13 Forensic Applications of Differential Scanning Calorimetry JESSE H. HALL and BRUCE CASSEL
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P e r k i n - E l m e r C o r p . , N o r w a l k , C o n n . 06856
O v e r the l a s t decade T h e r m a l A n a l y s i s has m a d e s i g n i f i c a n t c o n t r i b u t i o n s to a surprising n u m b e r of f i e l d s (1). In the m u l t i - f a c e t e d field of p o l y m e r s c i e n c e , it h a s p r o v i d e d a u n i v e r s a l t o o l f o r a s c e r t a i n i n g and p r e d i c t i n g the effect of p r e p a r a t i o n and t h e r m a l h i s t o r y on the p h y s i c a l p r o p e r t i e s of p o l y m e r s . One r e a s o n f o r t h i s i s that a g r e a t d e a l of i n f o r m a t i o n about the o v e r -all s t r u c t u r a l state of a c o m p l e x m a c r o m o l e c u l a r s y s t e m i s r e v e a l e d i n the heat c a p a c i t y . Thus, while conventional analytical t e c h n i q u e s give i n f o r m a t i o n about the t o t a l amount of v a r i o u s m o l e c u l a r s u b s t a n c e s and m o l e c u l a r s u b g r o u p s p r e s e n t i n the s a m p l e , t h e r m a l a n a l y s i s g i v e s quite d i f f e r e n t i n f o r m a t i o n about the s y s t e m . It i n d i c a t e s c h a n g e s i n the m o r p h o l o g y o r m o l e c u l a r o r d e r as a f u n c t i o n of c h a n g e s i n t e m p e r a t u r e . S p e c i f i c a l l y , the p h y s i c a l p h e n o m e n a s t u d i e d by d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) and its o t h e r t h e r m a l c o u s i n s a r e t h e s e : f i r s t , the heat c a p a c i t y , the p o w e r r e q u i r e d to heat the s a m p l e at the d e s i g n a t e d r a t e ; s e c o n d , first-order phase c h a n g e s s u c h as m e l t i n g ; t h i r d , s e c o n d - o r d e r phase changes s u c h as the g l a s s t r a n s i t i o n ; f o u r t h , t h e r m a l l y i n i t i a t e d heats of r e a c t i o n s u c h as cross-linking and o x i d a t i v e o r r e d u c t i v e d e g r a d a t i o n ; and, f i n a l l y , e v a p o r a t i o n o r s u b l i m a t i o n . T h e r m o m e c h a n i c a l a n a l y s i s ( T M A ) and t h e r m o g r a v i m e t r i c a n a l y s i s ( T G A ) m o n i t o r d i m e n s i o n a l and weight changes i n the s a m p l e as a f u n c t i o n of temperature. The temperatures of o c c u r r e n c e of t r a n s i t i o n s and r e a c t i o n s a r e c h a r a c t e r i s t i c of the m a t e r i a l s ; a n d , if the a s s o c i a t e d y - c o o r d i n a t e (heat c a p a c i t y , weight o r d i m e n s i o n ) i s m e a s u r e d q u a n t i t a t i v e l y , t h i s c a n be u s e d as a b a s i s f o r a n a l y t i -cal d e t e r m i n a t i o n . D e s p i t e the unique c a p a b i l i t i e s of the m e t h o d , r e l a t i v e l y few s t u d i e s have b e e n r e p o r t e d w h i c h s p e c i f i c a l l y d e a l with the use of t h e r m a l a n a l y s i s in the f o r e n s i c s c i e n c e s . P e r h a p s the m o s t 114
In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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u s e f u l study thus f a r has b e e n that of W. M. S. P h i l p ( 2 ) , of the T o r o n t o C e n t r e of F o r e n s i c S c i e n c e , who a n a l y z e d by D S C a l l of the c o m m o n s y n t h e t i c f i b e r s i n use today. He had o u t l i n e d a p r o c e d u r e f o r a n a l y s i s and tabulated the t e m p e r a t u r e s at w h i c h c h a r a c t e r i s t i c t h e r m a l events o c c u r i n s o m e 50 c o m m e r c i a l s y n t h e t i c f i b e r s . P h i l p ' s c o n c l u s i o n was that,using quantitative DSC, it i s p o s s i b l e to i d e n t i f y f r o m m i l l i g r a m q u a n t i t i e s of f i b e r the c l a s s of p o l y m e r (nylon, p o l y e s t e r , t r i a c e t a t e s , p o l y o l e f i n s , p o l y a c r y l o n i t r i l e s , and m o d a c r y l i c s ) . In a d d i t i o n , he noted that c e r t a i n d i f f e r e n t i a t i o n c a n be m a d e w i t h i n e a c h c l a s s , f o r e x a m ple, i n d i s t i n g u i s h i n g the v a r i o u s t y p e s of n y l o n s and o r l o n s . R a t h e r than d u p l i c a t e P h i l p ' s work, it i s the p u r p o s e of