Pesticides Identification at the Residue Level

6 Infrared Microtechniques Useful for Identification of Pesticides at the Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on August 7, 2018 at 04:11:03 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Microgram Level R O G E R C.

BLINN

American C y a n a m i d C o . , Princeton, N. J. 08540

Infrared for

spectrophotometry

identification

pounds. oped for

extending

material.

Factors

in minimizing

the

of

conveniently

however, potassium

devel-

are for

achieved

trapping

amounts

of

this sensitivity

the sample

is also advantageous. micro

com-

have been

of contamination

from

usefulness

organic

to microgram

in achieving

effects

is most

chromatography; paring

amounts

this utility involved

of pesticides

acterization effluents

has a long history of

milligram

In recent years microtechniques

Isolation

/

of

gas

Various

pellets

well as the use of micro multiple

internal

charthin-layer

chromatographic

procedures

bromide

discussed.

infrared by

and

for

pre-

are described,

as

reflectance.

T p h e subject of " I d e n t i f i c a t i o n of P e s t i c i d e s at the R e s i d u e L e v e l " is b o t h J

- t i m e l y a n d i m p o r t a n t i n this p e r i o d w h e n w e are a c t i v e l y r e v i e w i n g

the values a n d h a z a r d s of p e s t i c i d e usage, since s u c h a r e v i e w is d e p e n d ent u p o n r e l i a b l e a n a l y t i c a l d a t a . T h e r e f o r e , the r e p o r t i n g of u n c o n f i r m e d r e s i d u e d a t a is m i s l e a d i n g a n d c a n often result i n controversy. of i n f r a r e d s p e c t r o p h o t o m e t r y

T h e use

has p i o n e e r e d i n this i m p o r t a n t task of

c o n f i r m i n g the i d e n t i t y of p e s t i c i d e residues. I n f r a r e d s p e c t r o p h o t o m e t r y has a l o n g h i s t o r y of usefulness i n h e l p i n g to establish a n d to c o n f i r m the i d e n t i t y of o r g a n i c c o m p o u n d s .

Func-

t i o n a l g r o u p - a b s o r p t i o n b a n d c o r r e l a t i o n charts are w e l l k n o w n a n d h a v e b e e n u s e d r o u t i n e l y b y o r g a n i c synthesis chemists a n d b y analysts for characterizing compounds

of u n k n o w n i d e n t i t y . W h e r e a s y n t h e t i c a l l y

p r e p a r e d c o m p o u n d is not a v a i l a b l e for c o m p a r i s o n w i t h the u n k n o w n , i n f r a r e d d a t a i n c o n j u n c t i o n w i t h mass, u l t r a v i o l e t , a n d n u c l e a r m a g n e t i c 81

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

82

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

resonance s p e c t r a l d a t a c a n a l l o w the d e d u c t i o n of the u n k n o w n c o m p o u n d s s t r u c t u r e w i t h reasonable assurance. H o w e v e r , p o s i t i v e i d e n t i f i c a t i o n m u s t a w a i t exact c o m p a r i s o n of the v a r i o u s p r o p e r t i e s of

the

u n k n o w n w i t h those of a c o m p o u n d p r e p a r e d s y n t h e t i c a l l y b y a n u n equivocal procedure.

I n p r a c t i c e , exact i n f r a r e d s p e c t r a l c o m p a r i s o n

alone u s u a l l y constitutes c o n f i r m a t i o n of i d e n t i t y . T h e l o w cost of m a n y fine i n f r a r e d spectrophotometers

has

con-

t r i b u t e d to t h e i r a v a i l a b i l i t y to most p e s t i c i d e r e s i d u e analysts, as c o n trasted to mass a n d n u c l e a r m a g n e t i c resonance

spectrometers.

The

s e n s i t i v i t y i n h e r e n t i n i n f r a r e d measurements for i d e n t i f i c a t i o n purposes is o n l y e x c e e d e d b y mass s p e c t r o m e t r y . T h i s s e n s i t i v i t y has b e e n r e a l i z e d b y the d e v e l o p m e n t

of s u i t a b l e a n d p r a c t i c a b l e m i c r o t e c h n i q u e s , a n d

some of these h a v e b e e n a v a i l a b l e for at least 10 years. N o w , w h a t are the factors that c a n be v a r i e d i n o r d e r to a c h i e v e s e n s i t i v i t y ? T h e l a w d e s c r i b i n g the a b s o r p t i o n of e l e c t r o m a g n e t i c energy b y a b s o r b i n g substances i n s o l u t i o n , Beer's L a w , states that the a b s o r b ance of a s o l u t i o n is p r o p o r t i o n a l to the a b s o r p t i v i t y of the a b s o r b i n g c o m p o u n d , the distance the e n e r g y b e a m passes t h r o u g h the s o l u t i o n , a n d the c o n c e n t r a t i o n of the a b s o r b i n g c o m p o u n d i n the s o l u t i o n .

In

other w o r d s , s e n s i t i v i t y c a n be a c h i e v e d b y p l a c i n g the m a x i m u m n u m b e r of the a v a i l a b l e m o l e c u l e s of a c o m p o u n d i n the u s a b l e e n e r g y b e a m of the spectrophotometer.

T h i s c a n b e a c c o m p l i s h e d b y i n c r e a s i n g the

distance the energy b e a m passes t h r o u g h the s o l u t i o n , a n d / o r b y i n c r e a s i n g the c o n c e n t r a t i o n of the s a m p l e i n the s o l u t i o n b y d e c r e a s i n g the v o l u m e r e q u i r e m e n t s of the s p e c t r o p h o t o m e t e r s s a m p l e h o l d i n g accessory.

