13 Diffusional
Monitoring
A New Approach to Personal Sampling
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D. W. GOSSELINK, D. L. BRAUN, H. E. MULLINS, S. T. RODRIGUEZ, and F. W. SNOWDEN 3M, 3M Center, Occupational Health & Safety Products Division, St. Paul, MN 55144 Recent years have seen advances in collection and analytical methods to determine contaminant concentrations in the air. One of these innovations involves the sampling of Organic Vapors without the use of mechanical air pumps; in particular the monitoring of personal exposures. Philosophically, personal monitoring may perform either of two functions. It may, 1) Define a hazard by generating a record of environmental exposures for a representative worker, or it may be used to 2) Verify the safety of workers in areas of exposure, both extremely important reasons or justifications for the use of personal monitoring. The conceptual development of diffusion monitoring took place in the late 'oO's to early '70 s. The first paper, published in May of 1972, described diffusion monitoring. Our first product was placed on the market in 1974 and patents have been issued in December of 1975 and April, 1975 to 3M. A second product, the Organic Vapor Monitor, was introduced to the market in January of 1978. An additional patent on personal exposure monitoring was issued in July of 1978. Of course, on considering personal exposure monitoring by any method, i t is helpful to have a point or frame of reference. In this case, that point of reference is the established pump and tube method. It features a mechanical pump whose function is to draw a constant, measured flow of an air-vapor mixture through a tube, normally consisting of two sections, where the vapors are selectively adsorbed. The first section usually contains 100 mg of charcoal and the second, or back-up section, contains 50 mg of charcoal. The pump is powered by a rechargeable battery. Its flow is controlled by setting a dial and is measured by a stroke counter. To obtain valid results a l l components must be calibrated and function properly. Since it is mechanical, the entire operation is subject to mechanical problems. f
0097-6156/81 /0149-0195$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
196 In
this
paper,
data
is
c i t e d which demonstrates
d i f f u s i o n a l m o n i t o r i n g can be used i n c i r c u m s t a n c e s
that i n which
pumps a n d c h a r c o a l t u b e s w e r e p r e v i o u s l y u s e d . The d a t a c i t e d emphasizes the c o m p a r a b i l i t y of d i f f u s i o n a l m o n i t o r i n g to c h a r c o a l the methods yield
t u b e and pump. are d i f f e r e n t ,
comparable
results.
However, i t must be n o t e d t h a t and w i l l not i n each c i r c u m s t a n c e T h e most
conclusive tests
will
b e t h o s e w h i c h employ a t h i r d method ( e . g . , g a s c h r o m a t o g r a p h y , i n f r a r e d a n a l y s i s ) capable of a c c u r a t e l y d e t e r m i n i n g the contaminant,
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Theory
on
or
contaminants
concentration.
of Operation
The o p e r a t i o n o f t h e 3M P e r s o n a l M o n i t o r i n g S y s t e m i s b a s e d t h e p r i n c i p l e of d i f f u s i o n . The m o n i t o r i s c o m p r i s e d of
a v e l o c i t y b a r r i e r , a s t a t i c a i r c o l u m n , and a s o r b e n t l a y e r at the bottom of the a i r column. I t i s assumed t h a t t h e c o n t a m i n a n t v a p o r c o n c e n t r a t i o n , at the v e l o c i t y b a r r i e r , i s r e p r e s e n t a t i v e of the ambient c o n c e n t r a t i o n or c o n c e n t r a t i o n t h a t the worker i s exposed t o . M o l e c u l e s o f t h e c o n t a m i n a n t v a p o r e n t e r t h e chamber t h r o u g h t h e v e l o c i t y b a r r i e r and p r o c e e d (by d i f f u s i o n ) a t a f i x e d r a t e to the a c t i v e sorbent l a y e r . The p r o c e s s o c c u r s v e r y rapidly. The m o n i t o r r e p r e s e n t s a d y n a m i c , n o n - m e c h a n i c a l s y s t e m . It i s not p a s s i v e . The d r i v i n g f o r c e , i t s c o n t i n u e d o p e r a t i o n , i s b a s e d on t h e d i f f e r e n c e b e t w e e n t h e c o n t a m i n a n t c o n c e n t r a t i o n a t t h e v e l o c i t y b a r r i e r (assumed t o b e e q u a l t o t h e a m b i e n t c o n c e n t r a t i o n ) and t h e s o r b e n t s u r f a c e (the p r i n c i p l e of d i f f u s i o n ) . T h a t d i f f e r e n c e i s m a i n t a i n e d by t h e c o n t i n u o u s a d s o r p t i o n o f t h e c o n t a m i n a n t v a p o r s by t h e sorbent. Diffusion ( E q u a t i o n 1)
c o n t r o l l e d p r o c e s s e s a r e m e a s u r e d by P i c k ' s
w
=
D
τ
collected
«
W
on t h e
' sorbent
law.
