Gas Chromatographic Measurement and Identification of Pesticide

5 Gas Chromatographic Measurement and Identification of Pesticide Residues with Electron Capture, Microcoulometric, and

Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0104.ch005

Electrical Conductivity Detectors WILLIAM

E.

WESTLAKE

Department of Entomology, University of California, Riverside, Calif. 92502

The

gas chromatograph

qualitative

information

ance of the identity electron

capture

exceedingly

is not

of the compound

detector,

minute

while

quantities

tually useless for qualitative metric detection is widely

the

The

pesticide chlorine, with

unexcelled

confirmation

capture

together

assur-

being detected. for

The

detecting

The

is

sulfur,

retention

of the identity

of recorder

when the presence The detector

or

nitrogen.

time data,

electrolytic

conductivity

and is capable

of measuring

of an

degree

being or

spe-

specificity, of

measured.

is similarly iodine

organo-

is almost

offers a high

detector organic

and

responses

This

of the compound

vir-

microcoulo-

system has a high degree of specificity

used for

is indicated.

assurance

specific

of many compounds,

chlorine for

of giving

identification.

using the electron cific

capable

but can give a high degree of

specific

bromine.

C o m e r e m a r k s of a g e n e r a l n a t u r e r e g a r d i n g gas c h r o m a t o g r a p h y are ^

a p p r o p r i a t e before b e g i n n i n g a d i s c u s s i o n of specific d e t e c t i o n sys-

tems. T h i s p a p e r is not i n t e n d e d to b e a r e v i e w , b u t references are c i t e d w h e r e it seems a p p r o p r i a t e to s u p p o r t p a r t i c u l a r points. T h e i n t e n t is to s u m m a r i z e , i n a g e n e r a l w a y , w h a t the detectors w i l l a n d w i l l not d o a n d to m a k e k n o w n the a u t h o r s ideas r e g a r d i n g some advantages a n d d i s a d vantages i n their use. A l t h o u g h some m a y t h i n k o t h e r w i s e , a s u b s t a n t i a l m a j o r i t y of a n a l y t i c a l chemists w i l l s u r e l y agree that, s t r i c t l y s p e a k i n g , the gas c h r o m a t o g r a p h is not a n i n s t r u m e n t for q u a l i t a t i v e analysis. A s a q u a n t i t a t i v e 73

In Pesticides Identification; Biros, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

74

PESTICIDES I D E N T I F I C A T I O N

t o o l i t is often s u p e r b , a n d for the i s o l a t i o n of i n d i v i d u a l c o m p o n e n t s

of

a m i x t u r e i t is f r e q u e n t l y i n d i s p e n s a b l e . T h e d e v e l o p m e n t , d u r i n g t h e past 10 years, of detectors possessing some degree of specificity has m a d e i t p o s s i b l e , f r e q u e n t l y , to o b t a i n a reasonable assurance of the i d e n t i t y of the d e t e c t e d c o m p o u n d .

N o n e of the s o - c a l l e d specific detectors is i n -

f a l l i b l e , for, a l t h o u g h t h e y m a y h a v e a g r e a t l y e n h a n c e d response for a p a r t i c u l a r m o l e c u l a r species, t h e y w i l l also r e s p o n d to a lesser d e g r e e to others.

F o r absolute c e r t a i n t y w h e n d e t e r m i n i n g u n k n o w n

compounds,

therefore, some a d d i t i o n a l v e r i f i c a t i o n is essential. Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0104.ch005

T h e a v a i l a b i l i t y of detectors ( p a r t i c u l a r l y e l e c t r o n c a p t u r e )

capable

of g i v i n g a s i g n a l for as l i t t l e as a f e w p i c o g r a m s of s o m e c o m p o u n d s has r e s u l t e d i n the u n i v e r s a l a n d almost e x c l u s i v e use of the gas c h r o m a t o g r a p h for m e a s u r i n g p e s t i c i d e residues. W i t h o u t the gas c h r o m a t o g r a p h , m u c h of the r e s e a r c h o n p e s t i c i d e residues b e i n g c o n d u c t e d t o d a y w o u l d not b e possible because the m a n p o w e r r e q u i r e d to process the l a r g e s a m ples a n d to d o the c h e m i c a l w o r k necessary for other methods of analysis for residues i n the 0.1 p p m a n d l o w e r range does not exist. M a n y of the measurements b e i n g r e p o r t e d c o u l d not b e m a d e at a l l ( a t least, n o t i n a n y p r a c t i c a l m a n n e r ) b y means other t h a n gas c h r o m a t o g r a p h .

