A Versatile Test Atmosphere Generation and Sampling System - ACS

33 A Versatile Test Atmosphere Generation and Sampling System

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SHUBHENDER KAPILA, RAVINDER K. MALHOTRA, and CORAZON R. VOGT Environmental Trace Substances Research Center, University of Missouri, Columbia, MO 65201 In recent years, a number of industrially important chemicals have been investigated for their potential carcinogenic, mutagenic and teratogenic properties. In order to protect workers exposed to such chemicals, maximum exposure limits have been set by OSHA. Several methods for the analysis of these chemicals (pollutants) have been reported. These methods can be divided into two catagories, those involving direct monitoring of the atmosphere and those involving trapping of pollutants on or in suitable medium followed by determination with appropriate techniques. Due to their harmful effect (even at very low concentrations), the need for accurate and precise determination can not be overstated. Furthermore, in instances where legal implications are involved, the reliability of the methods is of utmost importance. The most widely accepted method of evaluating the accuracy and precision of an analytical procedure is to sample known concentrations of contaminants in the atmosphere. Thus an important aspect of analytical method development is the generation of test atmospheres that simulate the conditions (i.e., concentration range, humidity, temperature and interferences) found during the field sampling. S e v e r a l t e s t atmosphere g e n e r a t i n g systems have been r e p o r t e d ( 1 - 6 ) , B a s i c p r i n c i p l e s and t e c h n i q u e s e m p l o y e d h a v e b e e n d i s c u s s e d i n d e t a i l b y N e l s o n ( 7 ) , The methods a r e g e n e r a l l y d i v i d e d i n t o two c a t a g o r i e s : s t a t i c methods a n d d y n a m i c methods. I n s t a t i c method a known amount o f c o n t a m i n a n t i s i n t r o d u c e d i n t o a f i x e d volume o f a i r i n d e v i c e s such a s t e f l o n b a g s , gas s a m p l i n g b u l b s a n d g a s c y l i n d e r s , e t c . Dynamic methods i n v o l v e continuous i n t r o d u c t i o n o f contaminant ( a t a c o n t r o l l e d r a t e ) i n t o a s t r e a m o f a i r . S t a t i c methods a r e g e n e r a l l y much s i m p l e r t o c o n s t r u c t a n d u s e , h o w e v e r , t h e s e s u f f e r f r o m a number o f p r o b l e m s . Dynamic m e t h o d s , w h i l e more e l a b o r a t e a n d r e l a t i v e l y more e x p e n s i v e , o f f e r g r e a t e r f l e x i b i l i t y i n c o n c e n t r a t i o n r a n g e , samp l e volume and a r e a l s o l e s s a f f e c t e d by a d s o r p t i o n l o s s e s .

0097-6156/81/0149-0533$05.00/0 © 1981 American Chemical Society

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

534

CHEMICAL

HAZARDS

IN

T H E

WORKPLACE


The present study describes a dynamic system f o r generating known concentrations of analytes i n simulated atmoshperes cont a i n i n g organic contaminants, both gases and l i q u i d s , at three d i f f e r e n t (concentration) l e v e l s simultaneously under c o n t r o l l e d humidity. An i n - l i n e monitoring device i n the form of a gas chromatograph (GC) was incorporated i n t o the system. Incorpora t i o n of the GC with an appropriate detector enhances the v e r s a t i l i t y of the o v e r a l l system, making i t s u i t a b l e f o r most v o l a t i l e organics. Thus i t o f f e r s a d i s t i n c t advantage over the s p e c i f i c gas analyzers used i n some previous generation f a c i l i t i e s (5-8). D e s c r i p t i o n and Working of the Generation

