Automated Measurement of Atmospheric Trace ... - ACS Publications

2 Automated

Measurement

of Atmospheric Trace Gases

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch002

Diffusion-Based Collection and Analysis Purnendu K . Dasgupta Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409

The theoretical and practical aspects of automated diffusion-based collection and analysis systems are described. The design of ther-modenuders, wet denuders, and diffusion scrubbers is discussed. Theoretical considerations include the estimation of collection efficiency, inlet length necessary for laminar flow development, and particle transmission through the system as well as the choice of a given design for the intended application. Practical collection and analysis systems are described on the basis of literature accounts of thermally cycled denuders, ion-exchange and porous-membrane-based diffusion scrubbers, and wet denuders, both of simple tubular and annular geometry.

T H E D E T E R M I N A T I O N O F A T M O S P H E R I C T R A C E G A S E S has r e p r e s e n t e d a n area of active endeavor since the onset of h u m a n i t y ' s interest i n the c h e m i s try of the atmosphere. P r e s e n t - d a y practice ranges from s a m p l i n g the air w i t h a l i q u i d absorber c o n t a i n e d i n a b u b b l e r to m a k i n g d i r e c t spectroscopic measurements w i t h p a t h lengths of several k i l o m e t e r s , w i t h obvious a t t e n d ant differences i n cost, sensitivity, a n d r e l i a b i l i t y . G e n e r a l l y a p p l i c a b l e d i r e c t spectroscopic t e c h n i q u e s i n present use r e l y i n g o n a b s o r p t i o m e t r y i n c l u d e tunable d i o d e laser spectroscopy (J), differential optical absorption spectroscopy (2), a n d F o u r i e r transform i n f r a r e d spectroscopy (3). I n g e n e r a l , these methods are r e l a t i v e l y free f r o m interferences a n d thus represent the techniques of choice as reference procedures. H o w e v e r , t h e i r w i d e s p r e a d 0065-2393/93/0232-0041$13.50/0 © 1993 American Chemical Society

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

42

MEASUREMENT CHALLENGES IN ATMOSPHERIC CHEMISTRY

a p p l i c a t i o n is d e t e r r e d b y t h e i r b u l k a n d cost. O c c a s i o n a l l y , c o m p a c t a b s o r p t i o m e t r i c i n s t r u m e n t s of adequate sensitivity are possible for a specific gas, as is the case for ozone (4). O t h e r examples o f d e d i c a t e d affordable i n s t r u m e n t s for specific gases t y p i c a l l y i n v o l v e l u m i n o m e t r y , for e x a m p l e , p u l s e d fluorometry for S 0 (5) a n d c h e m i l u m i n o m e t r y for the d e t e r m i n a t i o n of N O u p o n reaction w i t h 0 (6) o r for the d e t e r m i n a t i o n of 0 u p o n reaction 2

3

3

w i t h C H (7). I n general, h o w e v e r , i f the gases of interest can be r e p r o d u c i b l y c o l l e c t e d i n a l i q u i d (typically aqueous) absorber, a great v a r i e t y of analytical alternatives (e.g., c o l o r i m e t r i c - f l u o r o m e t r i c - e l e c t r o c h e m i c a l d e t e c t i o n f o l l o w i n g w e t c h e m i c a l m a n i p u l a t i o n s or i o n - l i q u i d chromatography) b e c o m e a p p l i c a b l e . I n most cases, s u c h approaches a l l o w good specificity a n d respectable l i m i t s of d e t e c t i o n ( L O D s ) at a m o d e s t cost. Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch002

2

4

Continuous Gas-Liquid

Contactors

C o l l e c t i o n of an a t m o s p h e r i c trace gas for subsequent w e t analysis has b e e n classically a c c o m p l i s h e d b y u s i n g a b u b b l e r or a n i m p i n g e r . U s u a l l y , the l i q u i d v o l u m e is not the l i m i t i n g factor i n the analytical p r o c e d u r e ; i f the gas c o l l e c t i o n efficiency is not seriously sacrificed at the same s a m p l i n g rate, the a b i l i t y to u t i l i z e a smaller l i q u i d v o l u m e translates i n t o a greater analyte concentration a n d thus a l o w e r attainable L O D . S e v e r a l alternatives are s u p e r i o r to b u b b l e r s a n d i m p i n g e r s i n this respect. M o r e o v e r , these a l t e r natives p e r m i t continuous l i q u i d a n d gas flow, a l l o w i n g c o n t i n u o u s analysis of the l i q u i d stream. T h e earliest example of such a d e v i c e is a m u l t i t u r n glass c o i l i n w h i c h the s a m p l e d air causes the s i m u l t a n e o u s l y p u m p e d l i q u i d absorber to f o r m a film o n the i n t e r i o r walls of the c o i l . A g a s - l i q u i d separator follows, a n d t y p i c a l l y the isolated l i q u i d stream is fed to an a i r - s e g m e n t e d continuous-flow analyzer. F u l l y automated a m b i e n t a i r analyzers based o n this p r i n c i p l e w e r e r e p o r t e d m o r e t h a n two decades ago (8), a n d m o r e m o d e r n adaptations have b e e n r e p o r t e d , most notably for t h e c o n t i n u o u s m e a s u r e m e n t o f H 0 a n d H C H O (9, JO). 2

2

A continuous gas s c r u b b e r can be d e s i g n e d a r o u n d the V e n t u r i p r i n c i p l e as w e l l (11). N e b u l i z i n g the absorber l i q u i d b y u s i n g the sample air as p r o p e l l a n t is a m o n g the most efficient a n d ingenious strategies u t i l i z e d to d e s i g n continuous g a s - l i q u i d contactors. T h e n e b u l i z e d l i q u i d is t h e n c o l l e c t e d b y p r o v i d i n g e i t h e r an i m p a c t i o n site (12) or a h y d r o p h o b i c m e m b r a n e (13). A l l of these g a s - l i q u i d contactors i n v o l v e a flow r e g i m e n that is far from l a m i n a r . C o n s e q u e n t l y , a significant (and v a r i a b l e , d e p e n d i n g o n the d e s i g n of the device) fraction of the s i m u l t a n e o u s l y s a m p l e d a t m o s p h e r i c aerosol is s c r u b b e d as w e l l , a n d it finds its way i n the l i q u i d effluent. I f the analyte m e a s u r e d is not significantly present i n the aerosol a n d does not interact w i t h any o f the aerosol constituents, no p a r t i c u l a r p r o b l e m s are p o s e d . I f it does e i t h e r , h o w e v e r , a different strategy is n e e d e d because r e m o v a l of the


2.

DASGUPTA

43

Automated Measurement of Atmospheric Trace Gases

aerosol f r o m the sample gas b y p r e f i l t r a t i o n is g e n e r a l l y unacceptable because of

filter-induced

artifacts.

Diffusion Denuders I n d e e d , it is often i m p o s s i b l e to d i s t i n g u i s h b e t w e e n an analyte o r i g i n a t i n g from the aerosol a n d the same analyte o r i g i n a t i n g f r o m the gas phase after i n c o r p o r a t i o n i n t o a l i q u i d absorber: the i n a b i l i t y to d i s t i n g u i s h b e t w e e n particulate N H

4

+

a n d N H (g) or b e t w e e n particulate N 0 ~ a n d H N 0 3

3

3

(g),

once c o l l e c t e d i n t o an aqueous s o l u t i o n , can b e c i t e d as examples. I n s u c h cases, diffusion-based c o l l e c t i o n has p r o v e n to b e the o n l y a p p r o a c h for


r e l i a b l y m e a s u r i n g a gas i n the p r e s e n c e o f an aerosol. Because the diffusion coefficient o f a gas m o l e c u l e is t y p i c a l l y 4 orders of m a g n i t u d e larger t h a n the smallest a t m o s p h e r i c aerosol of significance (in t e r m s of mass c o n t r i b u t i o n ) , it is possible to effect essentially q u a n t i t a t i v e r e m o v a l of analyte gas molecules b y diffusion i n an a p p r o p r i a t e l y d e s i g n e d system. T h e simplest f o r m of a diffusion-based gas-aerosol d i s c r i m i n a t o r of this type is a tube w i t h its i n t e r i o r walls coated w i t h some substance that serves as an efficient sink for the gas m o l e c u l e s . U n d e r l a m i n a r flow c o n ditions a n d w i t h a v e r t i c a l d e p l o y m e n t of the t u b e to a v o i d gravitational settling o f t h e aerosol, the aerosol t r a n s m i s s i o n efficiency c a n be n e a r l y quantitative (14, 15). T h e s e devices w e r e suggested o r i g i n a l l y b y T o w n s e n d (16) , a n d the i n i t i a l interest was to r e m o v e c e r t a i n gases to b e t t e r study t h e associated particles. T h e r e m o v a l o f gases b y means of diffusion thus e v e n tually l e d to t h e t e r m "diffusion d é n u d e r " . I n 1949, G o r m l e y a n d K e n n e d y (17) p r o v i d e d a m a t h e m a t i c a l treatment c o n c e r n i n g the diffusion f r o m a stream f l o w i n g t h r o u g h a c y l i n d r i c a l t u b e . S e v e r a l r e c o m p u t a t i o n s of the o r i g i n a l G o r m l e y - K e n n e d y solution have b e e n m a d e , a n d the expression o f B o w e n et al. (18) is r e g a r d e d t h e most accurate (19). T h e f o l l o w i n g expression (equation 1) agrees w i t h the results o f B o w e n et a l . u p to four significant figures. 1 - /

= 0.81905 E T

3 6 5 6 8

* + 0.09753 *τ

22

+ 0.0325 «τ · * + 0.01544 56

961

β

3 0 5 μ

Ι Ό 7 Μ μ

·

(1)

w h e r e / i s the fraction c o l l e c t e d b y the dénuder a n d μ is a d i m e n s i o n l e s s quantity given by μ = irDL/ρ

(2)

w h e r e D is the diffusion coefficient of the gas, L is the l e n g t h o f t h e t u b e , and

Q is t h e v o l u m e t r i c flow rate. F o r most d é n u d e r systems d e s i g n e d to

collect a gas, / is h i g h , a n d o n l y the first t e r m o n t h e r i g h t side o f e q u a t i o n


44

MEASUREMENT C H A L L E N G E S IN ATMOSPHERIC CHEMISTRY


1 needs to be u s e d . T h e s e p r i n c i p l e s are r e g a r d e d as sufficiently s o u n d to p e r m i t reliable measurements of diffusion coefficients of gases (20, 21). T h e first r e p o r t e d a p p l i c a t i o n of a dénuder d e v i c e w i t h subsequent c h e m i c a l analysis i n v o l v e d the speciation of gaseous a n d particulate fluoride. T h i s study u s e d a dénuder system c o n t a i n i n g three c o n c e n t r i c tubes (22). F e w details w e r e g i v e n as to w h y such a design was chosen; d e n u d e r s of a n n u l a r g e o m e t r y w e r e not reinvestigated u n t i l the 1980s. U s i n g the s i m p l e single-tube geometry, C r i d e r et a l . (23) r e p o r t e d the first c o n t i n u o u s gas-aerosol d i s c r i m i n a t i n g analytical i n s t r u m e n t . Since this t i m e , r e p o r t e d designs have i n c l u d e d m u l t i p l e tubes o p e r a t e d i n p a r a l l e l (24), t w o c o n c e n t r i c tubes (with the i n n e r w a l l of the outer t u b e a n d the o u t e r w a l l of the i n n e r t u b e b e i n g suitably coated to serve as the sink surface) h e l d together b y one or two t r i p o i n t w e l d s w i t h the air b e i n g s a m p l e d t h r o u g h the a n n u l a r space (25), a n d e v e n a set of 12 c o n c e n t r i c tubes operated i n the m u l t i p l e a n n u l a r g e o m e t r y (26). Stevens et a l . (27) s i m i l a r l y d e s c r i b e d a c o n c e n t r i c d é n u d e r w i t h three glass a n n u l i , p r o t e c t e d o n the exterior b y a stainless steel sheath so as to r e n d e r the assembly less fragile. K o u t r a k i s et a l . (28) d e s c r i b e d the d e s i g n of a compact serial a c i d - a n d alkali-coated dénuder, c o m p l e t e w i t h an i n l e t i m p a c t o r , for the m e a s u r e m e n t of acidic aerosols a n d gases. A n o t h e r r e p o r t e d diffusion-based s a m p l e r utilizes flow i n the l a m i n a r - t u r b u l e n t t r a n sition r e g i o n (29, 30). A decisive j u d g m e n t o n the advantages a n d d i s a d v a n tages of the transition-flow reactor, i n c l u d i n g its general a p p l i c a b i l i t y to a variety of analytes, awaits f u r t h e r studies. T h r e e reviews d e s c r i b i n g applications of diffusion d e n u d e r s have b e e n p u b l i s h e d . T h e doctoral dissertation of F e r m (31) reflects c o n s i d e r a b l e exp e r i e n c e w i t h single-tube d e n u d e r s for the m e a s u r e m e n t of a v a r i e t y of species. T h e r e v i e w b y A l i et a l . (32) is extensive; it p r o v i d e s an excellent historical a n d theoretical b a c k g r o u n d a n d s u m m a r i z e s the l i t e r a t u r e based o n the type of analyte gas d e t e r m i n e d . T h e focus of the most r e c e n t r e v i e w , b y C h e n g (19), is diffusion batteries u s e d for size d i s c r i m i n a t i o n o f aerosols as w e l l as diffusion d e n u d e r s . Various p h y s i c a l designs are discussed i n some d e t a i l i n that r e v i e w . D i f f u s i o n d e n u d e r s have b e c o m e a r e l a t i v e l y c o m m o n tool for the p r e s ent-day a t m o s p h e r i c analytical chemist. I n a c o m p a r i s o n of m e t h o d s for n i t r o g e n species analysis c o n d u c t e d i n u r b a n L o s A n g e l e s i n 1985, some 8 out of 18 i n s t r u m e n t s d e p l o y e d to measure H N 0 u s e d a diffusion d é n u d e r (33). T h i s study also i n d i c a t e d the uncertainties associated w i t h prefilters a n d filter-based measurements. D e s p i t e the i n c r e a s i n g p o p u l a r i t y of diffusion d e n u d e r s , the use of a t y p i c a l diffusion dénuder is v e r y l a b o r - i n t e n s i v e . A t y p i c a l a p p l i c a t i o n involves w a s h i n g a n d coating the active surfaces of a dénuder t u b e , field s a m p l i n g , r e t u r n i n g to the laboratory, w a s h i n g a n d r e m o v i n g the coating u n d e r n o n c o n t a m i n a t i n g c o n d i t i o n s , a n a l y z i n g the w a s h solution for the c o l l e c t e d analyte, a n d t h e n b e g i n n i n g the cycle anew. Efforts have b e e n a n d c o n t i n u e to be made to fully automate diffusion-based c o l 3


2.

DASGUPTA

Automated Measurement of Atmospheric

45

Trace Gases

l e c t i o n a n d analysis, at least for specific analytes. T h e s e devices are discussed i n the r e m a i n d e r of this chapter; the c i t e d r e v i e w s give the g e n e r a l a p p l i cation of diffusion d e n u d e r s .

Operating and Design Considerations S o m e considerations c o m m o n to a l l diffusion-based c o l l e c t i o n - a n a l y s i s sys


tems are discussed i n the f o l l o w i n g sections.

Laminar Flow Development. I n a t y p i c a l a p p l i c a t i o n , it is not generally possible to sample isokinetically. E q u a t i o n s g o v e r n i n g the c o l l e c tion efficiency (e.g., equation 1) a p p l y o n l y u n d e r l a m i n a r flow c o n d i t i o n s . It is c o n s i d e r e d necessary therefore to leave a l e n g t h of the t u b i n g surface at the entrance d e l i b e r a t e l y uncoated to p e r m i t l a m i n a r flow to f u l l y d e v e l o p before actual c o l l e c t i o n occurs. F o r a s i m p l e t u b e , the m i n i m u m i n l e t l e n g t h , L j , necessary to fully d e v e l o p l a m i n a r flow ( w i t h i n about 98%) is g i v e n b y (34): L

{

= 0.05 dN*

w h e r e d is the d i a m e t e r of the t u b e a n d N expressed as

N

R c

(3)

c

R e

, the R e y n o l d s n u m b e r , can b e

= 4ρ /Μη)

(4)

Ρ

w h e r e Q is the v o l u m e t r i c flow rate a n d ρ a n d η are the d e n s i t y a n d the viscosity of the sample gas, respectively. E q u a t i o n s 2 a n d 4 can b e c o m b i n e d to L

{

= 0.2 Qp/(m\)

(5)

T h i s e q u a t i o n shows that at a g i v e n location (p a n d η constant), L is solely d e p e n d e n t o n the s a m p l i n g rate Q. F o r d r y air at 20 °C a n d 1 a t m a n d w i t h a density of 1.2 g / L a n d a viscosity of 1.8 Χ 10" P, e q u a t i o n 5 can b e rewritten {

4

L, = 6.2 ρ

(6)

w h e r e L is expressed i n c e n t i m e t e r s a n d Q i n liters p e r m i n u t e . I n l e t l e n g t h considerations for an a n n u l a r d é n u d e r are not f u n d a m e n t a l l y different. F o r most a n n u l a r geometries i n present use, the a n n u l a r gap is s m a l l r e l a t i v e to the radius of c u r v a t u r e ; this situation p e r m i t s the p a r a l l e l - p l a t e a p p r o x i m a t i o n . F o r a set of p a r a l l e l plates separated b y the distance x, S c h l i c h t i n g {


46

MEASUREMENT CHALLENGES IN ATMOSPHERIC

CHEMISTRY

(35) stated that t h e i n l e t l e n g t h necessary for l a m i n a r flow d e v e l o p m e n t can be approximated by L

= 0.04 x N

4

(7)

R e

F o r an a n n u l a r dénuder, χ = (d

+ di)/2

Q

where d

0

(8)

a n d d are the i n n e r d i a m e t e r o f the o u t e r t u b e a n d t h e o u t e r {


d i a m e t e r o f the i n n e r t u b e (rod), r e s p e c t i v e l y , a n d IV

Re

= 2 ρ / [ < τ τ η ( 4 + dd] Ρ

(9)

C o m b i n i n g equations 7 t h r o u g h 9 gives A

= 0.04 Q (d 9

0

- d}jl[mid

0

+ d$\

(10)

W h e n the i n l e t l e n g t h is expressed i n t e r m s o f n u m b e r o f "gap w i d t h s " , t h e difference b e t w e e n the flow i n a tube a n d the flow i n an a n n u l u s o f n a r r o w gap differs o n l y b y 2 5 % [(0.05 - 0.04)/0.05]. T h i s situation is a n i n d i c a t i o n that the g r o w t h of the l a m i n a r b o u n d a r y layers from the w a l l to t h e c e n t e r of the c h a n n e l is s i m i l a r i n b o t h cases. Because d u c t f r i c t i o n coefficients, a measure o f m o m e n t u m transfer, do not v a r y b y m o r e t h a n a factor o f 2 for ducts of r e g u l a r cross sections w h e n expressed i n t e r m s of h y d r a u l i c d i a m eters, the use of the i n l e t l e n g t h for tubes or p a r a l l e l plates can b e e x p e c t e d to b e a reasonable a p p r o x i m a t i o n for t h e i n l e t lengths o f o t h e r cross sections u n d e r l a m i n a r flow c o n d i t i o n s . I n the a n n u l a r dénuder, the d i m e n s i o n l e s s i n l e t l e n g t h for l a m i n a r flow d e v e l o p m e n t , L ' , can b e expressed as V F o r a g i v e n flow rate, V

= 2L /(d i

Q

- di)

(11)

is i n v e r s e l y r e l a t e d to t h e gap w i d t h o r to the

d i a m e t e r (d or d^j at constant gap w i d t h (36). Q

Because i t m a y not b e possible to p r o v i d e an i n l e t l e n g t h sufficient for f u l l d e v e l o p m e n t o f l a m i n a r flow i n some applications, t h e c o n s e q u e n c e s o f not m e e t i n g this r e q u i r e m e n t m a y b e w o r t h y of e x a m i n a t i o n . A l t h o u g h the lack o f a f u l l y d e v e l o p e d l a m i n a r flow profile m a y m a k e it i m p o s s i b l e to c o m p u t e diffusion coefficients f r o m the o v e r a l l fraction o f the gas that p e n etrates the dénuder, most practitioners are solely i n t e r e s t e d i n o b t a i n i n g a good c o l l e c t i o n efficiency for the analyte gas. O v e r the i n l e t r e g i o n , analyte gas c o l l e c t i o n efficiency is e x p e c t e d to increase s l i g h t l y r e l a t i v e to p r e d i c t i o n s based o n diffusive mass transfer u n d e r l a m i n a r flow c o n d i t i o n s . A c c o r d i n g to M e r c e r (37) a n d F r i e d l a n d e r (38), the l e n g t h r e q u i r e d for p a r a b o l i c flow



2.

