7 Ultraviolet Spectrophotometry in Residue Analysis; Spectra-Structure Correlations O S M A N M. ALY and S. D .
FAUST
I.
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Department of Environmental S c i ences, Rutgers University, N e w Brunswick, N. J. 08903
Ultraviolet
dues. A correlation ture of several correlation chromophore analytical
between provide
present procedures.
conjunction
bands
the UV spectrum
is discussed. clues
and may a limitation
in this region.
the
such
type to
of
when
taken
by IR, NMR, A discussion
for the analysis
level is also
in the in and
may lead to structural analyst.
of
design
of many groups
on interpretations However,
that have been utilized
cides on the submicrogram
analyst
struc-
of
general
the
obtained
UV spectra
posals of value to the pesticide methods
help
tool resi-
and the
Knowledge
about
with the information
mass spectroscopy,
a valuable of pesticide
The transparency
the near UV imposes absorption
is considered
the identification
pesticides
may
SUFFET
Drexel Institute of Technology, P h i l a delphia, P a .
spectrophotometry
as an aid for confirming
H.
proof
of
the
pesti-
presented.
l t r a v i o l e t s p e c t r o p h o t o m e t r y has b e e n , for some t i m e , one of the most ^
v a l u a b l e tools of p e s t i c i d e r e s i d u e analysis. A n y c o m p o u n d w i t h a
sufficiently w e l l - d e f i n e d a n d intense a b s o r p t i o n spectra is p o t e n t i a l l y d e tectable b y this s p e c t r o p h o t o m e t r i c means. I n some cases, s u i t a b l e c h r o mophores Gunther
m a y b e p r o d u c e d b y c h e m i c a l transformations. (1)
Blinn
h a v e r e v i e w e d the different p r o c e d u r e s t h a t h a v e
u t i l i z e d for r e s i d u e analysis at the m i c r o g r a m l e v e l .
UV
and been
spectropho-
t o m e t r y is c o n s i d e r e d one of t h e v a l u a b l e aids for i d e n t i f i c a t i o n of o r g a n i c pesticides. A l t h o u g h the u l t r a v i o l e t d a t a m a y not p r o v i d e the m u l t i t u d e of i n f o r m a t i o n that c a n b e g a i n e d f r o m other s p e c t r o p h o t o m e t r i c m e t h o d s s u c h as I R , N M R , or mass s p e c t r o m e t r y , it is often possible to r e v e a l subtleties of structure w h i c h these t e c h n i q u e s cannot r e v e a l
(2,3,4).
95
Biros; Pesticides Identification Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
96
PESTICIDES I D E N T I F I C A T I O N
D u r i n g t h e p a s t d e c a d e , t r e m e n d o u s advances h a v e b e e n a c h i e v e d i n the d e v e l o p m e n t of e m p i r i c a l correlations b e t w e e n the s t r u c t u r e of o r g a n i c m o l e c u l e s a n d t h e i r u l t r a v i o l e t s p e c t r a (4, 5 ) .
This paper dis-
cusses the c o r r e l a t i o n b e t w e e n the s p e c t r a of some pesticides i n the n e a r u l t r a v i o l e t r e g i o n (190 to 400 fi) a n d t h e i r s t r u c t u r e . S u c h i n f o r m a t i o n w o u l d b e of v a l u e to the r e s i d u e c h e m i s t f o r q u a l i t a t i v e a n d q u a n t i t a t i v e purposes.
A l s o , the a p p l i c a t i o n of U V s p e c t r o p h o t o m e t r y
for
residue
analysis o n the s u b m i c r o g r a m l e v e l is p r e s e n t e d .
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Electronic Transitions A b s o r p t i o n of q u a n t a of r a d i a t i o n i n t h e u l t r a v i o l e t r e g i o n results i n a s p e c t r a l t r a n s i t i o n i n w h i c h t h e electrons of m o l e c u l e s are e x c i t e d f r o m t h e i r g r o u n d state to h i g h e r e n e r g y levels. T h e e n e r g y of a p a r t i c l e w h i c h is c o n f i n e d to a v e r y s m a l l r e g i o n is r e s t r i c t e d to c e r t a i n values.