This

latter a c c o m p l i s h m e n t is v i e w e d w i t h m i x e d emotions b y m a n y e x p e r i e n c e d r e s i d u e chemists, for the efforts d i r e c t e d t o w a r d d e t e c t i n g less a n d less a n d t o w a r d m i n i a t u r i z i n g samples h a v e l o n g since r e a c h e d the p o i n t of no r e t u r n . T h e errors i n h e r e n t i n s a m p l i n g , p r o c e s s i n g , i n s t r u m e n t a t i o n , a n d operator inconsistencies increase r a p i d l y as levels go

below

0.1 p p m a n d , at the 1 p p b l e v e l , it is d o u b t f u l that r e p r o d u c i b i l i t y c a n be as g o o d as ± 1 0 0 % .

W h e n its l i m i t a t i o n s are r e c o g n i z e d a n d the d a t a

p r o p e r l y i n t e r p r e t e d , the gas c h r o m a t o g r a p h is a n almost i n d i s p e n s a b l e i n s t r u m e n t for the r e s i d u e c h e m i s t e n g a g e d i n research a n d for r e g u l a t o r y agencies that m u s t screen thousands of samples e a c h y e a r for the presence of m a n y different pesticides. A s the title of this p a p e r i n d i c a t e s , i t is c o n c e r n e d o n l y w i t h the use of the electron c a p t u r e , m i c r o c o u l o m e t r i c , a n d e l e c t r o l y t i c c o n d u c t i v i t y detectors for q u a n t i t a t i o n a n d i d e n t i f i c a t i o n of p e s t i c i d e residues. F u r t h e r b a c k g r o u n d i n f o r m a t i o n o n gas c h r o m a t o g r a p h i c detectors m a y b e f o u n d i n the p a p e r b y W e s t l a k e a n d G u n t h e r

(1).

Electron Capture Detector T h e electron c a p t u r e detector is the most extensively u s e d , b y f a r , of those e m p l o y e d for p e s t i c i d e r e s i d u e d e t e r m i n a t i o n s .

It is also the

w o r s t possible c h o i c e for i d e n t i f y i n g the d e t e c t e d c o m p o u n d s .

Introduced

b y L o v e l o c k a n d L i p s k y ( 2 ) i n 1960, the use of this detector has r e v o l u t i o n i z e d p e s t i c i d e r e s i d u e d e t e r m i n a t i v e p r o c e d u r e s . T h e a b i l i t y to detect


5.

WESTLAKE

Gas Chromatographic

75

Measurement

a n d m e a s u r e e x c e e d i n g l y m i n u t e a m o u n t s of m a n y pesticides ( p a r t i c u l a r l y the o r g a n o c h l o r i n e c o m p o u n d s ) i n a w i d e r a n g e of substrates c a u s e d its i m m e d i a t e a c c e p t a n c e as a n a n a l y t i c a l t o o l for p e s t i c i d e r e s e a r c h . T h i s detector, h o w e v e r , has the u n f o r t u n a t e p r o p e r t y of g i v i n g responses to a host o f c o m p o u n d s other t h a n pesticides a n d is, therefore, v i r t u a l l y useless for i d e n t i f i c a t i o n . W h e n the i d e n t i t y of the c o m p o u n d b e i n g m e a s u r e d is k n o w n a n d o p e r a t i n g p a r a m e t e r s e s t a b l i s h e d , i t is u n e x c e l l e d

for

q u a n t i t a t i v e investigations. T h e l i m i t a t i o n s of the e l e c t r o n c a p t u r e detector w e r e q u i c k l y r e c o g Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0104.ch005

