10
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Computerized Quantitative Analysis of Copolymers by IR Spectroscopy 1
2
E. G.BARTICK ,J. C. CORBETT, and G. L. McCLURE Perkin-Elmer Corporation, Norwalk, CT 06856
The use of a commercially available software package (QUANT) is described in the analysis of co polymer films by computer assisted infrared spec troscopy. Important features of the software are illustrated by the example of analysis of vinyl acetate/vinyl chloride copolymers. Critical as pects of method development are explained and error sources are examined. Calibration is re ported with a correlation coefficient of 0.9998. It is observed from the error analysis data on replicate samples that errors associated with sample preparation are significantly larger than those related to measurement and evaluation of the spectra. Rapid, a c c u r a t e a n a l y s i s o f copolymer products i s c r i t i c a l t o t h e e f f i c i e n c y a n d economy o f modern i n d u s t r i a l c o p o l y m e r production. Infrared spectroscopy i s a well e s t a b l i s h e d tech n i q u e f o r b o t h q u a l i t a t i v e and q u a n t i t a t i v e a n a l y s i s o f p o l y m e r i c m a t e r i a l s ( 1 , 2 ) . However, t h e c o u p l i n g o f r e l a t i v e l y l o w c o s t d a t a h a n d l i n g h a r d w a r e and s o f t w a r e t o a m i c r o p r o c e s s o r c o n t r o l l e d i n f r a r e d spectrophotometer i s a r e l a t i v e l y recent development. T h i s c o u p l i n g c o n s i d e r a b l y enhances t h e l e v e l o f p e r f o r m a n c e one c a n e x p e c t f r o m q u a n t i t a t i v e i n f r a r e d s p e c t r o s copy. The r e s u l t i s t h a t s u c h s y s t e m s g r e a t l y r e d u c e t h e e f f o r t , e x p e n s e , and t i m e r e q u i r e d f o r a g i v e n a n a l y s i s a n d s i m u l t a n e o u s l y p r o v i d e improved a c c u r a c y , r e l i a b i l i t y , and precision. This paper w i l l d e s c r i b e a r e c e n t l y developed, c o m m e r c i a l l y a v a i l a b l e s o f t w a r e system which w i l l be r e f e r r e d t o h e r e a f t e r as QUANT. I n t h e c o u r s e o f a p p l i c a t i o n r e s e a r c h w i t h t h e QUANT s o f t w a r e a w i d e v a r i e t y o f c o p o l y m e r s y s t e m s Current address: Raybestos Manhattan, Corporate Materials Laboratory, 75 E. Main Street, Stratford, CT 06497 To whom all correspondence should be addressed 1
2
0097-6156/82/0197-0185$06.00/0 © 1982 American Chemical Society Provder; Computer Applications in Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
COMPUTER APPLICATIONS IN APPLIED POLYMER SCIENCE
186
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have b e e n s u c c e s s f u l l y a n a l y z e d . Indeed, t h e t e c h n i q u e s and p r i n c i p l e s d e s c r i b e d i n t h i s paper a r e g e n e r a l l y a p p l i c a b l e . They may be a d a p t e d t o s u i t t h e a n a l y s i s o f many c o p o l y m e r s y s t e m s . However, f o r t h e p u r p o s e o f i l l u s t r a t i n g p a r t i c u l a r f e a t u r e s o f t h e QUANT s o f t w a r e , t h e p r e s e n t d i s c u s s i o n w i l l b e r e s t r i c t e d to the analysis o f v i n y l c h l o r i d e / v i n y l acetate co polymer samples. Experimental C o p o l y m e r m a t e r i a l s were p u r c h a s e d f r o m S c i e n t i f i c P o l y m e r P r o d u c t s , I n c . , O n t a r i o , New Y o r k . Samples were p r e p a r e d f r o m v i n y l c h l o r i d e / v i n y l a c e t a t e b u l k copolymers by e v a p o r a t i n g s o l u t i o n s o f t h e p o l y m e r s ( i n 2-butanone (MEK)) on p o t a s s i u m b r o m i d e windows. S p e c t r a o f t h e r e s u l t i n g f i l m s were o b t a i n e d on a P e r k i n - E l m e r M o d e l 683 r a t i o r e c o r d i n g d o u b l e - b e a m i n f r a r e d spectrophotometer. The i n s t r u m e n t was s e t t o o p e r a t e w i t h t h e w i d e s l i t p r o g r a m a n d a t i m e s 4 m u l t i p l i e