These
energies are s u b j e c t e d to q u a n t u m restrictions w h i c h a l l o w the m o l e c u l e ( o r e l e c t r o n s ) to h a v e o n l y c e r t a i n energies. G e n e r a l l y , these restrictions are i n c r e a s i n g l y i m p o r t a n t as the r e g i o n i n w h i c h the p a r t i c l e is free to m o v e b e c o m e s s m a l l e r . T h a t is, of the w h o l e r a n g e of energies,
fewer
a n d m o r e w i d e l y s p a c e d ones are a l l o w e d as the p a r t i c l e m o t i o n is m o r e r e s t r i c t e d . T h e r e f o r e , the electrons w h i c h are confined to the v o l u m e , or p e r h a p s , to a p a r t of the v o l u m e of the m o l e c u l e are subject to significant q u a n t u m restrictions or w i l l b e q u a n t i z e d . T h e s e restrictions a l l o w the p r o p e r t i e s of m o l e c u l e s to b e s t u d i e d b y the m e t h o d s of (6).
spectroscopy
T h e e n e r g y of a q u a n t u m of r a d i a t i o n is c a l c u l a t e d f r o m P l a n k s
equation AE
=
Kv
(1)
w h e r e A E is the energy of a q u a n t u m ( e r g s ) , K is P l a n k s constant (6.62 10"
27
X
e r g - s e c ) , v is the f r e q u e n c y of w a v e m o t i o n i n c y c l e s / s e c o n d , a n d
(2) w h e r e c is the v e l o c i t y of l i g h t ( 3 X
10
10
c m / s e c ) a n d A is the w a v e l e n g t h
in cm. T h e e n e r g y of a q u a n t u m of r a d i a t i o n h a v i n g w a v e l e n g t h s of 700, 400, 200, a n d 150 is, therefore, 2.85 X 13.24 X
10"
12
1 0 , 4.97 X 1 2
10 , 9.9 X 12
10" , a n d 12
ergs, r e s p e c t i v e l y .
W h e n a m o l e c u l e is i r r a d i a t e d b y h e t e r o c h r o m a t i c l i g h t , it absorbs o n l y the protons w h o s e energies are e q u a l to that r e q u i r e d for p e r m i s s i b l e e n e r g y transitions w i t h i n the m o l e c u l e .
F o r most l a r g e m o l e c u l e s , the
e l e c t r o n i c a b s o r p t i o n b a n d is v e r y c o m p l i c a t e d , a n d a r a t h e r b r o a d a b s o r p -
Biros; Pesticides Identification Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
7.
A L Y , FAUST, SUFFET
Spectra-Structure
97
Correlations
t i o n b a n d w i t h l i t t l e o r no d e t a i l is o b t a i n e d .
T h i s is p a r t i c u l a r l y t r u e
w h e n the m a t e r i a l is s t u d i e d i n s o l u t i o n . A l t h o u g h a b s o r p t i o n of u l t r a v i o l e t l i g h t results i n the e x c i t a t i o n of electrons f r o m t h e i r g r o u n d state, the n u c l e i w h i c h the electrons h o l d together d e t e r m i n e the s t r e n g t h of the b i n d i n g . T h u s , the c h a r a c t e r i s t i c e n e r g y of transition, a n d h e n c e the w a v e l e n g t h of a b s o r p t i o n , is r e l a t e d to the e l e c t r o n i c s t r u c t u r e of a g r o u p of atoms w i t h i n the m o l e c u l e . g r o u p p r o d u c i n g a b s o r p t i o n is c a l l e d a c h r o m o p h o r e .
The
S t r u c t u r a l changes
affecting a c h r o m o p h o r e c a n b e e x p e c t e d to m o d i f y its a b s o r p t i o n .