Facility

Generation system i s d i v i d e d i n t o a Primary and a Secondary d i l u t i o n module as shown i n Figure 1. Primary D i l u t i o n Module: The Primary D i l u t i o n Module i s used f o r d i l u t i o n of the analytes with n i t r o g e n . The volume of contaminant i s regulated by a very f i n e metering valve V 3 (Nupro V a l v e s ) . I t i s measured by rotameter R i and mixed with a known volume of n i trogen c o n t r o l l e d by a f i n e metering valve V 4 and measured by rotameter R 2 . The doped n i t r o g e n stream i s s p l i t i n t o two streams by a micro-valve V ^ Q ( S c i e n t i f i c Glass & Engineering Co.). The major p o r t i o n i s sent to a burner. The smaller p o r t i o n i s measured through rotameter R 3 and then f u r t h e r d i l u t e d with n i t r o g e n (rotameter R 4 ) . A d e s i r e d volume of t h i s d i l u t e d flow i s passed on to a four-port switching valve ( V a l c o ) . The contaminated a n a l y t e - n i t r o g e n stream can then be sent e i t h e r to the GC (Bendix Model 2500) through a s i x - p o r t sampling v a l v e V 2 (Valco) or to the secondary d i l u t i o n module. The sampling loop attached to the s i x - p o r t valve c o n s i s t s of a s t a i n l e s s s t e e l tube with a volume of 0.17 mL. The sample i n the loop i s i n j e c t e d i n t o the GC by switching the v a l v e , and the concentration of contaminant i n the n i t r o g e n stream i s determined by the f o l l o w i n g equation: r n"

a m t

o f

contaminant i n loop(g) s i z e of the loop (mL)

x

22.4xl0 M

3

X

where C^ = concentration of contaminant i n ^ M

= molecular weight of the contaminant

T

=

expt(l)

P

-

e x p t ( l ) pressure, mm

P 760 (in

X

T+273 273

1f|

6

ppm)

temperature, °C Hg

The amount of the contaminant i n the loop (g) i s determined from a c a l i b r a t i o n curve e s t a b l i s h e d at the time of the a n a l y s i s .


KAPILA

E T A L .

GC

Test Atmosphere

Carrier

Generation

535

gas Burner


To

Figure 1. Schematic of the test atmosphere generation system, various components are as follows: 1, air compressor (Gast model #5HCD); 2, dehumidifier; 3, activated charcoal filter (Alltech 8129); 4, molecular sieve and decimal filter (Alltech 8125); 5, humidifier with thermostated water bath (Neslab model G175); 6, digital mass flow meter (Matheson model 8240); 7, humidity monitor (YSI model 91HC); 8, humidity sensor (YSI double bobbin); 9, primary dilution chamber; 10, 11, secondary dilution chambers; PR, pressure regulators; R0-R7, rotameters with range of 5 mL to 150 mL/min (Brooks model AAA); V four-port switching valve (Valco valves); V , six-port sampling valve (Valco valves); V , double stem, very fine metering valve (Nupro #2SGD); V\-V , fine metering valves (Nupro #25G and 2MG); SPi-SPs, sampling ports. u

2

s

9


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The concentration of the contaminant can a l s o be c a l c u l a t e d from s p l i t r a t i o s and flow r a t e s using the f o l l o w i n g equation: C

C

n -

m

(

N

f l

f

+

C f

l Cf

±

)

(

2 Nf + Cf 2

6 2

>

X

1

0

where m = mole f r a c t i o n of contaminant


C f i = contaminant

at V 3

flow (R^), mL/min.

Nf-^ = n i t r o g e n flow ( R 2 ) , mL/min. Cf2 = d i l u t e d contaminant

flow ( R 3 ) , mL/min.

Nf2 = n i t r o g e n flow ( R 4 ) , mL/min. Secondary D i l u t i o n Module: The secondary d i l u t i o n module i s designed to produce c o n t r o l l e d flows of dry or humid a i r f o r the generation of t e s t atmosphere at three concentration l e v e l s s i multaneously. The a i r i s obtained through a compressor (Gast Model #5HCD) l o c a t e d approximately 150 f t from the f a c i l i t y . I t i s de-humidified by a r e f r i g e r a t i o n u n i t . Trace organics are r e moved by passing the a i r through a c t i v a t e d c h a r c o a l and mole c u l a r sieve f i l t e r s . The a i r flow i s measured by a rotameter Rg and a mass flow meter (Matheson model #8240). The p u r i f i e d a i r i s h u m i d i f i e d by sweeping i t over water at constant temperature i n a thermostated water bath, and passed through a condensor to remove excess water. R e l a t i v e humidity i s monitored by a hygrometer (YSI Model 91). The a i r and analyte-doped n i t r o g e n (from the primary d i l u t i o n module) are allowed to mix i n a d i l u t i o n chamber. The pressure i n s i d e the chamber i s maintained at one atmosphere. The exact c o n c e n t r a t i o n of the analyte i n the t e s t atmosphere, ( i f 2 to 5 ppm, depending on the p a r t i c u l a r contaminant) can be determined by d i r e c t a n a l y s i s of a i r taken from SP^. The port SP^ can a l s o be connected to a sampling manifold for c o l l e c t i o n of the contaminant on a s o l i d sorbent. A p o r t i o n of the a i r i s sampled and the unsampled p o r t i o n i s measured by rotameter R^o ^ mixed with the r e q u i r e d amount of p u r i f i e d a i r f o r second d i l u t i o n . The second sampling i s done at port S P 2 ; t h i r d d i l u t i o n and subsequent sampling are f e a s i b l e at port S P 3 . E v a l u a t i o n of the generation system was done with acetone and ethylene oxide as the t e s t contaminants. The a c t i v a t e d c h a r c o a l (Columbia JXC) was s e l e c t e d f o r c o l l e c t i o n of the a n a l y t e s . This adsorbent has been reported to give good r e s u l t s f o r ethylene oxide (9). The sampling tubes cons i s t e d of b o r o s i l i c a t e glass tubes (15 cm x 4 mm i.d.) packed with 400 mg of the c h a r c o a l (300 mg i n f r o n t s e c t i o n and 100 mg a n c