T h e w o r d " s o l u t i o n " m a y also refer to the essentially p u r e c o m -

p o u n d , e i t h e r as a film or as p a r t i c i p a t e d v e r y finely i n a m e d i u m . T h e other factor i n B e e r s L a w affecting s e n s i t i v i t y is the a b s o r p t i v i t y of the s a m p l e . T h e analyst, of course, has no c o n t r o l over this factor, b u t he c e r t a i n l y s h o u l d b e a w a r e of it. H e s h o u l d k n o w that a l a r g e r s a m p l e of the l o w - a b s o r p t i v i t y c y c l o d i e n e

pesticides, for e x a m p l e , w i l l be r e -

q u i r e d to a c h i e v e a satisfactory s p e c t r u m t h a n for the m o r e s t r o n g l y absorbing organophosphorus

pesticides.

O f course, the r e a l c h a l l e n g e to a c h i e v i n g s e n s i t i v i t y is i n e l i m i n a t i n g or m i n i m i z i n g the ever-present sources of interference.

A s the size of

the s a m p l e decreases into the m i c r o g r a m range, d e c r e a s i n g the i n t e r f e r ences at the same rate is i n c r e a s i n g l y difficult. T h e sources of interference are e v e r y w h e r e , a r i s i n g f r o m the s a m p l e , solvents, reagents, atmosphere, h a n d l i n g , a n d other s i m i l a r sources.

M a n y of these interferences c a n b e

m i n i m i z e d b y d u e care i n c h o o s i n g i s o l a t i o n p r o c e d u r e s ,

pre-washing

sorbents a n d glassware, p u r i f y i n g solvents a n d reagents, a n d p r o p e r m a n i p u l a t i v e t e c h n i q u e s . B u t c o m p l e t e e l i m i n a t i o n of a l l i n t e r f e r i n g m a t e rials is e x t r e m e l y difficult to a c c o m p l i s h .

T h u s , interferences d o


limit

6.

BLINN

Infrared

83

Microtechniques

the s e n s i t i v i t y t h a t c a n b e a c h i e v e d . C h e n a n d D o r i t y ( J ) r e c e n t l y d i s cussed the i m p o r t a n c e o f m i n i m i z i n g interferences i n m i c r o s a m p l i n g . W h a t o r d e r of s e n s i t i v i t y is a c h i e v a b l e i n the i n f r a r e d w i t h techniques a n d equipment presently available?

Several workers

the have

p r o p o s e d that the absolute l o w e r l i m i t of s e n s i t i v i t y is 10 n g of a c o m p o u n d w i t h m o d e r a t e l y s t r o n g a b s o r p t i v i t y values ( 2 , 3 ) .

However, be-

cause of the d i f f i c u l t y i n p l a c i n g a l l or most of the s a m p l e i n the a c t u a l u s a b l e e n e r g y b e a m of the spectrophotometer, a n a c t u a l l o w e r l i m i t of sensitivity is a b o u t o n e - t e n t h to o n e - h a l f of a m i c r o g r a m , t h a t is, a b o u t 100 to 500 n g (4, 5 ) .

F o r most w o r k e r s , this l i m i t w i l l b e a b o u t five to

ten m i c r o g r a m s . O n e does not h a v e to b e a p r o f e s s i o n a l i n f r a r e d spectroscopist to a t t a i n success i n the m i c r o g r a m r a n g e .

Students that h a v e

n e v e r p r e v i o u s l y u s e d a n i n f r a r e d spectrometer h a v e a c h i e v e d excellent spectra w i t h t e n m i c r o g r a m s or less of a p e s t i c i d e after a f e w h o u r s of i n s t r u c t i o n a n d p r a c t i c e , so m i c r o t e c h n i q u e s i n the i n f r a r e d are u s a b l e b y the p e s t i c i d e r e s i d u e analyst. A s stated p r e v i o u s l y , the p e s t i c i d e must b e i s o l a t e d f r o m the s a m p l e , w h e t h e r the s a m p l e is n a t u r a l w a t e r s , s o i l , or p l a n t or a n i m a l tissue. T h i s isolation must be virtually complete.

U s u a l l y , a c h r o m a t o g r a p h i c process

is u s e d to a c h i e v e this s e p a r a t i o n of the p e s t i c i d e f r o m n a t u r a l substances e x t r a c t e d f r o m the s a m p l e a n d f r o m other pesticides a n d f o r e i g n s u b stances r e s i d i n g i n the s a m p l e .

V a p o r phase a n d t h i n - l a y e r c h r o m a -

t o g r a p h y h a v e t r a d i t i o n a l l y b e e n the methods of c h o i c e for this p u r p o s e . W h e n u s i n g either of these c h r o m a t o g r a p h i c processes i n o r d e r to isolate a p e s t i c i d e for i n f r a r e d s c r u t i n y , i t is advantageous to subject the s a m p l e extract to a r i g o r o u s " c l e a n u p " p r i o r to c h r o m a t o g r a p h y .

W i t h a de-

creased a m o u n t of extraneous m a t e r i a l to be s e p a r a t e d f r o m the p e s t i c i d e , the c h r o m a t o g r a p h i c process w i l l be m o r e efficient. E v e n w i t h a r i g o r o u s c l e a n u p , a s a m p l e m a y r e q u i r e several c h r o m a t o g r a p h i c isolations b e f o r e reliable data can be obtained.