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T h e t h r e e m a j o r c o n t r i b u t o r s to e l e c t r o n i c s p e c t r a are the s i n g l e b o n d ( a - e l e c t r o n s ) , the m u l t i p l e b o n d ( ^ - e l e c t r o n s ) , a n d the u n s h a r e d e l e c t r o n pair
(n-electrons). C o m p o u n d s h a v i n g o n l y a - v a l e n c y electrons are s a t u r a t e d a n d
do
not a b s o r b i n the n e a r u l t r a v i o l e t r e g i o n . A l l the electrons of the m o l e c u l e are i n v o l v e d i n s i n g l e b o n d s , a n d t h e y cannot b e r e a r r a n g e d to a n e x c i t e d state w i t h o u t d i s r u p t i o n of the m o l e c u l a r b o n d i n g .
E x c i t a t i o n of
these
electrons r e q u i r e s h i g h - e n e r g y photons i n the f a r u l t r a v i o l e t r e g i o n (A 150 fi).
/
t r a n s i t i o n o f a n n - e l e c t r o n to t h e r i n g o r b i t a l ( 4 ) .
Acidification of a solution of compound containing the pyridine nucleus r e m o v e s this l o n g - w a v e l e n g t h b a n d f r o m t h e s p e c t r u m .
Substitution of
the p y r i d i n e r i n g i n t h e a o r /3 positions results i n a shift o f t h e 257-it
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b a n d to longer wavelengths.
N i c o t i n e ( F i g u r e 7 ) a n d anabasine are
t y p i c a l examples o f t h e ^ - s u b s t i t u t e d p y r i d y l c h r o m o p h o r e w h i c h absorbs at 262 ii. P a r a q u a t ( a q u e o u s ) absorbs at 256 /A. I n this m o l e c u l e are n o n o n b o n d e d
there
electrons b u t t h e y a r e i n v o l v e d i n t h e c a t i o n f o r m a -
t i o n , a n d t h e s p e c t r u m is v e r y s i m i l a r t o t h a t o f a n a c i d i c s o l u t i o n o f
200
240 WAVELENGTH,
280
320
mjj
Figure 7. Ultraviolet absorption spectra of pyridine, (in ethanol), paraquat, ana diquat (in water)
nicotine
Biros; Pesticides Identification Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
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110
PESTICIDES
IDENTIFICATION
WAVELENGTH,
Figure
8. Ultraviolet absorption spectra of simazine and atrazine (in water)
p y r i d i n e . T h e s p e c t r u m o f d i q u a t , o n the other h a n d , shows a d r a m a t i c b a t h o c h r o m e shift t o 308 tt s i m i l a r t o t h a t o b s e r v e d w i t h the a - t e r - a m i n o p y r i d y l derivatives
{24).
St/m-TRiAZiNE CHROMOPHORE.
T h e s p e c t r u m of s i m a z i n e a n d a t r a z i n e
i n w a t e r ( F i g u r e 8 ) shows a w e a k b a n d at 263 tt w h i c h is t o b e c o m p a r e d w i t h the IT — » TT* (255 tt) b a n d o f b e n z e n e . A n o t h e r intense b a n d at 220 tt is o b s e r v e d w h i c h is c h a r a c t e r i s t i c o f a l l t h e s u b s t i t u t e d st/ra-triazines (Table I X ) .
Ultraviolet Micro Spectrophotometry as an Aid to the Identification of Pesticide Residues R e c e n t l y Suffet ( 2 5 ) a n d F a u s t a n d Suffet ( 2 6 ) r e p o r t e d a n i n t e n sive s t u d y o n t h e s e p a r a t i o n a n d i d e n t i f i c a t i o n o f t h e p h o s p h a t e
ester
pesticides p a r a t h i o n , d i a z i n o n , a n d f e n t h i o n a n d t h e i r d e g r a d a t i o n p r o d ucts. A s u m m a r y o f a m i c r o s p e c t r o p h o t o m e t r i c u l t r a v i o l e t t e c h n i q u e that w a s u t i l i z e d as a n a i d i n i d e n t i f y i n g these c o m p o u n d s i s p r e s e n t e d here.
Biros; Pesticides Identification Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
7.