( 1 )
The
weight
the air
d i f f u s i o n c o e f f i c i e n t , the dimensions of the c o l u m n ( a r e a d i v i d e d by p a t h l e n g t h ) and t h e
is
equal to
the
product
of
static concentration
gradient. C^ i s t h e c o n c e n t r a t i o n a t t h e v e l o c i t y b a r r i e r w h i c h i s assumed t o c l o s e l y a p p r o x i m a t e t h e a m b i e n t c o n c e n t r a tion. C i s t h e c o n c e n t r a t i o n i n s i d e t h e m o n i t o r a t t i m e t = 0. n
The D ( A / L ) t e r m i s t h e ^ M o n i t o r S a m p l i n g R a t e and h a s t h e d i m e n s i o n s o f f l o w r a t e (cm / s e c ) . The e x p o s u r e t i m e i s measured by t . T h e r e f o r e , t h e w e i g h t c o l l e c t e d , W, i s e q u a l t o the f l o w or sampling r a t e times the ambient c o n c e n t r a t i o n times the t i m e . T h i s i s a dynamic, nature d r i v e n type of monitoring r a t h e r than a p a s s i v e system.
In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
13.
GOSSELiNK
Diffusional
ET AL.
Monitoring
197
The A v e r a g e D r i f t T i m e o r t h e t i m e i t t a k e s t h e m o n i t o r r e s p o n d t o c h a n g e s i n c o n c e n t r a t i o n c a n be c a l c u l a t e d f r o m E q u a t i o n 2:
to
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Where Τ = R e s p o n s e t i m e ( s e c . ) 2 D = T h e D i f f u s i o n C o e f f i c i e n t (cm / s e c . ) L = P a t h l e n g t h (cm) F o r a p a t h l e n g t h o f .65 cm and d i f f u s i o n r a t e o f 0.12, t h e r e s p o n s e t i m e ( A v e r a g e D r i f t T i m e ) i s l e s s t h a n two seconds. In o t h e r works, the m o n i t o r responds t o changes i n c o n c e n t r a t i o n o f most o r g a n i c s i n t h e a t m o s p h e r e i t i s s e n s i n g , w i t h i n two s e c o n d s . How i s t h e p r o c e s s a f f e c t e d by c h a n g e s i n c o n d i t i o n s ? The d i f f u s i o n c o e f f i c i e n t i s an i n v e r s e f u n c t i o n of p r e s s u r e , w h i l e concentration varies d i r e c t l y with pressure. The r e s u l t i s t h a t weight c o l l e c t e d i s c o n s t a n t w i t h r e s p e c t to changes i n pressure. S i m i l a r l y , the d i f f u s i o n c o e f f i c i e n t i s a f u n c t i o n o f a b s o l u t e t e m p e r a t u r e t o t h e 3 / 2 s power and c o n c e n t r a t i o n i s i n v e r s e l y p r o p o r t i o n a l t o t h e s q u a r e r o o t (T ) of the absolute temperature. (1) T
D = f (T
3 / 2
)
(3)
C = ί φ
(4)
Therefore, W = f(T
1 / 2
)
(5)
The n e t r e s u l t i s t h a t W i s c h a n g e s a p p r o x i m a t e l y 1% f o r each increment of 1 0 ° F above ( i n c r e a s i n g ) o r below (decreasing) 70°F. By rearranging E q u a t i o n ( 1 ) , s u b s t i t u t i n g f o r the sampling r a t e , Κ = D — and s o l v i n g f o r the e n v i r o n m e n t a l c o n c e n t r a t i o n , we a r r i v e a t E q u a t i o n ( 6 ) :
c
i =
IT
