Electrochemical Sensors, Sensor Arrays, and Computer Algorithms

17 Electrochemical Sensors, Sensor Arrays, and Computer Algorithms For Detection and Identification of Airborne Chemicals Joseph R. Stetter

Downloaded by CORNELL UNIV on July 11, 2012 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch017

Energy and Environmental Systems Division, Argonne National Laboratory, Argonne, IL 60439

Recent developments in the field of sensing airborne chemicals using electrochemical sensors and sensor arrays are reviewed. Such systems detect, identify, and quantify potential chemical hazards to protect the health and safety of workers and citizens. The application discussed in this review article is single chemicals at part-per-million levels in air. The sensor system consists of an array of sensors used In four modes of operation, and the data are interpreted by a computer algorithm. Pattern recognition techniques are being used to understand the information content of the arrays and to focus future experimental work. T h i s paper p r o v i d e s a b r i e f review and t e c h n i c a l p e r s p e c t i v e on r e c e n t developments i n t h e f i e l d o f s e n s i n g a i r b o r n e c h e m i c a l s u s i n g e l e c t r o c h e m i c a l s e n s o r s and sensor a r r a y s . S e l e c t i v e d e t e c t i o n of gases and v a p o r s i s c e n t r a l t o s o l v i n g t h e many i n d u s t r i a l and s o c i e t a l problems s u r r o u n d i n g hazardous c h e m i c a l s . F o r example, inexpensive sensors that can d e t e c t , identify, and q u a n t i f y otherwise i n v i s i b l e h a z a r d s a r e needed t o c h a r a c t e r i z e s o u r c e e m i s s i o n s , t r a c e the t r a n s p o r t o f c h e m i c a l s through the environment, measure l e v e l s o f human e x p o s u r e , d e s i g n c o s t e f f e c t i v e c l e a n - u p s t r a t e g i e s , and p r o t e c t t h e h e a l t h and s a f e t y o f b o t h workers and citizens. S o l v i n g these gas and vapor d e t e c t i o n problems w i l l r e q u i r e a v a r i e t y o f new s e n s o r s , s e n s o r systems, and i n s t r u m e n t s . Field d e t e c t i o n o f a i r b o r n e c h e m i c a l s can be somewhat a r b i t r a r i l y d i v i d e d i n t o t h r e e d i s t i n c t s i t u a t i o n s . The f i r s t case i s when a s p i l l o r l e a k r e s u l t s i n a s i n g l e compound o c c u r r i n g i n a i r f a r i n excess o f i t s background c o n c e n t r a t i o n . The second case i s when one o r s e v e r a l t r a c e c o n s t i t u e n t ( s ) o c c u r i n a complex background ("needlein-the-haystack" problem). The t h i r d case i s when a complete a n a l y s i s i s needed f o r a l l minor as w e l l as major c o n s t i t u e n t s o f a complex m i x t u r e . The f i r s t case i s the one s p e c i f i c a l l y addressed by the approaches d i s c u s s e d i n t h i s review a r t i c l e . The second and 0097-6156/86/0309-0299S06.00/0 © 1986 American Chemical Society

In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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t h i r d c a s e s , a l t h o u g h n o t d i s c u s s e d h e r e , w i l l be addressed near f u t u r e by a p o r t a b l e , i n e x p e n s i v e sensor t e c h n o l o g y .

i n the

The t h r e e c a t e g o r i e s o f gas d e t e c t i o n problems mentioned above are extremely important i n developing the i n i t i a l g u i d e l i n e s f o r s e n s o r s and i n s t r u m e n t s . Each type o f problem w i l l r e q u i r e c e r t a i n c a p a b i l i t i e s i n t h e s e n s o r s and the a l g o r i t h m r e q u i r e d t o i n t e r p r e t the d a t a . However, i t i s not r e a s o n a b l e t o expect a s i n g l e sensor or i n s t r u m e n t t o be developed f o r each c h e m i c a l o r s i t u a t i o n . One way t o i n c r e a s e t h e i n f o r m a t i o n content of sensors i s t o use them i n sensor a r r a y s . T h i s approach i s c o n t r i b u t i n g s i g n i f i c a n t l y t o solutions f o r t h e d e t e c t i o n problems p r e s e n t e d by t h e t h r e e categories. A l l sensor a r r a y s a r e not e q u a l l y p r o m i s i n g . So, w h i l e t h e t e c h n o l o g y i s b e i n g d e v e l o p e d , how i s one t o judge w h i c h avenues t o pursue and w h i c h t o a v o i d ? The d i s c u s s i o n s i n t h e f o l l o w i n g t h r e e s e c t i o n s p r o v i d e a u s e f u l framework f o r a p p r o a c h i n g t h i s problem o f sensor r e s e a r c h and i n s t r u m e n t development. Commentaries on r e c e n t d e s i g n s f o r e l e c t r o c h e m i c a l s e n s o r s , sensor a r r a y s , and a l g o r i t h m s f o r i d e n t i f i c a t i o n and q u a n t i f i c a t i o n o f a i r b o r n e c h e m i c a l s a r e p r e s e n t e d from t h e above p e r s p e c t i v e . Electrochemical