H o w e v e r , for e a c h process to w h i c h the

s a m p l e is subjected, there is a n i n e v i t a b l e loss of a p o r t i o n of the s a m p l e . T h e loss m a y b e m i n o r , b u t a p o r t i o n of the s a m p l e is lost f r o m infrared procedure.

the

T h e r e f o r e , the s t a r t i n g s a m p l e m u s t b e of sufficient

size so that there w i l l b e e n o u g h p e s t i c i d e i s o l a t e d via a l l of the i s o l a t i o n p r o c e d u r e s to a l l o w i n f r a r e d e v a l u a t i o n . T h e w e l l - d e s e r v e d p o p u l a r i t y of gas c h r o m a t o g r a p h y i n the

field

of p e s t i c i d e r e s i d u e analysis suggests that this t o o l b e u s e d for i s o l a t i o n purposes. T h e t r a p p i n g of gas c h r o m a t o g r a p h i c peaks for i n f r a r e d i d e n tification has b e e n a n d is b e i n g u s e d . S e v e r a l factors m u s t b e c o n s i d e r e d i n the successful use of gas c h r o m a t o g r a p h y for t r a p p i n g a n d the m e a n i n g f u l e v a l u a t i o n of the s p e c t r u m of the t r a p p e d m a t e r i a l .

The high

temperatures a n d the l a r g e , often c a t a l y t i c a l l y a c t i v e , surface areas that are e n c o u n t e r e d

i n the gas c h r o m a t o g r a p h i c process c a n c h a n g e


the

84


chemical nature of the sample. Gas chromatography of such compounds can

r e s u l t i n u s a b l e a n d r e l i a b l e e l u t i o n peaks for m e a s u r e m e n t s , b u t

c o u l d b e m i s l e a d i n g f o r i d e n t i f i c a t i o n purposes.

T h e larger amounts o f

m a t e r i a l n e e d e d for i n f r a r e d purposes c o m p a r e d w i t h a n a l y t i c a l studies makes the c h r o m a t o g r a p h i c process m o r e difficult a n d increases the p o s s i b i l i t y f o r c h e m i c a l a l t e r a t i o n o f the p e s t i c i d e .

Another factor to b e

c o n s i d e r e d w h e n t r a p p i n g a gas c h r o m a t o g r a p h i c p e a k i s t h a t the spec i f i c i t y o f a detector w i l l often o b s c u r e the s i m u l t a n e o u s e l u t i o n o f i n t e r f e r i n g substances f r o m the s a m p l e a n d the c o l u m n .

S u c h detectors as

t h e e l e c t r o n c a p t u r e , t h e r m i o n i c , flame p h o t o m e t r i c ,

microcoulometric,

a n d n i t r o g e n detectors r e s p o n d s e l e c t i v e l y to c e r t a i n types o f c o m p o u n d s a n d i n s e n s i t i v e l y o r not a t a l l t o others.

T h i s i s the reason t h a t these

detectors w e r e chosen f o r p e s t i c i d e r e s i d u e analysis. T h e r e f o r e , a detector response as a s h a r p p e a k f r o m a b a s e l i n e m a y e i t h e r b e c a u s e d b y the e l u t i o n o f a single c o m p o u n d

o r b y this c o m p o u n d

i n company

o t h e r m a t e r i a l t o w h i c h the detector i s u n r e s p o n s i v e .

with

T h e l a t t e r case

c o u l d b e u n s u i t a b l e for i n f r a r e d purposes. A n o t h e r source o f i n t e r f e r e n c e of this t y p e is f r o m c o l u m n " b l e e d . "

T h e s t a t i o n a r y phase o f a n y gas

c h r o m a t o g r a p h i c c o l u m n does possess a c e r t a i n a m o u n t o f v o l a t i l i t y a n d w i l l s l o w l y e l u t e a n d often collect at the exit p o r t .

T h e investigator

s h o u l d b e f a m i l i a r w i t h the i n f r a r e d s p e c t r u m o f his stationary phase. F i g u r e 1 shows the s p e c t r u m o f one o f the c o l u m n m a t e r i a l s u s e d i n p e s t i c i d e analysis, s i l i c o n e o i l . E v e r y o n e u s i n g i n f r a r e d t e c h n i q u e s s h o u l d k n o w this s p e c t r u m , as i t c a n arise as w e l l f r o m s t o p c o c k gease o n glassware

(I).

B e c a u s e o f t h e d e s t r u c t i v e n a t u r e a n d / o r extreme s e n s i t i v i t y o f c o m m o n l y u s e d detectors f o r p e s t i c i d e p r o b l e m s , the gas c h r o m a t o g r a p h u s e d for p e s t i c i d e analysis u s u a l l y is n o t s u i t a b l e f o r t r a p p i n g .

Although

splitters are a v a i l a b l e that c a n b e u s e d to c o n v e r t the a n a l y t i c a l i n s t r u 0.0 .10 UJ

g.20

r v / V

£.30

SUo

350

SILICONE OIL

.70 1.0 1.5 1 4000 3000

1

i

2000

1500

Figure 1.

i

1200 CM"!

i

i

1000

900

i

800

Infrared spectrum of silicone oil


1 700

6.

BLINN

Infrared

85

Microtechniques

m e n t for p r e p a r a t i v e purposes, this u s u a l l y is not satisfactory o w i n g to the lost i n s t r u m e n t t i m e for a n a l y t i c a l purposes a n d the m a n - h o u r s a n d f r u s t r a t i o n r e q u i r e d to a c h i e v e

conversion.