t h i s p r e s e n t study to show how t h i s a p p r o a c h c a n be extended t h r o u g h the use of s e c o n d - g e n e r a t i o n i n s t r u m e n t a t i o n . What a r e the p a r t i c u l a r d e m a n d s o r c o n s t r a i n t s inherent i n f o r e n s i c a n a l y s i s ? T h e r e a r e two b a s i c f u n c t i o n s of a n a l y t i c a l i n s t r u m e n t s i n f o r e n s i c s c i e n c e . T h e f i r s t i s to use t h e m to obtain i n f o r m a t i o n about a m a t e r i a l s u c h as i t s c h e m i c a l c o m p o s i t i o n , f o r e x a m p l e , f o r the p u r p o s e of s e t t l i n g patent r i g h t s o r for f i n d i n g the o r i g i n of a m a t e r i a l . T h e s e c o n d f u n c t i o n , w h i c h i s the m o r e a p p l i c a b l e to t h e r m a l a n a l y s i s , i s as a c o m p a r i t i v e technique to d e t e r m i n e i f two m a t e r i a l s a r e f r o m the s a m e o r i g i n . In t h i s c a s e , the m o s t c o m m o n p r o b l e m i s to e s t a b l i s h that an a r t i f a c t l i n k e d to the c r i m e - - u s u a l l y s o m e i t e m o v e r l o o k e d by the c r i m i n a l - - i s of i d e n t i c a l o r i g i n to a s i m i l a r s a m p l e i n the p o s s e s s i o n of the suspect. T h e s a m p l e s c o l l e c t e d i n the f i e l d a r e f r e q u e n t l y damaged, c o n t a m i n a t e d and d i m i n u t i v e , thus p o s i n g a c o n s i d e r a b l e c h a l l e n g e f o r the f o r e n s i c a n a l y s t . The p a r t i c u l a r v i r t u e s of t h e r m a l a n a l y s i s of s u c h s a m p l e s a r e these: f i r s t , a g r e a t d e a l of i n f o r m a t i o n c a n be g l e a n e d about a s a m p l e without d e s t r o y i n g the s a m p l e f o r f u r t h e r a n a l y s i s ; s e c o n d , the i n f o r m a t i o n i s c o m p l e m e n t a r y r a t h e r than d u p l i c a t i v e of s p e c t r o s c o p i c and c h r o m a t o g r a p h i c e v i d e n c e ; t h i r d , nonp o l y m e r i c m a t e r i a l s (e.g., dyes, s o i l , d r i e d blood) a r e noni n t e r f e r i n g ; f o u r t h , the m e t h o d i s r a p i d and does not r e q u i r e sample preparation. W h i l e as a g e n e r a l m e a n s of m a t e r i a l study the m e t h o d i s u n i v e r s a l i n i t s c a p a b i l i t i e s , it i s p r i m a r i l y i n the a n a l y s i s of p o l y m e r s that it i s u s e f u l f o r f o r e n s i c p u r p o s e s . T h e a r e a w h e r e it i s p e r h a p s m o s t n e e d e d and w h i c h a l s o d e m o n s t r a t e s the m e t h o d o l o g i c a l a p p r o a c h best i s that of f i b e r a n a l y s i s .
In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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Experimental T h e t h e r m a l a n a l y s i s l a b o r a t o r y u s e d i n t h i s study, a s s e e n i n f i g u r e s 1-3, i n c l u d e d the P e r k i n - E l m e r M o d e l D S C - 2 s c a n n i n g c a l o r i m e t e r , the M o d e l T G S - 1 t h e r m o g r a v i m e t r i c a n a l y z e r , the M o d e l T M S - 1 t h e r m o m e c h a n i c a l a n a l y z e r , and t h e M o d e l A D - 2 a u t o b a l a n c e . T h e s e i n s t r u m e n t s , b e s i d e s b e i n g a m o n g the m o s t s e n s i t i v e and quantitative on the m a r k e t , have c e r t a i n f e a tures which make them especially useful for forensic analysis. F o r i n s t a n c e , the s c a n n i n g c a l o r i m e t e r was u s e d with a f r e o n b a s e d c o o l e r ( T h e I n t r a c o o l e r II) w h i c h a f f o r d e d the f o l l o w i n g c a p a b i l i t i e s : (1) C o n t i n u o u s o p e r a t i o n f r o m - 7 0 ° C to 7 0 0 ° C without the need f o r h a r d w a r e c h a n g e s at high and low t e m p e r a t u r e s . (2) R e p r o d u c i b l e p r o g r a m c o o l i n g r a t e s of up to 8 0 ° C / m i n . o v e r the t r a n s i t i o n r a n g e o f the f i b e r s . (3) F r o s t - f r e e s h o c k c o o l i n g of 1 0 0 ° C / s e c . (over the s a m e r a n g e ) w i t h i n the c a l o r i m e t e r d r y box b y p l a c e m e n t o f s a m p l e on the i s o t h e r m a l c a l o r i m e t e r b l o c k . (4) H i g h r e s o l u t i o n and fast r e s p o n s e t i m e s f o r s h a r p p e a k s and s h o r t (
