3 An I n f r a r e d A n a l y s i s Method for the D e t e r m i n a t i o n of H y d r o c a r b o n s C o l l e c t e d o n
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Charcoal Tubes THOMAS C. THOMAS and ANDREW RICHARDSON III USAF Occupational and Environmental Health Laboratory, Brooks Air Force Base, San Antonio, TX 78235 In handling worldwide industrial hygiene problems f o r the A i r Force, our Laboratory receives a heavy work load of charcoal tube and vapor monitor air samples. With our work load increasing, and more methods being applied to gas chromatography, any method which would remove some of the work load from our overworked gas chromatographs would be welcomed. Any alternate methods would have to have similar sensitivities, be r e l a t i v e l y specific and able to analyze the samples quickly. We have found that infrared spectroscopy, when applied to hydrocarbon samples collected on charcoal tubes or vapor monitors, meets these requirements. To analyze charcoal tubes or vapor monitors for hydrocarbons by IR, the desorption solvent would have to be IR inactive i n the region 3100 to 2750 c m . Two such solvents: Freon 113 (1,1,2 trichloro-1,2,2-trifluoroethane) and perchloroethylene meet this requirement. Freon 113 desorbs hydrocarbons from charcoal at consistent % recoveries, however, does not meet the minimum desorption efficiency of >75% recommended by NIOSH (1_). However, a mixture of perchloroethylene and Freon 113 does provide a desorption efficiency >80% and can be analyzed by this IR method. Therefore, to check the p o s s i b i l i t y of using this IR procedure in our Laboratory, i t was decided to evaluate the procedure against the recommended NIOSH GC procedures. We have limited our study to the hydrocarbons: JP-4 aviation fuel and PD-680 cleaning solvent. In addition to evaluating the IR method for charcoal tubes and vapor monitors, we also compared the two methods on actual f i e l d samples. -1
R
Experimental Gas chromatography. A Varian Model 1800 gas chromatograph equipped with dual flame ionization detectors was used and This chapter not subject to U.S. copyright. Published 1981 American Chemical Society
In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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i t was coupled to a Hewlett-Packard 3350 Data System. A 10' X 1/8" s t a i n l e s s s t e e l column o f 10% 0V-101 on 80/100 mesh chromosorb W was used. The column, i n j e c t o r , and d e t e c t o r were a t 8 5 0 C , and 225°C, r e s p e c t i v e l y . Nitrogen c a r r i e r gas flow was at 20 ml/min; hydrogen gas flow was a t 35 ml/miη and a i r flow a t 400 ml/min. The charcoal tubes were broken open and the charcoal t r a n s f e r r e d i n t o a stoppered g l a s s t e s t tube. One m i l l i l i t e r of carbon d i s u l f i d e was p i p e t t e d i n t o each tube, and 1.5 ml of carbon d i s u l f i d e was p i p e t t e d i n t o each 3M vapor monitor badge. A f t e r 30 minutes, a l i q u o t s o f carbon d i s u l f i d e were i n j e c t e d i n t o the gas chromatograph and compared versus hydro carbon standards prepared i n carbon d i s u l f i d e . The t o t a l areas of the sample and standard peaks were measured by the data system. I n f r a r e d Spectroscopy. A Perkin-Elmer Model 283 I n f r a r e d Spectrometer was used. Normal o p e r a t i n g c o n d i t i o n s were used: normal s l i t , IX expansion, 12-minute scan speed (4000-200 wave numbers), and normal g a i n , i n accordance with Perkin-Elmer setup i n s t r u c t i o n s . A Beckman 2 cm path l e n g t h , near i n f r a r e d s i l i c a c e l l (holds 8 ml sample) was used to hold the desorbing s o l u t i o n i n the sample compartment ( F i g u r e 1). The charcoal tubes were broken open and the charcoal t r a n s f e r r e d to stoppered g l a s s t e s t tubes. When checking Freon 113 as a desorbant, 10 ml was p i p e t t e d i n t o each tube. A f t e r 30 