f o r p r e p a r a t i o n o f the sorbent must be shown to be c o n s i s t e n t w i t h s e v e r a l batches of prepared sorbent. The s t a b i l i t y of the coated sorbent alone may be a f a c t o r and must be t e s t e d . A f t e r a l l the developmental work i s completed, the s p e c i a l i z e d sorbent i s f r e q u e n t l y u s e f u l f o r o n l y one substance. Method Development Using S o l i d Sorbents F i g u r e 1 i l l u s t r a t e s the procedure t h a t has been used i n our l a b o r a t o r i e s to develop sampling and a n a l y t i c a l methods f o r substances that may be c o l l e c t e d on s o l i d sorbents. The general approach w i l l f i r s t be d i s c u s s e d , and then an example w i l l be given. In the general approach, s e v e r a l steps may be i s o l a t e d , but i n a c t u a l method development, these steps are i n t e r r e l a t e d . L i t e r a t u r e Search. Method development begins w i t h a l i t e r a ture search, g a t h e r i n g i n f o r m a t i o n such as p h y s i c a l and chemcial


12.

GUNDERSON AND F E R N A N D E Z

Solid Sorbent

Sampling

LITERATURE SEARCH


•

Physical Properties

•

Chemical Properties

•

Analytical Methods

•

Sampling Methods

SAMPLING METHOD DEVELOPMENT

VALIDATION STUDIES •

Generation of Test Atmospheres

•

Precision and Accuracy

•

Storage Stability

Figure 1.

Solid sorbent method development


189


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p r o p e r t i e s of the a n a l y t e , a n a l y t i c a l and sampling methods, t o x i c o l o g i c a l data, and i n d u s t r i a l usage and occurrence.


A n a l y t i c a l Method Development. From the p o i n t of view of s o r bent s e l e c t i o n , the important f a c t o r s to c o n s i d e r i n a n a l y t i c a l method development are sorbent/solvent c o m p a t i b i l i t y and the detect i o n l i m i t of the a n a l y t e . Sampling Method Development. S e l e c t i o n of a sampling medium i s f r e q u e n t l y the most lengthy process i n method development. Various sorbent/solvent combinations are f i r s t t e s t e d f o r p r e l i m i n a r y r e c o v e r i e s . Charcoal should be considered f i r s t because of i t s widespread use and a v a i l a b i l i t y . I f c h a r c o a l or s y n t h e t i c c h a r c o a l i s not acceptable, s i l i c a g e l and the porous polymers should be considered. I f porous polymers f a i l to g i v e s u c c e s s f u l r e s u l t s , then d e r i v a t i z a t i o n o f the a n a l y t e on a surface-coated sorbent should be considered. Whichever sorbent appears most promising should be t e s t e d at an e a r l y stage f o r storage s t a b i l i t y of spiked samples and breakthrough volumes w i t h t e s t atmospheres. V a l i d a t i o n Tests. A f t e r sampling and a n a l y t i c a l methods have been developed, v a l i d a t i o n t e s t s are performed. The c r i t e r i a f o r a c c e p t a b i l i t y were discussed e a r l i e r and are d e t a i l e d i n Reference 2. V a l i d a t i o n t e s t s are performed by c o l l e c i t n g s e t s of samples from generated atmospheres of a known c o n c e n t r a t i o n and then anal y z i n g these samples. Some s e t s of samples are used to determine p r e c i s i o n and accuracy of the method, and another s e t i s used to determine storage s t a b i l i t y . I f , at any stage of t e s t i n g and v a l i d a t i o n , the method f a i l s , developmental work must r e v e r t to an e a r l i e r stage. Example o f Method Development As an example of t h i s method development procedure, l e t us cons i d e r the case of diphenyl ( b i p h e n y l ) . The p h y s i c a l and chemical p r o p e r t i e s of d i p h e n y l are given i n F i g u r e 2. We estimated the vapor pressure to be 0.05 mm at 25°C ( t h i s i s e q u i v a l e n t to 70 ppm). Chemically, diphenyl i s a comparatively nonreactive compound, not s u b j e c t to h y d r o l y s i s , and r e l a t i v e l y n o n v o l a t i l e . These chara c t e r i s t i c s do not exclude any p a r t i c u l a r sorbents. The i n t e r ferences i n a workplace cannot be g e n e r a l i z e d because of the compound's widespread and v a r i e d usage. However, because the OSHA standard f o r diphenyl i s only 0.2 ppm, i t i s l i k e l y that i n t e r f e r ing m a t e r i a l s may c o e x i s t at concentrations of g r e a t e r magnitude. T h e i r presence may a f f e c t the c a p a c i t y of a sorbent and should be considered. An e a r l i e r study at SRI concluded that diphenyl could not be c o l l e c t e d on a c t i v a t e d coconut c h a r c o a l because recovery was poor. The recovery of diphenyl from coconut c h a r c o a l i s given below.



GUNDERSON AND F E R N A N D E Z

Solid Sorbent

Sampling

Chemical Structure C

1 2

H

1 0

mw 154.20 mp

69.2° C

bp

255.2° C 1mm at 70.6°C

Vapor pressure

5mm at 101.8° C 10mm at 117.0° C Soluble inethanol ether CCI

4

benzene CS

2

methanol Insoluble in water Use: heat transfer fluid OSHA standard: 0.2 ppm

Figure 2.

Properties of diphenyl



192 Solvent CS

Recovery (%)

60.2 30.1 15.1

35.7 32.8 29.1

Benzene

60.2

52.0

CH C1

60.2

Chemical Hazards in the Workplace - American Chemical Society

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