Generation of Test Atmospheres of Toxic Substances for Evaluation of

1 Generation of Test Atmospheres of Toxic Substances for Evaluation of A i r Sampling Methods C. CLARINE ANDERSON, E L L E N C. GUNDERSON, and D A L E M. COULSON SRI International, Menlo Park, CA 94025 BRUCE GOODWIN and K E N N E T H T. MENZIES Downloaded by FORDHAM UNIV on January 13, 2013 | http://pubs.acs.org Publication Date: April 22, 1980 | doi: 10.1021/bk-1980-0120.ch001

Arthur D. Little, Inc., Cambridge, MA 02140

The need for measuring worker exposure to various toxic materials in the workplace atmosphere demands that appropriate tested methods be available for determining the exposure levels. A c r i t i cal part of the protocol for testing methods is the preparation of test atmospheres of these toxic materials. For many types of samples, i t is frequently sufficient to test analytical procedures on spiked samples, but this is strictly not true for measurements of industrial hygiene samples. For instance, adsorption of compounds onto a sorbent such as charcoal or silica gel may be weaker when the material is deposited from solution than when it is adsorbed as a vapor from a moving air stream. Storage stability is also frequently affected by the method of deposition onto the collecting medium. Material may adhere to the sampling tube or filter cassette during sampling. High humidity effects cannot be adequately tested on spiked samples. Another important area of interest is the collection of materials that may be present in both particulate and vapor phases at the concentration levels of interest. Frequently, vapor pressure data are not available, and a determination of whether vapor/particulate mixtures must be measured can be made only by preparing and sampling test atmospheres. The preparation of synthetic atmospheres for nonreactive gases and vapors is relatively straightforward, but the preparation of fumes, aerosols, and particulates is considerably more difficult. For purposes of industrial hygiene sampling, a polydisperse aerosol containing respirable-size particles is required. This paper describes some of the techniques used to generate synthetic atmospheres of toxic materials. The work was part of a study supported by the National Institute for Occupational Safety and Health to develop and validate methods for sampling and analysis of various materials found in the workplace atmosphere and was a joint effort of SRI International and Arthur D. Little, Inc. Emphasis was placed on reproducible and reliable generation techniques that could be used for a wide variety of compounds. Unique methods were devised for certain difficult materials. Over 230 compounds were studied, including gases, vapors, fumes, and 0-8412-0539-6/80/47-120-001$05.00/0 © 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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a e r o s o l s . The a i r concentrations ranged from 0.05 to 15 mg/m f o r p a r t i c u l a t e s and from 0.05 to 2000 ppm f o r gases and vapors. We d e s c r i b e below a s p e c i a l d i l u t i o n system, which has been used to produce s e v e r a l c o n c e n t r a t i o n l e v e l s simultaneously, and a number of source generators, which supply a high c o n c e n t r a t i o n l e v e l f o r dilution.

Downloaded by FORDHAM UNIV on January 13, 2013 | http://pubs.acs.org Publication Date: April 22, 1980 | doi: 10.1021/bk-1980-0120.ch001

D i l u t i o n System Figure 1 diagrams a s e r i a l d i l u t i o n system designed by C. Ε. Lapple of SRI I n t e r n a t i o n a l . I t produces dynamically generated t e s t concentrations at three l e v e l s , each at a predetermined r a t i o to the adjacent one. The important components of the system are the primary chamber, which s u p p l i e s a high c o n c e n t r a t i o n of t e s t a i r , a mixing and d i l u t i o n channel, and three sampling chambers, from which samples are drawn. A l l d i l u t i o n and exhaust streams are metered through c r i t i c a l flow o r i f i c e s . A f i l t e r e d compressed a i r supply provides the d i l u t i o n a i r . A vacuum pump i s used f o r the exhaust streams. To s t a r t generation, the output from a source generator i s introduced i n t o the primary chamber. S u f f i c i e n t d i l u t i o n a i r , which may be h u m i d i f i e d , i s added to t h i s output to reduce the con c e n t r a t i o n to the h i g h e s t d e s i r e d l e v e l , shown i n Figure 1 as Οχ. A p o r t i o n of t h i s purposely contaminated a i r i s drawn i n t o the mix ing channel. Excess contaminated a i r passes out of the primary chamber i n t o a d i s p o s a l system. As the a i r moves down the mixing channel, a f r a c t i o n i s drawn through the f i r s t chamber at s p e c i f i e d volumetric flow r a t e , Qj. The c o n c e n t r a t i o n i n t h i s f i r s t chamber i s unchanged. Clean f i l t e r e d a i r i s next metered i n t o the channel at a r a t e Q , causing the f i r s t d i l u t i o n to a c o n c e n t r a t i o n C 2 . A p o r t i o n of t h i s a i r i s then p u l l e d through the second chamber. The next stage of d i l u t i o n i s achieved by adding a second amount of a i r at a r a t e Qb to the channel. Part of t h i s a i r , now at the lowest concentra t i o n , i s used to supply the t h i r d chamber. The remaining a i r from the channel i s removed by the l a s t meter at a r a t e Qg. The general r e l a t i o n s h i p f o r the d i l u t i o n r a t i o from chamber to chamber i s a