DASGUPTA


Trace Gases

47

to d e v e l o p is short c o m p a r e d to the l e n g t h r e q u i r e d for the p a r t i c l e c o n centration b o u n d a r y l a y e r to develop. A s such, the entrance l e n g t h s h o u l d have little influence o n the extent of particle d e p o s i t i o n o n the dénuder. H o w e v e r , the existence of some uncoated i n l e t l e n g t h is i m p o r t a n t , because otherwise particles deposited i n the entrance r e g i o n w i l l b e c o m e part of the sample analyzed. O n the other h a n d , l e a v i n g l o n g , u n c o a t e d lengths of a glass t u b e , from w h i c h d e n u d e r s are c o m m o n l y c o n s t r u c t e d , is h a r d l y a desirable state of affairs. S u c h a l e n g t h can easily acquire a static charge (particularly at l o w relative h u m i d i t i e s ) a n d can c o n t r i b u t e to significant loss of particles (39); the loss, p r e d i c t a b l y , is e v e n h i g h e r w h e n the surface is poly(tetrafluoroethylene) ( P T F E ) , k n o w n to be p a r t i c u l a r l y susceptible to a c q u i r i n g an electrostatic charge (40). O n the basis of these c o n f l i c t i n g c o n siderations, the p r u d e n t course of action appears to be to r e q u i r e several centimeters of uncoated entrance l e n g t h , a l t h o u g h this l e n g t h m a y not b e sufficient to fully d e v e l o p l a m i n a r flow. H o w e v e r , this discussion m a y b e i r r e l e v a n t to issues of l e a v i n g any uncoated lengths at a l l i n d e n u d e r s that are m a n u a l l y w a s h e d (with w a s h l i q u i d b e i n g s u b s e q u e n t l y analyzed). I n such cases, t y p i c a l l y b o t h the uncoated a n d coated p o r t i o n of the dénuder is w a s h e d (41).

Collection Efficiency. Single-Tube Denuders. F o r the appropriate design of a diffusion-based collection device for an i n t e n d e d a p p l i c a t i o n , the ability to estimate the collection efficiency a p r i o r i is of considerable h e l p . A l t h o u g h the theoretical soundness of the G o r m l e y - K e n n e d y e q u a t i o n (equation 1) is not q u e s t i o n e d , it is based o n the a s s u m p t i o n that the uptake p r o b a b i l i t y of the analyte gas at the w a l l is u n i t y ; that is, the w a l l is t r u l y a "perfect s i n k " , a n d e v e r y collision results i n uptake. T h i s a s s u m p t i o n is unrealistic. I n recent years, this issue has b e e n r e e x a m i n e d . M c M u r r y a n d S t o l z e n b u r g (42) s h o w e d for a l i q u i d - c o a t e d dénuder h o w the uptake p r o b ability (discussed b y the authors i n terms of the "mass a c c o m m o d a t i o n coefficient") can be evaluated from collection efficiency m e a s u r e m e n t s . M u r p h y a n d F a h e y (43) u t i l i z e d the mathematical solution o r i g i n a l l y d e v e l o p e d for h e m o d i a l y z e r s b y C o o n e y et a l . (44); this treatment assumes a constant uptake p r o b a b i l i t y that may be less than u n i t y . T o use the M u r p h y - F a h e y approach, h o w e v e r , this p r o b a b i l i t y m u s t b e p r e c i s e l y k n o w n . W h e n the dénuder active surface is an i n e r t porous m e m b r a n e such that the analyte m o l e c u l e m u s t diffuse across the pores to be t r a p p e d b y an absorber l i q u i d , o n l y a fraction of the m e m b r a n e surface is porous, a n d the pores m a y also be tortuous. C o n s e q u e n t l y , c o l l i s i o n at the m e m b r a n e surface is not synonymous w i t h uptake. C o r s i et a l . (45) d e v e l o p e d a n u m e r i c a l solution for the collection efficiency o b s e r v e d for such a m e m b r a n e - b a s e d diffusion dénuder, hereinafter r e f e r r e d to as a diffusion s c r u b b e r (DS). B o t h groups of researchers d e a l i n g w i t h the issue of less t h a n u n i t y u p t a k e p r o b ability reached the conclusion that this value m u s t be v e r y m u c h less than

American Cfaemjcaâ Sûciûty Library 115516th SUHW. Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; Washington. American Chemical Society: Washington, DC, 1993. O.C. 20011

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u n i t y before a significant d e p a r t u r e f r o m G o r m l e y - K e n n e d y b e h a v i o r b e comes e x p e r i m e n t a l l y d i s c e r n i b l e . T h e c o l l e c t i o n efficiency d e t e r m i n e d b y D a s g u p t a et a l . (46) for a h y d r o p h o b i c porous P T F E m e m b r a n e t u b e (2-μηι pores, 5 0 % surface porosity, 0 . 4 - m m - t h i c k wall) c o l l e c t i n g S O (dilute H 0 f l o w i n g as s c r u b b e r l i q u i d outside the tube) i n d e e d agrees w i t h i n 1% w i t h values c o m p u t e d d i r e c t l y f r o m the G o r m l e y - K e n n e d y e q u a t i o n at s a m p l i n g rates l o w e r t h a n 2 L / m i n (89 to 100% c o l l e c t i o n efficiency). A t h i g h e r s a m p l i n g rates, h o w e v e r , e x p e r i m e n t a l c o l l e c t i o n efficiencies w e r e l o w e r t h a n G o r m l e y - K e n n e d y values. O v e r a l l , for most p r a c t i c a l single-tube d e n u d e r s , w h e t h e r m e m b r a n e - b a s e d o r not, the G o r m l e y - K e n n e d y e q u a t i o n p r o v i d e s a r e l i a b l e starting estimate for d e v i c e d e s i g n . A gradual decrease of c o l l e c t i o n efficiency d u r i n g s a m p l i n g d u e to surface saturation is possible w i t h t y p i c a l sorbent-coated d e n u d e r s (47); w i t h automated diffusion d e n u d e r s - s c r u b b e r s this decrease w o u l d not appear to be an i m p o r t a n t issue i n a s m u c h as the c o l l e c t i n g m e d i u m is e i t h e r c o n t i n u o u s l y or c y c l i c a l l y r e n e w e d .


s

2

2

Annuhr Denuders. C o m p u t i n g the c o l l e c t i o n efficiency of a n n u l a r d e n u d e r s — w h i c h are b e c o m i n g increasingly p o p u l a r because of t h e i r a b i l i t y to m a i n t a i n near-quantitative c o l l e c t i o n efficiencies at h i g h s a m p l i n g rates w i t h i n a c o m p a c t d e s i g n — i s less straightforward. P o s s a n z i n i et a l . (25) i n t r o d u c e d the a n n u l a r g e o m e t r y to the present-day practice of diffusion-based s a m p l i n g a n d suggested an e m p i r i c a l e q u a t i o n to calculate the c o l l e c t i o n efficiency / : 1 - / = Α ^

(12)

Δ

w h e r e A a n d α are e m p i r i c a l constants a n d Δ is g i v e n b y Δ = μ(ά

0

+ di)/(d - dù

(13)

0

A l t h o u g h the precise values of A a n d α w e r e said to be d e p e n d e n t o n the exact p h y s i c a l design o f the d e v i c e , u n d e r conditions w h e r e the radius of c u r v a t u r e is large c o m p a r e d to the a n n u l a r gap (d » d - d j , A and α w e r e r e g a r d e d to be constants. F o r S 0 as the test gas i n t e t r a c h l o r o m e r curate(II)-coated glass a n n u l a r d e n u d e r s (Q = 1 to 40 L / m i n , Δ = 0.064 to 0.272, / = 0.81 to 1.00), t h e y r e p o r t e d A = 0.82 a n d α = 5.63. I n a later study d e a l i n g w i t h the c o l l e c t i o n of N 0 (g) w i t h K I - i m p r e g n a t e d a n n u l a r d e n u d e r s coated w i t h p o l y e t h y l e n e g l y c o l , the same value for A b u t a different value for α was r e p o r t e d . T h e m u c h l o w e r value for a , 1.12, was a t t r i b u t e d to p o o r sink efficiency (48). A l i et a l . (32) have c r i t i c i z e d the drawbacks of such e m p i r i c a l approaches. O n the basis of the o r i g i n a l w o r k of G o r m l e y (49) o n diffusion b e t w e e n p a r a l l e l plates, t h e y suggested that the p a r a l l e l - p l a t e a p p r o x i m a t i o n of an a n n u l a r dénuder w i l l l e a d to a f o r m of e q u a t i o n 12 w h e r e A = 0.91 a n d α = 3.77. T h e differences i n the actual c o l l e c t i o n efficiencies r e s u l t i n g from the different values of A a n d α can be Q

0

2

2


2.

DASGUPTA


substantial unless the dénuder is o p e r a t i n g at n e a r l y quantitative c o l l e c t i o n efficiency. M o r e r e c e n t l y , W i n i w a r t e r (50) has b u i l t o n the w o r k of S t e p h a n (51) b y d e a l i n g w i t h heat transfer i n a n n u l a r tubes a n d a p p l y i n g a F i c k i a n diffusion m o d e l to c o m p u t e c o l l e c t i o n efficiencies i n an a n n u l a r dénuder. T h e r e s u l t i n g differential equations w e r e solved b y the R u n g e - K u t t a m e t h o d . C o l l e c t i o n efficiencies thus c o m p u t e d for H N 0 , H C O O H , a n d C H C O O H agreed w e l l w i t h the e x p e r i m e n t a l data o b t a i n e d b y R o s e n b e r g et a l . (52). W i n i w a r t e r p r o v i d e d tabulated data a n d graphs to enable c o m p u t a t i o n of c o l l e c t i o n efficiencies for specific a n n u l a r dénuder d i m e n s i o n s a n d o p e r a t i n g c o n ditions. Coûtant et a l . (26) also d e v e l o p e d a n u m e r i c a l m e t h o d ; t h e y u s e d the parallel-plate a p p r o x i m a t i o n . T h e i r m o d e l takes i n t o account the n o n u n i t y uptake p r o b a b i l i t y , w h i c h can be i n p u t as a separate parameter. T h e software (written i n C p r o g r a m m i n g language for I B M a n d c o m p a t i b l e P C s ) is available f r o m the authors a n d p e r m i t s calculation of c o l l e c t i o n efficiencies for m u l t i p l e c o n c e n t r i c tubes as w e l l . 3


49

Trace Gases

3

N o n e of these approaches are applicable to d e n u d e r s of the a n n u l a r g e o m e t r y w h e r e o n l y one of the surfaces i n contact w i t h the s a m p l e d gas is effective i n uptake. M e m b r a n e - b a s e d D S devices of the reverse geometry, i n t r o d u c e d o r i g i n a l l y b y T a n n e r et a l . (53), u t i l i z e a m e m b r a n e t u b e c o n centrically s u s p e n d e d w i t h i n an outer jacket t u b e . T h e gas is s a m p l e d t h r o u g h the a n n u l a r space, a n d the analyte gas is c a p t u r e d b y c o l l e c t i o n t h r o u g h the pores i n the m e m b r a n e . W i t h this d e s i g n , o n l y the i n n e r surface of the annulus is effective as a sink. Solutions to the p a r a l l e l p r o b l e m for heat transfer are available. L u n d b e r g et a l . (54) p r e s e n t e d an extensive set of n u m e r i c a l solutions for cases w h e r e e i t h e r the o u t e r or the i n n e r surface of the annulus is the sink. T h e s e results can b e d i r e c t l y u s e d , w i t h the l i m i t a t i o n that the treatment assumes the p r o b a b i l i t y of u p t a k e to b e u n i t y . T h e fraction c o l l e c t e d , / (referred to as the p a r a m e t e r 6 b y L u n d b e r g et al.), is calculated as a function of the dimensionless axial p o s i t i o n X , w h e r e X i n this a p p l i c a t i o n w o u l d be g i v e n b y mi

x

d

0

(3)

+ dj

.

.

( 1 4 )

=

F i g u r e 1 graphically depicts the n u m e r i c a l data relevant_to o u r a p p l i c a t i o n l i s t e d b y L u n d b e r g et a l . D i f f e r e n t sets of curves o f / v s . X are p r o v i d e d for i n d i v i d u a l values of d ld . D i s c r e t e data w e r e p r o v i d e d i n the n u m e r i c a l tables of the o r i g i n a l w o r k ; to p r o d u c e the continuous traces i n F i g u r e 1, a c u b i c spline fitting was used. {

0

T h e e x p e r i m e n t a l collection efficiencies for H 0 (estimated G r a h a m ' s law diffusion coefficient is 0.18 c m / s ) for an aqueous s c r u b b e r N a f i o n (perfluorinated ionomer) m e m b r a n e D S ( L = 40 c m , d = 0.5 c m , a n d d = 0.06 cm) have b e e n e x p e r i m e n t a l l y d e t e r m i n e d b y two i n d e p e n d e n t a p 2

2

2

0


{

50


/7V ω -M

Ο Q)

ν

Ο Ο C0.1


ο

V

-7

-Μ

υ

D

0.01

0.1

Dimensionless ΑχίαΙ Position (χ) Figure 1. Collection efficiency (f)for a dénuder of annular geometry in which only the inner surface of the annulus is a sink as a function of axial position X (see equation 14). The curves are based on numerical data in reference 54. From top to bottom, the traces correspond to d,/d = 1.0,0.5, 0.25, 0.1, 0.05, and 0.02. 0

proaches (40). H e r e the sink surface is e x p e c t e d to b e efficient i n u p t a k e , a n d / h a s b e e n o b s e r v e d to b e 0.944 ± 0.017, 0.838 ± 0.013, 0.732 ± 0.006, a n d 0.638 ± 0.002 at Q = J ) . 5 , 1.0, 1.5, a n d 2.0 L / m i n , respectively. T h e c o r r e s p o n d i n g values for X are 0.86, 0.43, 0.29, a n d 0.22, r e s p e c t i v e l y . T h e djd v a l u e for this D S is 0.12; i n t e r p o l a t i n g b e t w e e n the curves for dJd = 0.1 a n d 0.25 i n F i g u r e 1 reveals that the t h e o r e t i c a l p r e d i c t i o n s are i n excellent a g r e e m e n t w i t h the e x p e r i m e n t a l data. 0

Q

W i t h porous m e m b r a n e D S devices o f this g e o m e t r y a n d for t h i n m e m branes w i t h l o w - t o r t u o s i t y pores (i.e., w h e r e the diffusion distance w i t h i n the pores is v e r y s m a l l c o m p a r e d to the radial diffusion distance i n t h e D S ) , good p r e d i c t i o n s for the c o l l e c t i o n efficiencies can be o b t a i n e d i f the n o m i n a l X a n d dJd values are b o t h m u l t i p l i e d b y the fraction o f the surface that is porous. F o r e x a m p l e , w i t h a diffusion s c r u b b e r based o n s u c h a m e m b r a n e t u b e (L = _40 c m , d = 0.5 c m , d = 0.045 c m , fractional porosity 0.4), the c o r r e c t e d X values for H 0 as sample gas are 0.32, 0.16, 0 . 1 1 , a n d 0.08, r e s p e c t i v e l y for Q = 0.5, 1.0, 1.5, a n d 2.0 L / m i n , a n d the c o r r e c t e d dJd v a l u e is 0.036. T h e c o l l e c t i o n efficiencies p r e d i c t e d f r o m F i g u r e 1 (interp o l a t i n g b e t w e e n dJd values o f 0.02 a n d 0.05) are i n g o o d a g r e e m e n t w i t h Q

0

v

2

2

Q

Q


2.

DASGUPTA


Trace Gases

51

the e x p e r i m e n t a l results (40): 0.661 ± 0.017, 0.371 ± 0.013, 0.310 ± 0.001, a n d 0.234 ± 0.024 at t h e respective flow rates. T h e efficient sink c r i t e r i a cannot b e o v e r e m p h a s i z e d , h o w e v e r . W h e n the e x p e r i m e n t a l data for f o r m a l d e h y d e for e i t h e r o f the t w o different m e m brane scrubbers are c o n s i d e r e d , t h e theoretical p r e d i c t i o n s significantly e x c e e d t h e e x p e r i m e n t a l c o l l e c t i o n efficiencies. T h e uptake o f f o r m a l d e h y d e at a n aqueous interface is c o n t r o l l e d b y its rate o f h y d r a t i o n to m e t h y l e n e glycol, a process that is a c i d - o r base-catalyzed. T h e c o l l e c t i o n efficiency significantly increases i n going f r o m p u r e water to 0.1 M H S 0 as a s c r u b b e r l i q u i d (55), b u t t h e uptake p r o b a b i l i t y still remains a c o n t r o l l i n g factor i n d e t e r m i n i n g t h e c o l l e c t i o n efficiency. O b v i o u s l y , i n such cases t h e o r e t i c a l p r e d i c t i o n s m e r e l y establish a n u p p e r l i m i t . Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch002

2

Particle Transmission.