A

preparative instrument

w h i c h is u s e d o n l y for i d e n t i f i c a t i o n purposes is p r e f e r a b l e . T h e a c t u a l t r a p p i n g p r o c e d u r e to b e u s e d is a m a t t e r of i n d i v i d u a l preference, since there are m a n y p u b l i s h e d p r o c e d u r e s a n d c o m m e r c i a l types a v a i l a b l e . T h e c a p i l l a r y - t y p e t r a p p i n g d e v i c e ( 6 , 7, 8 ) is a p p e a l i n g l y s i m p l e , a n d the c o n c e n t r a t e d t r a p p e d p e s t i c i d e is c o n f i n e d o n the s m a l l i n n e r surfaces of the t u b e f r o m w h i c h i t is q u i c k l y , e a s i l y , a n d , m o r e i m p o r t a n t , efficiently t r a n s f e r r e d to w h i c h e v e r i n f r a r e d m i c r o s a m p l i n g d e v i c e is to b e used. T h i n - l a y e r c h r o m a t o g r a p h y is a m o r e a d a p t a b l e p r o c e d u r e f o r the i s o l a t i o n of m i c r o g r a m q u a n t i t i e s of a p e s t i c i d e p r i o r to its i d e n t i f i c a t i o n b y i n f r a r e d e v a l u a t i o n , a n d the t h i n - l a y e r process aids i n the i d e n t i f i c a t i o n as w e l l .

L o c a t i n g the area i n w h i c h the p e s t i c i d e resides o n the d e v e l -

o p e d chromatogram can be a problem, however.

A sorbent w i t h

fluores-

cent i n d i c a t o r c a n b e u s e d for those c o m p o u n d s w h i c h q u e n c h

fluo-

rescence. A l t e r n a t i v e l y , the c h r o m a t o g r a p h y of standards i n a side c h a n n e l c a n b e u s e d for l o c a t i n g purposes

or a n a l i q u o t of the u n k n o w n

s o l u t i o n c a n b e c h r o m a t o g r a p h e d i n a side c h a n n e l for c o l o r i m e t r i c d e tection. O n c e l o c a t e d , the s o r b e n t f r o m that area c a n be s c r a p e d f r o m the p l a t e for e l u t i o n .

Elution

s h o u l d be

accomplished

with

a minimum

a m o u n t of as n o n p o l a r a solvent as possible, thus r e s t r i c t i n g the a m o u n t of c o - e l u t i n g interferences f r o m the sorbent.

V e r y p o l a r solvents

will

elute interferences f r o m the sorbent w h i c h are v e r y difficult to c l e a n u p {1,4).

T h e sorbent o n the t h i n - l a y e r p l a t e s h o u l d h a v e b e e n p r e - w a s h e d

w i t h the e l u t i n g solvent or a solvent of greater p o l a r i t y p r i o r to the c h r o m a t o g r a p h y , of course.

O n e s h o u l d not o v e r l o o k the p o s s i b i l i t y for

c h e m i c a l a l t e r a t i o n of the p e s t i c i d e o n the sorbent, as s e v e r a l types of c o m p o u n d s are subject to s u c h changes—e.g., p h e n o l s a n d amines. A l s o , v e r y p o l a r c o m p o u n d s are often difficult or i m p o s s i b l e to elute f r o m the sorbent.

A s i d e f r o m these l i m i t a t i o n s , t h i n - l a y e r c h r o m a t o g r a p h y is a d -

vantageous

for i s o l a t i n g pesticides p r i o r to i n f r a r e d e v a l u a t i o n .

s i m p l e e q u i p m e n t a n d t e c h n i q u e s r e q u i r e d a n d the r e l a t i v e

The

freedom

f r o m c h e m i c a l a l t e r a t i o n makes this a p p r o a c h u s e f u l to a l l . N o w as to the m i c r o t e c h n i q u e s w h i c h h a v e p r o v e d u s e f u l for i n f r a r e d s p e c t r o m e t r y , there are s e v e r a l types a v a i l a b l e i n the l i t e r a t u r e , s u c h as m i c r o c e l l s for solutions (9, 10),

m i c r o g r o o v e d plates for confined

(7, I I ) , s u s p e n d e d p a r t i c l e s on a m e m b r a n e filter (12,13), s p e c t r o m e t r y (14),

the v a r i o u s m i c r o p e l l e t p r o c e d u r e s (15,

20, 21, 22, 23, 24, 25),

a n d m u l t i p l e i n t e r n a l reflectance

films

microspecular 16,17,18,19,

(5,12,15,26,27).

T h e n , of course, there is m u l t i p l e scan interference s p e c t r o m e t r y

(28)

w i t h c o m p u t e r storage a n d h a n d l i n g of the d a t a . It is w i t h this t e c h n i q u e


86


t h a t significant gains i n s e n s i t i v i t y a n d s p e e d w o u l d a p p e a r p r o m i s i n g for the f u t u r e . B u t for the present, the use of m i c r o p o t a s s i u m b r o m i d e pellets a n d m u l t i p l e i n t e r n a l reflectance

are p r o v i n g most u s e f u l for

pesticide problems. O f a l l of the m i c r o s a m p l i n g t e c h n i q u e s d e s c r i b e d for i n f r a r e d spect r o m e t r y , the use of p o t a s s i u m b r o m i d e pellets of 1-2 m m i n d i a m e t e r offers the best o p p o r t u n i t y for p l a c i n g the m a x i m u m n u m b e r of m o l e c u l e s of the u n k n o w n i n the u s a b l e e n e r g y b e a m of the

spectrophotometer.

