Chemical Derivatization Techniques in Pesticide Analysis - ACS

12 Chemical Derivatization Techniques in Pesticide Analysis Advances and Applications W. P. COCHRANE Downloaded by RUTGERS UNIV on February 25, 2017 | http://pubs.acs.org Publication Date: October 30, 1980 | doi: 10.1021/bk-1980-0136.ch012

Laboratory Services Division, Agriculture Canada, Ottawa, Ontario, Canada K1A 0C5 In common with other analysts, the pesticide analyst has two major problems facing him during the analysis of a sample, namely, 1. The IDENTITY of the pesticide present 2. The AMOUNT of pesticide or its residue in the sample. In both these areas, chemical derivatisation has traditionally played a role and with the advent of gas chromatography an even more important role. The reasons for preparing a derivative suitable for GC analysis are many and varied and have been discussed thoroughly in a number of books and reviews (1-6). For convenience they are summarised in Table I . As can be seen, two different types of chemical derivatisation techniques are mentioned under Item 4 of Criteria, There is the chemical derivatisation of a pesticide as a pre-requisite of the method of analysis, e.g. esterification of the chlorophenoxy acids, as well as derivatisation as a method for confirmation of identity. The former must meet a l l the requirements associated with a practical, viable analytical procedure while for the latter the emphasis is on speed, ease of operation and reproducibility. Apart from the above reasons d e r i v a t i s a t i o n i s a l s o r e q u i r e d i n the analyses of many o f the newer p e s t i c i d e s which have one o r more f u n c t i o n a l groups which need p r o t e c t i o n i n order to f a c i l i t ate GC work. Also the i n c r e a s e d use o f high pressure l i q u i d chromatography (HPLC) n e c e s s i t a t e s p r e - or post-column d e r i v a t i s a t i o n since the choice o f HPLC detectors i s l i m i t e d compared with GC. The increased use o f d e r i v a t i s a t i o n r e a c t i o n s i s evident from the gradual increase i n the number o f p u b l i c a t i o n s d e a l i n g w i t h the subject. Figure 1 shows the y e a r l y v a r i a t i o n i n numbers o f publ i c a t i o n s d e a l i n g with d e r i v a t i s a t i o n i n p e s t i c i d e a n a l y s i s over the p e r i o d 1963 to 1978. While the i n t e r e s t i n d e r i v a t i s a t i o n techniques i n organophosphorus i n s e c t i c i d e a n a l y s i s has remained f a i r l y constant and a low l e v e l of a c t i v i t y , the OC i n s e c t i c i d e s underwent an i n c r e a s e d p e r i o d o f a t t e n t i o n from 1968-1972 which has s t a b l i z e d over the l a s t few years. I t i s i n the i n s e c t i c i d a l carbamate and h e r b i c i d e areas that an o v e r a l l steady increase i n the use o f d e r i v a t i s a t i o n r e a c t i o n s f o r q u a n t i t a t i v e and con0-8412-05 81 -7/80/47-136-231 $05.00/0 © 1980 American Chemical Society Harvey et al.; Pesticide Analytical Methodology ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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METHODOLOGY

Advantages Gained by Chemical D e r i v a t i s a t i o n and Some Reaction C r i t e r i a

Advantages: 1. 2. 3. 4. 5. 6. 7.

Β.

ANALYTICAL

Improved e x t r a c t a b i l i t y during the clean-up procedures. Change i n v o l a t i l i t y c h a r a c t e r i s t i c s . Increase i n thermal s t a b i l i t y of compounds. Improve chromatographic or s e p a r a t i o n behaviour. Increase i n s e n s i t i v i t y of d e t e c t i o n . Impart s e l e c t i v i t y . A i d i n c o n f i r m a t i o n of i d e n t i t y .

Criteria : 1. 2. 3. 4. 5. 6.

D e r i v a t i v e must be formed r a p i d l y . P r e p a r a t i o n should r e q u i r e minimum of manipulation. D e r i v a t i v e should be r e l a t i v e l y s t a b l e . Reaction must be q u a n t i t a t i v e and/or r e p r o d u c i b l e . Short (acceptable) GC r e t e n t i o n time f r e e from back ground or reagent i n t e r f e r e n c e s . Good s e n s i t i v i t y .

firmatory a n a l y s i s has o c c u r r e d . T h i s has been augmented i n r e cent years by i n t e r e s t i n the a n a l y s i s of f u n g i c i d e s , noteably e t h y l e n e t h i o u r e a and the e t h y l e n e b i s d i t h i o c a r b a m a t e s , as evidenced by the trend o f the o v e r a l l t o t a l curve. Since a number of reviews (4,5,7,8) cover the l i t e r a t u r e p r i o r to 1975, t h i s r e view w i l l h i g h l i g h t the more recent advances i n d e r i v a t i s a t i o n procedures or techniques as i l l u s t r a t e d w i t h a p p l i c a t i o n s from the 4 main groups o f p e s t i c i d e s , namely, h e r b i c i d e s , OCs, OPs and f u n gicides . Herbicides E s t e r i f i c a t i o n of the chlorophenoxy a c i d s to t h e i r r e s p e c t i v e methyl e s t e r s by reagents such as diazomethane, BF^/methanol or a c i d methanol mixtures have been f o r l o n g the standard procedure f o r the analyses of these compounds. Recently a number of workers have been i n v e s t i g a t i n g the use of new and more EC s e n s i t i v e e s t e r i f i c a t i o n reagents f o r general use. This has come about since the EC d e t e c t i o n of the methyl e s t e r s of MCPA or MCPB, which contain only one c h l o r i n e each, are very poor. A l s o the methyl e s t e r of MCPA has a very short r e t e n t i o n c l o s e to the s o l v e n t f r o n t when analysed i n c o n j u n c t i o n with 2,4-D, 2,4,5-T fenoprop, e t c . The a c i d s and the v a r i o u s e s t e r s i n v e s t i g a t e d are shown i n Table I I . In t r a n s e s t e r i f i c a t i o n work by Y i p (9) the 2 - c h l o r o e t h y l e s t e r was d i s c a r d e d s i n c e the background p a t t e r n of the d e r i v a t i v e gave too many peaks. However, Woodham et^ al. (10) used f