Sensors

Amperometry can be used t o d e t e c t and i d e n t i f y a i r b o r n e c h e m i c a l s , and d e v i c e s based on t h i s t e c h n o l o g y have e x i s t e d f o r s e v e r a l y e a r s f o r measuring such gases as CO (J_), NO ( 2 ) , N 0 (_2), H S ( 3 ) , a l c o h o l (4_), and h y d r a z i n e s (_5). F i g u r e 1 i s a schematic diagram o f an e l e c t r o c h e m i c a l d e t e c t o r system. The s e n s o r s a r e o p e r a t e d a t c o n s t a n t p o t e n t i a l ; t h u s , t h e g e n e r a l a n a l y t i c a l t e c h n i q u e can be c a t e g o r i z e d as chromoamperometry o r c o n s t a n t - p o t e n t i a l amperometry. 2

2

The sensor c o n s i s t s o f s i x major p a r t s ( s e e F i g u r e 1 ) : f i l t e r , membrane, w o r k i n g o r s e n s i n g e l e c t r o d e , e l e c t r o l y t e , c o u n t e r e l e c t r o d e , and r e f e r e n c e e l e c t r o d e . Each p a r t i n f l u e n c e s the overall performance characteristics of the sensor. Choosing c o n s t r u c t i o n m a t e r i a l s and sensor geometry i s c r i t i c a l and has a profound i n f l u e n c e on t h e a c c u r a c y , p r e c i s i o n , response time, s e n s i t i v i t y , background, n o i s e , s t a b i l i t y , l i f e t i m e , and s e l e c t i v i t y of the r e s u l t i n g sensor. The r e l a t i o n s h i p s among m a t e r i a l s o f c o n s t r u c t i o n , sensor geometry, and performance c h a r a c t e r i s t i c s o f these s e n s o r s a r e s t i l l p o o r l y d e f i n e d . A l t h o u g h a thorough d i s c u s s i o n o f these r e l a t i o n s h i p s i s beyond t h e scope o f t h i s a r t i c l e , t h e response mechanism o f an amperometric e l e c t r o c h e m i c a l c e l l s h o u l d be d i s c u s s e d . The response o f an amperometric the f o l l o w i n g e i g h t s t e p s :

gas sensor can be d e s c r i b e d by

1•

I n t r o d u c t i o n of the chemical filter,

2.

Diffusion membrane,

t o the sensor

of the reactant across

through t h e

the working

electrode



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3.

Dissolution electrolyte,

of

4.

D i f f u s i o n of interface,

5.

Adsorption

6.

Electrochemical reaction,

7.

Desorption

8.

D i f f u s i o n of the p r o d u c t s away from the r e a c t i o n zone,

the

the

electroactive

chemical

to

onto the e l e c t r o d e

of the p r o d u c t s ,

species

in

the

the e l e c t r o d e - e l e c t r o l y t e

surface,

and

Any of these s t e p s can be r a t e l i m i t i n g , thus d e t e r m i n i n g the ultimate sensitivity and response c h a r a c t e r i s t i c s of the e l e c t r o c h e m i c a l sensor. The parameters most f r e q u e n t l y observed to i n f l u e n c e the c h a r a c t e r i s t i c s of these s e n s o r s i n c l u d e sample f l o w r a t e , w o r k i n g e l e c t r o d e c o m p o s i t i o n , e l e c t r o l y t e , membrane t y p e , and e l e c t r o c h e m i c a l p o t e n t i a l of the s e n s i n g e l e c t r o d e . By c o n t r o l l i n g these parameters d u r i n g d e s i g n , the sensor e n g i n e e r can a c h i e v e the d e s i r e d sensor response c h a r a c t e r i s t i c s . The e i g h t r e a c t i o n s t e p s i n the sensor model i n c l u d e a v a r i e t y of c h e m i c a l and p h y s i c a l p r o c e s s e s , a l l of w h i c h are i n f l u e n c e d by the system components shown i n F i g . 1. The s e n s o r i s u s u a l l y d e s i g n e d so t h a t the k i n e t i c s of the p h y s i c a l p r o c e s s e s ( i . e . , mass t r a n s p o r t by d i f f u s i o n ) are l i m i t i n g , but i t i s p o s s i b l e to c o n s t r u c t s e n s o r s t h a t e x h i b i t performance c h a r a c t e r i s t i c s l i m i t e d by the k i n e t i c s of the c h e m i c a l / e l e c t r o c h e m i c a l processes. Step 2 i s usually limited by the p e r m e a b i l i t y of the membrane. I n c e r t a i n sensor d e s i g n s , the membrane i s e l i m i n a t e d t o avoid t h i s step. Step 4 r e f e r s to the d i f f u s i o n of the s o l v a t e d gas in the e l e c t r o l y t e to the electrode-electrolyte interface. D i f f u s i o n i n l i q u i d s i s o f t e n c o n s i d e r a b l y slower than d i f f u s i o n a c r o s s a membrane. I f the s e n s i n g e l e c t r o d e i s f l o o d e d w i t h e l e c t r o l y t e , the response i s slow because the gas must d i f f u s e through the e l e c t r o l y t e b e f o r e r e a c h i n g the r e a c t i o n s u r f a c e . T y p i c a l l y , i n c r e a s i n g the v o l u m e t r i c f l o w r a t e through the sensor i n c r e a s e s the mass t r a n s p o r t of the a n a l y t e to the w o r k i n g electrode, thereby increasing the observed signal (i.e., sensitivity). H i g h f l o w r a t e s a l s o decrease sensor response t i m e . Sensor response w i t h changing f l o w r a t e has been d i s c u s s e d (_1_, 6) • The s e l e c t i v i t y of the sensor can a l s o be improved by c o n t r o l l i n g the electrochemical p o t e n t i a l of the w o r k i n g e l e c t r o d e . For example, p r o p e r s e l e c t i o n of a Au e l e c t r o d e p o t e n t i a l w i l l a l l o w the d e t e r m i n a t i o n of N0 i n the presence of NO (2_, 7_). 2