n i z e d , a n d the s e a r c h w a s c o n t i n u e d for m o r e s u i t a b l e d e t e c t i o n systems, r e s u l t i n g i n the d e v e l o p m e n t of s e v e r a l alternate types, e a c h h a v i n g its advantages a n d d i s a d v a n t a g e s . I n the m e a n t i m e , efforts h a v e c o n t i n u e d to m a k e the e l e c t r o n c a p t u r e detector a q u a l i t a t i v e t o o l a n d to i m p r o v e its p e r f o r m a n c e a n d ease of m a i n t e n a n c e . T h e m a j o r i t y of the e l e c t r o n c a p t u r e detectors use t r i t i u m as a n i o n i z a t i o n source, a l t h o u g h s t r o n t i u m - 9 0 w a s a d o p t e d b y some m a n u f a c t u r e r s . B o t h h a v e the d i s a d v a n tage of b e i n g v o l a t i l e at e l e v a t e d t e m p e r a t u r e s , a n d o p e r a t i n g t e m p e r a tures are l i m i t e d to 2 2 5 ° C or less. E v e n t h e n , there is a g r a d u a l loss of r a d i o a c t i v i t y , a n d deposits of s a m p l e extractives a n d c o l u m n p a c k i n g m a t e r i a l s b u i l d u p r a p i d l y i n the detectors, m a k i n g t h e m i n o p e r a b l e i n a r e l a t i v e l y short t i m e . T h e recent i n t r o d u c t i o n of n i c k e l - 6 3 as a n i o n i z a t i o n source p e r m i t s o p e r a t i n g t e m p e r a t u r e s u p to 4 0 0 ° C , thus p r e v e n t i n g the b u i l d - u p of c o l u m n effluents i n the detector.

U n f o r t u n a t e l y , there are

disadvantages for some designs, i n c l u d i n g h i g h e r m i n i m u m d e t e c t a b l e a m o u n t s a n d , m o r e i m p o r t a n t i n the w r i t e r s o p i n i o n , v e r y l i m i t e d l i n e a r ranges of response. O n e s u c h detector that was tested h a d a u s a b l e l i n e a r r a n g e of a b o u t 100 to 500 p g of a l d r i n .

I n this instance, the m i n i m u m

d e t e c t a b i l i t y was a c t u a l l y l o w e r t h a n necessary b u t the response w a s so n o n l i n e a r outside this r a n g e that it was v i r t u a l l y useless.

A t least t w o

m a n u f a c t u r e r s n o w c l a i m to h a v e n i c k e l - 6 3 detectors that h a v e u s a b l e l i n e a r ranges a n d m i n i m u m d e t e c t a b i l i t y e q u i v a l e n t to or better t h a n the t r i t i u m detectors.

A n o t h e r t y p e of e l e c t r o n c a p t u r e detector that w i l l

operate at temperatures u p to 4 0 0 ° C uses a ' n o n r a d i o a c t i v e " source i n the f o r m of a n e l e c t r i c a l d i s c h a r g e . T h i s detector gives a response s i m i l a r to that of other e l e c t r o n c a p t u r e detectors a n d is b e i n g r o u t i n e l y u s e d b y a n u m b e r of laboratories. Efforts t o w a r d u s i n g the electron c a p t u r e gas c h r o m a t o g r a p h as a q u a l i t a t i v e t o o l h a v e c e n t e r e d first o n the use of t w o or m o r e different c o l u m n s that h o p e f u l l y w i l l g i v e different r e l a t i v e r e t e n t i o n times for the various e l u t i n g peaks a n d , w i t h the j u d i c i o u s use of s t a n d a r d s , s o m e assurance of the i d e n t i t y of u n k n o w n s . T h i s p r o c e d u r e c a n b e t i m e - c o n s u m i n g a n d , w h i l e h e l p f u l , s t i l l m a y not y i e l d a n y t h i n g p o s i t i v e . A p r e f e r able procedure

is to c h r o m a t o g r a p h a s a m p l e , t h e n c a r r y out


some

76


c h e m i c a l r e a c t i o n (e.g., o x i d a t i o n , r e d u c t i o n , d e h y d r o c h l o r i n a t i o n , r e a r r a n g e m e n t , o r a d d i t i o n ) to f o r m a d e r i v a t i v e t h a t w i l l h a v e a different r e t e n t i o n t i m e . A c o m p a r i s o n w i t h standards t r e a t e d i n the same m a n n e r c a n g i v e i n f o r m a t i v e d a t a . A n i l l u s t r a t i o n of this is the c o n v e r s i o n DDT

of

to D D E b y d e h y d r o c h l o r i n a t i o n . S u c h i n d i r e c t m e t h o d s of g e t t i n g

the e l e c t r o n c a p t u r e gas c h r o m a t o g r a p h to give q u a l i t a t i v e i n f o r m a t i o n m a y produce useful but not conclusive confirmatory data. T h e r e are t w o areas i n p e s t i c i d e r e s i d u e d e t e r m i n a t i o n s w h e r e the e l e c t r o n c a p t u r e detector is w i t h o u t e q u a l . T h e first is for the s c r e e n i n g Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0104.ch005

of samples b y r e g u l a t o r y agencies to e l i m i n a t e f r o m f u r t h e r c o n s i d e r a t i o n a l l b u t the s m a l l p e r c e n t a g e t h a t c o n t a i n above-tolerance pesticide(s).