c

2

Ci

Q2

+

Q

E

where C i and C 2 r e f e r to the concentrations i n adjacent chambers; Q , Q 2 , and Q E represent v o l u m e t r i c flow r a t e s . The flow patterns are shown i n Figure 2. In p r a c t i c e , the r a t i o s and flow r a t e s through the chambers are chosen according to sampling needs, and the exhaust r a t e i s determined by s o l v i n g the equation f o r Qg. This expression i s somewhat more complicated f o r more than a two-stage d i l u t i o n , but can e a s i l y be set up and worked out. The system i n use f o r the A

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

ANDERSON

E T AL.

Test Atmospheres

T o Exhaust

of Toxic

Substances

Filtered Compressed Air

1 DILUTION AIR SUPPLY MANIFOLD

I

I

METER


Dilution A.r

-

I [~METER

PRIMARY CHAMBER (C,)

From Source Generator EXHAUST AIR VACUUM MANIFOLD

/VACUUM\ I PUMP

Figure 1.

Exhaust Air

Schematic of three-stage dilution system; Q = volumetric flow rate and C = concentration.

Figure 2. Calculation of dilution ratios; C = concentration in first chamber, C = concentration in second chamber, and Q = volumetric flow rate. t


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methods v a l i d a t i o n study operates with a l l flow r a t e s equal to each other, so the d i l u t i o n r a t i o i s one-half. Figure 3 shows the exhaust a i r d i s p o s a l system. Several s a f e ty f e a t u r e s have been b u i l t i n t o i t s design. To prevent contaminat i o n of the surrounding atmosphere, excess a i r from the primary chamber and the exhaust streams from the d i l u t i o n system are f i l t e r e d , then f e d i n t o a combustion chamber where they are burned. The e n t i r e system i s maintained at 1 i n c h of water vacuum to p r e vent t o x i c m a t e r i a l s from escaping i n t o the l a b o r a t o r y . A compressed a i r e j e c t o r causes a negative pressure i n the system. A p r e s s u r e - s e n s i t i v e alarm connected to the primary chamber sounds an a l e r t i f the pressure r e t u r n s to normal. F i g u r e 4 shows the a c t u a l d i l u t i o n system. The tower at l e f t i s the primary chamber. I t s l a r g e volume i s important f o r use with a e r o s o l s because even at high a i r flow r a t e s the v e l o c i t y i s low, which allows time f o r s o l v e n t evaporation when wet a e r o s o l d r o p l e t s are introduced. Low v e l o c i t y a l s o helps prevent impaction of part i c l e s w i t h the chamber w a l l s . The three cone-shaped sampling chambers shown i n Figure 4 have a 1-inch i n t e r n a l diameter at the top and a 6-inch i n t e r n a l diame t e r at the base where samples are taken. The i n c r e a s i n g area creates gradual r e d u c t i o n of v e l o c i t y as the a i r flows down the chamber. Samples are withdrawn from the c y l i n d r i c a l s e c t i o n . I t s base i s provided w i t h f i t t i n g s f o r use with a v a r i e t y of samplers. A l l three of the t e s t chambers may be monitored with e i t h e r a t o t a l hydrocarbon a n a l y z e r or a gas chromatograph f i t t e d with a gas samp l i n g loop. Source

Generators

Source generators are used to supply the i n i t i a l high concent r a t i o n of analyte i n the t e s t a i r . In a d d i t i o n to using some commercially a v a i l a b l e generators, we developed s e v e r a l generators s p e c i f i c a l l y f o r t h i s program. The types of m a t e r i a l s generated and the general techniques used are l i s t e d below. Gases

D i l u t i o n of compressed

gas

Vapors

Dewpoint s a t u r a t i o n , then d i l u t i o n D e l i v e r y with i n f u s i o n pump In s i t u production from a precursor compound

Aerosols and aerosol/vapor mixtures

Atomization/spray d r y i n g Condensation of heated vapor

Metal oxide fumes

Thermal degradation of organic precursor compound a f t e r atomization


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ANDERSON

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Test Atmospheres

of Toxic

Substances

ALARM

PRIMARY CHAMBER

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AIR

CHAMBER

EJECTOR


FILTER MIXING CHANNEL

Figure 3.