Harvey et al.; Pesticide Analytical Methodology ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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Table I I :


Compound

2,4-D 2,4-DB 2,4,-DP 2,4,5-T 2,4,5-TB 2,4,5-TP MCPA MCPB MCPP 2,3,6-TBA Picloram Dicamba Fenac References

Chemical

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Techniques

D e r i v a t i v e s o f the Chiorophenoxyalky1 A c i d s and Some Other A c i d i c H e r b i c i d e s

2,2,2-trichloro e t h y l

2-chloro ethyl

2,2,2-trifluoro ethyl

X X X X

X

X

X

X

X X X

ΈΈΈ-

X X X X X X X X X X X

X X 9-14

9-14

14

13, 15

1/Pentafluorobenzyl a BCl^/2-chloroethanol reagent to determine residues o f 2,4-D i n s o i l , sediment and water samples while Gutenman and L i s k (11) used BF^/2-chloroethanol f o r e s t e r i f i c a t i o n o f MCPA i n s o i l e x t r a c t s . Woodham eit a l (10) found that although the 2 - c h l o r o e t h y l e s t e r had the d e s i r e d s e n s i t i v i t y , the r e a c t i o n produced l e s s i n t e r f e r e n c e and gave a l o n g e r r e t e n t i o n time, i t was s t i l l s u b j e c t to i n t e r ferences from c e r t a i n s o i l types ( F i g . 2A). A f t e r d i e t h y l e t h e r e x t r a c t i o n a clean-up step was added employing an a l k a l i n e wash t o remove i n t e r f e r i n g substances. The lower l i m i t o f s e n s i t i v i t y f o r t h i s method i n s o i l s i s approximately 0.01 ppm f o r 2,4-D ( F i g . 2B). More r e c e n t l y (12, 13) B C l ^ : 2-chloroethanol was used t o determine chlorophenoxy a c i d r e s i d u e s i n n a t u r a l waters. I t was found, as would be expected, that t h i s reagent produced l i t t l e or no product f o r 2,3,6-TBA and dicamba due to the s t e r i c hinderance o f σ-Cl atoms. The p r a c t i c a l l i m i t s o f d e t e c t i o n ranged from a lower l i m i t 0.01 ug/L f o r fenoprop t o a h i g h o f 2-5 ug/L f o r MCPB. Y i p (9) showed that t r i f l u o r o e t h a n o l i n c r e a s e d the ECD response o f 2,4,5-T and fenoprop but n o t MCPA o r 2,4-D. More r e c e n t l y , Mierzwa and Witek (14) found that a f t e r e s t e r i f i c a t i o n of 2,4-D and MCPA with 20% 2 , 2 , 2 - t r i c h l o r o e t h a n o l i n t r i f l u o r o a c e t i c an hydride i n the presence o f I^SO, a lower l i m i t o f 0.096 ppb 2,4-D and 0.06 ppb MCPA i n 1 l i t r e samples o f water was o b t a i n e d . As i n most of the d e r i v a t i s a t i o n r e a c t i o n s , removal o f excess


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1974

YEAR

Figure 1. Variation in number of publications dealing with chemical derivatization techniques for pesticides from 1962-1978 ((+) total (including fungicides); (A) herbicides; (\J) OCs (including Mirex, PCB's, etc.); (O) DP's)

RE T E N T I O N

T I M E (mm )

Journal of Agricultural and Food Chemistry

Figure 2. Chromatographic tracing of (Α) α 2,4-Ό fortified soil sample (2-chloroethyl ester) without alkali treatment and (B) samples with alkali pre-wash: (a) a blank soil sample; (b) soil sample fortified with 0.013 ppm 2,4-Ό acid; and (c) soil sample fortified with 0.667 ppm 2,4-Ό acid (\0)



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reagents i s mandatory p r i o r to ECD a n a l y s i s . I t was found that the s e n s i t i v i t y o f the e s t e r s i n c r e a s e d from the 2 - c h l o r o e t h y l to t r i f l u o r o e t h y l to the t r i c h l o r o e t h y l d e r i v a t i v e . With MCPA the t r i c h l o r o e t h y l e s t e r was 100 times more s e n s i t i v e than the 2 - c h l o r o e t h y l e s t e r , while with 2,4-D the i n c r e a s e was o n l y a f a c t o r o f 10. S i m i l a r l y , Chau and co-workers (15) have shown that the PFB e s t e r s are more EC s e n s i t i v e than the corresponding 2 - c h l o r o e t h y l e s t e r s . For example, the PFB e s t e r o f MCPA i s 2.5 times g r e a t e r than that obtained with the 2 - c h l o r o e t h y l e s t e r and 1000 times greater than MCPA methyl e s t e r . Normally, the e s t e r i f i c a t i o n r e a c t i o n with p e n t a f l u o r o b e n z y l bromide (PFBBr) i s much simpler t o perform s i n c e i t takes p l a c e i n acetone which i s e a s i l y evaporated a f t e r the r e a c t i o n . However, the PFBs tend t o have longer r e t e n t i o n times and t h i s reagent r e a c t s under b a s i c c o n d i t i o n s with both phenols and c a r b o x y l i c a c i d s . I t has been reported that an 0V-101/0V-210 column separated 9 PFB e s t e r s , o n l y the MCPA and 2,4-DP over-lapped,while a 5% DC-200 or 3% DC-11 column gave comp l e t e s e p a r a t i o n o f 10 PFB e s t e r s . O v e r a l l , i t would appear that the s e n s i t i v i t i e s o f the PFB and t r i c h l o r o e t h y l e s t e r s are e q u i v a l e n t , with the f i n a l choice depending upon background i n t e r ference from the sample and s u i t a b l e column s e p a r a t i o n c h a r a c t e r i stics . Organochlorine I n s e c t i c i d e s and Related Compounds Since w e l l - e s t a b l i s h e d chemical d e r i v a t i s a t i o n techniques a l ready e x i s t f o r the m a j o r i t y o f the organochlorine i n s e c t i c i d e s (5, T) i t i s not s u r p r i s i n g t h a t recent a c t i v i t y i n t h i s area has centred round compounds such as Kepone, Mirex, HCB, the PCBs, e t c . which c o - i n t e r f e r e i n both the i d e n t i f i c a t i o n and q u a n t i t a t i o n o f pesticide residues. Kepone (chlordecone) can be converted back t o Mirex by p e r c h l o r i n a t i o n ( F i g . 3) u s i n g a 4:1 r a t i o o f phosphorus p e n t a c h l o r i d e to aluminium c h l o r i d e i n carbon t e t r a c h l o r i d e a t 145 C f o r 3 h r s . i n a c l o s e d tube (16). To e l i m i n a t e any Mirex that may have been o r i g i n a l l y p r e s e n t , the s e p a r a t i o n o f Kepone was performed on a m i c r o - F l o r i s i l column p r i o r t o d e r i v a t i s a t i o n . S i m i l a r l y , a microF l o r i s i l column was u t i l i z e d a f t e r r e a c t i o n to remove e a r l y e l u t i n g peaks from the gas chromatograms, and e s p e c i a l l y when the t o t a l amount of Kepone present was l e s s than 25 ng. T h i s d e r i v a t i s a t i o n technique was found s e n s i t i v e t o ppb l e v e l s o f Kepone i n environmental and b i o l o g i c a l samples. For example, Figure 4 shows an o y s t e r e x t r a c t before and a f t e r d e r i v a t i s a t i o n (16) . The Kepone l e v e l averaged 0.07 ppm (upper chromatogram) i n t h i s p a r t i c u l a r sample which was run on a 4% SE-30/6% 0V-210 column. The r e a c t i o n was found t o be q u a n t i t a t i v e and 8 o f the more common 0C p e s t i c i d e s were found to disappear on r e a c t i o n or give der i v a t i v e s with GC r e t e n t i o n d i f f e r e n t than Mirex. Mirex has been a s s o c i a t e d with the c o n t r o l o f the imported f i r e ant i n the south-eastern USA, where i t was subsequently