levels of some

O n c e the r e t e n t i o n times a n d detector responses

of

the

sought c o m p o u n d s h a v e b e e n e s t a b l i s h e d , samples c a n b e i n j e c t e d , a n d those not s h o w i n g a response i n d i c a t i n g the presence of i l l e g a l q u a n t i t i e s of pesticides m a y b e e l i m i n a t e d f r o m f u r t h e r c o n s i d e r a t i o n . T h o s e s h o w i n g possible above-tolerance levels ( n o r m a l l y less t h a n 2 % of the t o t a l ) m a y t h e n b e subjected to i n d e p e n d e n t c o n f i r m a t o r y m e t h o d s to e s t a b l i s h definitely the i d e n t i t y of the c o m p o u n d ( s ) . indicating organochlorine

pesticides, for

Electron capture

responses

e x a m p l e , are often

routinely

c h e c k e d b y m i c r o c o u l o m e t r i c gas c h r o m a t o g r a p h y to establish w h e t h e r or n o t the c o m p o u n d ( s ) c o n t a i n c h l o r i n e . T h e c o m b i n a t i o n of r e t e n t i o n t i m e a n d c h l o r i n e content g i v e reasonable assurance of the i d e n t i t y of the compound.

O r g a n o p h o s p h o r u s pesticides m a y b e c h e c k e d u s i n g a p h o s -

p h o r u s - s e n s i t i v e detector, n i t r o g e n - c o n t a i n i n g pesticides w i t h a n i t r o g e n detector, etc. T h e other area i n w h i c h the e l e c t r o n c a p t u r e detector has t r e m e n d o u s use is i n research activities w h e r e a k n o w n c o m p o u n d is s t u d i e d u n d e r c o n t r o l l e d c o n d i t i o n s . I n this k i n d of s t u d y , the substrate c a n b e c h e c k e d f o r interferences, f o r t i f i e d c o n t r o l samples c a n b e u s e d to d e t e r m i n e r e sponse of the c o m p o u n d of interest i n the presence of the substrate extractives, a n d levels i n t h e treated samples t h e n d e t e r m i n e d w i t h a h i g h d e g r e e of r e l i a b i l i t y , p r o v i d e d the w o r k is d o n e b y a s k i l l e d r e s i d u e analyst w h o w i l l i m m e d i a t e l y r e c o g n i z e a n y i r r e g u l a r i t i e s . P e r h a p s t h e most u n f o r t u n a t e p r o p e r t y of the e l e c t r o n c a p t u r e gas c h r o m a t o g r a p h is t h a t almost a n y o n e c a n take one a n d , after a f e w h o u r s of i n s t r u c t i o n f r o m a salesman, c a n inject samples a n d o b t a i n r e c o r d e r responses.

T h i s has h a p p e n e d , for there w a s a n i m m e d i a t e r u s h to get

o n t h e b a n d w a g o n b y m a n y w o r k e r s i n various phases of p e s t i c i d e r e s e a r c h a n d r e l a t e d fields w h o h a d a b s o l u t e l y no t r a i n i n g i n p e s t i c i d e r e s i d u e analysis or i n the use of i n s t r u m e n t s s u c h as the gas graph.

chromato-

T h e r e s u l t is that there is a n u n k n o w n , b u t v e r y s u b s t a n t i a l ,

a m o u n t of d a t a i n the l i t e r a t u r e t h a t is of a b s o l u t e l y n o v a l u e a n d w h i c h , m o r e i m p o r t a n t , m a y b e m i s l e a d i n g . It w a s r e a l i z e d , b e l a t e d l y b y m a n y ,


5.

WESTLAKE

Gas Chromatographic

77

Measurement

that e l e c t r o n c a p t u r e responses are o n l y i n d i c a t o r s a n d c a n n o t b e a c c e p t e d until confirmed b y an accepted

independent method.