C e r t a i n l y , the best sensitivities h a v e b e e n r e a l i z e d b y this t e c h n i q u e .

The

e q u i p m e n t c o m m o n l y u s e d for p r e p a r i n g m i c r o p o t a s s i u m b r o m i d e pellets is s h o w n i n F i g u r e 2. T h e k e y to s e n s i t i v i t y is the a b i l i t y to transfer t h e m a x i m u m a m o u n t of c o m p o u n d to the m i n i m u m a m o u n t of p o t a s s i u m b r o m i d e to b e pressed i n t o the m i c r o p e l l e t .

Conventional mixing pro-

cedures u s i n g a m o r t a r a n d pestle, s u c h as is s h o w n i n F i g u r e 2, are s u b ject to v e r y l a r g e losses of c o m p o u n d to the surface area of the m i x i n g vessel w h e n o n l y 10 m g or less of p o t a s s i u m b r o m i d e are u s e d ( J , O n e of

the first attempts to resolve this p r o b l e m was the

i l l u s t r a t e d i n F i g u r e 3, w h i c h w a s d e v e l o p e d b y M c C a u l l e y (21) t h e n r e f i n e d b y C h e n (16).

18).

procedure and

W i t h this t e c h n i q u e , p o w d e r e d p o t a s s i u m

b r o m i d e was p a c k e d t i g h t l y i n the 1.5-mm orifice of the stainless steel disk. T h e s a m p l e i n s o l u t i o n of a v o l a t i l e solvent was a d d e d

dropwise

to the p o t a s s i u m b r o m i d e , a l l o w i n g the solvent to evaporate

between

a d d i t i o n s . A f t e r c o m p l e t i n g the a d d i t i o n s , the p e l l e t w a s pressed i n the c o n v e n t i o n a l m a n n e r for i n f r a r e d e v a l u a t i o n . T h i s p r o c e d u r e t h e o r e t i c a l l y offers c o m p l e t e transfer of the s a m p l e to the e n e r g y b e a m of the spectro-

Figure

2.

Equipment

useful in preparing

micro potassium

bromide


pellets

6.

BLINN

Figure

3.

photometer.

Infrared

Microtechniques

Illustration

of technique Chen

87

described (16)

by

McCaulley

(21)

and

P r a c t i c a l l y , this u s u a l l y does n o t h a p p e n since a p o r t i o n

of the s o l u t i o n is p r e f e r e n t i a l l y d r a w n f r o m the p o t a s s i u m b r o m i d e

be-

cause of the greater surface t e n s i o n of the m e t a l disk. T h e losses are often v e r y great, b u t w i t h d u e care a n d steady nerves, excellent s e n s i t i v i t y c a n b e r e a l i z e d w i t h this t e c h n i q u e . Another technique developed

to m i n i m i z e losses of c o m p o u n d

c o n t a i n e r surfaces is d e s c r i b e d b y C h e n a n d D o r i t y (1)

on

a n d b y de K l e i n

( 2 9 ) , i n w h i c h the s a m p l e is d e p o s i t e d i n a c a p i l l a r y tube. T h e p o w d e r e d p o t a s s i u m b r o m i d e c a n b e a d d e d to the c a p i l l a r y t u b e e i t h e r p r i o r to the a d d i t i o n of the s a m p l e or a f t e r w a r d s ; m i x i n g is a c c o m p l i s h e d i n the latter case b y s h a k i n g a n d l e t t i n g the s c o u r i n g a c t i o n of the p o t a s s i u m b r o m i d e r e d u c e r e s i d u a l t u b i n g surface losses. C u r r y et al. (18)

d e s c r i b e another t e c h n i q u e , s h o w n i n F i g u r e 4,

i n w h i c h t h e y " d i s p e n s e " a b o u t h a l f of a m i c r o l i t e r of a c h l o r o f o r m s o l u t i o n of the s a m p l e to the t i p of a s y r i n g e needle, " p i c k i n g u p " p o w d e r e d p o t a s s i u m b r o m i d e b y a d h e r i n g a c t i o n o n the n e e d l e t i p , d i s p e n s i n g a n other a l i q u o t of the s o l u t i o n to the p o w d e r , a n d e v a p o r a t i n g the c h l o r o form.

T h e " d i s p e n s i n g " of the s o l u t i o n is c o n t i n u e d u n t i l the e n t i r e

s a m p l e is t r a n s f e r r e d to the p o w d e r , w h i c h is t h e n pressed i n t o a m i c r o pellet.

This procedure

is t h e o r e t i c a l l y s o u n d b u t w o u l d seem to

be

n e r v e - w r a c k i n g i n o p e r a t i o n . A c t u a l l y , the greatest d i f f i c u l t y is i n a c h i e v i n g the i n i t i a l a d h e s i o n of the p o w d e r e d p o t a s s i u m b r o m i d e to t h e s y r i n g e needle.

O n c e a d h e r e d , the p o w d e r

w i l l stick to the n e e d l e t i p u n t i l

c o m p l e t e l y d r y of solvent. T w o t e c h n i q u e s for t r a n s f e r r i n g samples to p o t a s s i u m b r o m i d e p o w d e r are e s p e c i a l l y s u i t e d for use w i t h t h i n - l a y e r c h r o m a t o g r a p h y . p r o c e d u r e d e s c r i b e d b y de K l e i n (30)

is i l l u s t r a t e d i n F i g u r e 5.


A

It i n -

88


Figure 4.