The c o n s t r u c t i o n m a t e r i a l s of each sensor p a r t w i l l i n f l u e n c e its o p e r a t i n g c h a r a c t e r i s t i c s , as i l l u s t r a t e d i n the f o l l o w i n g examples. Choosing a Au r a t h e r than a P t e l e c t r o c a t a l y s t f o r the s e n s i n g e l e c t r o d e a l l o w s f o r s e l e c t i v e d e t e r m i n a t i o n of H S i n the presence of CO (8). Using a charcoal f i l t e r i n combination w i t h a 2


302


CO sensor a l l o w s d e t e c t i o n o f CO i n t h e presence o f hydrocarbons and other adsorbable contaminants. The membrane i s u s u a l l y chosen f o r i t s a b i l i t y t o protect the sensing e l e c t r o d e . However, i f i t has low p e r m e a b i l i t y t o a i r , t h e sensor w i l l have a slower response time. The e l e c t r o l y t e and counter e l e c t r o d e have a l s o been r e p o r t e d as i n f l u e n c i n g s e l e c t i v i t y and d e v i c e performance i n t h e d e t e r m i nation of hydrazines ( 5 ) and N 0 (9_), r e s p e c t i v e l y . Finally, m a t e r i a l s o f c o n s t r u c t i o n a r e t y p i c a l l y T e f l o n and h i g h - d e n s i t y plastics like polypropylene because such materials must be c o m p a t i b l e w i t h r e a c t i v e gases and c o r r o s i v e e l e c t r o l y t e s .


2

Amperometric s e n s o r s a r e important i n portable instrument design because they are r e l a t i v e l y small, i n e x p e n s i v e , and l i g h t w e i g h t , and u s e v e r y l i t t l e power t o g e n e r a t e s i g n i f i c a n t signals. They can d e t e c t p a r t - p e r - m i l l i o n (ppm) l e v e l s o f e l e c t r o c h e m i c a l l y a c t i v e gases and v a p o r s , can be e n g i n e e r e d t o have s i g n i f i c a n t s e l e c t i v i t y , and can be o p e r a t e d over a wide range o f temperatures. These excellent operating features make electrochemical c e l l s p a r t i c u l a r l y suited f o r portable instruments constructed with electrochemical c e l l s . With the proper choice of f i l t e r s , f l o w r a t e s , and p o t e n t i a l s , they can p r o v i d e s t a b l e and m e a n i n g f u l measurements i n a v a r i e t y o f f i e l d s i t u a t i o n s . Sensor A r r a y s E l e c t r o c h e m i c a l s e n s o r s respond t o a l i m i t e d number o f c h e m i c a l s , and each one responds w i t h l i m i t e d s e l e c t i v i t y . One way t o overcome these l i m i t a t i o n s , o r a t l e a s t t o improve t h e s e n s o r ' s c a p a b i l i t i e s , i s t o c o n s t r u c t s e n s o r a r r a y s (10, 1 1 ) . I n f o r m a t i o n i s c r e a t e d i n a s e n s o r by i t s r e a c t i o n t o a c h e m i c a l s t i m u l u s . T h i s r e a c t i o n o f t h e s e n s o r t o t h e type o f c h e m i c a l and i t s c o n c e n t r a t i o n creates; an a n a l y t i c a l s i g n a l t h a t i s decoded i n t o an e l e c t r o n i c s i g n a l by t h e sensor. I n o t h e r words, t h e e l e c t r o n i c s i g n a l c o n t a i n s the r e c o r d e d information. More s p e c i f i c a l l y , i f chemicals a r e t o be i d e n t i f i e d on t h e b a s i s o f t h e i r r e l a t i v e e l e c t r o c h e m i c a l r e a c t i v i t i e s , then the sensor a r r a y must be d e s i g n e d t o a c c o m p l i s h t h i s t a s k . I t must c o n t a i n s e n s o r s t h a t r e c o r d a d i f f e r e n t e l e c t r o c h e m i c a l response f o r each c h e m i c a l s p e c i e s . C o n v e r s e l y , the chosen s e n s o r s f o r t h e a r r a y must o n l y decode those c h e m i c a l s e x h i b i t i n g d i f f e r i n g r e a c t i v i t i e s on t h e g i v e n s e n s o r s . Thus, t o a c h i e v e t h e most e f f e c t i v e method, the s e n s o r a r r a y and t h e a n a l y t i c a l problem must be c o n s i d e r e d together. The o b j e c t i v e i s t o p e r f o r m b o t h q u a l i t a t i v e and q u a n t i t a t i v e analyses simultaneously. Each sensor y i e l d s a s p e c i f i c type and amount o f i n f o r m a t i o n . However, i f sensors a r e combined, t h e r a t i o s of s e n s i t i v i t i e s among d i f f e r e n t s e n s o r s p r o v i d e new a n a l y t i c a l information. Thus, t h e i n f o r m a t i o n content o f t h e a r r a y s i s g r e a t e r than t h a t developed by an i n d i v i d u a l sensor ( 1 2 ) . A sensor a r r a y c o n s i s t i n g o f f o u r e l e c t r o c h e m i c a l s e n s o r s and two h o t w i r e f i l a m e n t s has r e c e n t l y been e v a l u a t e d (13-15). The a r r a y was c o n s t r u c t e d t o b o t h i d e n t i f y and q u a n t i f y a t o x i c vapor i n a few minutes u s i n g a portable, battery-operated, lightweight instrument. The e l e c t r o c h e m i c a l s e n s o r s a r e combined w i t h h o t