4

Coarse particles ( > ~ 2 . 5 μηι) are p r e s e n t i n

all a t m o s p h e r i c samples. I f a diffusion-based c o l l e c t i o n system is n o t e q u i p p e d w i t h a n i m p a c t o r o r cyclone at t h e front e n d to r e m o v e t h e coarse particles, some coarse particle d e p o s i t i o n w i l l o c c u r i n t h e system. T h i s is especially t r u e for a n n u l a r d e n u d e r s (in w h i c h t h e i n n e r m e m b e r m u s t have some p h y s i c a l means o f attachment to the outer m e m b e r a n d such s t r u c t u r a l supports are o b l i g a t o r i l y present i n t h e flow path) a n d D S devices w i t h Ttype a i r inlets. L a r g e r particles are often d e r i v e d f r o m crustal sources a n d display significant a c i d - n e u t r a l i z i n g capacity. I f total a c i d i t y is to b e d e t e r m i n e d i n the a t m o s p h e r i c sample, significant errors can b e i n t r o d u c e d unless such particles are first r e m o v e d . P T F E - c o a t e d cyclones as a n i n t e g r a l part of t h e sample i n l e t appear to b e t h e best c h o i c e for r e m o v i n g t h e coarse particles (27, 56). R e l a t i v e to a m o r e polar surface l i k e glass, t h e a d s o r p t i o n of sticky gases l i k e H N 0 is m i n i m i z e d o n a P T F E o r P T F E - c o a t e d surface. A l t h o u g h s u c h a surface is m o r e p r o n e to p r o m o t e electrostatically i n d u c e d fine-particulate d e p o s i t i o n (vide infra), the residence t i m e w i t h i n the c y c l o n e is t y p i c a l l y b e l o w 1 s, a n d significant losses o f fine particles have not b e e n o b s e r v e d (27, 56). I n this c o n n e c t i o n , maintenance r e q u i r e m e n t s o f a n y i n s t r u m e n t w i t h a coarse-particle r e m o v a l system at t h e i n l e t n e e d to b e p o i n t e d out. A l t h o u g h t h e rest o f the system m a y w e l l b e c o n f i g u r e d to b e c o m p l e t e l y automated, w i t h o u t p e r i o d i c c l e a n i n g of the c y c l o n e t h e r e s u l t i n g data m a y b e subject to significant e r r o r . Because o f d i u r n a l t e m p e r a t u r e variations, errors m a y accrue, for e x a m p l e , f r o m c y c l i c d e p o s i t i o n a n d e v a p oration o f NH4NO3 as w e l l as f r o m loss o f a c i d gases d u e to g a s - p a r t i c l e interactions w i t h d e p o s i t e d coarse particles. 3

F o r fine particles, despite t h e fact that t h e major rationale b e h i n d dif fusive s a m p l i n g o f a gas is to achieve d i s c r i m i n a t i o n f r o m t h e c o n c u r r e n t l y present a t m o s p h e r i c aerosol, r e l a t i v e l y little attention has b e e n p a i d to ac tually c h a r a c t e r i z i n g the particle transmission t h r o u g h these systems. A s u m m a r y o f existing data has b e e n p r e s e n t e d (40). T h e o n l y t h o r o u g h charac-



52


terization o f p a r t i c l e losses that has a p p e a r e d i n the l i t e r a t u r e concerns the H a r v a r d - E n v i r o n m e n t a l P r o t e c t i o n A g e n c y a n n u l a r d é n u d e r system (39, 57). T h e system is e q u i p p e d w i t h a n i n t e g r a l i n l e t c y c l o n e o r i m p a c t o r . T h e first study (57) suggested that total p a r t i c l e losses are < ~ 3 % for the p a r t i c l e size 1.50 to 2.77 μπι. T h e m o r e recent a n d m o r e extensive study (39) dealt w i t h particles o v e r a m u c h greater size range a n d w i t h different degrees o f charge: n e u t r a l , w i t h B o l t z m a n n charge, or w i t h a single charge. F u r t h e r , the glass d é n u d e r surfaces w e r e e i t h e r u n c o a t e d o r coated w i t h N a C l o r c i t r i c a c i d . T h e loss increased i n going from the n e u t r a l to the c h a r g e d aerosol a n d was somewhat h i g h e r for an u n c o a t e d dénuder surface c o m p a r e d w i t h the coated surface, p r e s u m a b l y because of the greater t e n d e n c y o f the u n coated surface to acquire a static charge. T y p i c a l losses i n a single-stage u n c o a t e d dénuder i n the 0.10- to 0.86-μιη size range r a n g e d from 0.9 to 8 % for a n aerosol w i t h B o l t z m a n n charge d i s t r i b u t i o n ; a n a d d i t i o n a l 1.8 to 5 . 3 % was lost o n essential system components. A l t h o u g h the o v e r a l l extent o f the loss is still reasonably s m a l l , it may not b e insignificant i f the i n t e n t is to measure the aerosol c o m p o s i t i o n . T h e effect of electrostatic c h a r g i n g cannot be o v e r e m p h a s i z e d — d r a m a t i c increases i n p a r t i c l e d e p o s i t i o n o c c u r r e d w h e n the d é n u d e r was d r i e d w i t h a vigorous flow of c o m p r e s s e d air. W i t h P T F E tubes, m u c h m o r e susceptible to a c q u i r i n g a static charge, as m u c h as 3 to 1 5 % o f the aerosol, 0.1 to 1.09 μιχι i n d i a m e t e r a n d w i t h a B o l t z m a n n charge d i s t r i b u t i o n , can b e lost u n d e r d r y conditions m e r e l y b y passage t h r o u g h a 7 0 - c m - l o n g straight P T F E t u b e (40). C l e a r l y , l o n g i n l e t lines m u s t be a v o i d e d to m e a s u r e p a r t i c l e c o m p o sition along w i t h gases. U n f o r t u n a t e l y , t u b i n g materials that are c o n s i d e r e d i n e r t are also m o r e susceptible to a c q u i r i n g a static charge. T h e c h o i c e o f the sample c o n d u i t m a t e r i a l is also dictated b y the a d s o r p t i o n characteristics it displays t o w a r d the analyte gases of interest. F o r a v a r i e t y of p o l a r gaseous analytes, for e x a m p l e , H N 0 , H C 1 , a n d H 0 , glass i n l e t lines are u n a c ceptable. E v e n w h e n the c o n d u i t is e l e c t r i c a l l y c o n d u c t i v e , for e x a m p l e , stainless steel, the extent o f aerosol d e p o s i t i o n is acutely d e p e n d e n t o n the charge o n the aerosol (58); little c o n t r o l can b e exercised o v e r this situation. 3

2

2

I n any fully a u t o m a t e d i n s t r u m e n t o p e r a t i n g u n a t t e n d e d , it is also h i g h l y desirable that the i n s t r u m e n t p e r i o d i c a l l y zeros a n d calibrates itself. T h u s , aside f r o m the sample air, zero a n d calibrant gases m u s t b e accessible to the c o l l e c t i o n system i n l e t b y some a u t o m a t e d v a l v i n g a r r a n g e m e n t . T y p i cally, P T F E or perfluoroalkoxy ( P F A ) solenoid valves are u s e d to s w i t c h sample streams. H o w e v e r , v i r t u a l l y a l l available c o m m e r c i a l valves have right-angled passageways i n s i d e the v a l v e , a n d d e p o s i t i o n of a m b i e n t p a r ticulate matter occurs at the b e n d . O v e r a p e r i o d of t i m e , this b u i l d u p c o m p r o m i s e s sample i n t e g r i t y . S o m e valves also b e c o m e sufficiently w a r m u p o n e n e r g i z a t i o n to cause significant loss of l a b i l e analyte gases l i k e H O (55). A t h r e e - p o s i t i o n e l e c t r o p n e u m a t i c s l i d e r valve w i t h no sharp angular bends (59) m i n i m i z e s these p r o b l e m s . A n altogether different a n d possibly 2


a

2.

DASGUPTA


53

Trace Gases


s u p e r i o r approach is to p r o v i d e a T - c o n n e c t i o n at the v e r y b e g i n n i n g of the i n l e t l i n e . Z e r o or calibration gases are i n t r o d u c e d t h r o u g h this T - a r m as d e s i r e d b y appropriate valves c o n n e c t e d to these sources. T h e zero o r c a l i b r a t i o n gas flow is made greater than the sample flow. T h u s , d u r i n g zero or c a l i b r a t i o n p e r i o d s , no a m b i e n t air is d r a w n i n t o the s y s t e m ; r a t h e r , excess zero o r c a l i b r a t i o n gas is v e n t e d t h r o u g h the i n l e t .

Annular Geometry: Preferred in A l l Applications? T h e a n n u l a r g e o m e t r y is r a p i d l y r e p l a c i n g the single-tube g e o m e t r y i n most applications. C l e a r l y , the a n n u l a r design is capable of c o l l e c t i n g a greater analyte mass p e r u n i t t i m e . F o r automated c o l l e c t i o n - a n a l y s i s systems that operate o n a c y c l i c s o r p t i o n - d e s o r p t i o n protocol (e.g., t h e r m o d e n u d e r s a n d v i d e infra), the c o l l e c t e d analyte is generally d e s o r b e d as a p l u g w i t h i n a fixed t i m e w i n d o w . G r e a t e r c o l l e c t e d mass therefore translates to b e t t e r L O D s or t e m p o r a l r e s o l u t i o n or b o t h . O n the other h a n d , for a w e t d é n u d e r i n w h i c h a flowing s c r u b b e r l i q u i d c o n t i n u o u s l y wets the active surfaces a n d the effluent is t h e n taken for analysis, the advantages of an a n n u l a r g e o m e t r y , i n v i e w of its m o r e c o m p l e x c o n s t r u c t i o n , are less clear-cut unless the s c h e m e i n volves concentration of the dénuder l i q u i d effluent p r i o r to analysis. T h i s situation arises because most continuous flow-through analytical detectors are concentration-sensitive rather than mass-sensitive. T h e a b i l i t y of the annular geometry to collect a greater analyte mass is associated w i t h a larger active surface area. W i t h the reasonable a s s u m p t i o n that the m i n i m u m n e c essary s c r u b b e r l i q u i d flow rate to effectively present a c o n t i n u o u s l y r e n e w e d c o l l e c t i o n surface is d i r e c t l y p r o p o r t i o n a l to the active surface area, most of the advantage of the a n n u l a r g e o m e t r y disappears. F o r the e x a m p l e of the isoefficient dénuder trio c i t e d b y P o s s a n z i n i et a l . (25), the c o l l e c t i o n effic i e n c y of a single-tube d é n u d e r — L = 50 c m , d = 0.3 c m at Q = 1.7 L / m i n — i s i d e n t i c a l to that of two a n n u l a r d e n u d e r s — L — 10 c m , d = 1.0 c m , d = 1.3 c m at Q = 3.8 L / m i n a n d L = 20 c m , d = 3.0 c m , a n d d = 3.3 at Q = 20 L / m i n . T h e differences i n the analyte mass c o l l e c t e d p e r u n i t t i m e is reflected i n the ratio of the flow rates, 1:2.23:11.8. T h e c o r r e s p o n d i n g ratio of the active surface area, *n(d + d,)L, is 1:1.52:8.4. T h e c o l l e c t e d mass p e r u n i t surface area ratio, the s u p e r i o r i t y factor, is t h e n o b t a i n e d b y d i v i d i n g the first set of n u m b e r s b y the second a n d is 1:1.45:1.20. T h i s d e g r e e of i m p r o v e m e n t is h a r d l y w o r t h the a d d e d c o m p l e x i t y of the annular d e s i g n . H i g h e r s a m p l i n g rates are sometimes c o n s i d e r e d desirable to m i n i m i z e i n l e t losses. H o w e v e r , a h i g h flow dénuder is not essential for {

Q

{

0

0

this p u r p o s e . A n i n l e t m a n i f o l d can b e set u p w i t h a h i g h flow rate a n d s m a l l residence t i m e , a n d the dénuder, w i t h its o w n aspiration source, is t h e n c o n n e c t e d w i t h a v e r y short c o n d u i t to this m a n i f o l d . T h e s e considerations p e r t a i n largely to a u t o m a t e d systems. I n systems u t i l i z i n g m a n u a l c o l l e c t i o n a n d analysis, the a n n u l a r g e o m e t r y does have advantages, as e v i d e n t f r o m its c u r r e n t w i d e s p r e a d use.


54

M E A S U R E M E N T C H A L L E N G E S IN ATMOSPHERIC CHEMISTRY

Automated Collection-Analysis Systems T h e e x i s t i n g systems can be b r o a d l y classified into t w o groups: systems that deal w i t h a gaseous analyte, a n d systems that deal w i t h the analyte i n the l i q u i d phase. T o date, the first group is c o m p o s e d solely o f t h e r m o d e n u d e r s , devices that r e l y o n t h e r m a l l y c y c l e d s o r p t i o n - d e s o r p t i o n steps. T h e second g r o u p i n c l u d e s w e t diffusion d e n u d e r - s c r u b b e r s a n d devices that are c y c l i cally sorbent-coated a n d w a s h e d .


Thermodenuders S a m p l i n g a m b i e n t air t h r o u g h sorbents l i k e d i p h e n y l p h e n y l e n e oxide (Tenax) a n d t h e r m a l l y d e s o r b i n g the adsorbed c o m p o u n d s for gas c h r o m a t o g r a p h i c analysis are a m o n g the most established a n d useful practices for the d e t e r m i n a t i o n of organic c o m p o u n d s i n a m b i e n t air. T h e r m o d e n u d e r s r e p r e s e n t the e q u i v a l e n t approach w i t h diffusion-based s a m p l i n g . B r a m a n et a l . (47) r e p o r t e d the first such d e v i c e for the m e a s u r e m e n t of a t m o s p h e r i c N H a n d H N 0 , a n d a p p l i c a t i o n to a m b i e n t air analysis was r e p o r t e d b y M c C l e n n y et a l . (60). Tungsten(VI) oxide ( W 0 , often r e f e r r e d to as tungstic acid) is coated o n the i n s i d e walls of porous glass (Vycor) o r q u a r t z tubes b y e l e c t r i c a l l y h e a t i n g a c o n c e n t r i c a l l y s u s p e n d e d t u n g s t e n w i r e . T h e p o l y m e r i c b l u e W ( I V ) oxide i n i t i a l l y f o r m e d is c o n v e r t e d to the p r e f e r r e d y e l l o w W ( V I ) f o r m b y h e a t i n g the t u b e to 500 °C w h i l e passing oxygen t h r o u g h i t . I n the automated i n s t r u m e n t (60), the s a m p l e r i n l e t a l l o w e d , b y a p p r o p r i a t e l y c o n n e c t e d s o l e n o i d valves, the c h o i c e o f s a m p l e air, calibrant, a n d a p u r g e gas (20% 0 i n H e , u s e d d u r i n g the d e s o r p t i o n step). T h e sample air is d r a w n t h r o u g h a W 0 - c o a t e d d é n u d e r t u b e c o n t a i n i n g an i n t e g r a l h e a t i n g c o i l a n d t h e n t h r o u g h a catalyst t u b e c o n t a i n i n g a g o l d catalyst, also p r o v i d e d w i t h a h e a t i n g c o i l . A t y p i c a l operational c y c l e involves (1) 1 0 - 5 0 m i n o f s a m p l i n g of a m b i e n t air (Q = 1 L / m i n ) ; (2) i n t e gration of the baseline signal of a c h e m i l u m i n e s c e n c e - b a s e d N O . m o n i t o r for 10 m i n (this is c o n s i d e r e d b l a n k , a n d step 1 continues d u r i n g this t i m e ; (3) ceasing s a m p l i n g , s w i t c h i n g to p u r g e gas, a n d c o n n e c t i n g the c o n v e r t e r t u b e exit to the N O m o n i t o r b y a p p r o p r i a t e v a l v i n g , c o n t r o l l e d e l e c t r i c a l h e a t i n g o f b o t h the dénuder a n d the c o n v e r t e r , a n d r e c o r d i n g o f the resultant signal f r o m the N O . m o n i t o r (10 m i n ) ; a n d (4) c o o l i n g (10 m i n ) before the cycle is b e g u n anew. D u r i n g step 1, s a m p l e d N H a n d H N 0 are t a k e n u p b y the dénuder. W h e n it is heated i n step 3, H N 0 first comes off as N 0 , a n d at a h i g h e r t e m p e r a t u r e N H comes off w i t h o u t d e c o m p o s i t i o n . I n the presence o f the h e a t e d A u catalyst, b o t h are c o n v e r t e d to N O a n d are thus m e a s u r e d b y the NO m o n i t o r . T h e i n d i v i d u a l signals f r o m H N 0 a n d N H are t e m p o r a l l y separated; although t h e y are not b a s e l i n e - r e s o l v e d , the separation is c o n s i d e r e d adequate. T h e L O D for N H o r H N 0 was 70 parts p e r t r i l l i o n b y v o l u m e (pptrv) for a 2 0 - m i n sample. C o l l e c t i o n of p a r t i c u l a t e 3

3

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A

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DASGUPTA

Automated Measurement of Atmospheric Trace Gases

55

a m m o n i u m a n d nitrate w i t h a p a c k e d c o l u m n of W 0 - c o a t e d sand a n d a n a l ysis b y s i m i l a r t h e r m a l d e s o r p t i o n t e c h n i q u e s are also m e n t i o n e d i n these papers (47, 60), a l t h o u g h few details are g i v e n . A l t h o u g h the tungstic oxide t h e r m o d e n u d e r i n p r i n c i p l e is a n attractive means o f m e a s u r i n g t w o analyte gases of c o n s i d e r a b l e interest, field studies have i n d i c a t e d that u n k n o w n interferences can seriously c o m p r o m i s e the r e l i a b i l i t y of the data generated for e i t h e r N H o r H N 0 . I n i n t e r c o m p a r i s o n studies, the data generated b y this t e c h n i q u e have b e e n i n e r r o r b y as m u c h as a factor of 2 to 6. [See F o x et a l . (61), R o b e r t s et a l . (62), a n d A p p e l et al. (63)]. I n a m o r e d e t a i l e d subsequent study, R o b e r t s et a l . (64) m a d e a n u m b e r of i m p o r t a n t observations r e g a r d i n g this t e c h n i q u e as a p p l i e d to the m e a s u r e m e n t of b a c k g r o u n d levels of N H . T h e y f o u n d that n e i t h e r the b l u e W ( I V ) oxide nor the y e l l o w W ( V I ) oxide is best s u i t e d for the p u r p o s e ; r a t h e r , the b l u e - g r e e n oxide, p r e s u m a b l y of an i n t e r m e d i a t e o x i d a t i o n state, is the most desirable f o r m . A l k y l a m i n e s l i k e e t h y l a m i n e w e r e also f o u n d to b e taken u p b y such a dénuder. A l t h o u g h such amines are d e s o r b e d at a t e m p e r a t u r e significantly l o w e r than N H u n d e r i d e a l i z e d test c o n d i t i o n s , s i g nificant amounts of the amines are c o n v e r t e d to N H o n the active surface o f the dénuder at relative h u m i d i t y levels greater t h a n 2 5 % . C o n s e q u e n t l y , it is d o u b t f u l that such c o m p o u n d s can actually be differentiated f r o m N H u n d e r actual m e a s u r e m e n t conditions. M o s t i m p o r t a n t l y , c o n t i n u e d r e l i a b l e p e r f o r m a n c e o f the d é n u d e r is v e r y susceptible to o v e r h e a t i n g ; the r e c o v e r y o f N H decreases a n d the peak b e c o m e s i l l - d e f i n e d for an o v e r h e a t e d d é n u d e r . W i t h frequent c a l i b r a t i o n to ensure accuracy, the authors m e a s u r e d N H levels i n isolated regions i n the C o l o r a d o m o u n t a i n s a n d coastal C a l i fornia to b e 200 ± 80 a n d 360 ± 170 p p t r v , r e s p e c t i v e l y ; these are a m o n g the lowest N H concentrations r e p o r t e d for N H i n c o n t i n e n t a l air. D e s p i t e the apparent a p p l i c a b i l i t y , Roberts et a l . d i d not endorse the t e c h n i q u e . Because d e s o r p t i o n of N H occurs at t e m p e r a t u r e s near those that alter the surface, they c o n c l u d e d that the performance is susceptible to slow e v o l u tionary a n d occasionally catastrophic failure a n d that other alternatives s h o u l d b e sought. M u c h m o r e favorable results w e r e later r e p o r t e d w i t h a m o l y b d e n u m oxide dénuder of annular g e o m e t r y (65); this d é n u d e r is d e s c r i b e d i n a later section. 3