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Journal of Agricultural and Food Chemistry

Figure 4.

Electron-capture gas chromatograms of an oyster extract before and after derivatization ((PCl /AlCl ), 0.66 mg injected (16)) 5

3



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found i n non-target organisms i n c l u d i n g man (17). S i m i l a r l y , Mirex and/or photomirex (8-monohydromirex) have been i d e n t i f i e d as major contaminants i n f i s h from Lake Ontario (18), cormorant eggs (19) and Canadian Human milk (20). Mirex and photomirex are p a r t i c u l a r l y d i f f i c u l t to determine i n the presence o f l a r g e q u a n t i t i t e s o f PCB without p r i o r s e p a r a t i o n since t h e i r r e t e n t i o n times are s i m i l a r to that of major heptachlorobiphenyls on most GC columns. Lane e t a l (21) s t u d i e d the p h o t o l y t i c and o * - i r r a d i a t i o n of duck egg t i s s u e s c o n t a i n i n g n a t u r a l l y o c c u r i n g Mirex. Although 7 d e r i v a t i v e s were obtained on p h o t o l y s i s and 8 from ϊ - i r r a d i a t i o n the major product from both r e a c t i o n s was i d e n t i f i e d as photomirex ( F i g . 3 ) . In t h i s instance UV p h o t o l y s i s was c a r r i e d out at 254 nm on the a c t u a l egg homogenate and caused a 36% de crease a f t e r 48 h r s . whereas f-irradiation r e s u l t e d i n a 64% l o s s of Mirex. I r r a d i a t i o n , t h e r e f o r e , could be used as a c o n f i r m a t o r y t e s t so long as the r e s u l t i n g chromatograms do not become too complex f o r i n t e r p r e t a t i o n . As p o s t u l a t e d i n a previous r e view (5), Mirex can a l s o be confirmed by r e d u c t i o n o f the gem-dichloro methylene group with C r C l - (22) . In p r a c t i s e , three major peaks are obtained, the r a t i o s of which were found to vary w i t h the CrCl^/acetone r a t i o and temperature o f r e a c t i o n . None of these products appear to be photomirex. Reaction was c a r r i e d out overnight at 55-60°C or room temperature f o r 32 h r s . or l o n g e r . While photomirex could be completely reacted to give a major p r o duct with a d i f f e r e n t r e t e n t i o n time than Mirex, Kepone only r e acted p a r t i a l l y . Lewis et_ al^ (23) employed a d i f f e r e n t approach i n that M i r e x - c o n t a i n i n g human t i s s u e e x t r a c t s were subjected to a diethylamine a s s i s t e d p h o t o l y s i s a t wavelength> 280 nm to s e l e c t i v e l y e l i m i n a t e PCB i n t e r f e r e n c e s i n PCB/Mirex mixtures. A 100 min. p h o t o l y s i s of an A r o c h l o r 1260/Mirex mixture i n 10 ml hexane c o n t a i n i n g diethylamine r e s u l t e d i n complete e l i m i n a t i o n of the i n t e r f e r i n g h e p t a c h l o r o b i p h e n y l component and only a 0-5% l o s s of Mirex. However, some i n t e r f e r e n c e s appear t o remain i n the photomirex region of the chromatograms. A l s o i t has been pointed out by Mes and co-workers (20) t h a t although Mirex can be i d e n t i f i e d by GC, u s i n g diethylamine a s s i s t e d p h o t o l y s i s to l i m i t PCB i n t e r f e r e n c e , q u a n t i t a t i o n at low l e v e l s (0.01-0.1 ppm) r e main questionable s i n c e the workers found poor agreement between t h e i r GC and mass spectrometry data. Other i n d i r e c t approaches to the PCB/Mirex problem have been the p e r c h l o r i n a t i o n of the PCBs to decachlorobiphenyl with Sb CI,.-containing reagents (24) and n i t r a t i o n of the PCBs w i t h a 1:1 mixture o f cone. H^SO^/ fuming HNO^ (25) . Shown i n Figure 5 i s a chromatogram o f an ex t r a c t of a h e r r i n g g u l l egg from Lake Ontario a f t e r clean-up on a 1% d e a c t i v a t e d F l o r o s i l column; note that Mirex i s only partially r e s o l v e d from a h e p t a c h l o r o b i p h e n y l peak. The lower t r a c i n g i s the same e x t r a c t a f t e r a 30 min. n i t r a t i o n at 70 C. Mirex and photomirex are the only major c o n s t i t u e n t s to s u r v i v e n i t r a t i o n . HCB, ^-nonachlor and small amounts o f mono- and d i hydromirex compounds c o n s t i t u t e most of the minor peaks remaining


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RETENTION TIME (MINS.)