Illustration of technique described by Curry et al. (18)

volves s c r a p i n g the sorbent f r o m a r o u n d the c h r o m a t o g r a p h i c spot of interest, p l a c i n g a " d a m " of p o w d e r e d p o t a s s i u m b r o m i d e a r o u n d the t i p of the spot, e l u t i n g the c o m p o u n d w i t h a v o l a t i l e solvent, a n d a l l o w i n g the solvent to s p i l l o v e r to the " d a m " of p o t a s s i u m b r o m i d e w h e r e i t is a l l o w e d to d r y . T h e other t e c h n i q u e i n v o l v e s u s i n g the c o m m e r c i a l l y a v a i l a b l e " w i c k s t i c k " a n d is d e s c r i b e d b y K r o h n e r a n d K e m m n e r

(5).

T h e w e d g e of p o t a s s i u m b r o m i d e is d i p p e d at its base into a s o l u t i o n of the c o m p o u n d i n a v o l a t i l e solvent. T h e s o l u t i o n migrates u p the w e d g e to the t i p , w h e r e i t evaporates, a n d the c o m p o u n d is c o n c e n t r a t e d i n the tip.

T h e t i p is t h e n b r o k e n f r o m the w e d g e a n d pressed i n t o a m i c r o -

p e l l e t . T h e use of " w i c k s t i c k s " is s i m p l e , r e l i a b l e , a n d r e c o m m e n d e d

for

p r e p a r i n g m i c r o p o t a s s i u m b r o m i d e pellets. A l l of these m i c r o t e c h n i q u e s are successful i n t r a n s f e r r i n g the s a m p l e to the p o t a s s i u m b r o m i d e m i c r o p e l l e t , b u t losses of s a m p l e are i n h e r e n t f o r e a c h of t h e m . T h e s e losses c a n b e v e r y great w i t h o u t a t t e n t i o n to detail.

E l i m i n a t i o n of w a t e r a b s o r p t i o n b a n d s is also q u i t e difficult to

accomplish.

A f u r t h e r d i s a d v a n t a g e of this p r o c e d u r e is the d i f f i c u l t y

i n r e c o v e r y of the s a m p l e for e v a l u a t i o n b y other means.

B u t the ex-

c e l l e n t s e n s i t i v i t y a c h i e v a b l e does r e c o m m e n d this s a m p l i n g p r o c e d u r e for m i c r o q u a n t i t i e s of m a t e r i a l . I n F i g u r e 6 is p r e s e n t e d the p r i n c i p l e b y w h i c h i n f r a r e d spectra are o b t a i n e d b y m u l t i p l e i n t e r n a l reflectance.

T h e i n f r a r e d e n e r g y is


6.

BLINN

Infrared

Figure 5.

89

Microtechniques

Illustration

of technique

described by de Klein

(30)

d i r e c t e d i n t o the entrance face of the i n f r a r e d t r a n s p a r e n t c r y s t a l l i n e plate.

T h i s e n e r g y is reflected r e p e a t e d l y f r o m the i n n e r faces of the

top a n d b o t t o m surfaces as i t transverses the p l a t e , emerges t h r o u g h the exit face, a n d is d i r e c t e d i n t o the spectrophotometer.

T h e s a m p l e is

p l a t e d as a film o n the top a n d b o t t o m surfaces of the p l a t e . A t e a c h t o t a l l y i n t e r n a l reflection there is a p e n e t r a t i o n of the

electromagnetic

field i n t o the r a r e r m e d i u m b e y o n d the reflecting interface. A n y s a m p l e i n c o n t a c t w i t h the surface is p e n e t r a t e d a f e w m i c r o n s , a n d a s p e c t r u m results. flectance

T h e most u s e f u l c r y s t a l l i n e m a t e r i a l for m u l t i p l e i n t e r n a l r e spectrometry

is K R S - 5 , a m i x t u r e of

thallous i o d i d e . W h e n u s i n g a 50- X

20- X

thallous bromide

a b o u t 15 to 20 fig are r o u t i n e l y a c h i e v e d w i t h stronger a b s o r b i n g

Figure 6.

and

1-mm p l a t e , sensitivities of

Pathway of infrared energy beam in multiple internal ance plate

com-

reflect-


90


4 . 9 fiq A B A T E ® MOSQUITO LARVACIDE 0.0

.10 .20 .30 .40 .50

MICRO MULTIPLE INTERNAL REFLECTANCE K R S - 5 12.7 X 5 X I mm PLATE

.701.0l.5b_ 4000 3000

Figure

pounds.

1.5 mm. POTASSIUM BROMIDE PELLET

2000

1500

_L

1000

1200 CM"'

7. Infrared spectra of Abate mosquito ternal reflectance and micro potassium

I

900

larvicide bromide

800

700

by multiple inpellet

T h e r e is n o w a m i c r o v e r s i o n o f this t e c h n i q u e i n t r o d u c e d b y

W i l k s Scientific C o . u s i n g a 12.7- X 5- X 1-mm p l a t e w h i c h w i l l a l l o w sensitivities o f a b o u t 3 to 5 jig. T h e great a d v a n t a g e o f m u l t i p l e i n t e r n a l reflectance is t h e ease o f a p p l y i n g t h e s a m p l e to t h e surface o f t h e r e flectance

p l a t e . I t is just s t r e a k e d o r d o t t e d as a s o l u t i o n o n t o t h e s u r -

faces, a l l o w i n g t h e solvent to evaporate.