A n outstanding

i l l u s t r a t i o n of m i s i n t e r p r e t e d e l e c t r o n c a p t u r e gas c h r o m a t o g r a p h i c r e sponses is f o u n d i n the m u l t i t u d e of reports of D D T residues i n e v e r y t y p e of e n v i r o n m e n t i n e v e r y p a r t of the w o r l d . It is o n l y n o w b e c o m i n g e v i d e n t that m a n y of these r e p o r t e d D D T residues w e r e , i n fact, p o l y chlorinated

biphenyls, and

compounds

h a v e b e e n m i s t a k e n for D D T w h e n s u i t a b l e c o n f i r m a t o r y

it may

be

found

eventually that

other


p r o c e d u r e s w e r e not used.

The Microcoulometric Detector O n e of the most u s e f u l detectors for gas c h r o m a t o g r a p h i c d e t e r m i n a t i o n of p e s t i c i d e residues, the m i c r o c o u l o m e t r i c detector, w a s b y C o u l s o n et al (3,4)

and described by Coulson (5).

developed

T h i s w a s the first

system to b e d e v e l o p e d s p e c i f i c a l l y for p e s t i c i d e r e s i d u e d e t e r m i n a t i o n . C a s s i l (6)

d e s c r i b e d the i n s t r u m e n t a n d its a p p l i c a t i o n i n the d e t e r m i n a -

t i o n of T h i o d a n i n the presence of D D T . O v e r the e n s u i n g years, the m i c r o c o u l o m e t r i c d e t e c t i o n system b e c a m e v i r t u a l l y a necessity i n a n y l a b o r a t o r y d e t e r m i n i n g o r g a n o c h l o r i n e or s u l f u r - c o n t a i n i n g o r g a n i c p e s t i cides.

I n a d d i t i o n to its use as a p r i m a r y t o o l for analysis, the detector

has v e r y w i d e use as a c o n f i r m a t o r y p r o c e d u r e for e l e c t r o n c a p t u r e d e tector responses a p p e a r i n g to b e o r g a n o c h l o r i n e pesticides. ficity

T h e speci-

for c h l o r i n e , or s u l f u r , as the case m a y be, a l l o w s the o p e r a t o r a

h i g h degree of confidence i n the results o b t a i n e d a n d has l e d to the extensive use of this d e t e c t i o n system for c o n f i r m a t o r y purposes b y

the

U . S. F o o d a n d D r u g A d m i n i s t r a t i o n a n d other r e g u l a t o r y agencies. D u r i n g the past eight years, m a n y i m p r o v e m e n t s h a v e b e e n i n the d e t e c t i o n system.

A n i m p r o v e d t i t r a t i o n c e l l a n d a better

t r o m e t e r a n d p y r o l y s i s f u r n a c e i n c r e a s e d the response

made elec-

many-fold and

s i m p l i f i e d the m a i n t e n a n c e p r o b l e m s associated w i t h the v a r i o u s c o m ponents.

T h e latest m o d e l , r e c e n t l y a n n o u n c e d , is c o m p l e t e l y n e w , w i t h

the e x c e p t i o n of the t i t r a t i o n c e l l , a n d e m b o d i e s a m i n i a t u r i z e d p y r o l y s i s f u r n a c e a n d i m p r o v e d c o u l o m e t e r a n d p o w e r s u p p l y as w e l l as a c o m pletely new cabinet configuration. A newly-designed pyrolysis tube, it is c l a i m e d , has r e s u l t e d i n a system t h a t is insensitive to changes i n gas flow rates a n d gives i n c r e a s e d response.

D o u b l i n g or h a l v i n g the o x y g e n

flow rate does n o t a p p r e c i a b l y affect response, n o r d o s u b s t a n t i a l changes i n the c a r r i e r gas flow rate. T h i s is i n s h a r p contrast to a m i n i a t u r i z e d p y r o l y s i s system a n n o u n c e d b y G u i f f r i d a a n d Ives ( 7 )

that gave s u b -

s t a n t i a l changes i n response w i t h c h a n g i n g gas flow rates. Tests w i t h the n e w p r o d u c t i o n m o d e l s h o w that the o x y g e n used for c o m b u s t i o n m u s t be s a t u r a t e d w i t h w a t e r v a p o r to g i v e m a x i m u m a n d


78


consistent response. a n d G u n t h e r (8)