r ( n o i s e f i l t e r ) . The r e s u l t i n g spectral data contained a s i g n a l - t o - n o i s e r a t i o o f a p p r o x i m a t e l y 3000:1. A l l i n s t r u m e n t o p e r a t i o n s a n d d a t a c o l l e c t i o n were c o n t r o l l e d b y a P e r k i n - E l m e r I n f r a r e d D a t a S t a t i o n . S p e c t r a were r e c o r d e d a n d s t o r e d on a m i c r o f l o p p y d i s k b y means of a general purpose s p e c t r o s c o p i c m a n i p u l a t i o n software system c a l l e d PECDS. Q u a n t i t a t i v e e v a l u a t i o n was p e r f o r m e d s u b s e q u e n t l y on t h e c o l l e c t e d s p e c t r a l d a t a u s i n g a n a l y t i c a l methods c r e a t e d w i t h p r o g r a m s o f t h e QUANT s o f t w a r e s y s t e m f o r t h e I n f r a r e d D a t a Station. E v a l u a t i o n p r o c e d u r e s i n v e s t i g a t e d i n c l u d e d peak h e i g h t a n d peak a r e a t e c h n i q u e s . A b s o r b a n c e r a t i o s were r e l a t e d t o c o n c e n t r a t i o n r a t i o s by c a l i b r a t i o n w i t h standards r a n g i n g f r o m 2% t o 17% v i n y l a c e t a t e i n t h e c o p o l y m e r . M u l t i p l e s a m p l e s o f a n o m i n a l 13% v i n y l a c e t a t e c o n c e n t r a t i o n were r u n f o r e v a l u a t i o n b y t h e s o f t w a r e as an i n d i c a t o r o f t h e p r e c i s i o n o f t h e analysis. V i n y l a c e t a t e c o n c e n t r a t i o n s were c a l c u l a t e d on t h e b a s i s o f t h e c a r b o n y l band w i t h a maximum a b s o r b a n c e a t 1738 c m a n d a b a s e l i n e absorbance d e r i v e d from l i n e a r i n t e r p o l a t i o n between a b s o r b a n c e v a l u e s a t 1850 a n d 1650 cm-1. V i n y l c h l o r i d e c o n c e n t r a t i o n s were c a l c u l a t e d o n t h e b a s i s o f t h e n e t a b s o r b a n c e o f t h e C - C l band w i t h a maximum a b s o r b a n c e a t 691 cm-1 w i t h r e s p e c t t o a b a s e l i n e drawn h o r i z o n t a l l y f r o m t h e a b s o r b a n c e a t 750 cm-1. Peak a r e a f o r t h e v i n y l a c e t a t e c a r b o n y l a b s o r p t i o n was e v a l u a t e d b e t w e e n 1752 a n d 1723 cm-1. p a k a r e a f o r v i n y l c h l o r i d e was e v a l u a t e d b e t w e e n 750 a n d 658 cm" . - 1
e
1
R e s u l t s and D i s c u s s i o n D e s c r i p t i o n o f Software. The QUANT s o f t w a r e p a c k a g e i s composed o f t h r e e m a j o r p r o g r a m s , SNGLE, RATIO, a n d MULT. E a c h o f t h e s e p r o g r a m s i s d e s i g n e d f o r optimum p e r f o r m a n c e f o r p a r t i c u l a r types o f q u a n t i t a t i v e a p p l i c a t i o n s . SNGLE i s
Provder; Computer Applications in Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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10.
BARTICK ET AL.
Quantitative
IR Analysis
of
187
Copolymers
d e s i g n e d f o r a n a l y s i s o f s i n g l e component s y s t e m s w i t h s a m p l e s o f d e f i n e d p a t h l e n g t h , and a p p r o p r i a t e l y w i l l be e x c l u d e d f r o m t h i s discussion. RATIO i s d e s i g n e d f o r two component s y s t e m s o f determined o r indeterminate pathlength. MULT i s d e s i g n e d f o r s i m u l t a n e o u s a n a l y s i s o f two t o t e n components and i n c l u d e s ma t r i x techniques i n the data processing. A fundamental hypothesis o f t h e RATIO s o f t w a r e i s t h a t bands c a n be f o u n d a t t r i b u t a b l e t o e a c h o f t h e components, a n d t h a t t h e s e bands a r e n o t s i g n i f i c a n t l y o v e r l a p p e d b y bands o f t h e o t h e r r e s p e c t i v e component. I t f u r t h e r m o r e assumes t h a t , by p r o p e r c h o i c e o f b a s e l i n e f r e q u e n c i e s , t h e p r o p e r amount o f b a c k g r o u n d a b s o r b a n c e may be sub t r a c t e d f r o m t h e peak a b s o r b a n c e i n t h e v i c i n i t y o f t h e b a n d s u c h t h a t t h e n e t a b s o r b a n c e m e a s u r e d f o r a p a r t i c u l a r component i s r e l a t e d t o t h e c o n c e n t r a t i o n o f t h a t component by a f i r s t o r d e r dependence o n l y . This implies that the equation o f the c a l i b r a t i o n f u n c t i o n f o r e a c h component must be e x p r e s s e d as i n e q u a t i o n 1 ( a a n d b) i n s t e a d o f e q u a t i o n 2 ( a a n d b ) , and t h a t t h e p l o t s o f t h e c a l i b r a t i o n f u n c t i o n s f o r e a c h o f t h e i n d i v i d u a l compo nents would pass through t h e o r i g i n . a)