minutes, the s o l u t i o n was mixed and a n a l y z e d . When checking the perch!oroethylene/Freon 113 mixture, 0.5 ml o f p e r c h l o r o e t h y l e n e was i n d i v i d u a l l y p i p e t t e d i n t o each tube. A f t e r 30 minutes, 9.5 ml o f Freon 113 was added and each tube mixed on a vortex s t i r r e r . For the 3M vapor monitors, 0.5 ml of p e r c h l o r o e t h y l e n e was p i p e t t e d i n t o each monitor and allowed 30 minutes f o r d e s o r p t i o n . The monitors were then c a r e f u l l y r i n s e d with Freon 113 through a g l a s s funnel i n t o a 25 ml graduated c y l i n d e r . The f i n a l volume was a d j u s t e d to 10 ml with Freon 113 and mixed. The s o l u t i o n s were i n d i v i d u a l l y poured i n t o the s i l i c a c e l l , the c e l l placed i n the IR sample compartment and a scan made from 3130 to 2750 wave numbers. A s l o p i n g l i n e was drawn across the r e s u l t i n g s p e c t r a from the 3130 to the 2750 wave number ( F i g u r e 2 ) . The peak i n t e n s i t i e s were measured at 2958, 2926, and 2855 wave numbers. For each standard, the i n t e n s i t i e s a t these three wave numbers were measured and the sum p l o t t e d versus c o n c e n t r a t i o n . The sum from each sample was then compared to the standard curve ( F i g u r e 3 ) . Hydro carbon standards were prepared by i n j e c t i n g known amounts o f JP-4 and PD-680 i n t o a 5% p e r c h l o r o e t h y l e n e - 95% Freon 113 mixture.
In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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3.
THOMAS A N D RICHARDSON
Figure 1.
IR Analysis
of Hydrocarbons
Infrared cell used to hold extraction solutions
In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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CHEMICAL
mg. JP-4 'per tube Figure 3.
2
JP-4 A viation fuel standard curve
In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
3.
THOMAS A N D RICHARDSON
IR Analysis
of
Hydrocarbons
41
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Scope o f the Study A n a l y t i c a l Desorption Recovery. S p i k i n g with known amounts of JP-4 and PD-680 was done i n two ways. F i r s t , known amounts were i n j e c t e d i n t o the c e n t e r o f the f r o n t p o r t i o n o f the charcoal tube (SKC, NIOSH approved c h a r c o a l t u b e s ) . Secondly, the vapor monitors (3M 3500 o r g a n i c vapor monitors) were s p i k e d by i n j e c t i n g known amounts through the c e n t e r e l u t r i a t i o n p o r t of the monitor cap, onto a p i e c e o f f i l t e r paper placed between the e l u t r i a t i o n cap and the d i f f u s i o n p l a t e o f the vapor monitor ( 2 ) . The f i l t e r paper was removed a f t e r 24 hours and analyzed T o r any unvaporized p o r t i o n . The monitor was analyzed as d e s c r i b e d e a r l i e r . The s p i k i n g was done a t t h r e e l e v e l s , roughly corresponding t o 0.5X, 1 .OX, and 2.OX t h e o c c u p a t i o n a l standard o f these hydrocarbons, f o r a 10 l i t e r a i r volume. Recovery was i n i t i a l l y checked using o n l y Freon 113 as a desorbant, however, when r e c o v e r i e s were found t o be below 75% (Table I ) , the Freon 113/perchloroethylene mixture was e v a l u a t e d . A n a l y t i c a l P r e c i s i o n . A d d i t i o n a l l y , t h e a n a l y t i c a l method was t e s t e d t o assure t h a t i t s p r e c i s i o n was a c c e p t a b l e . T h i s t e s t was performed a t three d i f f e r e n t l e v e l s as b e f o r e , except t h a t the s p i k i n g was done d i r e c t l y i n t o known amounts o f the Freon 113/perchloroethylene mixture. S i x samples a t each o f the three c o n c e n t r a t i o n s were used t o form the b a s i c s t a t i s t i c a l s e t of data. The samples were compared t o p r e v i o u s l y prepared standards. F i e l d Sampling. An o p p o r t u n i t y arose where a c t