A L Y , FAUST, SUFFET
Spectra-Structure
A B e c k m a n D K - 2 A spectrophotometer ard benzene vapor spectrum.
111
Correlations
was calibrated with a stand-
T h e spectrophotometer
w a s adjusted for
the h i g h e s t r e s o l u t i o n w i t h a t o l e r a b l e signal-to-noise r a t i o . S i n c e h i g h r e s o l u t i o n d e m a n d s a n a r r o w slit w i d t h , i n s t r u m e n t s e n s i t i v i t y w a s set as h i g h as possible. A s a m a x i m u m s i g n a l response w a s n e e d e d , a l o w t i m e constant w a s set, a n d a r e l a t i v e l y s l o w s c a n n i n g s p e e d w a s u s e d . A 1-cm B e c k m a n m i c r o c e l l ( a p e r t u r e v o l u m e o f 5 0 / J ) o f f u s e d s i l i c a w i n d o w s w i t h a r a n g e o f 220 to 2500 tt w a s u t i l i z e d .
A variable-beam
c o n d e n s e r s e r v e d t o attenuate the reference b e a m . A n a c t u a l w a v e l e n g t h cutoff at 230 tt w a s o b s e r v e d f o r t h e m i c r o c e l l . I n o r d e r t o l o w e r t h e Downloaded by FUDAN UNIV on January 16, 2017 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0104.ch007
cutoff p o i n t , a m a t c h e d 1-cm s t a n d a r d s i l i c a c e l l filled w i t h solvent w a s c e n t e r e d b e h i n d t h e reference b e a m attenuator.
This allowed a lower
cutoff a t 205 it. E t h a n o l , 9 5 % , w a s chosen b e c a u s e of a l o w cutoff p o i n t , 205 tt, a n d a h i g h d i s s o l u b l e c a p a c i t y f o r t h e c o m p o u n d s o f interest. T h e samples w e r e c o l l e c t e d f r o m G L C c o l u m n s b y t w o t e c h n i q u e s . A system w i t h K B r as a n absorbent was u s e d to collect G L C peaks f r o m single-column operation
with
a
flame
i o n i z a t i o n detector.
This was
adapted from a n original design for d u a l column operation ( 2 7 ) . It consisted o f c o l u m n effluent s p l i t t e r , c o l l e c t i o n t u b e , a n d f r a c t i o n collector. T h e s p l i t t i n g ratio o f t h e flow w a s 1:1.7, d e t e c t o r : o u t p u t .
T h i s system
w a s chosen f o r t h e f o l l o w i n g reasons: (a)
T h e r e is a d i r e c t G L C t o m i c r o s p e c t r o p h o t o m e t r i c
w i t h m i n i m u m intermediate handling.
operation
T h i s eliminates contamination
problems i n h a n d l i n g microgram quantities. T h e c o l l e c t i o n o f s e v e r a l peaks d u r i n g o n e c h r o m a t o g r a p h i c r u n
(b)
is possible. (c)
T h e ease o f o p e r a t i o n a n d r e p e a t a b i l i t y .
(d)
C o l l e c t e d fractions c a n b e d i s s o l v e d a n d r e c h r o m a t o g r a p h e d .
(e)
R e c o v e r i e s of 5 0 to 7 5 % h a v e b e e n o b t a i n e d .
T h e G L C peak collected o n the K B r m a y b e inserted into the m i c r o cell b y either of t w o techniques: b y p o u r i n g the p o w d e r directly into a Table I X . Common Name Atratone Simeton Prometon Simazine Propazine Trietazine Atrazine
Absorption Maxima for Some syw-Triazine Pesticides" Chemical
Name
A
2-Ethylamino-4-isopropylamino-6-methoxy-striazine 2,4-Bis(ethylamine)-6-methoxy-s-triazine 2 , 4 - B i s (isopropy l a m i n o - 6 - m e t h o x y ) - s - t r i a z i n e 2-Chloro-4,6-bis(ethylamino)-s-triazine 2-Chloro-4,6(isopropylamino)-s-triazine 2-Chloro-4-