33.

KAPILA

E T

Test Atmosphere

AL.

Generation

537

i n the back s e c t i o n ) . The sampling was done f o r one hour p e r i o d with vacuum pump (Gast Model #1031). Methanol and carbon d i s u l f i d e were used to desorb acetone and ethylene oxide, r e s p e c t i v e l y . Before proceeding to sampling, desorption e f f i c i e n c i e s were caref u l l y determined. Sampling: A sampling r a t e of approximately 55 mL/min was maintained by the use of c r i t i c a l o r i f i c e constructed i n the l a b o r atory. The schematic of the o r i f i c e i s shown i n Figure 2A. The o r i f i c e s were made by s i l v e r s o l d e r i n g a 0 . 5 m m o . d . (0.25 mm i d ) s t a i n l e s s s t e e l tubing i n t o a d r i l l e d out 1/8 i n c h o.d. s t a i n l e s s s t e e l rod, which i n turn was soldered i n t o a male connector (1/4 i n c h swagelok to 1/16 i n c h NTP). An o r i f i c e constructed i n t h i s manner was found to give a maximum flow of 300 mL/min. To obtain the d e s i r e d flows (250 and 55 mL/min), s t a i n l e s s s t e e l wires of various diameters were i n s e r t e d i n t o the 0.25 mm i . d . tubings and, i n some cases, the tubings were s l i g h t l y crimped. The o r i f i c e s were then connected i n t o the sampling manifold (Figure 2B). Each o r i f i c e constructed i n the above manner was c a l i b r a t e d with a rotameter and manometer. The set-up used f o r o r i f i c e c a l i b r a t i o n i s shown i n Figure 3, The uncorrected flow readings were obtained from the rotameter c a l i b r a t i o n curve. The c o r r e c t e d flows were then c a l c u l a t e d using the f o l l o w i n g equation: F

cor

= F

V

cal P

( ^ ) '

1

/

2

where: F

= c o r r e c t e d flow cor F - = flow read from the c a l i b r a t i o n curve cal P^ = pressure reading from manometer P^

= atmospheric pressure during rotameter

calibration

The c a l i b r a t i o n of the o r i f i c e s was done before and each sampling run to ensure accurate flow r a t e s .

after

Results and D i s c u s s i o n The simulated t e s t atmosphere with high l e v e l s of contaminant (2 to 5ppm) were analyzed by both the i n - l i n e GC ( i n the primary d i l u t i o n module) and by d i r e c t i n j e c t i o n of a i r taken from the sampling port ( i n the secondary d i l u t i o n module). The purpose of the l a t t e r was to e s t a b l i s h the r e l i a b i l i t y of the i n - l i n e GC. The i n - l i n e monitor was p a r t i c u l a r l y important i n t e s t atmosphere with low contaminant concentration because i n t h i s case, d i r e c t i n j e c t i o n of the a i r i s not f e a s i b l e . Table I compares the r e s u l t s obtained by the i n - l i n e GC and d i r e c t a i r i n j e c t i o n f o r ethylene oxide and acetone. Test atmospheres c o n t a i n i n g acetone, a r e l a t i v e l y innocous l i q u i d , and ethylene oxide, a mutagenic gas (at room temperature)


CHEMICAL


538

HAZARDS

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Sampling

Figure 2. A. schematic of the orifices used in sampling: 1, l/4"-l/16" male connector; 2, l/8"-o.d. (o. 1/32" i.d.) stainless steel tube; 3, 2.5-mm i.d. stainless steel capillary; 4, stainless steel wire. B. Schematic of the sampling manifold.