u a l f i e l d samples c o u l d be analyzed by both the i n f r a r e d and gas chromatographic methods. A t Robins AFB, G e o r g i a , workers were i n s p e c t i n g and r e p a i r i n g the i n t e r i o r and e x t e r i o r o f C-141 a i r c r a f t f u e l tanks. They were exposed o n l y t o JP-4 f u e l fumes. D u p l i c a t e charcoal tubes o r vapor monitors were attached t o each worker, one on each l a p e l . Samples were drawn through the charcoal tubes a t 0.20 t o 0.26 1pm by p o r t a b l e pumps attached to the worker's b e l t . Because o f s l i g h t v a r i a t i o n s , the t o t a l volumes c o l l e c t e d f o r the d u p l i c a t e s were c l o s e , but n o t e x a c t l y the same i n a l l cases. Samples were then l a b e l e d and shipped t o o u r Laboratory f o r a n a l y s i s by both methods. Results and D i s c u s s i o n Recovery S t u d i e s . As can be seen from Table I , Freon 113 desorbed both hydrocarbons with c o n s i s t e n c y , however, a t l e v e l s l e s s than 75% r e c o v e r y . The d e s o r p t i o n e f f i c i e n c y was t h e same, whether t h e sample was allowed 30 minutes o r 24 hours f o r d e s o r p t i o n . One s u r p r i s i n g f a c t was t h a t the NIOSH approved gas chromatographic technique f o r PD-680 a l s o showed a d e s o r p t i o n recovery a t l e s s than 75%; t h i s was rechecked s e v e r a l times
In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
CHEMICAL
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to c o n f i r m t h i s f i n d i n g . The documentation o f NIOSH v a l i d a t i o n t e s t s f o r petroleum d i s t i l l a t e s showed an average recovery o f 95.7 +7.44% ( 3 ) . Our study showed a recovery f o r JP-4 c l o s e to 100% by NIOSH approved method. It was found t h a t i n i t i a l d e s o r p t i o n with 0.5 ml o f p e r c h l o r o e t h y l e n e i n c r e a s e s the recovery to g r e a t e r than 80%. It i s t h e o r i z e d t h a t p e r c h l o r o e t h y l e n e i n c r e a s e s the d e s o r p t i o n of unsaturated and aromatic components t h a t would not desorb i n t o pure Freon 113. There were s e v e r a l reasons why the i n i t i a l d e s o r p t i o n was accomplished with 0.5 ml p e r c h l o r o e t h y l e n e and then brought up to 10 ml with Freon 113. Using a 2 cm i n f r a r e d c e l l , p e r c h l o r o e t h y l e n e i s not completely IR i n a c t i v e i n the region o f i n t e r e s t . I t has a weak peak a t approximately 2870 cm-1, which would cause problems i f pure p e r c h l o r o e t h y l e n e were used. Secondly, use o f Freon 113 lowers the i n h a l a t i o n hazard to the a n a l y s t . Table II l i s t s the r e c o v e r i e s from charcoal tubes a t d i f f e r e n t l e v e l s using the Freon 113/perchloroethylene mixture. Table III l i s t s the r e c o v e r i e s from the 3M vapor monitors using the Freon 113/perchloroethylene mixture. Poor recovery was a l s o obtained f o r PD-680 using the NIOSH GC method. S t a t i s t i c a l S t u d i e s . The present suggested standard f o r a i r monitoring accuracy i s t h a t the a b s o l u t e t o t a l e r r o r (sampling and a n a l y s i s ) should be l e s s than 25% i n a t l e a s t 95% of samples analyzed a t the l e v e l o f the standard ( 1 ) . T h i s i m p l i e s t h a t the t r u e c o e f f i c i e n t o f v a r i a t i o n o f the t o t a l e r r o r should be no g r e a t e r than 0.128 d e r i v e d as f o l l o w s : CV = 0.25/1.96 = 0.128. The number 0.128 i s the l a r g e s t t r u e p r e c i s i o n f o r a net e r r o r a t + 2 5 % a t the 95% c o n f i d e n c e l e v e l . The number 1.96 i s the a p p r o p r i a t e t - s t a t i s t i c from the t d i s t r i b u t i o n a t the same confidence l e v e l . Since the c o e f f i c i e n t o f v a r i a t i o n o f pump e r r o r i s assumed to be 5%, a method should have a CV a n a l y s i s