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c a t a l y s t f i l a m e n t s t o a l l o w t h e e l e c t r o c h e m i c a l s e n s o r s t o respond t o a broad range o f c h e m i c a l s . Gases a r e drawn o v e r the f i l a m e n t and i n t o t h e e l e c t r o c h e m i c a l s e n s o r , where p y r o l y s i s fragments c r e a t e d by t h e h o t f i l a m e n t s a r e d e t e c t e d by t h e e l e c t r o c h e m i c a l sensors. E l e c t r o a c t i v e gases and vapors a r e sensed d i r e c t l y , and n o n e l e c t r o a c t i v e gases and vapors ( e . g . , benzene) a r e d e t e c t e d a f t e r p y r o l y s i s over t h e heated f i l a m e n t ( 1 0 ) . I n t h i s manner, a n e a r l y u n i v e r s a l and v e r y nonselective d e t e c t o r i s c r e a t e d t h a t i s a compromise between w i d e s p r e a d response and h i g h s e l e c t i v i t y . F o r example, t h e p h o t o i o n i z a t i o n d e t e c t o r (PID) c a n d e t e c t p a r t - p e r - b i l l i o n l e v e l s o f benzene but cannot d e t e c t methane. C o n v e r s e l y , t h e flame i o n i z a t i o n d e t e c t o r (FID) can detect p a r t - p e r - b i l l i o n l e v e l s of methane b u t does not d e t e c t c h l o r i n a t e d compounds l i k e C C l ^ v e r y e f f e c t i v e l y . By combining the f i l a m e n t and e l e c t r o c h e m i c a l s e n s o r , a l l o f these c h e m i c a l s can be d e t e c t e d but o n l y a t p a r t - p e r - m i l l i o n l e v e l s and above. Because most c h e m i c a l vapors have t o x i c exposure l i m i t s above 1 ppm ( a few such as h y d r a z i n e s have l i m i t s below 1 ppm), t h i s s e n s i t i v i t y i s adequate f o r the i n i t i a l applications. Several cases of e l e c t r o c h e m i c a l s e n s o r s b e i n g used a t t h e s u b - p a r t - p e r - m i l l i o n l e v e l have been r e p o r t e d (3_, 1 6 ) . The f i l a m e n t and e l e c t r o c h e m i c a l sensor form t h e b a s i c gas sensor r e q u i r e d f o r d e t e c t i n g a wide v a r i e t y o f c h e m i c a l s i n a i r , but w i t h l i t t l e o r no s e l e c t i v i t y . The next step i s t o use an a r r a y of such s e n s o r s i n a v a r i e t y o f ways (modes) t o o b t a i n the i n f o r m a t i o n r e q u i r e d t o perform the q u a l i t a t i v e a n a l y s i s of an unknown a i r b o r n e c h e m i c a l . The f o u r e l e c t r o c h e m i c a l s e n s o r s were c a r e f u l l y chosen and have two w o r k i n g e l e c t r o d e s of Au and two o f P t . One Au and one P t electrode are operated at anodic potentials to facilitate o x i d a t i o n s , and t h e o t h e r two a r e o p e r a t e d a t c a t h o d i c p o t e n t i a l s t o f a c i l i t a t e reductions. When an e l e c t r o a c t i v e gas passes t h r o u g h t h i s a r r a y , t h e h a l f - w a v e p o t e n t i a l o f the c h e m i c a l s p e c i e s i s n o t measured. However, by comparing t h e s i g n a l s from t h e s e n s o r s i n t h e a r r a y , one c a n t e l l whether t h e h a l f - w a v e p o t e n t i a l i s above o r below 1.0 V v s . t h e s t a n d a r d hydrogen e l e c t r o d e ( n h e ) . Thus, t h e a r r a y s i g n a l s from these s e n s o r s , w h i l e n o t measuring the thermodynamic h a l f - w a v e p o t e n t i a l , do p r o v i d e a s e t of c h e m i c a l parameters r e l a t e d t o t h e v a p o r ' s e l e c t r o c h e m i c a l p r o p e r t i e s . Hence, t h e term "chemical parameter s p e c t r o m e t r y " was chosen t o d e s c r i b e this technique. Under m i c r o p r o c e s s o r c o n t r o l , a c o n s t a n t c o n c e n t r a t i o n o f t h e unknown vapor ( e . g . , 100 ppm benzene i n a i r ) i s passed through t h e a r r a y , and t h e s t e a d y s t a t e s i g n a l from t h e f o u r s e n s o r s i s read w i t h ( 1 ) the P t f i l a m e n t a t 900°C, ( 2 ) t h e Rh f i l a m e n t a t 900°C, ( 3 ) the Rh f i l a m e n t a t 1000°C, and ( 4 ) no f i l a m e n t s on. T h i s procedure g e n e r a t e s 16 s i g n a l s f o r each gas o r vapor as shown i n F i g u r e 2 f o r two r e p r e s e n t a t i v e c h e m i c a l s , benzene and c y c l o h e x a n e . The l a r g e s t s i g n a l i s s e t e q u a l t o 1.0, and t h e o t h e r c h a n n e l s a r e s c a l e d a c c o r d i n g l y , making t h e h i s t o g r a m c o n c e n t r a t i o n independent i n t h e l i n e a r range o f t h e s e n s o r s . A wide range o f compounds has been i n v e s t i g a t e d a t c o n c e n t r a t i o n s of 3 ppm t o s e v e r a l hundred p a r t s p e r m i l l i o n , each p r o d u c i n g a unique response p a t t e r n s ( 1 4 ) . Once t h e