Vacuum

T H E WORKPLACE

Tubes

Pump

B

Figure 3. System used for calibration of the critical orifices: 1, calibrated rotameter; 2, 5, vacuum/pressure gauge; 3, sampling manifold; 4, orifice; 6, vacuum pump.


33.

KAPILA

E T

Test Atmosphere

AL.

Generation

539


was generated. One-hour sampling on s o l i d sorbent was done at about 55 mL/min i n both humid and dry c o n d i t i o n s a f t e r two succe s s i v e d i l u t i o n s . (No sampling was done at the t h i r d l e v e l , i . e . , at S P 3 , because of the l a c k of a t h i r d i d e n t i c a l pump). The r e s u l t s obtained with acetone as the t e s t contaminant are shown i n Tables I I and I I I . Since acetone i s l e s s hazardous to h e a l t h than ethylene oxide, the high concentrations shown on the t a b l e s were chosen to evaluate the performance of the system at these l e v e l s . Recoveries were c l o s e to 100% i n both dry and humid a i r .

Table I A n a l y s i s of Generated Test Atmospheres Ethylene oxide

Acetone

%

I n - l i n e GC ng/ml a i r

Direct i n j ng/ml a i r

error

162.5 43.3 20.3 5.9

159.8 42.7 20.5 5.8

1.7 1.4 1.0 1.7

Sampling

Direct i n j . ng/ml a i r

error

1320.0 251.2

1330.0 246.0

0.07 1.7

Table I I of Acetone i n Dry Atmosphere, RH < 12%

Acetone 420 ppm Amount added (mg)( ) 2

3.34 3.39 3.12 2.94 2.90

Amount rec. (mg)(3) 3.97 4.03 3.71 3.50 3.45

x = 103.2

a = 4.9

Acetone 230 Percent Recovery

Amount added . (mg)' '

108.4 108.9 100.2 99.1 99.5

1.66 1.70 1.81 1.83 1.88

cv = 0.04

%

I n - l i n e GC ng/ml a i r

Amount rec..^. (mg)

x = 101.8

(1) lowest accurate hygrometer measurement (2) determined by i n - l i n e GC (3) c o r r e c t e d f o r a desorption e f f i c i e n c y of

(

r

ppm Percent Recovery

1.75 1.70 1.75 1.94 1.99 a = 3.9

105.4 100.0 96.6 106.0 101.1 cv =

84%


0.03

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Table I I I Sampling

of Acetone i n Humid Atmosphere, RH = 85%

Acetone 580 ppm Amount Amount added ^ rec. Percent (mg)' ' (mg)' ' Recovery


1

4.59 4.64 4.76 4.64 4.34 x = 94.58

4.47 4.37 4.73 4.38 3.80

97.4 94.2 99.4 94.4 87.5

a = 4.5

cv = 0.04

Acetone 285 Amount Amount added rec. (mg)^ (mg) 2.05 2.09 2.30 2.24 2.26 x = 98.44


1.91 1.86 2.37 2.24 2.33 a = 5.8

93.1 89.0 103.0 100.0 103.1 cv =

0.05

(1) determined by i n - l i n e GC (2) c o r r e c t e d f o r a desorption e f f i c i e n c y of 84% The ethylene oxide experiments were done at four concentrat i o n l e v e l s s t a r t i n g with a low of about 3 ppm to a high of 44 ppm. (Concentrations greater than 44 ppm ethylene oxide were not attempted because i t was f e l t that the exhaust system i n the laboratory at the time of the experiment may not be s u f f i c i e n t to handle s a f e l y any higher concentrations of the ethylene oxide.) Sampling at the low concentrations was done only under dry c o n d i t i o n s (RH < 12%) whereas, sampling at high concentrations was done both i n dry and humid atmospheres. The r e s u l t s are summarized i n Tables IV, V and VI. The percent recovery of ethylene oxide i s greater than 88% and l i t t l e or no change i n recovery i s observed i n dry and humid c o n d i t i o n s .