A f t e r t h e s p e c t r u m is r e c o r d e d ,

the s a m p l e c a n b e w a s h e d f r o m t h e surfaces f o r a n y f u r t h e r e v a l u a t i o n d e s i r e d . T h e o n l y r e a l d i f f i c u l t y w i t h s a m p l e p r e p a r a t i o n is w i t h those c o m p o u n d s w h o s e c r y s t a l l i n e s t r u c t u r e makes i n t i m a t e contact w i t h t h e surface difficult a n d those m a t e r i a l s w h i c h react w i t h t h e t h a l l o u s b r o m i d e o r i o d i d e . T h e s p e c t r u m r e s u l t i n g f r o m m u l t i p l e i n t e r n a l reflectance is s i m i l a r b u t s l i g h t l y different f r o m t r a n s m i s s i o n spectra, since a b s o r p t i o n is greater at t h e l o n g e r w a v e l e n g t h s . F i g u r e 7 shows t h e spectra o b t a i n e d f r o m 4.9 fig o f A b a t e m o s q u i t o l a r v i c i d e , u s i n g b o t h t h e m i c r o m u l t i p l e i n t e r n a l reflectance a n d m i c r o p o t a s s i u m b r o m i d e p e l l e t t e c h n i q u e s . T h e t e c h n i q u e o f C u r r y et al (18), d i s p e n s i n g t h e s o l u t i o n o f t h e i n s e c t i c i d e onto t h e p o t a s s i u m b r o m i d e


6.

BLINN

Infrared

91

Microtechniques

p o w d e r adhered to a syringe needle, was used. I n F i g u r e 8 is shown the s p e c t r a o f 9.8 fig o f t e c h n i c a l D D T o b t a i n e d i n t h e same m a n n e r , a n d F i g u r e 9 shows s i m i l a r l y p r e p a r e d s p e c t r a f o r 24.6 fig o f e n d r i n . T h e s e three figures w e r e p r e p a r e d t o i l l u s t r a t e t h e sensitivities a c h i e v a b l e , t h e superior sensitivity of the micro potassium bromide pellet technique, a n d the greater s e n s i t i v i t y a c h i e v e d f o r the h i g h l y a b s o r b i n g o r g a n o p h o s phorus pesticide than for the organochlorine

compounds.

I n c o n c l u s i o n , three factors a r e v e r y i m p o r t a n t t o m i c r o t e c h n i q u e s in infrared spectrophotometry.

F i r s t , i n o r d e r to g a i n s e n s i t i v i t y , a n effi-

c i e n t transfer o f t h e s a m p l e t o t h e u s a b l e e n e r g y b e a m o f t h e s p e c t r o p h o t o m e t e r m u s t b e a c h i e v e d . M i c r o t e c h n i q u e s s t r i c t l y l i m i t the a m o u n t of a l l o w a b l e c o n t a m i n a t i o n f r o m s u c h sources as t h e s a m p l e , solvents, sorbents, reagents, a t m o s p h e r e , h a n d l i n g , a n d t h e l i k e . S e c o n d l y , start the i s o l a t i o n p r o c e d u r e w i t h sufficient s a m p l e so t h a t there w i l l b e e n o u g h o f the finally i s o l a t e d p e s t i c i d e to a l l o w a s p e c t r u m . R e m e m b e r that e a c h h a n d l i n g step i n t h e i s o l a t i o n p r o c e d u r e s a n d t h e m i c r o i n f r a r e d techniques results i n some loss o f t h e sought p e s t i c i d e . T h i r d a n d last, i n f r a r e d s p e c t r o p h o t o m e t r y

is o n l y o n e t o o l u s e d

b y t h e p e s t i c i d e r e s i d u e analyst, a n d h e cannot b e e x p e c t e d t o b e c o m e 9.8 fJLQ TECHNICAL DDT 0.0 .10 .20 .30

MICRO MULTIPLE INTERNAL REFLECTANCE KRS-5 12.7 X 5 X I mm PLATE

.40 .50

.4050-

1.5mm POTASSIUM BROMIDE PELLET

.701.01.5b I 4000 3000

Figure 8.

2000

1500

1200 CM-I

1000 900

800

Infrared spectra of technical DDT by multiple reflectance and micro potassium bromide pellet

700

internal


92

PESTICIDES IDENTIFICATION

24.6 fig ENDRIN 0.0 .10 .20 .30 .40 .50

WITH TRACE OF MINERAL OIL MICRO MULTIPLE INTERNAL REFLECTANCE K R S - 5 1 2 . 7 X 5 X 1 mm PLATE

4000 3000

Figure 9.

Infrared spectra of endrin by multiple internal reflectance and micro potassium bromide pellet

as s k i l l e d as t h e p r o f e s s i o n a l spectroscopist.

B u t h e c a n achieve

suc-

c e s s f u l results i n t h e 5 - 1 0 jig r a n g e i f h e uses t h e s i m p l e s t t e c h n i q u e s c o m m e n s u r a t e w i t h t h e objectives o f t h e p r o b l e m .

Literature Cited (1) (2) (3) (4) (5) (6)

C h e n , J. Y. T . , Dority, R .W.,"Contamination C o n t r o l i n Infrared M i c r o analysis," Annual Meeting of the Association of Official Analytical Chemists, 83rd, Washington, D. C., October 14, 1969. M a s o n , W. B . , "Infrared Microspectrophotometry," Microchem. J. (Symp. Ser.) (1961) 1 , 2 9 3 - 3 1 0 . Traber, W. F., "'Thinking Small' w i t h Microanalysis Techniques," Ind. Res. (October 1966), pp. 80-85. F a h r , E., Rohlfing,W.,"Infrared Spectroscopy i n Biochemistry a n d Clinical Chemistry," Z. Anal. Chem. (1968) 243, 43-48. Krohner, P., Kemmner, G . , "Methods a n d Detection L i m i t s in the IR-Spectroscopic Investigation of M i c r o Quantities," Z . Anal. Chem. (1968) 243, 80-92. E d w a r d s , R . A., Fagerson, I. S., "Collection of Gas Chromatographic Fractions for Infrared Analysis," Anal. Chem. (1965) 3 7 , 1630.