ηίΖ ·Ί , ° 88 from Lake Ontario after cleanup on a t lor ml column; (B) same extract after nitration (represents about twice as much sample as A) Conditions: & χ 4 mm glass column with 1% SP-2100 on Supelcoport 100/120 mesh; flow rate 40 mL/min Argon/methane; oven 190°C; Ni detector. (

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f

a

h e r r i n g

g u l 1

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i n the chromatogram. Also c i s - and trans-chlordane can be q u a n t i t a t e d a f t e r n i t r a t i o n (25) . HCB i s recovered i n low y i e l d s (*Ί0%) p r i m a r i l y due to v o l a t i l i s a t i o n r a t h e r than r e a c t i o n with the a c i d mixture. S i m i l a r l y , l i n d a n e r e c o v e r i e s are e r r a t i c . A f t e r a c o l l a b o r a t i v e study i n v o l v i n g sediment, carp, e e l and g u l l egg samples, t h i s n i t r a t i o n technique was found r e l i a b l e f o r Mirex and photomirex a t l e v e l s ^ 1 0 ppb i n the presence o f 1,000-fold greater l e v e l s of PCB (26). Hexachlorobenzene (HCB) has a t t r a c t e d i n c r e a s e d a t t e n t i o n due to i t s common occurence i n human b l o o d , milk and adipose t i s s u e , w i l d l i f e samples, wheat and l a k e water. Various approaches ( F i g . 6) to the chemical c o n f i r m a t i o n o f HCB have i n cluded the p r e p a r a t i o n of a pentachlorophenyl p r o p y l e t h e r when t r e a t e d with KOH i n 1-propanol (27) . The d e r i v a t i v e had a l o n g e r GC r e t e n t i o n time than HCB. Another two step approach by H o l d r i n e t (28) i n v o l v e d r e a c t i o n of HCB w i t h 0.2N KOH i n e t h y l e n e g l y c o l under r e f l u x to produce pentachlorophenol which i s then e s t e r i f i e d with diazomethane to give the PCP methyl ether d e r i v a t i v e . I t was o r i g i n a l l y demonstrated by Baker (29) that HCB i s converted to monoethoxypentachlorobenzene when r e f l u x e d w i t h sodium e t h o x i d e . More r e c e n t l y , i t has been shown (30) that ex tended treatment ( i . e . g r e a t e r than 6 hrs.) w i t h NaOEt w i l l f u r t h e r convert the monoethoxy d e r i v a t i v e to diethoxy compounds. Since the p u r i f i e d diethoxy d e r i v a t i v e obtained from a ρrep-scale r e a c t i o n gave 2 major chromatographic peaks, i t i s suggested a mixture of isomers i s produced. S i m i l a r l y , C r i s t and co-workers (31)confirmed HCB i n adipose t i s s u e by the p r e p a r a t i o n of s e v e r a l d e r i v a t i v e s from v a r i o u s a l c o h o l s . I t was found that r e a c t i o n with a K0H/2p r o p a n o l / p y r i d i n e mixture f o r 10 min. at 100 C produced mono-isopropoxy pentachlorobenzene while a f t e r 30 min. b i s - i s o p r o p o x y t e t r a chlorobenzene was formed. The d i - s u b s t i t u t e d d e r i v a t i v e was l e s s subject to f u r t h e r s u b s t i t u t i o n than the mono-derivative and r e p r o d u c i b i l i t y of the r e a c t i o n was more e a s i l y c o n t r o l l e d such that a lower l e v e l of 5 ppb HCB could be confirmed i n f a t t y samples. The f l e x i b i l i t y of t h i s r e a c t i o n was shown by the use of other a l c o h o l s (EtOH, 1-PrOH, 1-BuOH) and the c o n f i r m a t i o n of 0.3 ppm HCB i n r a t adipose t i s s u e c o n t a i n i n g 160 ppm A r o c h l o r 1016. Since d e r i v a t i s a t i o n to the di-isopropoxy d e r i v a t i v e was not f e a s i b l e due to i n t e r f e r e n c e from an A r c c h l o r peak, con f i r m a t i o n based on the mono-substituted d e r i v a t i v e was p o s s i b l e . Organophosphorus and Carbamate I n s e c t i c i d e s Most OP i n s e c t i c i d e s may be determined d i r e c t l y by GC u s i n g the p h o s p h o r u s - s e l e c t i v e flame photometric detector (FPD) which helps to minimize clean-up. However, i t must be emphasised that the FPD i s o n l y a " s e l e c t i v e d e t e c t o r f o r phosphorus (at 540 nm) or sulphur (at 394 nm) and not an "element s p e c i f i c d e t e c t o r " . This i s i l l u s t r a t e d i n F i g . 7 which shows the 4 o x i d a t i o n products 1 1


PESTICIDE A N A L Y T I C A L M E T H O D O L O G Y


240

R= Ethyl, 1 - P r o p y l . 2 - P r o p y l or tert.-Butyl

Figure 6.

Reactions used for the confirmation and/or quantitation of hexachlorobenzene (HCB)

Figure 7. Characteristics of methidathion and its sodium hypochlorite oxidation products on 1% DEGs at 200°C with flame photometric detection (P-mode)