Table IV . of Ethylene Oxide i n Dry Atmosphere, RH < 12% (

Sampling

Ethylene oxide 6.0 Amount added (ug)( ) 2

38.7 39.0 40.1 39.0 x = 96.2

ppm

Amount rec. (ug)

Percent Recovery

40.0 38.6 35.6 36.6

103.4 99.0 88.8 93.6

a = 2.7

cv = 0.02

Ethylene oxide 3.0 Amount added (ug)^ 18.2 17.0 18.7 18.2 x = 88.3

(1) lowest accurate hygrometer measurement (2) determined by i n - l i n e GC (3) c o r r e c t e d f o r a desorption e f f i c i e n c y of

Amount rec. (ug)^


16.1 15.1 16.2 16.4 a = 1.3

88.4 88.4 86.6 89.9 cv =

94%


0.01

33.

KAPILA

Test Atmosphere

E T A L .

Sampling


Ethylene oxide 40 ppm Amount

(yg)

(yg)< >

264.6 266.0 266.9 255.0 250.3

264.1 266.9 260.7 253.9 239.6

x

A

= 98.4

2

O = 1.7

Percent Recovery 99.8 99.2 97.7 99.6 95.7 cv = 0.01

Ethylene oxide 22 ppm Amount added

Amount rec.

(yg)^)

< >

120.4 122.7 134.7 131.1 132.3

109.3 109.3 119.0 119.0 121.2

91.1 89.1 88.3 99.1 91.6

o = 4.2

cv = 0.04

x = 91.89

2

Percent Recovery

(1) determined by i n - l i n e GC (2) c o r r e c t e d f o r a desorption e f f i c i e n c y of 84%

Tables II-VI f u r t h e r confirmed the accuracy of the i n - l i n e GC as a monitor. However, i t should be pointed out that f o r atmospheres of very high concentrations i . e . , at l e v e l s where the amount of contaminants i n the sampling loop exceeds the l i n e a r range of the detector, the i n - l i n e GC would be of l i t t l e use. Conclusion In t h i s paper, we have presented a system f o r generating contaminated t e s t a i r . We have shown that the system can generate t e s t a i r a t two l e v e l s ( r e a d i l y expandable to three l e v e l s by the use of a t h i r d pump) with reasonable accuracy. A broad



542

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range of concentrations ( 3 . 0 ppm ethylene oxide and 5 8 0 ppm acetone) have been obtained. I t i s reasonable to assume that concentrations higher than what have been demonstrated may be obtained. However, i f the analyte i s c o l l e c t e d i n a s o l i d sorbent, the c o l l e c t i o n c a p a c i t y of the sorbent must not be exceeded and that once adsorbed, can be desorbed with e f f i c i e n c y greater than 8 0 % . The i n - l i n e GC i n the primary d i l u t i o n module i s shown to give accurate r e s u l t s and therefore the concentration of the contaminant can be determined a t any time during sampling by means of a four-port switching v a l v e . The system permits r a p i d (about 2 min) adjustment of generated concentration l e v e l s by the use of f i n e metering valves i n conjunction with w e l l - c a l i brated rotameters and the i n - l i n e GC. However, the e q u i l i b r a t i o n of the atmosphere i n the secondary d i l u t i o n module takes s l i g h t l y longer. The system a l s o permits a wide range of humidity l e v e l s to be set by v a r y i n g the temperature of the water bath.

Literature Cited 1. 2.

Nelson, G.O.; Griggs, K.S Rev. Sci. Instr. 1968, 39, 927. Nelson, G.O.; Hodgkins, D.J. Am. Ind. Hyg. Assoc. J. 1972,

3.

Pella, P.A.; Hughes, E.E.; Taylor, J.K. Am. Ind. Hyg. Assoc. J.

4. 5.

Ellgehausen, D. Anal. Lett. 1975, 8, 11. Dietrich, M.W.; Chapman, L.A.; Mieure, J.P. Am. Ind. Hyg. Assoc. J. 1978, 39, 385. Vincent, W.J.; Hahn, K.J.; Ketcham, N.H. Am. Ind. Hyg. Assoc. J. 1979, 40, 512. Nelson, G.O. "Controlled Test Atmospheres: principles and techniques"; Ann Arbor Science Publishers, Inc. Ann Arbor, MI

33,

110.

1975, 36,

6. 7.

755.

1971.

8. 9.

Nelson, G.O.; Swisher, L.W.; Taylor, R.D.; Bigler, B.E. Am. Ind. Hyg. Assoc. J. 1975, 36, 49. Qazi, A.H.; Ketcham, N.H. Am. Ind. Hyg. Assoc. J. 1977, 38,

635.

RECEIVED September 29, 1980.


A Versatile Test Atmosphere Generation and Sampling System - ACS

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