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FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS FILTER

ELECTROCHEMICAL GAS OUTLET

MEMBRANE

CELL


F i g u r e 1 C o n c e p t u a l model o f an amperometric gas/vapor sensor (WE = w o r k i n g e l e c t r o d e , E l ' y t e = e l e c t r o l y t e , RE - r e f e r e n c e e l e c t r o d e , and CE = c o u n t e r e l e c t r o d e ) .

F i g u r e 2 N o r m a l i z e d response o f sensor a r r a y t o benzene and cyclohexane.


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h i s t o g r a m i s known f o r a g i v e n compound, i t can be used i n f r a r e d spectrum o r f i n g e r p r i n t t o i d e n t i f y the c h e m i c a l .

like

an


D u r i n g o p e r a t i o n of t h i s sensor a r r a y , s i g n i f i c a n t i n f o r m a t i o n is gathered by the f o u r e l e c t r o c h e m i c a l sensors about t h e electrochemical activity of the c h e m i c a l and i t s pyrolysis products. Comparing t h e sensor r e a d i n g s taken w i t h t h e Rh f i l a m e n t at 900°C v s . 1000°C r e v e a l s whether f o r m a t i o n o f e l e c t r o a c t i v e products i s a h i g h l y a c t i v a t e d or a nonactivated process. S i m i l a r l y , comparing sensor r e a d i n g s f o r t h e P t v s . Rh f i l a m e n t s p r o v i d e s a comparison of the c a t a l y t i c a c t i v i t y of P t v s . Rh f o r t h e pyrolytic formation of e l e c t r o - o x i d i z a b l e or e l e c t r o - r e d u c i b l e compounds. Thus, sensor r e a d i n g s taken t o g e t h e r p r o v i d e a s e t of parameters r e l a t e d t o t h e s p e c i f i c e l e c t r o c a t a l y t i c and c a t a l y t i c p r o p e r t i e s of t h e i n d i v i d u a l c h e m i c a l s . An automated i n s t r u m e n t i n c o r p o r a t i n g t h i s sensor a r r a y and o p e r a t i n g procedure was c o n s t r u c t e d (15) to e v a l u a t e and demonstrate i t s u t i l i t y f o r gas d e t e c t i o n . Using microprocessor c o n t r o l , the i n s t r u m e n t s e n s o r s c o u l d be z e r o e d , c a l i b r a t e d , o r operated i n any one o f t h r e e modes of o p e r a t i o n — " u n i v ( e r s a l ) , " "select," or "ident(ify)." I n u n i v e r s a l mode, the i n s t r u m e n t d i s p l a y s a s i m p l e bar graph of any contaminant p r e s e n t i n t h e a i r t o w h i c h t h i s sensor a r r a y responds ( 1 4 ) , and t h i s g r a p h i c d i s p l a y can be used t o l o c a t e leaks. I n t h e i d e n t i f y mode, a p p r o x i m a t e l y 500 cm of v a p o r / a i r m i x t u r e t r a p p e d i n a sample bag i s a n a l y z e d ( i . e . , a h i s t o g r a m i s generated). A f t e r a n a l y s i s , t h e user i s g i v e n a r e p o r t of t h e gas identity and c o n c e n t r a t i o n , t h e percentage of the s h o r t - t e r m exposure l i m i t (STEL), number and name of o t h e r compounds i n t h e d e v i c e l i b r a r y t h a t a r e " c l o s e " t o the unknown, and t h e " e u c l i d e a n " d i s t a n c e of the unknown from the c l o s e s t l i b r a r y e n t r y . These latter two p i e c e s of i n f o r m a t i o n r e l a t e t o the r e l i a b i l i t y of t h e a n a l y s i s performed by t h e u n i t . The i n t e n t of t h i s e x e r c i s e has been t o demonstrate t h a t t h e a r r a y d e v i c e can p r o v i d e s u f f i c i e n t i n f o r m a t i o n t o i d e n t i f y a c h e m i c a l hazard and t o d e c i d e whether t h e a n a l y s i s s h o u l d be b e l i e v e d . Algorithms f

The system a l g o r i t h m i s the h e a r t of the a r r a y d e v i c e s a b i l i t y t o p r o v i d e the needed i n f o r m a t i o n r a p i d l y and i n a u s e f u l format. The q u a n t i t a t i v e i n f o r m a t i o n can be e a s i l y o b t a i n e d from the s t r o n g e s t l i n e a r d a t a channel u s i n g t r a d i t i o n a l c a l i b r a t i o n t e c h n i q u e s . More s u b t l e t y i s needed t o o b t a i n t h e q u a l i t a t i v e i n f o r m a t i o n d e s c r i b e d p r e v i o u s l y . How t h a t i s done i s the s u b j e c t o f t h e f o l l o w i n g section. The s i m p l e s t form of p a t t e r n comparison ( e u c l i d e a n d i s t a n c e i n 16-diraensional space) was used t o d e s i g n and b u i l d an o p e r a t i o n a l p o r t a b l e gas m o n i t o r i n g u n i t ( 1 5 ) . W i t h the l i m i t e d computer power of a p o r t a b l e i n s t r u m e n t , any one of about a dozen gases c o u l d be i d e n t i f i e d i n l e s s than one minute of c o m p u t a t i o n a l t i m e . This a l g o r i t h m was e v a l u a t e d u s i n g a d a t a s e t f o r r e p e a t e d runs o f 16 d i f f e r e n t c h e m i c a l s i n 2 d i f f e r e n t sensor a r r a y s ( 1 4 ) . The r e s u l t s