Figure 4.

Exhaust air disposal system

Three-stage dilution system


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Vapor Generator. Figure 5 shows a simple vapor generator used f o r m a t e r i a l s with a c c u r a t e l y known vapor pressures. The threenecked f l a s k c o n t a i n i n g the chemical i s placed i n a c o n t r o l l e d tem perature bath. Nitrogen i s bubbled through the heated l i q u i d and passes out through the thermostated condenser. The vapor-saturated n i t r o g e n i s then d i l u t e d with a i r . The concentration of vapor i n the a i r i s c a l c u l a t e d using the vapor pressure at the temperature of the condenser and the flow r a t e s of n i t r o g e n and d i l u t i o n a i r . A s p e c i a l adaptation was used f o r mercury vapor. Mercurycoated Monel screens were placed i n both the f l a s k and the conden ser; a c o n c e n t r a t i o n range of 0.05 to 0.2 mg of mercury per cubic meter was used f o r e v a l u a t i n g the method f o r c o l l e c t i o n of mercury on s i l v e r e d Chromosorb P. In S i t u Generator. Very r e a c t i v e substances such as s t i b i n e must o f t e n be generated i n s i t u . A method f o r preparing s t i b i n e , reported by Gunn et a l . [ J . Phys. Chem., 64, 1334 (I960)], was modified to permit continuous generation at a c o n t r o l l e d r a t e . The experimental apparatus i s shown i n Figure 6. A concen t r a t e d b a s i c s o l u t i o n of potassium antimony t a r t r a t e c o n t a i n i n g sodium borohydride i s used. A s u f f i c i e n t amount of t a r t a r i c a c i d i s added to prevent h y d r o l y s i s and p r e c i p i t a t i o n of the r e s u l t i n g antimony compound. The s o l u t i o n i s d e l i v e r e d with a syringe d r i v e under the surface of a s o l u t i o n of 4 Ν s u l f u r i c a c i d , and s t i b i n e i s produced i n the a c i d i c medium. The net r e a t i o n i n the f l a s k i s 3BHi; + 4H Sb0 3

3

+ 3H

+

+

3H B0 3

3

+ 3H 0 + 4SbH 2

3

+

Nitrogen sweeps the gaseous r e a c t i o n products out of the f l a s k i n t o the d i l u t i o n / s a m p l i n g system. Although the r e a c t i o n i s not q u a n t i t a t i v e , i t was found that r e p r o d u c i b l e concentrations of s t i b i n e could be generated, even at d i f f e r e n t syringe d r i v e d e l i v e r y r a t e s , A e r o s o l Generator. The atomizer shown i n F i g u r e 7 was de signed to produce a e r o s o l s of c e r t a i n organic m a t e r i a l s that are s o l u b l e only i n e a s i l y vaporized solvents such as isopropanol or toluene. D i f f i c u l t i e s were encountered when using other n e b u l i z e r s because of c o n t i n u a l concentration of the s o l u t i o n r e s u l t i n g from evaporation of the s o l v e n t , making i t impossible to p r e d i c t or con t r o l the t e s t a i r c o n c e n t r a t i o n and the p a r t i c l e s i z e . In F i g u r e 7, the base s e c t i o n has been enlarged f o r c l a r i t y . A s o l u t i o n of the analyte i s g r a v i t y fed at a c o n t r o l l e d r a t e i n t o a small chamber at the base. Four small o r i f i c e s connect the cham ber to an annular opening surrounding a small n o z z l e . Compressed a i r f o r c e d through the nozzle atomizes the s o l u t i o n . Large drop l e t s impacting on the top and s i d e s of the c y l i n d e r are c o l l e c t e d i n a trough at the c y l i n d e r base and d r a i n out i n t o a c o l l e c t i o n r e s e r v o i r . Remaining a e r o s o l d r o p l e t s pass through a cyclone; those l e s s than 3 ym i n diameter emerge from the cyclone i n t o the


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Li J Τ

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Figure 5. Vapor saturator

THERMOSTATTED BATH

To Dilution/Sampling System

Nitrogen (1 Liter — Per Minute)

Rotameter

Magnetic Stirrer

Figure 6.

s

V

r i n

9

e

D r i v e

Stibine generator

To Primary J Chamber

Cyclone

Solution Reservior \ / \\

Generation of Test Atmospheres of Toxic Substances for Evaluation of

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