A
b)
A
1
= (C k )X
b)
A
2
= (C k )X
x
= (C^+b^X
b)
A
2
= (C k
1
1
2
2
(1)
2
2 +
b )X 2
(2)
Aj, and A a r e t h e o b s e r v e d a b s o r b a n c e s a t t h e a n a l y t i c a l wavenumbers V]_ and V2. C\ and C2 r e p r e s e n t t h e c o n c e n t r a t i o n s o f components 1 and 2. X i s t h e common p a t h l e n g t h . b i and b2 a r e the e f f e c t i v e absorbances which r e s u l t from p a r t i c u l a r c h o i c e s i n b a s e l i n e s t h a t w o u l d b e m e a s u r e d a t wavenumbers v i and \>2 i f t h e c o n c e n t r a t i o n o f components 1 a n d 2 were e x t r a p o l a t e d t o z e r o concentration, k i a n d k2 a r e p r o p o r t i o n a l i t y c o n s t a n t s . I f the b a s e l i n e s a r e chosen c o r r e c t l y then t h e r a t i o o f n e t absorbance A l t o n e t a b s o r b a n c e A2 becomes what i s shown i n e q u a t i o n 3. 2
A /A 1
= (k /k )(C /C )(X/X) = k(C C )
2
1
2
1
2
1
I f t h e analogous r a t i o i s formed w i t h equations r e s u l t i s e q u a t i o n 4. A /A x
2
= C k +b )X = C k +b 1
1
(c k 2
1
2 +
1
b )x 2
1
c k 2
2 +
b
(3)
2
2a a n d 2b t h e
1
(4)
2
An u n f a v o r a b l e s i t u a t i o n a r i s e s i f b a s e l i n e s f o r t h e bands a r e n o t c h o s e n s u c h t h a t t h e i n t e r c e p t s b i and b2 a r e n o t c l o s e t o zero. Then t h e a b s o r b a n c e r a t i o A1/A2 i s n o t a l i n e a r f u n c t i o n o f c o n c e n t r a t i o n r a t i o C1/C2. Then t h e a d v a n t a g e s a s s o c i a t e d w i t h l i n e a r e q u a t i o n s a r e no l o n g e r a v a i l a b l e f o r p u r p o s e s o f c a l i b r a t i o n o f s t a n d a r d s and c a l c u l a t i o n o f unknowns. The RATIO s o f t w a r e f u n c t i o n s on t h e b a s i s t h a t t h e a b s o r b a n c e r a t i o i s a
Provder; Computer Applications in Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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188
COMPUTER APPLICATIONS IN APPLIED POLYMER SCIENCE
l i n e a r f u n c t i o n i n order to take advantage of the l i n e a r l e a s t squares t e c h n i q u e t o p r o c e s s the d a t a . T h e r e f o r e , i t i s impor t a n t t h a t b a s e l i n e s be c h o s e n c a r e f u l l y so t h a t b]^ and b2 a r e negligibly small. The RATIO s o f t w a r e can d e a l w i t h o v e r l a p o f t h e bands i f o n l y one o f t h e bands i s o v e r l a p p e d . The s i t u a t i o n o f s i n g l e o v e r l a p i s observed i n the v i n y l c h l o r i d e / v i n y l a c e t a t e system. As can be s e e n i n F i g u r e s 1, 2 and 3, t h e c a r b o n y l band o f v i n y l a c e t a t e a t 1737 cm-1 i e s s e n t i a l l y f r e e o f o v e r l a p by bands a t t r i b u t a b l e t o ^ v i n y l c h l o r i d e ( 3 ) . However, t h e v i n y l c h l o r i d e band a t 691 cm i s o v e r l a p p e d t o some e x t e n t by weak a b s o r p t i o n s o f v i n y l a c e t a t e i n t h e 600 t o 700 cm-1 r e g i o n . I f b a s e l i n e s are chosen such t h a t i n t e r c e p t s are s m a l l the absorb a n c e s o f t h e bands a t 1738 and 691 c n r l s h o u l d be a l i n e a r c o m b i n a t i o n o f t e r m s s u c h as shown i n e q u a t i o n s 5 and 6. s
A
1738 = f l V A ^
k
A
691
k
C
C
= ( 2 VC
+ k
X
5
C
3 VA^
X
6
I f t h e a b s o r b a n c e r a t i o i s f o r m e d by d i v i d i n g e q u a t i o n 6 e q u a t i o n 5 t h e r e s u l t i s e q u a t i o n 7. A
/ A
k
C
691 1738 = 2 VC k
+ k
C
3 VA
=