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obtained from the macro-scale r e a c t i o n of n e u t r a l i s e d sodium hypo c h l o r i t e w i t h methidathion (Supracide ) (32). G e n e r a l l y , o x i d a t i o n of the P-S groups i n such i n s e c t i c i d e s as malathion, d i a zinon, p a r a t h i o n , f e n i t r o t h i o n , e t h i o n , e t c . y i e l d t h e i r r e s p e c t i v e oxons. A s i n g l e oxon d e r i v a t i v e peak i s the r e s u l t of h y p o c h l o r i t e o x i d a t i o n of methidathion a t the ng l e v e l but 4 p r o ducts were observed at the macro-scale. Although t h i s chromato gram was obtained u s i n g the Ρ-mode o f the FPD only one peak, namely methidaoxon,peak 2, contains phosphorus. The other 3 p r o ducts c o n s i s t of 2 t h i a d i a z o l i n o n e r i n g s l i n k e d by methylene, s u l f i d e or d i s u l f i d e groupings r e s p e c t i v e l y . A l s o a 10:1 r e s ponse r a t i o was obtained when e q u i v a l e n t amounts of the sulphide and d i s u l p h i d e were recorded on the S- and Ρ-channels,respectively, of the FPD. O x i d a t i o n of phorate w i t h NaOCl or ^ °2 phorate oxon s u l f o x i d e and not phorate oxon. Recently the q u a n t i t a t i v e o x i d a t i o n of oxydemeton methyl with KMhO^ to i t s corresponding s u l f o n e has been used f o r i t s determination i n a v a r i e t y o f p l a n t and animal t i s s u e s down to a l i m i t of 0.01 ppm (33). An a l t e r n a t i v e approach to the a n a l y s i s of oxydemeton metyl, which contains a d i a l k y l s u b s t i t u t e d s u l f o x i d e moiety, i s v i a t r i f l u o r a c e t y l a t i o n ( F i g . 8) (34) . Reaction of oxydemeton methyl with t r i f l u o r a c e t i c anhydride at 100° f o r 15 min. r e s u l t s i n a mono-trifluoroacetoxy d e r i v a t i v e which thermally degrades oncolumn to give 2 peaks, the c i s - and trans-isomers of dehydrooxydemeton-methyl. Other compounds w i t h a s u l f o x i d e moiety that can give mono-or di-TFA d e r i v a t i v e s s u i t a b l e f o r confirmatory purposes are d a s a n i t , mesurol s u l f o x i d e , nemocur s u l f o x i d e , a l d i c a r b s u l f o x i d e , counter s u l f o x i d e and oxycarboxin. Due to the poor chromatographic c h a r a c t e r i s t i c s o f s u l f o x i d e s , a n a l y s i s was p r e v i o u s l y performed f o l l o w i n g o x i d a t i o n to t h e i r r e s p e c t i v e s u l fones. Now with mesurol, a l l 3 o x i d a t i o n products plus phenols can be analysed simultaneously. Of course, t r i f l u o r a c e t y l a t i o n and methylation of NH-containing OPs and carbamates are p o s s i b l e but these procedures have been e x t e n s i v e l y covered i n previous reviews (5, T) . 2

r

e

s

u

l

t

s

i

n

However, two other areas are o f i n t e r e s t ( F i g . 9) namely, a) d e r i v a t i s a t i o n of phenols or amines to phosphorus-containing compounds and b) on-column methylation. In the former, phosphory l a t i o n of 7 a l c o h o l s and 12 phenols with d i e t h y l chlorophosphate i n the presence o f t r i e t h y l a m i n e at 60-70 C i n benzene has been reported (35). A l i p h a t i c a l c o h o l s , such as methanol, butanol or isoamyl a l c o h o l r e a c t i n s t a n t a n e o u s l y while phenol and eresoIs r e q u i r e ca 1-1 1/2 h r s . and x y l e n o l s 3-4 h r s . f o r best r e s u l t s . The r e s u l t i n g a l k y l or a r y l d i e t h y l phosphates were s e l e c t i v e l y detected at the 10-25 ng l e v e l with the P-mode FPD. Similarly, Jacob et_ a l (36) used dimethyl t h i o p h o s p h i n i c c h l o r i d e i n the presence of excess t r i e t h y l a m i n e to convert primary a l i p h a t i c , aromatic and h e t e r o c y c l i c amines to the corresponding N-dimethylt h i o p h o s p h i n i c amides. The excess reagent was e a s i l y removed by t r e a t i n g the r e a c t i o n mixture w i t h methanol-sodium hydrogen c a r -


242

PESTICIDE

CH O 3

ANALYTICAL METHODOLOGY

Ο

s

A p ,° O.ÏCF3 CH (/ S.CH .CH.S.CH .CH

CH

CH 0" S.CH .CH .S.CH .CH 3

N

2

2

2

V

3

3

2

3

ΔΙ

OXYDEMETON METHYL CH3O

CH 0 2


2

JQ N

S.CH=CH.S.CH CH 2

3

0.5 f sd

u Journal of Agricultural and Food Chemistry

Figure 8. Reaction of oxydemeton methyl with trifluoroacetic anhydride (TFAA) and the GC characteristics of the two products (cis- and trans-dehydro-oxydemeton methyl) on 3% OV-17 at 190°C (34)

PHOSPHORYLATION

α)

Ο R-OH + CI-P-IOCjH,). 1

5

1

3

/

b§-70*C

Ο R-0-P(OC,H_L + HCI 2 52

R = alkyl or aryl.

b)

S R'-NH + CI-P(OCH ) 2

2

3

J

1

2

—

3

-,

-20fo + 20°C

S R-NH-P-(OCH ) + HCI 3

J

2

l

R = primary aliphatic, aromatic or heterocyclic amines

Figure 9.