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Interestingly, m u c h of the early interest i n d e v e l o p i n g a u t o m a t e d t h e r m o d e n u d e r s c e n t e r e d o n the d e t e r m i n a t i o n of aerosol-phase analytes r a t h e r than gases. L i n d q v i s t (66) d e s c r i b e d a system for the d e t e r m i n a t i o n o f aerosol H S 0 i n w h i c h the analytical a n d regeneration step of the d é n u d e r was f u l l y automated b u t the transfer of the dénuder t u b e b e t w e e n the c o l l e c t i o n system a n d the analyzer was m a n u a l . A s discussed s u b s e q u e n t l y , this step is not difficult to automate. T h e strategy b e h i n d the c o l l e c t i o n of aerosols b y a dénuder t y p i c a l l y involves the r e m o v a l of a l l gases that m a y pose an interference b y an appropriate p r e d e n u d e r . T h e aerosol is t h e n t h e r m a l l y c o n v e r t e d to a gas (either a phase transition or d e c o m p o s i t i o n i n t o gaseous 2

4


56


products), a n d this gas is taken u p b y a dénuder that is t h e r m a l l y d e s o r b e d d u r i n g the analysis step. I n L i n d q v i s t ' s i n s t r u m e n t , two serial glass (Pyrex) p r e d e n u d e r s (one coated w i t h oxalic a c i d to r e m o v e N H , a n d the o t h e r coated w i t h a l u m i n a i m p r e g n a t e d w i t h A g N 0 , H B 0 , N a H C 0 , a n d tartaric acid) d e s i g n e d to r e m o v e S Q , H S , C H S H , C H S C H , a n d C H S S C H are used. T h e p r i n c i p a l dénuder is a 7 3 . 3 - X 0.45-em q u a r t z t u b e , first solution-coated w i t h an acetone solution o f M n ( N 0 ) a n d P d C l a n d later h e a t e d to 800 °C i n s e q u e n t i a l H , air, a n d H e streams to o b t a i n a coating of M n 0 - P d O . D u r i n g c o l l e c t i o n , this t u b e is m a i n t a i n e d at 138 °C to v o l atilize the aerosol H S 0 , w h i c h is c o l l e c t e d as sulfate. D u r i n g analysis, the dénuder is s u p p l i e d w i t h a small flow o f h e l i u m a n d h e a t e d to 800 ° C , w h e r e u p o n the sulfate decomposes to S 0 . T h e exit gas f r o m the dénuder is m e r g e d w i t h a small flow of H that t h e n flows t h r o u g h a q u a r t z t u b e h e a t e d to 1000 °C. T h e r e s u l t i n g H S is c o l l e c t e d o n a silver w o o l trap. W h e n this step is c o m p l e t e , the s i l v e r w o o l is flash-heated to 600 ° C , a n d the l i b e r a t e d H S is fed to a gas c h r o m a t o g r a p h e q u i p p e d w i t h a p h o t o i o n ization detector. T h e e n t i r e analytical cycle r e q u i r e s 12 m i n . Samples of 60 m i n p r o v i d e an L O D o f 60 n g / m (15 p p t r v gas-phase equivalent) i f i m m e d i a t e l y analyzed. B l a n k s increase u p o n storage, a n d the attainable L O D increases b y an o r d e r o f m a g n i t u d e u p o n 24 h o f storage; this observation c l e a r l y underscores the advantages o f a fully a u t o m a t e d system w i t h no elapsed t i m e b e t w e e n c o l l e c t i o n a n d analysis. U n d e r a m b i e n t c o n d i t i o n s , the relative standard d e v i a t i o n (rsd) of the m e a s u r e m e n t p r o c e d u r e is r e p o r t e d to b e 17%. 3

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L i n d q v i s t s u b s e q u e n t l y d e s c r i b e d a s i m i l a r system for the d e t e r m i n a t i o n o f H N 0 (67). H N 0 was collected o n an A l ( S 0 ) - c o a t e d q u a r t z dénuder. T h e t h e r m a l l y d e s o r b e d NO was d e t e r m i n e d b y gas c h r o m a t o g r a p h y - p h o t o i o n i z a t i o n detection. S u b s e q u e n t l y T a n n e r et a l . (68) s i m p l i f i e d a n d fully automated the o v e r a l l system configuration. A 5 1 - X 0 . 4 - c m q u a r t z dénuder t u b e was solution-coated w i t h 2 0 % w / v A l ( S 0 ) a n d u s e d at a v e r y l o w Q (0.1 L / m i n ) . T h e d e s o r p t i o n step i n v o l v e d h e a t i n g to 500 °C for 1 m i n ; the l i b e r a t e d N O ^ was d e t e r m i n e d b y a c h e m i l u m i n e s c e n c e m o n i t o r . A l t h o u g h laboratory results w e r e attractive, field i n t e r c o m p a r i s o n s w i t h a n u m b e r o f o t h e r methods i n d i c a t e d l o w a n d variable results; the reasons for this discrepancy c o u l d not b e i d e n t i f i e d w i t h certainty. 3

3

2

4

3

x

2

4

3

T h e N e t h e r l a n d s E n e r g y R e s e a r c h F o u n d a t i o n ( E C N ) has m a d e m a n y significant c o n t r i b u t i o n s to the progress of a t m o s p h e r i c analytical c h e m i s t r y , p a r t i c u l a r l y i n the area of automated diffusion d e n u d e r - b a s e d analytical systems. A s early as 1981, the E C N g r o u p , i n collaboration w i t h K l o c k o w a n d N i e s s n e r , w h o o r i g i n a l l y d e v e l o p e d the t h e r m o a n a l y t i c a l strategy for the m e a s u r e m e n t o f strong acids, d e s c r i b e d a serial seven-section d é n u d e r syst e m to p e r f o r m measurements o f H N 0 , N H , H S 0 , a m m o n i u m n i t r a t e , a n d a m m o n i u m sulfate (69). T h e first section was coated w i t h N a F a n d was r e p o r t e d to retain H N 0 (g), the second section was coated w i t h H P 0 a n d 3

3

2

4

3


3

4

2.

DASGUPTA


57

Trace Gases

r e t a i n e d N H (g), the t h i r d section was coated w i t h N a O H to retain S 0 3

2

(a

p o t e n t i a l interfèrent), the fourth section was coated w i t h N a F a n d h e a t e d to 130 °C to convert H S 0 2

into the vapor phase a n d r e t a i n it as w e l l as to

4

dissociate N H N 0

3

a n d to retain the r e s u l t i n g H N 0 , the fifth section was

coated w i t h H P 0

4

to retain N H e v o l v e d from the f o u r t h section, the sixth

4

3

3

3

section was coated w i t h N a F a n d heated to 230 °C to dissociate a m m o n i u m sulfates a n d retain the r e s u l t i n g H S 0 , a n d the s e v e n t h section c o n t a i n e d 2

4

an H P 0 - c o a t e d t u b e to collect the N H e v o l v e d i n the sixth section. T h e 3

4

3

system was not automated; it w o u l d be i n d e e d difficult to automate s u c h an i n v o l v e d scheme. S l a n i n a et a l . (70) subsequently d e s c r i b e d a fully a u t o m a t e d c o m p u t e r Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch002

c o n t r o l l e d t h e r m o d e n u d e r system for the m e a s u r e m e n t o f sulfuric a c i d a n d a m m o n i u m sulfate aerosol. A s i n L i n d q v i s t ' s w o r k , sulfur gases are r e m o v e d b y t w o serial glass p r e d e n u d e r s , one coated w i t h K C 0 2

a n d the o t h e r w i t h

3

activated carbon. T h e m a i n d e n u d e r s are 5 0 - X 0 . 6 - c m q u a r t z tubes s o l u t i o n coated i n i t i a l l y w i t h C u ( N 0 ) , d r i e d a n d t h e n fired at 900 °C i n a n i t r o g e n 3

2

stream to o b t a i n a m i x e d C u - C u O coating. T w o s e q u e n t i a l C u - C u O dénuder tubes are u s e d : T h e first is m a i n t a i n e d at 120 °C a n d the second at 240 °C for the respective c o l l e c t i o n of aerosol sulfuric a c i d a n d a m m o n i u m sulfates. D u r i n g analysis, the tubes are heated, one at a t i m e , to 800 ° C , a n d the l i b e r a t e d S 0 is m e a s u r e d b y a flame p h o t o m e t r i c sulfur analyzer. T h e tubes 2

are t h e n regenerated w h i l e still hot b y i n j e c t i n g a small v o l u m e of clean a i r , a n d the cycle is started anew. T h e i n s t r u m e n t is e q u i p p e d w i t h a second p a i r o f C u - C u O dénuder tubes that are u s e d for s a m p l i n g w h i l e the

first

pair goes t h r o u g h the analytical cycle. P r o v i s i o n s for p e r i o d i c a u t o m a t e d calibration w i t h S 0 as a c a l i b r a t i o n standard are also p r o v i d e d . T h e system 2

p r o v i d e s an L O D o f 20 n g / m for a 1-h sample a n d 100 n g / m for a 5 - m i n 3

sample for e i t h e r H S 0 2

3

or ( N H ) S 0 .

4

4

2

4

A l t h o u g h this t h e r m o d e n u d e r c o l l e c t i o n - a n a l y s i s system represents an i n t e r e s t i n g a n d ingenious advance i n the art of a t m o s p h e r i c analysis, i t d i d not m e a n i n g f u l l y solve the i n t e n d e d m e a s u r e m e n t p r o b l e m . T h e p r e m i s e o f the t h e r m a l differentiation step is that H S 0 a n d ( N H ) S 0 aerosols exist 2

4

4

2

4

as an external m i x t u r e . I n l i g h t of w h a t is p r e s e n t l y k n o w n about the h e t erogeneous oxidation of S 0 , this p r e m i s e is incorrect. I n t e r n a l m i x t u r e s , 2

that is, aerosols c o n t a i n i n g H S 0 2

4

i n various stages of n e u t r a l i z a t i o n w i t h

N H , cannot be t h e r m a l l y differentiated to h y p o t h e t i c a l constituent c o m 3

ponents i n the d e s c r i b e d m a n n e r . O n the basis o f subsequent findings, i t is d o u b t f u l that ( N H ) S 0 a n d N H H S 0 , e v e n w h e n d i s c r e t e l y p r e s e n t i n 4

2

4

4

4

the same sample, can be t h e r m a l l y differentiated. F u r t h e r , the t h e r m a l d e c o m p o s i t i o n of m o r e volatile a m m o n i u m salts (e.g., N H N 0 ) can p r o d u c e 4

negative interferences i n H S 0 2

4

3

measurements w i t h t h e r m a l d e n u d e r s (41).

T o b e fair, science advances o n faltering steps; I have b e e n j u s t as g u i l t y o f m a k i n g the i d e n t i c a l erroneous assumption (71, 72).


58


I n any case, the d e v e l o p m e n t of the automated H S 0 - ( N H ) S 0 d e t e r m i n a t i o n system was c l e a r l y an i m p o r t a n t step t o w a r d the s u b s e q u e n t l y r e p o r t e d t h e r m o d e n u d e r system for the m e a s u r e m e n t of S 0 (73). I n this i n s t r u m e n t , a q u a r t z p r e d e n u d e r , solution-coated w i t h N a H S 0 - A g S 0 , removes i n t e r f e r i n g sulfur gases, notably H S . T h i s r e m o v a l is f o l l o w e d b y a C u - C u O - c o a t e d dénuder as d e s c r i b e d i n a p r e c e d i n g section. D u r i n g the analysis step, the sulfate r e s u l t i n g from the c o l l e c t e d S 0 is t h e r m a l l y d e c o m p o s e d to S 0 a n d m e a s u r e d w i t h a p u l s e d fluorescence i n s t r u m e n t . U n d e r field conditions, this setup p e r m i t t e d an L O D of 40 p p t r v o f S 0 for a 3 0 - m i n sample (Q = 0.5 L / m i n ) . A n L O D almost an o r d e r of m a g n i t u d e b e t t e r was attainable w i t h the flame p h o t o m e t r i c detector i n the laboratory. T h e r e p r o d u c i b i l i t y of the t e c h n i q u e was m e a s u r e d w i t h p a r a l l e l t r i p l i c a t e d e n u d e r s a n d r a n g e d f r o m 2 to 5 % r s d . 2

4

4

2

4

2

4

2

4

2

2

2


2

C o n t i n u i n g applications of t h e r m o d e n u d e r s t y p i c a l l y u t i l i z e the a n n u l a r geometry. K e u k e n et a l . (74) d e s c r i b e d a v a n a d i u m p e n t o x i d e coated q u a r t z a n n u l a r d é n u d e r system. T h e dénuder i t s e l f a n d the o v e r a l l setup of the system are s h o w n i n F i g u r e s 2 a n d 3. T h e dénuder is coated b y an aqueous s l u r r y of V 0 a n d d r i e d at 70 °C. T h e dénuder (d = 2.8 c m , d = 2.5 c m , a n d L = 25 cm) e x h i b i t e d essentially quantitative efficiency for c o l l e c t i n g N H at Q = 10 L / m i n . F o l l o w i n g the c o l l e c t i o n p e r i o d ( 1 0 - 2 0 m i n ) , a m o t o r i z e d screw d r i v e moves a p r e h e a t e d o v e n (700 °C) a r o u n d the dénuder ( F i g u r e 3) a n d stops w h e n the dénuder is h a l f c o v e r e d (position 1). A f t e r 5 m i n , the o v e n is m o v e d again, this t i m e to cover the entrance h a l f of the dénuder as w e l l (position 2). D u r i n g s a m p l i n g , a m m o n i a is essentially r e m o v e d c o m p l e t e l y b y the first h a l f of the dénuder. U n k n o w n n i t r o g e n c o n t a i n i n g gaseous interferences are, o n the other h a n d , r e m o v e d ineffic i e n t l y a n d are sorbed a p p r o x i m a t e l y to the same extent o n the first a n d the second h a l f of the dénuder. W h e n the dénuder is h e a t e d , b o t h N H a n d any other n i t r o g e n - c o n t a i n i n g c o m p o u n d s are c o n v e r t e d to NO a n d m e a s u r e d i n t u r n b y a c h e m i l u m i n e s c e n c e - t y p e N O m o n i t o r . T h u s , the peak o b t a i n e d i n p o s i t i o n 1 corresponds to the interferences o n l y , a n d the peak o b t a i n e d i n p o s i t i o n 2 corresponds to b o t h N H a n d the interferences. T h e N H signal is d e t e r m i n e d b y difference. F o r a 1 0 - m i n sample, the L O D is ~ 1 5 0 p p t r v w i t h a t y p i c a l r s d of 5 % u n d e r a m b i e n t m e a s u r e m e n t c o n d i t i o n s . I n p a r a l l e l measurements w i t h a H P 0 - c o a t e d s i n g l e - t u b e dénuder, a w e t annular dénuder (vide infra) a n d a t u n a b l e d i o d e laser s p e c t r o m e t e r , v e r y good agreement was r e p o r t e d . 2

Q

5

{

3

3

x

x

3

3

3

4

E a r l i e r , a less c o n v e n t i o n a l a n n u l a r g e o m e t r y was r e p o r t e d b y L a n g f o r d et a l . (65) for the d e t e r m i n a t i o n of N H . F o l l o w i n g t h e i r p r e v i o u s w o r k w i t h tungstic oxide coated d é n u d e r tubes (62, 64), t h e y sought a m o r e r u g g e d a n d r e p r o d u c i b l e means of fabricating m e t a l oxide coated d e n u d e r s . T h e s e goals w e r e m e t i n a g e o m e t r y i n w h i c h a W or M o r o d is c o n c e n t r i c a l l y p l a c e d w i t h i n a q u a r t z t u b e (d = 0.32 c m , d = 0.40 c m , a n d active L = 30.5 cm); this g e o m e t r y is analogous to that o f the D S d e s c r i b e d b y T a n n e r 3

{

0



QUARTZ TUBE 6 mm id.

AIR IN

3

QUARTZ TUBE 36mm. id. RESISTANCE WIRE

Figure 2. Vanadium pentoxide coated annular thermodenuder for the determination of NH . (Reproduced permission from reference 74. Copyright 1989 Pergamon Press.)

-COATED ANNULAR 1.5mm


with

TO PUMP

3

CD

ο

i

3

Co

C

Ο

ζ/5

Ό >


TURN-OVER-SWITCH * MICROSWITCHES

CALIBRATION VALVE

Figure 3. Instrument schematic for the thermodenuder of Figure 2. (Reproduced with permission from reference 74. Copyright 1989 Pergamon Press.)

Vdc

500 ppm NO GAS MIXTURE"


2.