T h i s was reported b y Coulson (5) for the first m o d e l p r o d u c e d

and by Barkley

b u t has b e e n l a r g e l y

o v e r l o o k e d b y users of this e q u i p m e n t . T h e latest p r o d u c t i o n m o d e l of the m i c r o c o u l o m e t r i c d e t e c t i o n syst e m has a m i n i m u m d e t e c t a b l e l i m i t of a b o u t 1 n g of c h l o r i n e b u t , i n the w r i t e r s o p i n i o n , the m i n i m u m for p r a c t i c a l use is a b o u t 3 n g . T h i s estim a t e is b a s e d u p o n responses o b t a i n e d b y D o h r m a n n I n s t r u m e n t s p e r s o n n e l w i t h s t a n d a r d solutions of

pure lindane.

These

results

were

o b t a i n e d i n m o d e I o p e r a t i o n (gas flow into the c e l l b e t w e e n the elecDownloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0104.ch005

t r o d e s ) rather t h a n m o d e I I o p e r a t i o n (gas flow i m p i n g i n g d i r e c t l y o n the sensor e l e c t r o d e ) , a m o r e " s e n s i t i v e " m o d e c u r r e n t l y b e i n g u s e d o n last y e a r s m o d e l detector i n o u r laboratories. M o d e I is p r e f e r r e d , for o p e r a t i n g p a r a m e t e r s are m u c h less c r i t i c a l . I n 1966, M a r t i n ( 9 )

a n n o u n c e d a t i t r a t i o n c e l l for the m i c r o c o u l o -

m e t r i c system that is specific for n i t r o g e n after p y r o l y s i s of the s a m p l e u n d e r r e d u c i n g c o n d i t i o n s d u r i n g w h i c h the n i t r o g e n is c o n v e r t e d a m m o n i a . A l b e r t (10)

e x t e n d e d this w o r k , a n d C a s s i l et al. (11)

to

have

m o r e r e c e n t l y r e p o r t e d o n the use of this detector for d e t e r m i n i n g o r g a n o n i t r o g e n pesticides a n d h a v e c o m p a r e d its p e r f o r m a n c e w i t h t h e elect r o l y t i c c o n d u c t i v i t y detector,

finding

t h e m e q u a l . T h e s e authors f o u n d

b o t h detectors u s e f u l i n the range of 3 to 200 n g of n i t r o g e n , p e r m i t t i n g t h e m e a s u r e m e n t of as little as 0.05 p p m of o r g a n o n i t r o g e n

pesticides

i n 100-gram samples. B u r c h f i e l d a n d his associates (12, 13, 14)

reported a procedure i n -

v o l v i n g r e d u c t i v e p y r o l y s i s of the effluent gases w h e r e b y

compounds

c o n t a i n i n g c h l o r i n e , s u l f u r , o r p h o s p h o r u s f o r m H C 1 , H S , or P H , respec2

tively.

3

B y u s i n g a n a l u m i n u m o x i d e s c r u b b e r f o l l o w i n g the p y r o l y s i s

stage, the system was specific for p h o s p h o r u s .

H o w e v e r , the d e t e c t i o n

system appears to b e i n f e r i o r to other a v a i l a b l e detectors for p h o s p h o r u s a n d w i l l p r o b a b l y h a v e little or no use for this p u r p o s e .

The Electrolytic Conductivity Detector T h e e l e c t r o l y t i c c o n d u c t i v i t y detector for gas c h r o m a t o g r a p h y d e v e l o p e d b y C o u l s o n (15, 16, 17),

was

w h o d e s c r i b e d m o d e s of o p e r a t i o n

for the d e t e c t i o n of c h l o r i n e , s u l f u r , or n i t r o g e n , b u t d i d not establish the r e l i a b i l i t y of the detector for p e s t i c i d e r e s i d u e analysis or the m i n i m u m d e t e c t a b i l i t y for e a c h m o l e c u l a r species. C a s s i l et al. (11)

described

the use of the detector for d e t e r m i n i n g residues of c a r b a m a t e pesticides a n d c o m p a r e d its response w i t h that of the m i c r o c o u l o m e t r i c detector, as m e n t i o n e d e a r l i e r , finding t h e m e q u a l i n response a n d s e l e c t i v i t y a n d u s a b l e o v e r a r a n g e of 3 to 200 n g of n i t r o g e n . A n i m p r o v e d p y r o l y s i s t u b e w a s d e s c r i b e d , a n d n i c k e l w i r e or t u r n i n g s was u s e d as the catalyst


5.