6.

BLINN

Infrared

Microtechniques

93

(7) Kabot, F . , "How to Collect Fractions from the Model 800 Series Gas Chromatograph," Perkin-Elmer Corp., G . C. Newsletter (1967) 3 (1), 1-4. (8) Oadland, R. K., Glock, E., Bodenhamer, N . L., " A Simple Technique for Trapping Gas Chromatographic Samples from a Capillary Column for Mass Spectrometry or Re-Chromatography on Another Column," J. Chromatog. Sci. (1969) 7, 187-189. (9) Crosby, N . T . , Laws, E . Q., "The Use of Infrared Spectroscopy in the Analysis of Pesticide Residues," Analyst (1964) 89, 319-327. (10) Price, G . C., Sunas, E . C., Williams, J. F . , "Micro Cell for Obtaining Normal Contrast Infrared Solution Spectra at the Five Microgram Level," Anal. Chem. (1967) 39, 138-140. (11) Mills, A . L . , "Infrared Identification of Microgram Quantities of Heroin Hydrochloride," Anal. Chem. (1963) 35, 416. (12) Hannah, R. W . , Dwyer, J. L . , "Analysis of Suspended Particulates with Membrane Filters and Attenuated Total Reflection," Anal. Chem. (1964) 36, 2341-2344. (13) Sloane, H . J . , "Infrared Differential Technique Employing Membrane Filters," A n a l . Chem. (1963) 35, 1556-1558. (14) Sloane, H . J., Johns, T., Cadman, W . J . , Ulrich, W . R., "Infrared Examination of Microsamples. Application of a Specular Reflectance System," Appl. Spectry. (1965) 19, 130-135. (15) Blinn, R. C., "Infrared Techniques Useful in Residue Chemistry," J. Assoc. Offic. Agr. Chemists (1965) 48, 1009-1017. (16) Chen, J. Y. T., "Micro-KBr Technique of Infrared Spectrometry," J. Assoc. Offic. Agr. Chemists (1965) 48, 380-384. (17) Chen, J. Y. T . , Gould, J. H . , "Micro-AgCl Technique of Infrared Spectrometry," Appl. Spectry. (1968) 22, 5-7. (18) Curry, A. S., Read, J. F . , Brown, C., Jenkins, R. R., "Micro Infrared Spectroscopy of Gas Chromagraphic Fractions," J. Chromatog. (1968) 38, 200-208. (19) Garner, H . R., Packer, H . , "New Techniques for the Preparation of KBr Pellets from Micro-Samples," Appl. Spectry. (1968) 22, 122-123. (20) Hayden, A . L., Brannen, W . L . , Craig, N . R., " A Micro-Extraction Technique with Compounds Isolated from Thin-Layer Chromatograms," J. Pharm. Sci. (1968) 57, 858-860. (21) McCaulley, D . F . , " A n Approach to Separation, Identification, and Determination of at Least Ten Organophosphate Pesticide Residues in Raw Agricultural Products," J. Assoc. Offic. Agr. Chemists (1965) 48, 659-665. (22) Mount, D . I., Boyle, H . W . , "Parathion—Use of Blood Concentrations to Diagnose Mortality of Fish," Environ. Sci. Technol. (1969) 3, 11831185. (23) Payne, W . R., Jr., Cox, W . S., "Micro-Infrared Analysis of Dieldrin, E n drin, and other Chlorinated Pesticide Residue in Complex Substrates," J. Assoc. Offic. Agr. Chemists (1966) 49, 989-996. (24) Robbins, J. D . , Bakke, J. E., Fjelstul, C. E., "Practical Micro-KBr Disk Techniques for Infrared Spectrometry," 157th Meeting, ACS, Minneapolis, Minn., April 14, 1969. (25) Sterling, K. J . , "Preparation of Potassium Bromide Disks for Infrared Microanalysis by Using a Half-Inch Die," Anal. Chem. (1966) 38, 1804. (26) Hermann, T . S., "Identification of Trace Amounts of Organophosphorous Pesticides by Frustrated Multiple Internal Reflectance Spectroscopy," Appl. Spectry. (1965) 19, 10-14.


94 (27) (28) (29) (30)

PESTICIDES

IDENTIFICATION

W i l k s Scientific C o r p . (South Norwalk, C o n n . ) , "Internal Reflectance Spectrometry," V o l . 1, 40 pp., 1965. L o w , M. J . D . , " A p p l i c a t i o n of Multiple-Scan Interferometry to the Measurement of Infrared Spectra," Appl. Spectry. (1968) 22, 463-471. de K l e i n , W . J., "Infrared-Spectroscopic Identification of Compounds Separated by Gas Chromatography, using a Potassium Bromide M i c r o -pellet Technique," Z . Anal. Chem. (1969) 246, 294-297. de K l e i n , W . J., "Infrared Spectra of Compounds Separated by T h i n L a y e r Chromatography using a Potassium Bromide Micro-pellet T e c h nique," Anal. Chem. (1969) 41, 667-668.

RECEIVED

June 29,

1970.


Pesticides Identification at the Residue Level

Recommend Documents