DASGUPTA

61

Automated Measurement of Atmosphenc Trace Gases

et al. (53) a n d reduces the o v e r a l l t h e r m a l mass of the system. H e a t i n g u n d e r a flow of 0 at 300 to 400 °C produces M o ( I V ) oxide w i t h i n 15 m i n , a n d the d e s i r e d M o ( V I ) oxide is p r o d u c e d u p o n f u r t h e r h e a t i n g to 450 ° C . (The W r o d produces similar coatings of the c o r r e s p o n d i n g (IV) a n d (VI) oxides b u t at h i g h e r temperatures.) A f t e r 1 h , the flow is r e v e r s e d a n d the assembly is heated for another h o u r to p r o d u c e a m o r e e v e n coating. T h e e n t i r e p r o c e d u r e r e q u i r e s 3 h a n d produces an o p t i m u m oxide coating 0.25 m m t h i c k . S a m p l i n g is t y p i c a l l y c o n d u c t e d for 30 m i n at Q = 0.9 L / m i n . 2

D e t a i l e d studies o f the d e s o r p t i o n b e h a v i o r o f N H f r o m a l l four oxide surfaces [W(IV), W ( V I ) , M o ( I V ) , a n d Mo(VI)] w e r e r e p o r t e d (65). T h e W 0 , M o 0 , a n d W 0 surfaces a l l d e s o r b N H as N H at respective t e m p e r a t u r e s of —280, 300, a n d 350 °C a n d thus r e q u i r e the u s u a l h e a t e d A u c o n v e r t e r for the c o n v e r s i o n of N H to N O p r i o r to m e a s u r e m e n t b y a c h e m i l u m i nescence-type N O , m o n i t o r . I n contrast, the M o 0 surface shows t w o d i s t i n c t d e s o r p t i o n steps at 350 a n d 380 °C; the first corresponds to the d e s o r p t i o n of N H as N H , a n d the second peak has b e e n s h o w n to b e N O , the d é n u d e r surface i t s e l f acting as the necessary oxidative c o n v e r t e r . A t the l o w N H levels of interest, the second peak amounts to ~ 3 0 % of the total N H s o r b e d (the figure is essentially constant for a g i v e n dénuder, b u t i n d i v i d u a l d e n u d e r s r e q u i r e separate calibrations), a n d this situation allows sufficient s e n sitivity for the system to be operated w i t h o u t an a d d i t i o n a l oxidative c o n v e r t e r . T h e M o 0 surface also takes u p N 0 a n d H N 0 , w h i c h are b o t h d e s o r b e d as N O at 200 a n d 200 to 250 ° C , r e s p e c t i v e l y . T h e feasibility of t h e r m a l separation of sorbed N 0 a n d H N 0 was not i n v e s t i g a t e d i n this w o r k ; rather, u s i n g a p r o g r a m that r a p i d l y heats the d é n u d e r to 250 °C a n d t h e n gradually raises the t e m p e r a t u r e to 400 ° C , a c o m p l e t e separation o f the N 0 - H N 0 a n d the N H peaks (the rejection ratio of the N H peak f r o m N 0 is greater t h a n 5000:1) is o b t a i n e d . A flow of N is u s e d d u r i n g the d e s o r p t i o n a n d the c o o l i n g steps. 3

2


2

3

3

3

3

3

3

3

3

3

3

2

2

2

3

3

3

3

3

2

2

L i k e the W 0 dénuder, the M o 0 dénuder is u n a b l e to d i s t i n g u i s h b e t w e e n N H a n d amines; h o w e v e r , the p r e d o m i n a n c e of the f o r m e r i n most m e a s u r e m e n t situations may make this l i m i t a t i o n of little c o n s e q u e n c e . T h e L O D is e s t i m a t e d to be 1 0 - 2 0 p p t r v of N H ; d u r i n g extensive field studies, concentrations as l o w as 50 p p t r v w e r e m e a s u r e d a n d constitute the lowest r e p o r t e d gaseous N H concentrations thus far. A t l o w levels v i r t u a l l y e v e r y surface shows some uptake of N H ; to a v o i d this situation, the e n t i r e t r a i n is m a i n t a i n e d at 50 °C ( i n c l u d i n g the dénuder, d u r i n g the c o l l e c t i o n step, to p r e v e n t adsorption o n the q u a r t z walls). T h i s practice does cause some c o n c e r n about interference f r o m the p o t e n t i a l evaporative dissociation f r o m aerosol N H N 0 ; h o w e v e r , side-by-side studies w i t h c i t r i c a c i d coated s i m p l e t u b u l a r denuders indicate that the evaporation kinetics is slow, a n d the o b s e r v e d extent of artifactual N H is l i k e l y less t h a n 3%. O v e r a l l , the M o 0 t h e r m o d e n u d e r represents an elegant c o m b i n a t i o n of good d e s i g n a n d c l e v e r 3

3

3

3

3

3

4

3

3


3

62


use o f c h e m i s t r y , a n d the studies clearly reflect the a t t e n t i o n to d e t a i l b e stowed b y its inventors. K l o c k o w et a l . (75) d e s c r i b e d t w o separate a u t o m a t e d t h e r m o d e n u d e r systems for the simultaneous d e t e r m i n a t i o n of H N 0 a n d N H N 0 . O n e p r o b l e m i n the d e t e r m i n a t i o n of H N 0 is that most d é n u d e r coatings t r i e d for this p u r p o s e also take u p some o t h e r n i t r o g e n - c o n t a i n i n g c o m p o u n d s , p r e s u m a b l y organic i n nature. A l t h o u g h such c o m p o u n d s are not c o l l e c t e d w i t h h i g h efficiency, the NO e v o l u t i o n f r o m these species d u r i n g the t h e r m a l d e s o r p t i o n step cannot be t e m p o r a l l y separated f r o m that o f H N 0 b y any reasonable p r o g r a m m e d t e m p e r a t u r e steps, a n d the r e s u l t i n g i n t e r f e r e n c e can b e significant. T h e first system d e v i s e d a t t e m p t e d to solve this p r o b l e m b y u s i n g a train of three serial annular q u a r t z d e n u d e r s s i m i l a r to that d e s c r i b e d e a r l i e r (74). A l l t h r e e d e n u d e r s are solution-coated w i t h M g S 0 . T h e serial dénuder train incorporates valves at the c o n n e c t i n g p o i n t b e t w e e n each dénuder such that d u r i n g the d e s o r p t i o n step each d é n u d e r can be separately h e a t e d a n d its effluent can b e a n a l y z e d b y a c h e m i l u m i n e s c e n c e type N O m o n i t o r . D u r i n g the c o l l e c t i o n step (Q = 5 L / m i n ) , the first two d e n u d e r s , o p e r a t i n g at r o o m t e m p e r a t u r e , separately collect H N 0 p l u s u n k n o w n interference a n d the u n k n o w n interference o n l y . T h e t h i r d d é n u d e r is o p e r a t e d at 150 °C to volatilize N H N 0 a n d collect the r e s u l t i n g H N 0 . D u r i n g the d e s o r p t i o n step, each t u b e is h e a t e d i n t u r n to 700 ° C , a n d the e v o l v e d N O ^ is detected. T h e difference b e t w e e n the signals f r o m tubes 1 a n d 2 is taken to be H N 0 , a n d the signal f r o m t u b e 3 is t a k e n to b e N H N 0 . T h e system is p r o v i d e d w i t h provisions for a u t o m a t e d d e t e c t o r calibration w i t h c y l i n d e r N O . A second t r a i n of three s e q u e n t i a l d e n u d e r s is p r o v i d e d to alternate the c o l l e c t i o n - a n a l y s i s cycles b e t w e e n the t w o trains a n d thus i m p r o v e t e m p o r a l r e s o l u t i o n . W i t h a 3 0 - m i n sample, L O D s of 40 p p t r v a n d 100 n g / m w e r e r e p o r t e d for H N 0 a n d N H N 0 , r e s p e c t i v e l y . 3

4

3

3

x


3

4

x

3

4

3

3

3

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3

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3

T h e s e c o n d system d e s c r i b e d i n this p a p e r (75) u t i l i z e s single-tube d e n u d e r s o p e r a t i n g at Q = 0.66 L / m i n . T h e first a n d t h i r d d e n u d e r s are m a d e o f glassy c a r b o n (50 X 0.6 cm) a n d are coated w i t h a m e t h a n o l i c suspension of B a S 0 a n d a w a t e r - m e t h a n o l solution of N a F , r e s p e c t i v e l y . T h e m i d d l e d é n u d e r is a glass t u b e (50 X 1.5 cm) c o n t a i n i n g a n i n s e r t e d r o l l e d sheet o f a c t i v a t e d - c a r b o n - i m p r e g n a t e d filter paper. A c c o r d i n g to K l o c k o w et a l . , the B a S 0 - c o a t e d glassy carbon dénuder does not collect the u n k n o w n o r ganic n i t r o g e n - c o n t a i n i n g interference(s) to a significant extent a n d collects H N 0 selectively. T h e n i t r o g e n - c o n t a i n i n g interferents are r e m o v e d b y the activated c a r b o n dénuder, a n d the N H N 0 is t h e n v o l a t i l i z e d a n d c o l l e c t e d i n the N a F - c o a t e d dénuder m a i n t a i n e d at 140 °C. D u r i n g the analysis step d e n u d e r s 1 a n d 3 are s e q u e n t i a l l y heated to 700 ° C , the l i b e r a t e d N O is m e a s u r e d , a n d the d e n u d e r s r e s p e c t i v e l y y i e l d measures o f H N 0 and N H N 0 . F o r 2 4 - h samples, respective L O D s of 20 p p t r v of H N 0 a n d 60 n g / m of N H N 0 w e r e r e p o r t e d . 4

4

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T h e general advantages of t h e r m o d e n u d e r systems i n c l u d e facile p r e c o n c e n t r a t i o n , a u t o m a t i o n , a n d the ease w i t h w h i c h t h e y can b e c o u p l e d to a v a r i e t y of detectors d e s i g n e d to h a n d l e gas-phase samples. T h e fact that all f l o w i n g fluids are i n the gas phase simplifies o v e r a l l system d e s i g n . O n the negative side, t h e r m o d e n u d e r s f r e q u e n t l y have h i g h analytical b l a n k s , t h e y c o n s u m e a large a m o u n t of electrical p o w e r , a n d substantial c o o l - d o w n times are r e q u i r e d for a n e w s a m p l i n g cycle to b e g i n unless p a r a l l e l s a m p l i n g trains are u s e d instead.


Wet Effluent Denuders-Scrubbers W e t effluent diffusion-based c o l l e c t i o n devices can be s u b d i v i d e d into t h r e e g r o u p s — c o n v e n t i o n a l coated d e n u d e r s , i n w h i c h the c o a t i n g - w a s h i n g analysis steps are f u l l y automated; m e m b r a n e - b a s e d diffusion scrubbers; a n d w e t diffusion d e n u d e r s . T h e r e is o n l y one report of an example of the first type. Bos (76) describes a system i n w h i c h a single-tube glass d é n u d e r is coated i n situ w i t h a m e t h a n o l i c solution of c i t r i c a c i d , the dénuder is d r i e d w i t h clean a i r , a m b i e n t air is s a m p l e d for a preset p e r i o d of t i m e , the d é n u d e r surface is w a s h e d d o w n w i t h w a t e r , a n d the washings are sent (for the d e t e r m i n a t i o n of the c o l l e c t e d N H ) to an a i r - s e g m e n t e d flow analysis system r e l y i n g o n the i n d o p h e n o l b l u e c h e m i s t r y , a n d t h e n the cycle is b e g u n anew. S u c h an o b v i o u s l y c o m p l e x a r r a n g e m e n t cannot be r e p l i c a t e d from the v e r y l i m i t e d d e s c r i p t i o n p r o v i d e d i n the b r i e f p a p e r b y Bos (76). S h o u l d it be possible to operate s u c h i n s t r u m e n t s r e l i a b l y over l o n g periods of t i m e , t h e y clearly m a y have m a n y applications. N o f u r t h e r reports o n this t e c h n i q u e have a p p e a r e d i n the decade since Bos p u b l i s h e d his w o r k , a n d the m e t h o d is not i n present use. 3

Diffusion Scrubbers D i f f u s i o n scrubbers are m e m b r a n e - b a s e d d e n u d e r s i n w h i c h the sample air flows o n one side of a m e m b r a n e a n d a suitable s c r u b b e r l i q u i d flows o n the other side. T h e analyte gases of interest are c o l l e c t e d i n the s c r u b b e r l i q u i d , a n d the effluent is subjected to analysis. T h e simplest g e o m e t r y is that of a c o n v e n t i o n a l single-tube dénuder. A i r is s a m p l e d t h r o u g h a t u b u l a r m e m brane w h i l e the s c r u b b e r l i q u i d is p u m p e d i n a c o u n t e r c u r r e n t fashion t h r o u g h an external jacket t u b e s u r r o u n d i n g the m e m b r a n e t u b e .

Ion-Exchange Membrane D S Devices. was wet in a was

T h e first D S r e p o r t e d (77)

based o n a perfluorosulfonate cation-exchange m e m b r a n e t u b e (Nafion, i n t e r n a l d i a m e t e r 700 μπι, w a l l thickness 75 μιη, 30 c m long) c o n t a i n e d glass jacket. D i l u t e H S 0 was p u m p e d t h r o u g h the jacket, a n d the D S u s e d for s a m p l i n g N H . Ion-exchange m e m b r a n e s are h y d r o p h i l i c , a n d 2

4

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d u r i n g the s a m p l i n g the t u b e is moist o n the i n t e r i o r w a l l as w e l l . A m m o n i a is c a p t u r e d at the m e m b r a n e surface as N H , w h i c h migrates across the m e m b r a n e to the s c r u b b e r l i q u i d . N a f l o n is a m e m b r a n e of c o n s i d e r a b l e structural strength a n d inertness (aside f r o m its ion-exchange properties). S m a l l n e u t r a l polar m o l e c u l e s , for e x a m p l e , H 0 a n d H C H O , a n d small cations show good transport properties across N a f i o n . It s h o u l d be possible to sample l o w - m o l e c u l a r - w e i g h t a m i n e s , for e x a m p l e , C H N H , b u t as the cations b e c o m e larger, m o r e h y d r o p h o b i c , or b o t h , t h e i r diffusion coefficients i n the m e m b r a n e decrease m a r k e d l y . A c i d gases, w h i c h l e a d to characteristic anions, cannot generally b e s a m p l e d w i t h N a f i o n because the D o n n a n b a r r i e r i n h i b i t s the transport o f anions across the cation-exchange m e m b r a n e . 4

+

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H o w e v e r , a c i d gases constitute a s i n g u l a r l y i m p o r t a n t class of analytes to the a t m o s p h e r i c c h e m i s t . It is logical to attempt t h e i r c o l l e c t i o n w i t h a D S based o n an anion-exchange m e m b r a n e . T h e major p r o b l e m is the lack of the availability of suitable anion-exchange m e m b r a n e tubes. P h i l l i p s a n d D a s g u p t a (78) s t u d i e d a D S based o n a P T F E m e m b r a n e t u b e i n t o w h i c h v i n y l b e n z y l c h l o r i d e is radiation-grafted a n d t h e n q u a t e r n i z e d (79). W i t h a D S b u i l t f r o m a 2 5 - e m - l o n g (3000 μΐΏ i . d . , 100-μιτι wall) t u b e , the c o l l e c t i o n of H N 0 was s t u d i e d w i t h o n - l i n e U V d e t e c t i o n . T h e D of H N 0 d e t e r m i n e d from the d e p e n d e n c e o f / o n Q was i n good agreement w i t h o t h e r D values i n the l i t e r a t u r e . T h e s c r u b b e r l i q u i d was a solution of K S 0 a n d sulfamic a c i d ; the latter was i n c o r p o r a t e d to m i n i m i z e the interference f r o m N 0 a n d H O N O i n the d e t e r m i n a t i o n of H N 0 . (A D S based o n an i o n exchange m e m b r a n e cannot b e u s e d w i t h p u r e w a t e r as the s c r u b b e r l i q u i d i f the c o l l e c t e d analyte species is an i o n that is b o u n d o n the ion-exchange sites. T h e s c r u b b e r l i q u i d m u s t contain i o n i c species of suitable d i s p l a c i n g p o w e r i n sufficient c o n c e n t r a t i o n to displace the c o l l e c t e d analyte i o n f r o m the ion-exchange site.) A l t h o u g h this w o r k p r o v e d i n p r i n c i p l e the a p p l i c a b i l i t y of an anion-exchange m e m b r a n e based D S , it d i d not r e p r e s e n t a practical means of m e a s u r i n g a t m o s p h e r i c H N 0 ; the d i r e c t U V d e t e c t i o n m e t h o d p r o v i d e d n e i t h e r the necessary sensitivity nor selectivity. O t h e r p r o b l e m s m a y o c c u r i n d e a l i n g w i t h the simultaneous c o l l e c t i o n a n d analysis of a v a r i e t y o f a c i d gases w i t h an anion-exchange m e m b r a n e based D S . A n i o n exchange sites t e n d to catalyze the oxidation of sulfite to sulfate a n d n i t r i t e to nitrate because o f the greater affinity of the i o n exchanger for the m o r e o x i d i z e d f o r m . T h e first oxidation does not present a major p r o b l e m ; c o l l e c t e d S 0 is often d e l i b e r a t e l y o x i d i z e d to sulfate before d e t e r m i n a t i o n . H o w e v e r , the second o x i d a t i o n does c o m p l i c a t e the differentiation of H O N O f r o m HN0 . 3

3

H N 0 3

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Ion-exchange m e m b r a n e s are h y d r o p h i l i c . T h e y p r e s e n t an active s u r face that has the same uptake p r o b a b i l i t y for the analyte gas as m a y b e o b t a i n e d w i t h a w e t dénuder w i t h the same aqueous s c r u b b e r l i q u i d . T h u s , w i t h s c r u b b e r l i q u i d s that are efficient sinks, these d e n u d e r s t y p i c a l l y e x h i b i t G o r m l e y - K e n n e d y b e h a v i o r i n terms o f c o l l e c t i o n efficiency. T h e transport


2.