Phosphorylation reactions of alcohols and amines


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bonate and the d e r i v a t i v e s were w e l l separated on a SE-30 o r OV-17 c a p i l l a r y column. Although a lower l i m i t of d e t e c t i o n of 0.5 pg f o r N - d i m e t h y l t h i o p h o s p h i n y l a n i l i n e was obtained with a rubidium sulphate AFID, an FPD d e t e c t o r c o u l d a l s o be used. T h i s d e r i v a t i s a t i o n technique was a p p l i e d to twenty aromatic o r h e t e r o c y c l i c amines as w e l l as 6 amino a c i d e s t e r s . However, i t should be p o i n t e d out that dimethylthiophosphinic c h l o r i d e r e a c t s and i s as s e n s i t i v e , i f not more so, to moisture problems than TFAA. Since the r e s u l t i n g product c o n t a i n s Ρ i t too gives a FPD/AFID response. The general subject of on-column p y r o l y t i c methylation has been r e c e n t l y reviewed (37) and, i n p a r t i c u l a r , t h i s technique has been the subject of a number of papers d e a l i n g w i t h the i d e n t i f i c a t i o n and q u a n t i t a t i o n of many organophosphorus p e s t i c i d e s and r e l a t e d d i a l k y l phosphorothioates (38-AO) as w e l l as carbamate p e s t i c i d e s (41-43^. GLC on-column t r a n s e s t e r i f i c a t i o n of OP i n s e c t i c i d e s at 260 C u s i n g trime thylphenylammonium hydroxide i n methanol, r e s u l t s i n s h o r t - c h a i n t r i a l k y l phosphates which can be q u a n t i t a t e d using the P-mode FPD ( F i g . 10). The exact s t r u c t u r e of the t r i a l k y l phosphate formed i s dependent upon the s t r u c t u r e of the o r i g i n a l OP i n s e c t i c i d e . Parathion i s t r a n s e s t e r i f i e d to give diethylmethylthiophosphate (DEMTP) while azinphos methyl y i e l d s t r i m e t h y l dithiophosphate (TMDTP). The minimum detectable l i m i t f o r malathion was 400 pg. The technique was a p p l i e d to the determination of chlorphoxim at l e v e l s o f 10 ppb i n f i s h and 0.1 ppb i n water. I t has been suggested that methanolic TMPAH q u a n t i t a t i o n of d i a l k y l phosphorothioates and phosphorodithioates could provide a convenient and s a f e a l t e r n a t i v e to the use o f diazomethane where methylation i s the f i n a l step i n OP residue a n a l y s i s (40). S i m i l a r l y , on-column methylation has been a p p l i e d to carbamate p e s t i c i d e s c o n t a i n i n g an a c t i v e N-H group. Wien and Tanaka (41) showed t h a t N - a r y l carbamates are methylated on-column with t r i me thy 1 a n i l i n i urn hydroxide-methanol to give the i n t a c t N-methyl and N - a r y l d e r i v a t i v e s . On the other hand N-methyl, 0 - a r y l c a r bamates such as c a r b a r y l or carbofuran y i e l d e d o n l y the methyl ethers o f t h e i r r e s p e c t i v e phenols. T h i s work has now been ex tended to s u l f u r - c o n t a i n i n g carbamates such as methomyl, methiocarb, a l d i c a r b , e t c . (42-43). Here the oxime h y d r o l y s i s products of these carbamates are chromâtοgraphed as the 0-methyl oximes. T h i s r e a c t i o n has been a p p l i e d to the a n a l y s i s o f oxamyl (lower l e v e l 0.5 ug i n 50 g sample) i n s o i l u s i n g the S-mode FPD. However, i t must be p o i n t e d out that to date, the a p p l i c a t i o n of the phosphorylation and on-column methylation techniques have had only l i m i t e d a p p l i c a t i o n . Phenols and

Fungicides

Pentachlorophenol (PCP) i s widely used as a p e s t i c i d e i n a g r i c u l t u r e and as a wood p r e s e r v a t i v e i n i n d u s t r i a l and domestic products. PCP together with other chlorophenols which a r i s e as


PESTICIDE

244

ON-COLUMN

ο)

ANALYTICAL

METHYL AT ION

S NQ Q>-OP-(OCH ) 3

2

2

T M

6

^

c

H

-

N0 Q OCH CH30P(OCH ) >

2

3 +

3

S b) ( C H 0 ) . p . S - Ç H . C 0 E t CH C0 Et 2


CHC0 Et CHC0 Et

2

2

2

TMPT

Methyl Parathion

3

METHODOLOGY

2

2

S (CH 0) -PSCH, 3

2

2

TMDTP

Malathion

c) ^ ^ - N - C - O C H - M e

+

™ £ •

σ

MeO NCOCHMe

Propham

d)

Me NC-C=NOCNHMe SCH-, 2

Oxamyl

Figure 10.

HN

Examples

TMAH

M e N C - C = NOMe 2

SCH

3

Oxamyl methoxime

of on-column methylation reactions of organophosphorus and carbamate pesticides

NH II

S

Figure

11.

Acylation reactions of ethylenethiourea (ETU) with dichloroacetic anhydride (DC A A ) and dichloroacetyl chloride (DCACl)