WESTLAKE

Gas Chromatographic

79

Measurement

i n s t e a d of the n i c k e l s u l f a t e - t r e a t e d q u a r t z w o o l suggested b y C o u l s o n . V a r i o u s a l k a l i e s w e r e tested b e t w e e n the p y r o l y s i s f u r n a c e a n d detector, b a r i u m oxide on 10/16 mesh perlite being preferred. P a t c h e t t (18)

m a d e a d e t a i l e d s t u d y a n d e v a l u a t i o n of the e l e c t r o -

l y t i c c o n d u c t i v i t y detector a n d d e v e l o p e d t e c h n i q u e s to p e r m i t d e p e n d a b l e a n d c o n t i n u o u s use of the detector near a l o w e r l i m i t of d e t e c t i o n of 0.1 n g of n i t r o g e n . H e m a d e a s u b s t a n t i a l i m p r o v e m e n t i n detector r e sponse t h r o u g h m o d i f y i n g the detector c e l l b y p l a c i n g a T e f l o n insert


i n the i n l e t t u b e to r e d u c e s o r p t i v e losses of a m m o n i a . H e also p r o v e d the necessity for u s i n g h i g h - p u r i t y h y d r o g e n gas a n d m a i n t a i n i n g h i g h purity deionized water between Cassil recommend

p H 7.0 a n d 7.5.

the use of h y d r o g e n

B o t h Patchett a n d

as the c a r r i e r gas to o b t a i n

o p t i m u m consistent results. W e s t l a k e et al

(19)

u s e d the e l e c t r o l y t i c c o n d u c t i v i t y detector

for

d e t e r m i n i n g residues of A C D 1 5 M , a t r i a z i n e h e r b i c i d e ( A l l i e d C h e m i c a l C o . ) i n sweet c o r n g r a i n a n d plants (stalks a n d l e a v e s ) . T h e y also d e m onstrated the s e p a r a t i o n of a m i x t u r e of eight t r i a z i n e h e r b i c i d e s a n d the absence of i n t e r f e r r i n g responses at the r e t e n t i o n t i m e of the c o m p o u n d either b y n a t u r a l l y - o c c u r r i n g p l a n t c o m p o n e n t s or r e l a t e d pesticides.

A

major peak, f r o m a n u n i d e n t i f i e d c o m p o n e n t of the c o r n p l a n t extractives, was o b s e r v e d at a r e t e n t i o n t i m e a b o u t three times that of A C D 1 5 M .

It

was not present i n the c o r n g r a i n . I n f o r m a t i o n to date i n d i c a t e s that s u c h a n interference is r a r e , b u t this serves to i l l u s t r a t e the necessity for v i g i l a n c e at a l l times, e v e n w h e n u s i n g a detector as specific as this one, to g u a r d against i n c o r r e c t i n t e r p r e t a t i o n s of the r e c o r d e r responses.

In gen-

e r a l , the e l e c t r o l y t i c c o n d u c t i v i t y detector is r e m a r k a b l y free f r o m i n t e r ferences f r o m n a t u r a l l y - o c c u r r i n g c o m p o n e n t s

i n p l a n t extractives a n d

a n y t h i n g other t h a n o r g a n o n i t r o g e n c o m p o u n d s , b u t there are f r e q u e n t exceptions. T h e e l e c t r o l y t i c c o n d u c t i v i t y detector has b e e n u s e d to d e t e r m i n e o r g a n i c i o d i n e w i t h excellent success b y W e s t l a k e (20),

as w e l l as c h l o -

r i n e i n o r g a n o c h l o r i n e pesticides, o p e r a t i n g i n the r e d u c i n g m o d e

to

y i e l d H I or H C 1 as the d e t e c t e d p r o d u c t . T h e m i n i m u m d e t e c t a b i l i t y for c h l o r i n e is a p p r o x i m a t e l y e q u a l to that of the c u r r e n t m i c r o c o u l o m e t r i c d e t e c t i o n system. C o u l s o n (4, 15)

c o m p a r e d the responses of the elec-

trolytic conductivity, microcoulometric, and electron capture

detectors

for o r g a n o c h l o r i n e c o m p o u n d s i n v a r i o u s extractives a n d f o u n d the first t w o a p p r o x i m a t e l y e q u a l a n d the electron c a p t u r e detector u n s a t i s f a c t o r y because of h i g h b a c k g r o u n d . O n e i m p o r t a n t a d v a n t a g e of the e l e c t r o l y t i c c o n d u c t i v i t y detector is its s i m p l i c i t y . It r e q u i r e s no a m p l i f i c a t i o n of s i g n a l , thus e l i m i n a t i n g the electrometer necessary for the other systems. I n the r e d u c i n g m o d e , it is