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of the analyte from one side of the m e m b r a n e to the o t h e r takes place i n the c o n d e n s e d phase. F o r t y p i c a l analyte species, t h e relaxation t i m e , g i v e n b y f/D 1 b e i n g the m e m b r a n e thickness a n d D b e i n g the analyte diffusion coefficient i n the m e m b r a n e matrix, is of the o r d e r of 0.75 to 3 m i n for 5 0 to ΙΟΟ-μιη-thick m e m b r a n e s w h e n the analyte species is not r e t a i n e d i n the m e m b r a n e b y electrostatic forces. T h i s c o n d i t i o n m a y b e t h e d e t e r m i n i n g factor i n c o n t r o l l i n g the response times of c o n t i n u o u s analyzers based o n such m e m b r a n e - b a s e d collectors. N o n a q u e o u s or m i x e d aqueous s c r u b b e r l i q u i d s can also be u s e d w i t h an ion-exchange m e m b r a n e , s h o u l d t h e i r use be d e e m e d b e n e f i c i a l for the c o l l e c t i o n of certain analytes. T r a n s p o r t across an ion-exchange m e m b r a n e t u b e does not r e l y o n passage t h r o u g h pores; the m e m b r a n e s are therefore not easily " f o u l e d " . H o w e v e r , i n d e s i g n i n g a s i m p l e D S based o n t u b u l a r ion-exchange m e m b r a n e s , it is difficult to achieve the c o n c u r r e n t goals o f h a v i n g a m e m b r a n e t u b e large e n o u g h i n d i a m e t e r to sample a i r u n d e r conditions o f l o w N , t h i n e n o u g h to have r a p i d transport of analytes across i t , a n d yet structurally s t r o n g e n o u g h to resist d e f o r m a t i o n as the s c r u b b e r l i q u i d is p u m p e d i n an a n n u l a r space necessarily small to m i n i m i z e the i n t e g r a t i o n v o l u m e . O t h e r available h y d r o p h i l i c m e m b r a n e tubes, for e x a m p l e , c e l l u l o s i c tubes u s e d for dialysis, are l i k e l y to have the same limitations. N e v e r t h e l e s s , r e c e n t w o r k (40, 80) shows that successful designs can i n d e e d be a c h i e v e d w i t h ion-exchange m e m b r a n e s , a n d response times m a y actually b e b e t t e r for u n c h a r g e d a n alytes or analytes that are c h a r g e d s i m i l a r l y to the m e m b r a n e matrix rather than for analytes that are r e t a i n e d o n the m e m b r a n e b y i o n exchange. m9


Trace Gases

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R e

Porous Membrane D S Devices. T h e a p p l i c a b i l i t y o f a s i m p l e t u b u l a r D S based o n a porous h y d r o p h o b i c P T F E m e m b r a n e t u b e was d e m onstrated for the c o l l e c t i o n of S 0 (dilute H 0 was u s e d as t h e s c r u b b e r l i q u i d , a n d c o n d u c t o m e t r i c d e t e c t i o n was used) (46). T h e parameters of available t u b u l a r m e m b r a n e s that are i m p o r t a n t i n d e t e r m i n i n g the o v e r a l l b e h a v i o r o f s u c h a d e v i c e i n c l u d e the f o l l o w i n g : F i r s t , t h e fractional surface porosity, w h i c h is t y p i c a l l y b e t w e e n 0.4 a n d 0.7 a n d represents the p r o b a b i l i t y of an analyte gas m o l e c u l e e n t e r i n g a p o r e i n the e v e n t o f a c o l l i s i o n w i t h the w a l l . S e c o n d , w a l l thickness, w h i c h is t y p i c a l l y b e t w e e n 25 a n d 1000 μπι a n d d e t e r m i n e s , together w i t h the p o r e tortuosity (a m e a s u r e o f h o w c o n v o l u t e d the p a t h is from one side of the m e m b r a n e to the other), the o v e r a l l diffusion distance f r o m one side of t h e w a l l to the o t h e r . I f u p t a k e p r o b a b i l i t y at the a i r - l i q u i d interface i n the p o r e is not the c o n t r o l l i n g factor, t h e n i t e m s 1 a n d 2 together d e t e r m i n e the c o l l e c t i o n efficiency. T h e transport of the analyte gas m o l e c u l e takes place w i t h i n t h e pores, i n t h e gas phase. T h i s process is far faster than the situation w i t h a h y d r o p h i l i c m e m b r a n e ; the relaxation t i m e is w e l l b e l o w 100 m s , a n d the o v e r a l l response t i m e m a y i n fact be d e t e r m i n e d b y l i q u i d - p h a s e diffusion i n the b o u n d a r y l a y e r w i t h i n the l u m e n of the m e m b r a n e t u b e , b y l i q u i d - p h a s e d i s p e r s i o n w i t h i n the 2

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analytical system, or b y b o t h . C o n s e q u e n t l y , the response times o f porous m e m b r a n e D S based analyzers are u n l i k e l y to be l i m i t e d b y the m e m b r a n e response t i m e . T h e t h i r d i m p o r t a n t m e m b r a n e p a r a m e t e r is the p o r e size; it is t y p i c a l l y 0.02 to 5 μιη. T o g e t h e r w i t h the contact angle of the s c r u b b e r l i q u i d o n the m e m b r a n e surface ( w h i c h is a c o m b i n e d measure o f the h y d r o p h o b i c i t y of the m e m b r a n e a n d the surface tension o f the l i q u i d ) , this p a r a m e t e r d e t e r m i n e s the pressure at w h i c h the s c r u b b e r l i q u i d w i l l u n d e s i r a b l y seep t h r o u g h the m e m b r a n e . L i q u i d s w i t h l o w surface tensions (e.g., methanol) cannot be u s e d w i t h porous m e m b r a n e s — t h e y r e a d i l y leak t h r o u g h the m e m b r a n e . W i t h water, the pressure necessary to force it t h r o u g h a 0.4m m - t h i c k P T F E m e m b r a n e w i t h 2-μπι pores is —10 p s i ; the necessary p r e s sure is m o r e t h a n an o r d e r o f m a g n i t u d e greater for a m u c h t h i n n e r , s i m i l a r m e m b r a n e w i t h 0.02-μπι pores. I n g e n e r a l , m e m b r a n e s w i t h > 2 - μ π ι p o r e size leak m u c h too easily a n d are unusable for D S applications. A l t h o u g h the first porous m e m b r a n e s c r u b b e r was d e m o n s t r a t e d w i t h P T F E m e m branes w i t h 2-μπι pores (46), the pressure tolerance t o w a r d l i q u i d leakage for s u c h a m e m b r a n e c o n t i n u a l l y decreases w i t h use, p r e s u m a b l y because of s o i l i n g that c o m p r o m i s e s the h y d r o p h o b i c i t y of the m e m b r a n e surface. C o n s e q u e n t l y , the m e m b r a n e e v e n t u a l l y leaks. A l t h o u g h this p r o b l e m has b e e n solved i n d e p e n d e n t l y b y two research groups as d e s c r i b e d i n the f o l l o w i n g section, s u c h solutions w e r e d i c t a t e d b y t u b u l a r m e m b r a n e s t h e n a v a i l a b l e — a series of i n e r t porous p o l y p r o p y l e n e m e m b r a n e tubes w i t h m a n y attractive features (0.2-μπι pores, surface porosity 0.7, diameters 0.2^6 m m ) have since b e c o m e available ( A c c u r e l , E n k a A G , W u p p e r t a l , G e r m a n y ) a n d may p e r m i t the fabrication of attractive porous m e m b r a n e D S devices o f the s i m p l e t u b u l a r geometry. A few other characteristics of a porous m e m b r a n e D S s h o u l d b e p o i n t e d out h e r e . T h e c o l l e c t i o n efficiency is d e p e n d e n t o n the pores b e i n g o p e n a n d available. D e p o s i t i o n of particulate m a t t e r o n the m e m b r a n e , s m a l l as it m a y b e , can r e d u c e the m e m b r a n e c o l l e c t i o n efficiency d u r i n g c o n t i n u e d use. F o r essentially the same reason, solutions c o n t a i n i n g significant amounts of d i s s o l v e d solids cannot b e u s e d as s c r u b b e r l i q u i d s because o f the e v a p orative d e p o s i t i o n of the s o l i d i n the pores. O n the o t h e r h a n d , s h o u l d analyte-bearing particles deposit o n the m e m b r a n e (e.g., i f a n ( N H ) S 0 particle is d e p o s i t e d d u r i n g the d e t e r m i n a t i o n of N H ) , the m e m b r a n e b e haves as a filter, a n d there is little p r o b a b i l i t y that the contents o f the p a r t i c l e w i l l b e i n c o r p o r a t e d into the s c r u b b e r l i q u i d (although, a d m i t t e d l y , it w i l l interact w i t h the i n c o m i n g sample gas). T h e fate of the p a r t i c l e , i f d e p o s i t e d o n a w e t t e d m e m b r a n e surface, w o u l d o b v i o u s l y b e q u i t e different. 4

2

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Annular Geometry D S Devices. T h e leakage p r o b l e m s associated w i t h a 2-μΓη-ροΓβ P T F E m e m b r a n e t u b e w e r e solved b y T a n n e r et a l . (53) b y a d o p t i n g the a n n u l a r geometry. A i r flows t h r o u g h the a n n u l a r space o f


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a jacket t u b e w h i l e l i q u i d flows t h r o u g h the c o n c e n t r i c a l l y p l a c e d m e m b r a n e tube. T h e m e m b r a n e t u b e is f i l l e d w i t h a s o l i d P T F E filament to r e d u c e the i n t e r i o r h o l d u p v o l u m e . T h e d e s i g n is s h o w n i n F i g u r e 4. T h i s config uration results i n v e r y little l i q u i d pressure o n the m e m b r a n e a n d m i n i m i z e s the p r o b a b i l i t i e s of leaks. T a n n e r et a l . (53) u s e d the porous m e m b r a n e D S to collect H 0 after p r e r e a c t i n g the sample air w i t h N O to r e m o v e 0 (and t h e r e b y e l i m i n a t e 0 - i n d u c e d artifact H 0 formation). F i e l d a n d laboratory results w e r e comparable w i t h parallel i m p i n g e r c o l l e c t i o n . M a n u a l c o l l e c t i o n of the s c r u b b e r effluent a n d off-line analysis was u s e d i n this w o r k ; n e v e r theless, T a n n e r et a l . clearly r e c o g n i z e d that the D S - b a s e d a p p r o a c h can p r o v i d e a clear advantage o n l y i f the s c r u b b e r effluent is c o u p l e d to a c o n tinuous analysis system. 2

2

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2

2

W o r k i n T a n n e r et al.'s laboratory w i t h the 2-μιη-ροΓβ P T F E m e m b r a n e s i n d i c a t e d , o n the other h a n d , l o n g - t e r m r e p r o d u c i b i l i t y p r o b l e m s aside f r o m issues of leakage. T h e y b e l i e v e d that m i n o r changes i n p o r e structure m a y be o c c u r r i n g i n this e x t r e m e l y flexible a n d p l i a b l e m e m b r a n e w h e n k e p t u n d e r t e n s i o n ; these changes m a y be reflected i n changes i n c o l l e c t i o n ef ficiency. I n o u r laboratory, t h i n - w a l l e d m e m b r a n e s o f m i n i m u m tortuosity (straight pores) of v e r y small p o r e size w e r e therefore sought to p r o v i d e good a n d r e p r o d u c i b l e collection efficiency a n d i m m u n i t y from leaks. T h e o n l y m e m b r a n e that m e t these c r i t e r i a ( C e l g a r d X - 2 0 , porous p o l y p r o p y l e n e , surface porosity 0.4, p o r e size 0.02 μπι, w a l l thickness 25 μπι) was available i n a m a x i m u m i n t e r n a l d i a m e t e r of 400 μπι. T h e a m o u n t o f air that can b e d r a w n t h r o u g h such a m e m b r a n e is o b v i o u s l y l i m i t e d , a n d thus the a n n u l a r g e o m e t r y , s i m i l a r to that d e s c r i b e d b y T a n n e r et a l . (53), was also a d o p t e d (55). A glass jacket t u b e (to m a i n t a i n a l i n e a r configuration) is l i n e d w i t h a 5m m i . d . P T F E t u b e a n d terminates i n p o l y p r o p y l e n e T-joints at e i t h e r e n d . A 4 0 - c m l e n g t h of a 400-μιη i . d . m i c r o p o r o u s C e l g a r d t u b e , filled w i t h a 300-μπι i . d . n y l o n m o n o f i l a m e n t fishing l i n e a n d p r o v i d e d w i t h P T F E c o n n e c t i n g tubes at the t e r m i n i is s u s p e n d e d c o n c e n t r i c a l l y i n s i d e the jacket t u b e b y appropriate seals at the T-fittings. A i r flows i n a n d out t h r o u g h the p e r p e n d i c u l a r arms o f the T-joints, a n d the s c r u b b e r l i q u i d is p u m p e d t h r o u g h the m e m b r a n e c o u n t e r c u r r e n t to the airflow. T o m i n i m i z e p a r t i c l e d e p o s i t i o n a n d p e r m i t l a m i n a r flow d e v e l o p m e n t , the m e m b r a n e begins ~ 7 c m from the T - p o r t . T h e results of some 10 person-years of effort to d e v e l o p a u t o m a t e d i n s t r u m e n t s p r o g r a m m e d w i t h p e r i o d i c zero a n d calibrate f u n c tions based o n s u c h a D S a n d i n t e n d e d for the m e a s u r e m e n t of H C H O , H 0 , a n d S 0 have b e e n d e s c r i b e d (55) a n d are s u m m a r i z e d b r i e f l y h e r e . 2

2

2

E a c h i n s t r u m e n t is d e d i c a t e d to a specific analyte a n d shares the f o l l o w i n g c o m m o n features. T h e sample i n l e t allows m i c r o p r o c e s s o r - p r o g r a m m e d selection o f sample, zero, a n d calibrant gas. R a t h e r t h a n b e i n g p r o g r a m m e d for continuous aspiration of the s a m p l e d a i r , the i n s t r u m e n t is p r o g r a m m e d to alternate b e t w e e n sample a n d zero w i t h p e r i o d i c (every 4



68

MEASUREMENT C H A L L E N G E S IN ATMOSPHERIC

CHEMISTRY

ill! 1

i

ι 1

1 « 1 I1 ll Π 11

1 1 I

1II II M M

Figure 4. Diffusion scrubber device for H O removal from air, consisting of the following: A , Teflon tubing adaptor; B, Teflon Swagelok union; C , 6-mm o.d. glass tube; D, air outlet; E, porous Teflon tube (2 mm i.d. x 30 cm, 70% porosity); F, 8-mm i.d. glass tube; G, scrubber solution with concentric Teflon displacement rod; H, air inlet; and I, Teflon tube connection to porous Teflon tube. Cross section inset: a, 8-mm o.d. (6-mm i.d.) glass tube; b, air stream; c, porous Teflon tube; d, H 0 scrubber solution; and e, Teflon displacement rod. (Reproduced from reference 53. Copyright 1986 American Chemical So ciety.) 2

z

2


2.

DASGUPTA

69

Trace Gases

h or longer) e a l i b r a n t - z e r o cycles. T h e sample p e r i o d s are not q u i t e l o n g e n o u g h to reach plateau response; the signal for each analytical cycle is therefore r e p r e s e n t e d b y a peak f o l l o w e d b y a r e t u r n to the baseline, w h i c h establishes the l i q u i d - p h a s e zero. T h e s c r u b b e r l i q u i d s u s e d are 0.1 M H S 0 , w a t e r , a n d 5 μ Μ H C H O for the H C H O , H 0 , a n d S 0 i n s t r u m e n t s , respectively, a n d t h e y are p u m p e d t h r o u g h the D S at flow rates of 4 0 - 8 5 μΐ^/τηίη. Total evaporative loss of the s c r u b b e r l i q u i d ranges u p to a m a x i m u m of 6 μ ί / η τ ί η for c o m p l e t e l y d r y air (Q = 2 L / m i n ) . A p p r o p r i a t e reagents are a d d e d v i a T-joints at m i c r o f l o w rates to the s c r u b b e r effluent. I n each case a fluorescent p r o d u c t is f o r m e d a n d m o n i t o r e d w i t h a filter fluorometer. F o r f o r m a l d e h y d e , the reaction w i t h acetylacetone a n d a m m o n i u m acetate to p r o d u c e d i a c e t y l d i h y d r o l u t i d i n e i n an i n - l i n e h e a t e d r e actor is used (81). F o r H 0 , the p e r o x i d a s e - m e d i a t e d oxidation of 4 - h y droxyphenylacetate f o l l o w e d b y the i n t r o d u c t i o n of a m m o n i a t h r o u g h a p e r m e a t i v e m e m b r a n e reactor is u s e d (82). F o r S(IV), the reaction w i t h alkaline 9-IV-aeridinylmaleimide i n a water~IV,]V-dimethylformamide m e d i u m to p r o d u c e the c o r r e s p o n d i n g fluorescent sulfonate is u s e d (83). T h e operational a n d performance characteristics are s h o w n i n T a b l e I ; t y p i c a l calibration runs a n d field data are s h o w n for the H 0 i n s t r u m e n t i n F i g u r e s 5 a n d 6. Various c a l i b r a t i o n sources have b e e n d e v e l o p e d for p r o v i d i n g calibrant gases to the D S i n s t r u m e n t s (84-87). N o m e a n i n g f u l interferences for any of the t e c h n i q u e s w e r e f o u n d except for that f r o m 0 for H 0 — t h i s interference does not occur i n the laboratory w i t h clean c a l i b r a n t gases a n d a clean D S , b u t it occurs w i t h a m b i e n t air a n d c l e a r l y increases w i t h the degree of soiling of the e n t i r e inlet system. A l t h o u g h none of the i n s t r u m e n t s u n d e r w e n t a field i n t e r c o m p a r i s o n study after t h e i r d e v e l o p m e n t was c o m p l e t e , the H C H O a n d H 0 i n s t r u m e n t s have b e e n u s e d i n various i n t e r 2



4

2

2

2

2

2

2

2

3

2

2

2

2

Table I. Typical Operating Parameters and Performance Specifications

Analyte

Sample (Calibrate)/ Zero Periods (min)

Lag Time" (min)

Rise Time (min)

Detection Limit (pptrv)

Linearity Limit (ppbv)

HCHO H 0 S0

3/7 1/4 3/6

6.3 1 1.6

1.7 0.8 1.9

100 30 175

S00 30* 300

2

2

2

b

0

d

rf

N O T E : All values were obtained with 40-em-long scrubbers (jacket tube i.d. 5 mm) at sampling rates of 2.0 L/min. "Time between the valve switching from zero to calibrate gas and the onset of change at instrument output. *Time required for signal output to change from 10 to 90% of the plateau value. Based on three times the noise level when zero air is being sampled. Experiments were conducted in all cases at concentrations less than or equal to three times the quoted detection limit. ^Highest concentration tested; linearity may extend further. Response was less than linear at higher concentrations. c

e


70


5.6

e


e

ppbv

4.1

1.8

yyii 0

60

120

Time,

min

Figure 5. A calibration sequence for the H 0 instrument. (Reproduced with permission from reference 55. Copyright 1988 Pergamon Press.) 2

2

c o m p a r i s o n studies at different stages i n t h e i r d e v e l o p m e n t a n d h a v e g e n erally fared w e l l (88-91). T h e lessons of the f i e l d studies d e a l i n g w i t h the s a m p l i n g o f a m b i e n t air a l l have to do w i t h the d e p o s i t i o n o f particulate m a t t e r a n d h o w such d e p o s i t i o n affects c o l l e c t i o n efficiencies i n the porous m e m b r a n e D S devices a n d analyte losses i n the i n l e t lines a n d the valves. A strategy for the facile field d e t e r m i n a t i o n of c o l l e c t i o n efficiencies u s i n g t w o serial D S devices has b e e n d e v e l o p e d (55). H o w e v e r , a l t h o u g h p e r i o d i c r e c a l i b r a t i o n , a p a r t o f the r o u t i n e p r o t o c o l , can to a large extent correct for any analyte loss or loss of c o l l e c t i o n efficiency i n the s c r u b b e r itself, losses p r i o r to the s c r u b b e r cannot be c o r r e c t e d for. A u n i q u e D S w i t h t w o C e l g a r d t u b u l a r m e m b r a n e s was u s e d b y S i g g (92) to collect two i n d e p e n d e n t l i q u i d effluents for the s i m u l t a n e o u s m e a s u r e m e n t o f organic peroxides a n d h y d r o g e n p e r o x i d e ; S i g g u s e d a differe n t i a l analytical s c h e m e s i m i l a r to that r e p o r t e d i n reference 90. S u c h devices have b e e n u s e d successfully i n a n u m b e r of b a l l o o n flights (92). R e c e n t l y , the a n n u l a r g e o m e t r y porous m e m b r a n e D S c o u p l e d to a fluorometric d e t e c t i o n system was s h o w n to b e a p p l i c a b l e to t h e sensitive m e a s u r e m e n t o f a m b i e n t a m m o n i a (59). T h e d e t e c t i o n s c h e m e is b a s e d o n



8:00

72

am

2

2

9:00

Time

10:00

11:00

12:00

pm

1:0

Figure 6. Field data from H 0 measurement, Greater Los Angeles, August 1986. The second peak from left is 72 pptrv. (Reproduced with permission from reference 55. Copyright 1988 Pergamon Press.)