306


i n d i c a t e d t h a t t h e a r r a y s were c o r r e c t 52 o f 62 and 40 o f 48 t i m e s , r e s p e c t i v e l y , u s i n g t h e averages o f t h e d a t a s e t s as r e f e r e n c e libraries. Using a w e i g h t v e c t o r t h a t emphasized channel 16 improved t h e percent correct identifications t o 97.5%. The e u c l i d e a n d i s t a n c e o f t h e i n d i v i d u a l p a t t e r n v e c t o r from i t s l i b r a r y entry i s r e l a t e d t o the correctness of the i d e n t i f i c a t i o n . This d e m o n s t r a t i o n o f a u s e f u l a r r a y c o n f i r m s th'e p r a c t i c a l i n f o r m a t i o n content o f t h e a r r a y and t h e c o n f i d e n c e w i t h w h i c h t h i s i n f o r m a t i o n can be r o u t i n e l y e x t r a c t e d from the d a t a s e t . To i n t e r p r e t the s i g n a l s generated by sensor arrays, computerized data-handling techniques are required. Although " f i n g e r p r i n t s " can o f t e n be r e c o g n i z e d by t h e t r a i n e d eye as u n i q u e , such qualitative information must be t r a n s l a t e d r a p i d l y and efficiently into a q u a n t i t a t i v e measure o f how unique t h e fingerprint i s . A d a t a s e t c o n t a i n i n g 22 i n d i v i d u a l chemical responses was e v a l u a t e d u s i n g p a t t e r n r e c o g n i t i o n t e c h n i q u e s ( 1 7 ) . Such p a t t e r n recognition algorithms are very useful i n the q u a n t i t a t i v e e v a l u a t i o n o f t h e i n f o r m a t i o n content o f a p a r t i c u l a r a r r a y o r d a t a s e t . I n t h i s s t u d y , c o r r e l a t i o n was found among most p a i r s o f d a t a channels: o n l y f i v e u n c o r r e l a t e d channels were found. However, even though t h e d a t a channels e x h i b i t e d h i g h c o r r e l a t i o n , t h e p a t t e r n s were s u f f i c i e n t l y unique t o i d e n t i f y each of t h e 22 compounds i n t h e d a t a s e t . These r e s u l t s p r o v i d e d t h e f i r s t q u a n t i t a t i v e measure o f t h e "uniqueness" o f t h e i n f o r m a t i o n g e n e r a t e d by t h i s a r r a y and method ( 1 7 ) . The i n f o r m a t i o n content i n such systems i s p o t e n t i a l l y l a r g e and s h o u l d be s u f f i c i e n t n o t o n l y t o i d e n t i f y s i n g l e c h e m i c a l s i n a i r , but a l s o t o i d e n t i f y mixtures of chemicals. To r e s o l v e m i x t u r e s o f N compounds unambiguously, N independent parameters y i e l d i n g N simultaneous independent e q u a t i o n s a r e needed. An e a r l i e r work ( 1 1 ) suggests a p r a c t i c a l approach t o t h i s problem. The minimum number o f parameters P r e q u i r e d t o i d e n t i f y compounds N on t h e b a s i s o f t h e presence o r absence o f s i g n i f i c a n t s i g n a l s i n v a r i o u s channels i s :

Z

" !=!

(N-D!I!