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metabolites of the OP and carbamate i n s e c t i c i d e s , and phenoxyacids h e r b i c i d e s , have been v a r i o u s l y determined i n t a c t using FID d e t e c t i o n or,more commonly> as t h e i r e t h e r s , e s t e r s or s i l y l der i v a t i v e s (7) i n environmental, a g r i c u l t u r a l and mammalian samples. The more commonly used d e r i v a t i v e i s the methyl e t h e r , using d i a zomethane, a f t e r e x t r a c t i o n and/or an acid/base p a r t i t i o n i n g c l e a n up s t e p . However, r e c e n t l y Edgerton and Moseman (44) compared previous methods f o r the residue a n a l y s i s of PCP i n urine and found an Alumina clean-up column f o l l o w i n g d e r i v a t i s a t i o n was necessary f o r l e v e l s below 30 ppb. A l s o a 1 h r . c l o s e d system a c i d h y d r o l y s i s step using HC1 gave as much as a 1 7 - f o l d higher PCP l e v e l than d i d other methods, i n d i c a t i n g PCP forms conjugates i n u r i n e . The h y d r o l y s i s , methylation, column clean-up p r o cedure allowed l e v e l s of 1 ppb PCP to be detected i n u r i n e . Also a lower l e v e l of 0.01 ppm PCP i n fish,shrimp and o y s t e r s has been obtained a f t e r e t h y l a t i o n with diazoethane followed by a F l o r i s i l clean-up (45) . When the e t h y l a t i o n procedure was used on seawater e x t r a c t s i n t e r f e r e n c e s were observed at concentrations lower than 0.01 ppb u s i n g ECD-GLC. Formation of the amyl d e r i v a t i v e using diazopentane, i n c r e a s e d the GC r e t e n t i o n time s u f f i c i e n t l y to separate PCP from e a r l y e l u t i n g peaks. Concentrations of PCP as low as 2 ppt c o u l d be detected on sea water. I n t e r e s t i n g l y , the same workers a l s o u t i l i z e d HPLC with UV d e t e c t i o n of the f r e e phenol with clean-up to o b t a i n d e t e c t i o n l i m i t s o f 5 ppm i n t i s s u e s and 2 ppb i n sea water. The l i m i t of PCP acetate i s about 1-2.5 pg w i t h a Ni-ECD (46, 47) which gave a 5 ppb l i m i t of d e t e c t a b i l i t y f o r PCP i n adipose t i s s u e (47). In the l a s t few years a number o f r e p o r t s have appeared on an e x t r a c t i v e p e n t a f l u o r o b e n z y l a t i o n procedure f o r phenols and c a r b o x y l i c a c i d s to enhance t h e i r e l e c t r o n - c a p t u r i n g p r o p e r t i e s . Here the e x t r a c t i o n and d e r i v a t i z a t i o n of the phenol or a c i d i s accomplished i n one step by the p a r t i t i o n of the phenol (or acid) anion from the aqueous phase as an i o n - p a i r with a quaternary ammonium i o n i n t o an organic phase which contains the d e r i v a t i z a t i o n reagent (48-51). This technique has been a p p l i e d to the e x t r a c t i v e p e n t a f l u o b e n z y l a t i o n of 2,4-D, and MCPA i n water w i t h a d e t e c t i o n l i m i t of 1-3 ug/L (52). A l s o , the p r i n c i p l e has been a p p l i e d to the e x t r a c t i v e d e r i v a t i z a t i o n of ethylene t h i o u r e a (ETU) i n water. Even though ETU can be GC chromâtοgraphed i n t a c t on Versamid 900 and Carbowax 20 M packed columns (52, 53) or various c a p i l l a r y columns (54), by f a r the most popular approach has been the a l k y l a t i o n of the t h i o c a r b o x y l group and i n some i n s t a n c e s , double d e r i v a t i z a t i o n i n v o l v i n g the NH-group ( 7 ) . D e r i v a t i z a t i o n has r e s u l t e d i n improved GC c h a r a c t e r i s t i c s and i n c r e a s e d s e n s i t i v i t y using EC d e t e c t i o n . However, these procedures i n v a r i a b l y require lengthy r e a c t i o n times at r e f l u x temperatures to achieve acceptable y i e l d s . A l s o , under these c o n d i t i o n s any parent e t h y l enebisdithiocarbamate f u n g i c i d e residue that i s present i s con v e r t e d to ETU to a c e r t a i n extent. Figure 11 shows the r e a c t i o n sequence f o r the room temperature e x t r a c t i v e N - a c y l a t i o n of ETU



246

PESTICIDE

ι 0

1 2

Retention

1 4 Time

ANALYTICAL

METHODOLOGY

1 6 (min.)

Figure 12. (A) Standard of dichloroacetyl-ETU (4.0 ng); (B) blank apple juice extract; (C) apple juice extract spiked with 1 ppm ETU after extractive derivatiza tion with DCA A. Conditions: 6' X 2 mm i.d. glass column with 3% OV-330 on 80-100 mesh Chromosorb 750; flow rate, 30 mL/min helium; oven, 200°C; NP alkali flame ionization detector.



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from water u s i n g d i c h l o r o a c e t i c anhydride i n C ^ C l ^ . To e f f e c t e f f i c i e n t e x t r a c t i o n , a c e t o n i t r i l e was used (as phase t r a n s f e r agent) i n a 1:10 r a t i o w i t h the water. Reaction i s complete i n 3 min. Although S - a l k y l a t i o n c o u l d be p o s t u l a t e d as a p o s s i b l e r e a c t i o n pathway, t h i s was discounted a f t e r the d i - a c y l a t e d d e r i v a t i v e was prepared from the mono-product u s i n g d i c h l o r o a c e t y l c h l o r i d e . NMR and GC/MS a n a l y s i s showed that e l i m i n a t i o n of HC1 together w i t h r i n g - c l o s u r e occurs on GC i n j e c t i o n to give a sharp, s i n g l e peak on 0V-17 o r 0V-330 columns. O r i g i n a l l y , t h i s e x t r a c t i v e a c y l a t i o n procedure was used as a r a p i d s c r e e n i n g procedure f o r the presence o f ETU i n water samples a t the 0.ΟΙ Ο.05 ppm l e v e l (56) and has now been extended t o f r u i t and vegetable j u i c e s . Figure 12 shows the chromatographic c h a r a c t e r i s t i c s o f the ETU d e r i v a t i v e on an 0V-330 column and d e t e c t i o n u s i n g NP heated-bead d e t e c t o r . A l s o i l l u s t r a t e d i s the before and a f t e r chromatograms o f a spiked apple j u i c e sample a t the 1 ppm l e v e l . In c o n c l u s i o n , i t i s obvious that chemical d e r i v a t i z a t i o n continues and w i l l continue t o p l a y an important r o l e i n both q u a l i t a t i v e and q u a n t i t a t i v e a n a l y s i s o f p e s t i c i d e s , t h e i r r e s i dues and m e t a b o l i t e s . One o f the major advantages being that d e r i v a t i z a t i o n gives an improvement i n s e l e c t i v i t y as a r e s u l t of the formation o f a c h a r a c t e r i s t i c d e r i v a t i v e which responds s e l e c t i v e l y t o c e r t a i n GC and HPLC d e t e c t o r s .

LITERATURE CITED 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Drozd, J., J . Chromatogr., 1975, 113, 303-356. Ahaya, S. J . Pharm. Sci., 1976, 65, 163-182. Blau, Κ., King, G.S., Eds. "Handbook of Derivatives for Chromatography", Heyden and Sons Ltd., London, 1977. Cochrane, W.P., Purkayastha. Toxicol. Environ. Chem. Reviews, 1973, 1, 137-268. Cochrane, W.P., J . Chromatogr. Sci., 1975, 13, 246-253. Nickolson, J.D. Analyst 1978, 103, 1-28 (Part I); ibid 1978, 103, 193-222 (Part 2). Cochrane, W.P., Chau, A.S.Y. "Advances in Chemistry Series" American Chemical Society, Washington, D.C. 1971, 104, 11-27. Khan, S.U. Res. Rev. 1975, 59, 21-50. Yip, G., J.Assoc. Offic. Anal. Chem., 1971, 54, 343-45. Woodham, D.W., Mitchell, W.G., Loftis, C.D., and Collier, C.W. J. Agric. Food Chem., 1971, 19, 186-188. Gutenmann, W.H., and Lisk, D.J., J. Assoc. Offic. Anal. Chem., 1964, 47, 353-354. Agemain, Η., Chau, A.S.Y., J . Assoc. Offic. Anal. Chem., 1977, 60, 1070-76. Chau, A.S.Y., Terry, K., J. Assoc. Offic. Anal. Chem. 1975, 58, 1294-1300. Mierzwa, S., Witek, S., J. Chromatogr. 1977, 136, 105-111. Chau, A.S.Y., Terry, K., J. Assoc. Offic. Anal. Chem., 1976, 59, 633-36. American Chemical Society Library 1155 16th St. N. W. Harvey et al.; Pesticide Analytical Methodology ACS Symposium Series; American Chemical Washington, DC, 1980. Washington, D. C.Society: 20036