80


h i g h l y specific f o r h a l i d e o r , w i t h a l k a l i s c r u b b e r , f o r n i t r o g e n .

I n the

o x i d a t i v e m o d e , i t is q u i t e specific f o r s u l f u r o r c h l o r i n e .

Summary E a c h o f t h e detectors d i s c u s s e d has a n e x t r e m e l y i m p o r t a n t p l a c e i n pesticide residue determinations. W h e n the limitations a n d capabilities of e a c h a r e r e c o g n i z e d a n d t h e responses e v a l u a t e d b y e x p e r i e n c e d p e r s o n n e l , a reasonable assurance of t h e i d e n t i t y o f m a n y pesticides c a n b e Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0104.ch005

o b t a i n e d . N o n e o f t h e detectors a r e c o m p l e t e l y specific a n d , f o r p o s i t i v e i d e n t i f i c a t i o n , v e r i f i c a t i o n b y a n i n d e p e n d e n t m e t h o d is r e q u i r e d . U n f o r t u n a t e l y , m a n y investigators f a i l e d to r e a l i z e ( a n d this is s t i l l t r u e o f s o m e ) t h a t t h e gas c h r o m a t o g r a p h c a n a n d does l i e , a n d a t r e m e n d o u s v o l u m e o f d a t a o f d o u b t f u l m e r i t has b e e n p u b l i s h e d . T h e m o r e experienced

one becomes w i t h the technique, t h e more

skeptical he

b e c o m e s a b o u t t h e v a l i d i t y o f t h e gas c h r o m a t o g r a p h i c responses.

Only

b y m a i n t a i n i n g this a t t i t u d e c a n v a l i d d a t a b e d e r i v e d f r o m gas c h r o m a t o g r a p h y o f p e s t i c i d e residues.

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)

Westlake, W . E., Gunther, F . A . , Residue Rev. (1967) 18, 175. Lovelock, J. E., Lipsky, S. R., J. Am. Chem. Soc. (1960) 82, 431. Coulson, D . M . , Cavenaugh, L . A . , Anal. Chem. (1960) 32, 1245. Coulson, D . M . , De Vries, J. E., Walther, B., J. Agr. Food Chem. (1960) 8, 399. Coulson, D . M . , Advan. Pest Control Res. (1962) 5, 153. Cassil, C. C., Residue Rev. (1962) 1, 37. Guiffrida, L . , Ives, N . F . ,J.Assoc. Offic. Agr. Chemists (1969) 52, 541. Barkley, J. H . , Gunther, F . A., unpublished report, 1964. Martin, R. L., Anal. Chem. (1966) 38, 1209. Albert, D . K., Anal. Chem. (1967) 39, 1113. Cassil, C. C., Stanovick, R. P., Cook, R. F . , Residue Rev. (1969) 26, 63. Burchfield, H . P., Johnson, D . E., Rhoades, J. W . , Wheeler, R. J.,J.Gas Chromatog. (1965) 3, 28. Burchfield, H . P., Rhoades, J. W . , Wheeler, R. J., J. Agr. Food Chem. (1965) 13, 511. Burchfield, H . P., Wheeler, R. J., J. Assoc. Offic. Agr. Chemists (1966) 49, 651. Coulson, D . M . , J .Gas Chromatog. (1965) 3, 134. Ibid., (1966) 4, 285 Coulson, D . M . , "Advances in Chromatography" Vol.III,p. 197, Marcel Dekker, New York, 1966. Patchett, G. G., J. Chromatog. Sci. (1970) 8, 155. Westlake, W . E . , Westlake, A . , Gunther, F . A . , J. Agr. Food Chem. (1970) 18, 685. Westlake, A . , unpublished data.

RECEIVED

June 12,

1970.


Gas Chromatographic Measurement and Identification of Pesticide

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