3

ν* Ο

u (ft φ

u c

C

(Λ


I

I

c

Ο

Ό >

72


the reaction of a m m o n i a w i t h o - p h t h a l d i a l d e h y d e a n d sulfite i n a h e a t e d i n l i n e reactor to p r o d u c e 1-sulfonatoisoindole (93). G e t t i n g a n analyte i n t o the l i q u i d phase often p e r m i t s a greater l a t i t u d e i n m a n i p u l a t i o n s t h a n is o t h e r w i s e possible. F o r e x a m p l e , the selective d e s t r u c t i o n o f H 0 o v e r organic peroxides w i t h a M n 0 catalyst has b e e n d e m o n s t r a t e d (91, 94). T h e w o r k o n a m m o n i a also i l l u s t r a t e d a facile i n - l i n e p r e c o n c e n t r a t i o n p r o c e d u r e g e n 2

2


2

erally a p p l i c a b l e to D S - b a s e d analyzers. T h e l i q u i d - p h a s e p o r t i o n o f the analytical system is s h o w n i n F i g u r e 7. C a r r i e r w a t e r (W) is d e i o n i z e d b y a s m a l l i n - l i n e i o n exchanger (C) that flows t h r o u g h the D S . T h e o - p h t h a l d i a l d e h y d e reagent (O) a n d buffered sulfite reagent (B) are a d d e d to the D S effluent. T h e stream flows t h r o u g h m i x i n g c o i l (M) a n d t h e n t h r o u g h reaction c o i l (R) i n h e a t e d b l o c k (H). T h e s t r e a m , d e b u b b l e d b y t u b u l a r porous m e m b r a n e (T), is d e t e c t e d b y fluorescence detector (F). Inset a shows the c o n v e n t i o n a l m o d e ; b shows the p r e c o n c e n t r a t i o n m o d e . T h e D S is c o n n e c t e d b e t w e e n the points A a n d A ' . I n the p r e c o n c e n t r a t i o n m o d e , a sixp o r t loop valve (V) is i n s t a l l e d b e t w e e n A a n d A ' a n d the D S is c o n t a i n e d w i t h i n its loop. D u r i n g the concentration step, the flow (W) bypasses the D S a n d establishes the system l i q u i d - p h a s e blank. M e a n w h i l e , air is s a m p l e d t h r o u g h the D S , any evaporative loss b e i n g m a d e u p b y the l i q u i d i n the r e s e r v o i r (W). D u r i n g the m e a s u r e m e n t m o d e , the valve is s w i t c h e d a n d the contents of the D S are flushed to the system. F o r a 3 - m i n - 2 - m i n l o a d - i n j e c t cycle, the L O D is 45 p p t r v of N H , a l t h o u g h accuracy at these 3

Figure 7. The DS-based ammonia analyzer liquid-phase schematic. Insets a and b show in-line or in-loop (preconcentration) configurations, respectively. (Reproduced from reference 59. Copyright 1989 American Chemical Society.)


2.

DASGUPTA


73

Trace Gases

levels can be c o m p r o m i s e d b y major variations i n relative h u m i d i t y unless corrections are m a d e . I n s t r u m e n t performance at l o w levels is s h o w n i n F i g u r e 8. A D S - b a s e d N H i n s t r u m e n t , s i m i l a r to that j u s t d e s c r i b e d , was a m o n g 3

several others tested i n a E u r o p e a n i n t e r c o m p a r i s o n study i n R o m e i n 1989 a n d was f o u n d to p e r f o r m w e l l (95).


1.0

ppbv

Figure 8. Performance of the DS-based ammonia instrument at low levels ofNH (g). (Reproduced from reference 59. Copyright 1989 American Chemical Society.) 3

M a n y specific a n d h i g h l y sensitive f l u o r o m e t r i c a n d e l e c t r o c h e m i c a l d e t e c t i o n methods for various analytes are available. T h e c o m b i n a t i o n of such d e t e c t i o n schemes w i t h a D S - b a s e d c o l l e c t i o n system p r o v i d e s a c o m b i n a t i o n of sensitive a n d affordable i n s t r u m e n t a t i o n for a t m o s p h e r i c m e a surements. A step-by-step construction a n d o p e r a t i o n m a n u a l for a D S - b a s e d f l u o r o m e t r i c H 0 analyzer is available (94). W i t h a change i n the reagents, the c a l i b r a t i o n source, a n d the conditions of the fluorometric m e a s u r e m e n t , such an i n s t r u m e n t is r e a d i l y reconfigured for a different analyte. T h e 1992 fabrication cost of a c o m p l e t e D S i n s t r u m e n t that u t i l i z e s fluorometric d e tection a n d i n c l u d e s a thermostated c a l i b r a t i o n source from c o m m e r c i a l l y available components is a p p r o x i m a t e l y $12,000. 2

2

Diffusion Scrubber Coupled-Ion Chromatography.

O v e r the

past d e c a d e , i o n c h r o m a t o g r a p h y (IC) has b e c o m e the t e c h n i q u e of choice for the d e t e r m i n a t i o n of anions. E v e n w i t h c o n v e n t i o n a l sorbent-coated d e n u d e r s for a c i d gases, the actual d e t e r m i n a t i o n is most c o m m o n l y c o n d u c t e d b y I C . T h e r e c o g n i t i o n of this fact l e d to the c o u p l e d D S - I C system (96). T h e system is s h o w n schematically i n F i g u r e 9. Items A t h r o u g h H constitute


74


A


.®—u

Figure 9. Configuration of the DS-IC system: A , clean air input; B , mass-flow controller; C , permeation device chamber; D and H, vents; E, needle valve-rotameter; F , needle valve; G , mass-flow meter; I, diffusion scrubber; /, scrubber liquid reservoir; K , needle valve-rotameter; L , suction pump; M , injection valve; N, peristaltic pump; O , eluentflow; P, downstream chromato graphic components; and Q, sample loop. (Reproduced from reference 96. Copyright 1989 American Chemical Society.)

the c a l i b r a t i o n source. S a m p l e - z e r o - c a l i b r a n t gas is d r a w n t h r o u g h the D S ( C e l g a r d m e m b r a n e a n d a n n u l a r geometry). A m i l d l y p n e u m a t i c a l l y p r e s s u r i z e d (1-2 psi) r e s e r v o i r (J) contains 1 m M H 0 , the s c r u b b e r l i q u i d . It is c o n n e c t e d to the D S m e m b r a n e , a n d the effluent l i q u i d is aspirated b y peristaltic p u m p (N) t h r o u g h the ΙΟΟ-μΙ, l o o p (Q) of c h r o m a t o g r a p h i c i n j e c t i o n valve (M) at a rate of 16 μΙ-./πιΐη. T h i s a r r a n g e m e n t is p r e f e r r e d over p u m p i n g the l i q u i d t h r o u g h the D S because it maintains a constant effluent flow rate f r o m the D S e v e n w h e n the extent of the s c r u b b e r l i q u i d evapo ration varies because o f v a r i a b l e i n l e t air h u m i d i t y . E v e r y 6 m i n (the a p p r o x i m a t e t i m e r e q u i r e d to p e r f o r m the chromatography), valve M switches for a l o n g e n o u g h p e r i o d (30-60 s) to c o m p l e t e l y inject the c o l l e c t e d sample i n t o the c h r o m a t o g r a p h i c system. T h u s a n e w c h r o m a t o g r a m , r e p r e s e n t i n g the p r e c e d i n g c o l l e c t i o n p e r i o d , is o b t a i n e d e v e r y 6 m i n . T h e r e is ~ 1 0 % l i q u i d - p h a s e c a r r y o v e r f r o m one sample to the next. T h e actual i n t e g r a t i o n 2

2


2.

DASGUPTA


75

Trace Gases

t i m e can b e effectively larger than the n o m i n a l t i m e r e s o l u t i o n for sticky analyte gases l i k e H N 0 because o f a d s o r p t i o n - d e s o r p t i o n o n i n l e t surfaces. I n the o r i g i n a l w o r k (96), the system p e r f o r m a n c e was p r i m a r i l y i l l u s t r a t e d w i t h S O as the test gas, a n d the L O D was r e p o r t e d to b e b e t t e r t h a n 20 p p t r v o f S 0 . M u c h w o r k has b e e n d o n e w i t h this system i n t h e analysis o f a m b i e n t air. T h e use o f d i l u t e H 0 as a s c r u b b e r l i q u i d ensures the c o m p l e t e oxidation of c o l l e c t e d S(IV) to S(VI) a n d t h e r e b y i m p r o v e s the L O D for S 0 . A t the same t i m e this s c r u b b e r l i q u i d does not significantly o x i d i z e c o l l e c t e d n i t r i t e to nitrate. A representative i o n c h r o m a t o g r a m of a m b i e n t air is s h o w n i n F i g u r e 1 0 — i d e n t i t i e s o f peaks a t h r o u g h g are d u e to u n r e s o l v e d organic acids, H C 1 , H O N O , C O , u n k n o w n , S 0 (320 p p t r v ) , a n d H N 0 (-700 pptrv), respectively. C l e a r l y , the system is a p p l i c a b l e for the simultaneous 3

a

2

2

2

2

2

3


a

Figure 10. An ambient air ion chromatogram (A) is shown in comparison with a standard liquid-phase ion chromatogram (B). (Reproduced from reference 96. Copyright 1989 American Chemical Society.)


76


d e t e r m i n a t i o n of a n u m b e r of acid gases. I n a i r b o r n e applications w h e r e significant changes i n the i n l e t pressure relative to the c a l i b r a t i o n conditions are i n v o l v e d a n d s a m p l i n g is c o n d u c t e d at a constant mass flow rate, pressure corrections are necessary; the m e t h o d has b e e n r e p o r t e d (97). T h e D S - I C i n s t r u m e n t has u n d e r g o n e a i r b o r n e i n t e r c o m p a r i s o n studies for the d e t e r m i n a t i o n of S 0 w i t h satisfactory performance (98). 2

M o r e r e c e n t l y , L i n d g r e n (99) u t i l i z e d a s i m i l a r c o n d u c t o m e t r i c D S - I C i n s t r u m e n t for the m e a s u r e m e n t of a t m o s p h e r i c H Q ; an L O D of 20 p p t r v was r e p o r t e d . T h e r a p i d response t i m e of the i n s t r u m e n t c o u l d r e a d i l y d e m onstrate the washout of H C 1 b y p r e c i p i t a t i o n . T h e d e t e c t i o n of c e r t a i n a n alytes (e.g., N 0 ~ ) is m o r e sensitively a c c o m p l i s h e d w i t h U V d e t e c t i o n t h a n w i t h c o n d u c t o m e t r y . T a n d e m c o n d u c t i v i t y a n d U V d e t e c t i o n w e r e u s e d for the d e t e r m i n a t i o n of H O N O (100); particulate n i t r i t e does not c o m m o n l y occur, a n d this w o r k u t i l i z e d a 1-m-long C e l g a r d m e m b r a n e c o i l e d i n t o a 0.3- X 4 0 - c m h e l i x s u s p e n d e d inside a 6 - m m - d i a m e t e r jacket t u b e . T h e l o n g e r m e m b r a n e l e n g t h p e r m i t s a greater c o l l e c t i o n efficiency, a n d its h e lical f o r m maintains the compactness, a l t h o u g h it does increase the extent of p a r t i c l e d e p o s i t i o n . T h e L O D for H O N O is 24 to 16 p p t r v at Q = 0.5 to 2.1 L / m i n , r e s p e c t i v e l y . T h e interference from N 0 is ~ 0 . 0 2 5 % o n a m o l a r basis, w i t h or w i t h o u t a d d e d N O or S 0 . T h e i n s t r u m e n t has b e e n u s e d extensively for a m b i e n t m e a s u r e m e n t s . H O N O m a x i m a at the m e a s u r e m e n t location w e r e c l e a r l y associated w i t h traffic. Persistent d a y t i m e c o n c e n t r a tions o f 1 0 0 - 5 0 0 p p t r v w e r e also o b s e r v e d , m u c h h i g h e r than c o u l d be accounted for b y interference from N 0 . B o t h the H C 1 a n d H O N O i n s t r u ments u s e d i n t e g r a l c a l i b r a t i o n sources based o n the e q u i l i b r i u m v a p o r p r e s sure o f the a c i d o v e r the c o r r e s p o n d i n g a m m o n i u m salts.


2

2

2

2

Dasgupta et al.

Miscellaneous Diffusion Scrubber Applications.

(101) c o m p a r e d t w o D S - b a s e d schemes for d e t e r m i n i n g S 0 , b o t h i n v o l v i n g reactions i n w h i c h the c o l l e c t e d analyte is c o n v e r t e d b y a p p r o p r i a t e c h e m istry into some o t h e r m o r e detectable c o m p o u n d . I n the first s y s t e m , the C e l g a r d - m e m b r a n e - b a s e d a n n u l a r g e o m e t r y D S is u s e d w i t h a w a t e r s c r u b ber. T h e c o l l e c t e d S(IV) i n the s c r u b b e r effluent is o x i d i z e d to sulfate b y the e n z y m e sulfite oxidase w i t h the simultaneous p r o d u c t i o n of an e q u i v a l e n t a m o u n t of H 0 . T h i s H O is t h e n d e t e c t e d b y the p r e v i o u s l y m e n t i o n e d fluorometric p r o c e d u r e i n v o l v i n g the o x i d a t i o n o f 4-hydroxyphenylacetate. O b v i o u s l y , the H 0 m u s t be r e m o v e d from the sample gas for this t e c h n i q u e to w o r k ; this was successfully a c c o m p l i s h e d w i t h an i n t e r n a l l y s i l v e r e d glass t u b e (catalytic d e c o m p o s i t i o n p r e d e n u d e r for H 0 ) w i t h o u t r e m o v i n g the S 0 . T h e second system u s e d a s i m p l e t u b u l a r D S w i t h a 2-μΓη-ροΓβ P T F E m e m b r a n e . T h i s scheme is u n i q u e i n that the m e a s u r e d analyte r e m a i n s i n the gas phase a n d the jacket v o l u m e t h r o u g h w h i c h the l i q u i d flows is not constrained; leakage p r o b l e m s are o b v i a t e d for these reasons. A 2.5 μ Μ solution of H g ( N 0 ) flows i n the jacket, a n d S 0 reacts w i t h H g at the 2

2

2

2

2

z

2

2

2

2

2

3

2

2

2


2 +

2.

DASGUPTA


Trace Gases

77

g a s - l i q u i d interface to p r o d u c e Hg(II) a n d Hg(0). T h e latter is p r e s e n t i n the exit gas from the D S a n d is c o l l e c t e d o n a g o l d c o i l . A f t e r c o l l e c t i o n for any d e s i r e d p e r i o d o f t i m e , t h e c o i l is flash-heated e l e c t r i c a l l y , a n d t h e l i b e r a t e d H g is m e a s u r e d b y a c o m m e r c i a l c o n d u c t o m e t r i c g o l d f i l m H g sensor. L O D s for the two systems w e r e comparable (80-170 p p t r v for system 1 a n d 6 0 - 1 5 0 p p t r v for system 2), a n d n o major interferences w e r e f o u n d for e i t h e r (as l o n g as H O was p r e r e m o v e d i n system 1). P a r a l l e l a m b i e n t measurements i n the 400- to 1000-pptrv range correlated w i t h an r o f 0.88 (r, l i n e a r c o r r e l a t i o n coefficient). I f u s e d as a s o l u t i o n , the e n z y m e is r e l a t i v e l y expensive, a n d the H 0 - t r a n s l a t i o n system can b e m o r e practical i f a n i m m o b i l i z e d sulfite oxidase e n z y m e is u s e d . W i t h the H g - t r a n s l a t i o n system, diffusion o f the Hg(0) i n a n d out o f the l i q u i d phase increases the i n s t r u m e n t response t i m e to ~ 5 m i n , e v e n t h o u g h o n l y 1 m i n o f s a m p l i n g t i m e was typically used. 2

z

2


2

2

T h e D S represents a n attractive collection interface to devise c o n t i n u ous-flow analytical systems e v e n w h e n d i s c r i m i n a t i o n b e t w e e n gas a n d p a r ticles is not a p a r t i c u l a r l y i m p o r t a n t issue. I n u n p u b l i s h e d w o r k , a C e l g a r d m e m b r a n e - b a s e d a n n u l a r geometry D S was u s e d w i t h a n absorbance detector (based o n a g r e e n - l i g h t - e m i t t i n g d i o d e e m i t t i n g at 555 nm) to d e t e r m i n e N 0 ; t h e w e l l - k n o w n G r i e s s - S a l t z m a n c h e m i s t r y was u s e d (102). T h e G r i e s s - S a l t z m a n reagent was d i r e c t l y p u m p e d at 13 μΐ^/ππη t h r o u g h the D S , a n d the absorbance o f the D S effluent was m o n i t o r e d . N o r m a l l y i t w o u l d n o t b e p e r m i s s i b l e to p u m p a reagent w i t h a significant a m o u n t o f dissolved solids because o f evaporative deposition as m e n t i o n e d . H o w e v e r , the uptake o f N 0 b y p u r e water is essentially n e g l i g i b l e ; t h u s , c o m p l e t e h u m i d i f i c a t i o n o f the i n l e t s a m p l e - z e r o air can b e a c h i e v e d b y passing i t t h r o u g h a porous m e m b r a n e i m m e r s e d i n w a t e r w i t h o u t significant loss o f N 0 . A t the same t i m e , H O N O is r e m o v e d . T h e h u m i d i t y o f the air stream p r e c l u d e s any evaporation o f the s c r u b b e r l i q u i d a n d p e r m i t s the v e r y l o w flow rate. O t h e r w i s e , evaporative losses a n d consequent v a r i a t i o n i n b l a n k values w o u l d greatly c o m p r o m i s e t h e r e l i a b i l i t y o f the data. T h e l o w flow rate a n d t h e i n t r i n s i c sensitivity o f this c o l o r i m e t r i c m e t h o d allows, w i t h i n e x p e n s i v e e q u i p m e n t , a n L O D o f 250 p p t r v o f N 0 w i t h 1 0 - m i n t i m e resolution. 2

2

2

2

Diffusion-Scrubber-Based

Analyzers; Limitations and Future

Considerations. A s may b e e v i d e n t from the foregoing discussion, C e l gard-based porous m e m b r a n e a n n u l a r g e o m e t r y D S devices have thus far b e e n t h e mainstay o f D S - b a s e d analyzers. C o l l e c t i o n efficiency o f porous m e m b r a n e devices does decrease w i t h particle d e p o s i t i o n . I n the short t e r m , the l u m e n o f the fiber needs to b e flushed w i t h a w e t t i n g solvent a n d d r i e d t h o r o u g h l y before reuse e v e r y 2 4 - 4 8 h , d e p e n d i n g o n the p a r t i c l e l o a d i n g of the air s a m p l e d . O v e r a longer t e r m , the i n s i d e o f the jacket needs to b e w a s h e d as w e l l to r e m o v e d e p o s i t e d material. T h e D S c o n s t r u c t i o n d e s c r i b e d i n reference 96 uses a d e m o u n t a b l e m e m b r a n e that c a n b e r e p l a c e d i n