where t h e compounds may be p r e s e n t i n c o m b i n a t i o n s o f up t o A components. F o r example, f o r N = 100 c h e m i c a l s i n m i x t u r e s where A - 3 o r 4, then P (number o f independent c h a n n e l s ) must be M 8 o r >22, r e s p e c t i v e l y . To i d e n t i f y any one o r any combination o f thousands o f c h e m i c a l s w i t h a sensor a r r a y i s a p r o d i g i o u s t a s k . Experimentally, such i d e n t i f i c a t i o n w i l l demand a l a r g e number o f sensors and o p e r a t i n g modes, r e s u l t i n g i n extended measurement t i m e s . Designing a system f o r 99.99% o f t h e expected o r most f r e q u e n t occurrences would r e s u l t i n h i g h r e l i a b i l i t y and a s i m p l e r d a t a - g a t h e r i n g system ( s e n s o r a r r a y ) . The a v a i l a b l e a l g o r i t h m s and t h e sensor a r r a y a r e a


17.

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c l o s e l y connected team. The a r r a y must be d e s i g n e d t o c o l l e c t the i n f o r m a t i o n t h a t c o n t a i n s the unambiguous answer t o the sensing problem, and the a l g o r i t h m must e x t r a c t t h i s i n f o r m a t i o n r a p i d l y and accurately. I f p r o p e r l y d e s i g n e d , the a l g o r i t h m w i l l r e q u i r e a minimum amount of c o m p u t a t i o n a l time on a s m a l l computer and w i l l m i n i m i z e the d e s i g n r e q u i r e m e n t s of the sensor a r r a y . W h i l e the a l g o r i t h m t h a t i n t e r p r e t s the sensor response must be e f f i c i e n t , i t must a l s o e x t r a c t the d e s i r e d i n f o r m a t i o n unambiguously. This requirement i s a s u b s t a n t i a l c h a l l e n g e when w o r k i n g w i t h e l a b o r a t e a r r a y s , l a r g e d a t a s e t s , and c o m p l i c a t e d responses ( e . g . , m i x t u r e s of c h e m i c a l s ) . As p r e v i o u s l y mentioned, computer programs can be used t o e v a l u a t e the "uniqueness" of the d a t a set produced by a g i v e n a r r a y for a g i v e n s e t of c h e m i c a l s . This c a p a b i l i t y i s extremely i m p o r t a n t because a l g o r i t h m s can be d e s i g n e d and used to measure a r r a y redundancy ( 1 1 ) , thereby p r o v i d i n g the i n f o r m a t i o n r e q u i r e d t o custom d e s i g n a r r a y s to s o l v e s p e c i f i c problems. I n o t h e r words, c o m p u t e r - a i d e d - d e s i g n (CAD) of a r r a y s i s p o s s i b l e . F u r t h e r , because the d i f f e r e n c e between known and unknown p a t t e r n s can be r a p i d l y c a l c u l a t e d , portable instruments can indeed be used f o r r a p i d i d e n t i f i c a t i o n i n the f i e l d . F i n a l l y , i t must be s t r e s s e d t h a t p a t t e r n r e c o g n i t i o n does not c r e a t e i n f o r m a t i o n t h a t the s e n s o r s d i d not g e n e r a t e . However, computer a l g o r i t h m s can o f t e n make the i n f o r m a t i o n c o n t a i n e d i n such d a t a s e t s more o b v i o u s and c e r t a i n l y e a s i e r t o d i s p l a y and use. Thus, the f o c u s f o r hardware i s t o d e s i g n a r r a y s t h a t g e n e r a t e more i n f o r m a t i o n , and the r o l e of algorithms i s to guide instrument development and e v a l u a t e the s u c c e s s of the e x p e r i m e n t . Conclusions