248

16. 17. 18. 19. 20. 21. 22. Downloaded by RUTGERS UNIV on February 25, 2017 | http://pubs.acs.org Publication Date: October 30, 1980 | doi: 10.1021/bk-1980-0136.ch012

23. J. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.

PESTICIDE ANALYTICAL METHODOLOGY

Moseman, R.F., Ward, M.K., Crest, H.L. and Zehr, R.D., J. Agric. Food Chem., 1978, 26, 965-968. Anonymous, Pest. Chem. News, 1976, 4, 15 Kaiser, K.L.E., Science, 1974, 185, 523-525. Zitko,V., Bull. Environ. Contam. Toxicol., 1976, 16, 399-405. Mes, J., Davies, D.J., and Miles, W., Bull. Environ. Contam. Toxicol. 1978, 19, 564-750. Lane, R.H., Grodner, R.M., and Graves, J . L . , J . Agric. Food Chem., 1976, 24, 192-193. Chau, A.S.Y., Carron, J.M. and Tse, H., J . Assoc. Offic. Anal. Chem., 1978, 61, 1475-1480. Lewis, R.G., Hanish, R.C., MacLeod, K.E., and Sovocool, G.W., Agric. Food Chem., 1976, 24, 1030-1035. Hallet, D.J., Norstrom, R.J., Onuska, F.I., Comba, M.E. and Sampson, R., J. Agric. Food Chem., 1976, 24, 1189-1193. Holdrinet, M.V.H., Bull. Environ. Contam. Toxicol., 1979, 21, 46-52. Norstrom, R.J., Won, H.T., Holdrinet, M.V.H., Calway, P.G., and Naftel, C.D., J . Assoc. Offic. Anal. Chem., 1980 (in press). Collins, G.B., Holmes, D.C., Wallen, Μ., J . Chromatogr. 1972, 69, 198-200. Holdrinet, M.V.H., J. Assoc. Offic. Anal. Chem., 1974, 57, 580-584. Baker, B.E., Bull. Environ. Contam. Toxicol., 1973, 10, 279-284. Rosewell,K.T. and Baker, B.E., Bull. Environ. Contam. Toxicol., 1979, 21, 470-477. Crist, H.L., Moseman, R.F., and Noneman, J.W., Bull. Environ. Contam. Toxicol., 1975, 14, 273-279. Singh, J., and Cochrane, W.P., J . Assoc. Offic. Anal. Chem., 1979, 62, 751-756. Thirtin, J.S., Olson, T.J. and Wagner, K., J . Agric. Food Chem., 1977, 25, 573-576. Greenhalgh, R., King, R.R., and Marshall, W.D., J . Agric. Food Chem., 1978, 26, 475-480. Deo, P.G., and Howard, P.H., J . Assoc. Offic. Anal. Chem., 1978, 61, 210-213. Jacob,K., Falkner, C., and Vogt, W., J.Chromatogr., 1978, 167, 67-75. Kossa, W.C., MacGee, J., Ramachandran, S. and Webber, A.J., J. Chromatogr. Sci., 1979, 17, 177-187. Dale, W.E., Miles, J.W., and Churchill II, F.C., J . Assoc. Offic. Anal. Chem., 1976, 59, 1088-1093. Miles, J.W., and Dole, W.E., J . Agric. Food Chem., 1978, 26, 480-482. Churchill II, F.C., Ku, D.N., and Miles, J.W., J . Agric. Food Chem., 1978, 26, 1108-1172. Wein, R.G., and Tanaka, F.S., J . Chromatogr., 1977, 130, 55-63.


12.

42. 43. 44. 45. 46.


47. 48. 49. 50. 51. 52. 53. 54. 55. 56.

COCHRANE

Chemical Derivatization Techniques

249

Bromilow, R.H., and Lord, K.A., J. Chromatogr., 1976, 125 495-502. Bromilow, R.H., Analyst, 1976, 101, 982-85. Edgerton, T.R. and Moseman, R.F., J. Agric. Food Chem., 1979, 27, 197-199. Faas, L.F. and Moore, J.C., J . Agric. Food Chem., 1979, 27, 554-557. Kriggsman, W., Van de Kamp, C.G., J. Chromatogr., 1977, 131, 412-416. Ohe, T., Bull. Environ. Contam. Toxicol., 1979, 22, 287-292. Gyllenhaal, O., Brotell, H. and Hartvig, P., J. Chromatogr., 1976, 129, 295-302. Davis, B., Anal. Chem., 1977, 49, 832-834. Rosenfeld, J.M., and Crocco, J.L., Anal. Chem., 1978, 50, 701-704. Gyllenhaal, O., J . Chromatogr., 1978, 153, 517-520. Akerbolm, M., 4th Intern. Congress of Pest. Chem. (IUPAC), Zurich, 1978, Paper No. VI-702. Otto, S., Keller, W., and Dresher, J . Environ Sci. Health, 1977, B12, 179-191. Farrington, D.S., and Hopkins, Analyst, 1979, 104, 111-116. Hirvi, T., Pyysalo, H., and Savolainen, K., J. Agric. Food Chem., 1979, 27, 194-195. Singh, J., Cochrane, W.P. and Scott, J., Bull. Environ. Contam. Toxicol., 1979, 23, 470-474.

RECEIVED February 7,

1980.


Chemical Derivatization Techniques in Pesticide Analysis - ACS

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