78


m i n u t e s . S o m e , h o w e v e r , m a i n t a i n that p e r i o d i c c l e a n i n g is sufficient a n d that m e m b r a n e r e p l a c e m e n t is n o t necessary e v e n i n y e a r - l o n g field use (103). I n any case, t h e n e e d for p e r i o d i c c l e a n i n g does d e t e r from l o n g - t e r m u n a t t e n d e d use. A l l a m b i e n t a i r studies c a r r i e d o u t thus far w i t h D S - b a s e d analyzers have b e e n c a r r i e d o u t w i t h o u t a n y attempts to r e m o v e coarse particles f r o m t h e sample, for e x a m p l e , w i t h a c y c l o n e o r a n i m p a c t o r , p r i o r to e n t r y to t h e D S . E x p e r i e n c e indicates that t h e large m a j o r i t y o f t h e particles d e p o s i t e d i n t h e D S are coarse particles, a n d p a r t i c l e d e p o s i t i o n e x p e r i m e n t s (40) c o n f i r m this t r e n d . P r e r e m o v a l o f coarse particles m a y greatly r e d u c e t h e f r e q u e n c y o f c l e a n i n g necessary. T h e effect o f the i n l e t g e o m e t r y was also s t u d i e d for 0 . 1 - to 3-μπι particles i n these e x p e r i m e n t s — as m a y b e i n t u i t i v e l y guessed, t h e extent o f loss is larger w i t h a T - t y p e t h a n a Y - t y p e air i n l e t geometry. M u c h o f this d e p o s i t i o n occurs i n t h e entrance r e g i o n a n d n o t o n t h e m e m b r a n e itself. T h e o n l y c o m m e r c i a l v e r s i o n o f the D S p r e s e n t l y available (Analytek, Umeâ, Sweden) u t i l i z e s P T F E e n d fittings i n w h i c h air flows i n - o u t t h r o u g h ports at an angle o f 45° relative to the m a i n b o d y o f t h e D S . T h i s g e o m e t r y s h o u l d have d e p o s i t i o n losses e q u a l to o r less t h a n those o f the Y - i n l e t s . H o w e v e r , m i n i m u m d e p o s i t i o n was o b s e r v e d w i t h a straight i n l e t system w h e r e t h e tubes c o n n e c t i n g to t h e t e r m i n i e n t e r a n d exit t h r o u g h t h e jacket walls ( F i g u r e 11). T h i s d e s i g n has b e c o m e t h e one o f choice i n m y laboratory.

PTFE FEP

Naff on

Polymer Jacket

fill

Figure 11. The straight inlet DS.

F r e q u e n t l y , a major l i m i t a t i o n o f D S - b a s e d c o l l e c t i o n systems is that t h e y operate at substantially subquantitative c o l l e c t i o n efficiencies at t h e t y p i c a l s a m p l i n g rates u s e d . T h i s situation increases t h e p r o b a b i l i t y o f e r r o r because o f large t h e r m a l variations that affect diffusive transport. F o r these reasons, s h o u l d w e t d e n u d e r s (vide infra) p r o v e to b e v i a b l e c o n t i n u o u s c o l l e c t i o n devices, t h e y m a y w e l l replace D S - b a s e d systems. T h e i r a b i l i t y to m o r e q u a n t i t a t i v e l y r e m o v e gases m a y also s p u r t h e d e v e l o p m e n t o f c o m b i n e d g a s - p a r t i c l e analyzer systems i n w h i c h , for e x a m p l e , the a c i d gases are r e m o v e d b y t h e dénuder a n d a n a l y z e d ; t h e particles are t h e n c o l l e c t e d b y the i m p a c t o r e q u i v a l e n t o f a w e t dénuder, a n d the a c i d i t y associated w i t h


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the aerosol is d e t e r m i n e d . D e s p i t e the bounties p r o m i s e d b y the w e t d é n u d e r , i n c e r t a i n cases, the D S m a y r e m a i n superior. N a f i o n - m e m b r a n e - b a s e d D S systems of the geometry s h o w n i n F i g u r e 11 can collect gases l i k e H 0 or H C H O w i t h efficiencies comparable to those attainable b y a s i n g l e - t u b e w e t dénuder of same l e n g t h , display no change i n c o l l e c t i o n efficiency o v e r t i m e (no pores to b e blocked), a n d are m o r e easily interfaced to the l i q u i d phase analyzer system. U s i n g slightly larger N a f l o n m e m b r a n e tubes a n d a g e o m e t r y w h e r e the air is s a m p l e d inside the m e m b r a n e a n d the s c r u b b e r l i q u i d is f l o w i n g outside, L i n d (80) s h o w e d excellent c o l l e c t i o n efficiency for H N 0 (g), c o u p l e d s u c h a D S to an a u t o m a t e d I C system, a n d has u s e d it successfully i n field experiments. 2

2


3

Wet Denuders A s the n a m e i m p l i e s , w e t d e n u d e r s are devices i n w h i c h the d é n u d e r active surfaces are c o n t i n u o u s l y w e t t e d b y the s c r u b b e r l i q u i d , a n d i n a t r u e c o n tinuous analyzer the effluent is c o n t i n u o u s l y r e m o v e d . I n m a n y ways, the w e t d é n u d e r represents a l l of the desirable f e a t u r e s — a c o n t i n u o u s l y r e n e w e d surface, the p o s s i b i l i t y of u s i n g any s c r u b b e r l i q u i d , a n d the best collection efficiency that a g i v e n s c r u b b e r l i q u i d w i l l a l l o w for a g i v e n geo m e t r y . F u r t h e r , i f the l i q u i d film is i n d e e d always present, the d é n u d e r w a l l for a l l practical purposes is c o m p o s e d of the l i q u i d , a n d r e l a t i v e l y few restrictions are p l a c e d o n the construction m a t e r i a l of the dénuder. W i t h a c o n d u c t i v e s c r u b b e r l i q u i d , electrostatic p a r t i c l e losses are also e x p e c t e d to b e m i n i m u m . I n its simplest f o r m , a w e t dénuder is a t u b e w i t h the s c r u b b e r l i q u i d flowing d o w n as a u n i f o r m f i l m along its i n n e r w a l l a n d the l i q u i d b e i n g c o l l e c t e d at the b o t t o m w i t h o u t a major d i s r u p t i o n of the b o u n d a r y layer of the air flow, w h i c h proceeds from the b o t t o m u p . W a t e r o r a n aqueous solution is most c o m m o n l y the s c r u b b e r l i q u i d of choice for d e t e r m i n i n g a v a r i e t y o f a t m o s p h e r i c gases o f interest. W a t e r has a r e l a t i v e l y large surface t e n s i o n , a n d it is p a r t i c u l a r l y difficult therefore to m a i n t a i n a u n i f o r m film. F u r t h e r , u n l i k e the l i q u i d flow w i t h a D S , the l i q u i d flow is not i n a closed system; t h e r e m a y b e attendant difficulties i n m a i n t a i n i n g an i n f l o w - o u t f l o w balance i n the face of a variable extent of evaporative loss. Significant progress has already b e e n m a d e t o w a r d a continuous-flow w e t dénuder. T h e w e t t e d w a l l c o l u m n ( W W C ) of F e n d i n g e r a n d G l o t f e l t y (104), based o n the o r i g i n a l w o r k of E m m e r t a n d P i g f o r d (105), u t i l i z e s a m e c h a n i c a l l y e t c h e d glass tube a n d U - s h a p e d c o p p e r w i r e pieces a l o n g the u p p e r i n t e r i o r walls to d i s r u p t the surface t e n s i o n of the w a t e r p u m p e d f r o m the top a n d make a u n i f o r m film. T h e air i n l e t - o u t l e t g e o m e t r y , i n r e g a r d to p o t e n t i a l particle losses, was far f r o m that d e s i r e d i n a w e t d é n u d e r (although it was of little c o n s e q u e n c e to these authors, w h o w e r e solely interested i n achieving gas-liquid equilibration). M o s t importantly, the a i r - l i q u i d flow-rate ratio i n the F e n d i n g e r a n d G l o t f e l t y W W C was o f the


80



o r d e r o f 100 (air flow rate 5 0 - 2 0 0 m L / m i n a n d w a t e r flow rate 1 . 4 - 2 . 0 m L / m i n ) , whereas i n a w e t d é n u d e r a i r - l i q u i d flow rate ratios 2 orders o f m a g n i t u d e greater (air flow rate 1000-2000 m L / m i n a n d l i q u i d flow rate 100-200 μι,/min) w o u l d b e desirable to achieve m e a n i n g f u l p r e c o n c e n t r a t i o n factors. I have a t t e m p t e d to adapt a c h e m i c a l l y or m e c h a n i c a l l y e t c h e d glass t u b e to air i n l e t - o u t l e t geometries that have l o w p r o b a b i l i t i e s of p a r t i c l e loss. W i t h desirable a i r - l i q u i d flow-rate ratios as stated, the i n t e g r i t y o f a c o n t i n uous film c o v e r i n g the surface, i f f o r m e d i n the first p l a c e , was s h o r t - l i v e d . I n v a r i a b l y , w i t h i n a few hours d r y areas w o u l d be apparent, a n d i n the worst case a single n a r r o w stream o f w a t e r w o u l d flow d o w n one w a l l . I f the d e p o s i t i o n o f particles was not a factor or was i n fact d e s i r e d , the d e s i g n w o u l d be m u c h s i m p l e r . M o s t l i k e l y , such a d e v i c e w o u l d operate i n the t u r b u l e n t flow m o d e ; the theoretical aspects have b e e n d i s c u s s e d b y L u c e r o (106) i n e x e m p l a r y d e t a i l for a h y p o t h e t i c a l d e v i c e i n w h i c h gaseous sample m o l e c u l e s , particles, or b o t h f r o m a t u r b u l e n t gas stream are d i s s o l v e d into a l a m i n a r film l i q u i d stream that flows i n t u r n to a c h r o m a t o g r a p h s a m p l i n g valve. T h e first r e p o r t e d w e t dénuder o p e r a t e d i n discrete cycles a n d was of annular geometry; analysis of the s c r u b b e r l i q u i d was c a r r i e d out off-line (107). T h e dénuder (d — 4.5 c m , d = 4.2 c m , a n d L = 30 cm) is s h o w n i n F i g u r e 12, a n d the c o m p l e t e setup is s h o w n i n F i g u r e 13. T h e t w o c o n c e n t r i c tubes are h e l d together b y P T F E spacers, a n d d u r i n g o p e r a t i o n the a n n u l a r space is c h a r g e d w i t h 25 m L of s c r u b b e r l i q u i d a n d the outer t u b e is rotated at 40 r p m b y a m o t o r - d r i v e n b e l t . T h i s o p e r a t i o n results i n an aqueous film, —0.5 m m t h i c k , o n the surfaces d e f i n i n g the a n n u l u s . A p a i r of d e n u d e r s is u s e d i n parallel i n the field i n s t r u m e n t , each w i t h Q = 32 L / m i n . O n e s c r u b b e r l i q u i d is a 0.5 m M H C O O H - H C O O N a buffer o f p H 3.7, a n d the other is a phosphate-buffered ( p H 7) s o l u t i o n c o n t a i n i n g 4h y d r o x y p h e n y l a c e t a t e , peroxidase, a n d f o r m a l d e h y d e . T h e first s o l u t i o n is u s e d for the c o l l e c t i o n of H C 1 , H N 0 , a n d N H , a n d the second is u s e d for the c o l l e c t i o n of S 0 a n d H 0 . A t the c o m p l e t i o n of the s a m p l i n g c y c l e (40 m i n ) , the dénuder p a i r is t i l t e d b y an e l e c t r i c jack, a n d the d é n u d e r effluents are p u m p e d out to i n d i v i d u a l autosampler tubes b y a p e r i s t a l t i c p u m p . T h e d e n u d e r s are w a s h e d w i t h 2 m L of the appropriate s c r u b b e r l i q u i d s before t h e y are c h a r g e d w i t h the n e w s c r u b b e r solutions, a n d t h e n the s a m p l i n g cycle is repeated. A l l of these operations are f u l l y a u t o m a t e d . T h e first dénuder i n l e t is e q u i p p e d w i t h a cyclone to r e m o v e coarse particles. T h e second dénuder i n l e t is e q u i p p e d w i t h provisions for p r e m i x i n g N O w i t h the s a m p l e d air to e l i m i n a t e 0 interference i n H 0 m e a s u r e m e n t (53). A t the stated flow rate, a l l of these analyte gases are c o l l e c t e d w i t h 9 0 % + efficiency, a n d the attainable L O D s are stated to b e 290, 80, 130, 190, a n d 7 p p t r v for N H , H N 0 , H C 1 , S 0 , a n d H 0 , r e s p e c t i v e l y . I n field i n t e r c o m p a r i s o n w i t h other t e c h n i q u e s , no major interferences for any o f the analytes o f interest w e r e apparent. Q

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3

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3

3

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Figure 13. Wet annular dénuder experimental arrangement. (Reproduced with permission from reference 107. Copyright 1988 Pergamon Press.)

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83

E x p e r i m e n t s have b e e n c o n d u c t e d i n m y laboratory o n various s i n g l e t u b e w e t dénuder designs (108). T h e s e devices have b e e n c o u p l e d to a n I C for s e m i c o n t i n u o u s analysis. T h e basic system d e s i g n is l a r g e l y the same as that d e s c r i b e d i n reference 96, except that an e i g h t - p o r t d u a l stack s l i d e r valve is u s e d for c h r o m a t o g r a p h i c i n j e c t i o n . S u c h a valve is c o n f i g u r e d to have t w o i n j e c t i o n loops; w h i l e one is i n the l o a d m o d e , the o t h e r is i n the inject m o d e . E a c h o f the loops contains a short p r e c o n c e n t r a t i o n c o l u m n . T h e effluent from the w e t dénuder is p u m p e d b y a p e r i s t a l t i c p u m p t h r o u g h the p r e c o n c e n t r a t i o n column(s). T h e t y p i c a l i n p u t l i q u i d flow rate is 1 0 0 - 7 0 0 μί«/ηπη. ( U n l i k e i n reference 96, the flow resistance of the l o o p c o n t a i n i n g the p r e c o n c e n t r a t o r c o l u m n is too h i g h for the c o l l e c t e d effluent to b e as p i r a t e d t h r o u g h i t . T h e c o l u m n m u s t b e p u m p e d , a n d s o m e losses o f l a b i l e analytes l i k e N 0 " o n passage t h r o u g h p o l y v i n y l chloride) ( P V C ) p u m p t u b i n g have b e e n observed.) I n the face o f variable evaporative losses (up to 30 μ ί / η π η for d r y air at Q = 2 L / m i n ) , the choice is e i t h e r to let some of the effluent l i q u i d be w a s t e d a n d p u m p an air-free stream to the p r e c o n c e n t r a t o r or to p u m p the e n t i r e effluent l i q u i d a n d some air t h r o u g h the p r e c o n c e n trator. T h e latter alternative was chosen because it was f o u n d that as l o n g as the I C e l u e n t was degassed, the s m a l l amounts of air o n the p r e c o l u m n are d i s s o l v e d u n d e r pressure i n the e l u e n t a n d do not cause any q u a n t i t a t i o n or r e p r o d u c i b i l i t y p r o b l e m s i f sufficient r e s t r i c t i o n is a d d e d to the detector c e l l exit to p r e v e n t b u b b l e formation. T h e use of the p r e c o n c e n t r a t o r c o l u m n s also allows r e l a t i v e l y large l i q u i d flow rates t h r o u g h the dénuder, a n d this d e s i g n facilitates c o m p l e t e w e t t i n g of the dénuder surface. A l t h o u g h the use of preconcentrators is generally a p p l i c a b l e for i o n i c analytes, this setup is not a p p l i c a b l e to analytes l i k e H 0 a n d H C H O . 2

2

2

T h e w e t dénuder designs are s h o w n i n F i g u r e 14. F i g u r e 14a shows an i n t e r n a l l y t h r e a d e d (7 m m , one t h r e a d p e r m i l l i m e t e r ) d é n u d e r that was fabricated from a glass-filled P T F E t u b e ( 1 / 4 - i n c h i . d . , 1 / 2 - i n c h o.d.). L i q u i d p u m p e d i n t h r o u g h the w a l l at the top follows the t h r e a d a n d is c o l l e c t e d b y aspiration at the b o t t o m . It is not k n o w n i f the u n e v e n w a l l h i n d e r s l a m i n a r flow d e v e l o p m e n t . T h e second dénuder, s h o w n i n F i g u r e 14b, c o n tains a porous p o l y p r o p y l e n e m e m b r a n e ( 5 . 5 - m m i . d . , 1.5-mm wall) j a c k e t e d b y a P V C t u b e . L i q u i d is forced t h r o u g h the m e m b r a n e w a l l s , collects at a w e l l at the b o t t o m , a n d is aspirated from t h e r e . T h e d e s i g n s h o w n i n F i g u r e 14c contains a r o l l e d sheet of a t h i n polycarbonate m e m b r a n e ( N u c l e p o r e ) w i t h i n a 9 . 4 - m m - i . d . P V C or P F A T e f l o n t u b e . T h e l i q u i d is i n t r o d u c e d s i m u l t a n e o u s l y t h r o u g h four s y m m e t r i c a l l y p l a c e d n e e d l e apertures at the top p e r i p h e r y of the t u b e a n d c o l l e c t e d at the b o t t o m i n a fashion s i m i l a r to that i n F i g u r e 14b. N o n e o f these designs are w e t t e d w i t h p u r e w a t e r r e l i a b l y o v e r l o n g p e r i o d s . H o w e v e r , a s o l u t i o n of a n o n i o n i c fluorocarbon surfactant (0.3% Z o n y l F S N ) passed t h r o u g h a m i x e d - b e d i o n exchanger is satisfactory. T h e L O D s attainable b y the I C system suffer b y a factor o f 2 to 3 w i t h c o n t i n u e d i n j e c t i o n o f the surfactant-containing samples, r e l a t i v e



84


(a)

(b)

(c)

Figure 14. Experimental wet dénuder designs. (Reproduced from 108. Copyright 1991 American Chemical Society.)

(d) reference

to p u r e l y aqueous samples, because of increased baseline noise. It was possible to use p u r e w a t e r i n the dénuder s h o w n i n F i g u r e 14c w i t h the p o l y carbonate f i l m r e p l a c e d b y a tissue paper. H o w e v e r , this setup was not s t u d i e d i n d e p t h because of concerns about the i n t e g r i t y of c o l l e c t e d samples. T h e response t i m e f r o m d é n u d e r c was excellent (

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