and F u t u r e Work

Experiments to date have shown t h a t a portable instrument i n c o r p o r a t i n g a t h o u g h t f u l l y chosen a r r a y of s e n s o r s can d e t e c t , i d e n t i f y , and q u a n t i f y a wide v a r i e t y of c h e m i c a l s i n a i r . A l s o , p a t t e r n r e c o g n i t i o n techniques are b e i n g used t o u n d e r s t a n d the i n f o r m a t i o n content of the a r r a y s and t o f o c u s f u t u r e e x p e r i m e n t a l work. Development of s m a l l e r , more s e n s i t i v e , and more r e l i a b l e e l e c t r o c h e m i c a l s e n s o r s w i l l expand the a p p l i c a t i o n s of the system described here. P r o g r e s s i n the a p p l i c a t i o n of sensor a r r a y s t o gas a n a l y s i s w i l l be made t h r o u g h i n c r e a s i n g l y independent d a t a c h a n n e l s u s i n g n o v e l c o m b i n a t i o n s of s e n s o r s and o p e r a t i n g modes. Computational approaches w i l l be m o d i f i e d t o s u i t s p e c i f i c types of sensor a r r a y s and t o make e c o n o m i c a l use of c o m p u t a t i o n a l space f o r p o r t a b l e i n s t r u m e n t a p p l i c a t i o n s . The p r i m a r y c h a l l e n g e s of the near f u t u r e w i l l be t o s o l v e the " n e e d l e - i n - t h e - h a y s t a c k " problem and t o proceed to complex m i x t u r e a n a l y s i s u s i n g a p l u r a l i t y of sensor r e s p o n s e s . Acknowledgments Work performed f o r the U.S. Coast Guard t h r o u g h an agreement w i t h the U.S. Department of Energy, under C o n t r a c t DTCG23-84-F-20045.


308


Literature Cited 1. 2. 3. 4. 5. 6.


7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Bay, H. W.; Blurton, K. F.; Sedlak, J. M.; Valentine, A. M. Anal. Chem. 1974, 46, 1837. Blurton, K. F.; Sedlak, J. M. Talanta. 1976, 23, 811. Stetter, J. R; Sedlak, J. M.; Blurton, K. F. J. Chromatog. Sci. 1977, 15, 125. Blurton, K. F.; Bay, H. W.; Lieb, H. C.; Oswin, H. G. Nature. 1972, 240, 52. Stetter, J. R.; Blurton, K. F.; Valentine, A. M.; Tellefsen, K. A. J. Electrochem. Soc. 1978, 125, 1804. Stetter, J. R.; Tellefsen, K. A.; Saunders, R. A.; DeCorpo, J. J. Talanta. 1979, 26, 799. Zaromb, S.; Stetter, J. R.; Attard, G. J. J. Electroanal. and Interfacial Electrochem. 1983, 148, 289. Blurton, K. F.; Sedlak, J. M. Talanta. 1976, 23, 445. Stetter, J. R.; Cromer, R. B. U.S. Patent 4 302 315, 1981. Stetter, J. R.; Zaromb, S.; Findlay, M. W., Jr. Sensors and Actuators. 1985, 6, 269. Zaromb, S.; Stetter, J. R. Sensors and Actuators. 1984, 6, 225. Hirshfeld, T.; Callis, J. B.; Kowalski, B. R. Science. 1984, 116, 312. Stetter, J. R.; Zaromb, S.; Penrose, W. R.; Findlay, M. W.; Otagawa, T.; Sincali, A. J. Proc. Hazardous Materials Spills Conf., J. Ludwigson, ed., 1984. Stetter, J. R.; Penrose, W. R.; Zaromb, S.; Christian, D.; Otagawa, T. Proc. 2nd Annual Tech. Seminar on Chem. Spills, 1985. Stetter, J. R.; Penrose, W. R.; Zaromb, S.; Stull, J. O. Anal. Instrum. 1985, 21, 163. Rogers, P. M.; Stetter, J. R.; Lau, Y-K; Cromer, R. B. Proc. JANNAF Environmental and Safety Subcommittee, 1980, pp. 1-22. Stetter, J. R.; Jurs, P. C.; Rose, S. L.; Stull, J. O. Proc. 1985 JANNAF Safety and Environmental Protection Subcommittee, 1985.

RECEIVED December 4, 1985


Electrochemical Sensors, Sensor Arrays, and Computer Algorithms

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