The Application of Materials Science Methods to Forensic Problems

7 The Application of Materials Science Methods to Forensic Problems—Principles, Serial Number Recovery, and Paper Identification BILL

C.

GIESSEN,

DONALD

E.

P O L K , and

JAMES

A.

W.

BARNARD

Downloaded by UNIV LAVAL on March 14, 2013 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0013.ch007

Institute of C h e m i c a l Analysis, A p p l i c a t i o n s , a n d F o r e n s i c Science, Northeastern U n i v e r s i t y , Boston, Mass. 02115

M a t e r i a l s science is a relatively young d i s c i p l i n e which emerged in the 1960's to provide a comprehensive view and approach to the study of m a t e r i a l s . I t i n c l u d e s aspects of s o l i d s t a t e p h y s i c s , chemistry, metallurgy, ceramics and other fields of science and engineering. Recent work has shown that m a t e r i a l s science can make a substantial and growing c o n t r i b u t i o n to c r i m i n a l i s t i c s . As a r e s u l t , f o r e n s i c m a t e r i a l s science may become a field of its own i n the future. In the f o l l o w i n g s e c t i o n s , we o u t l i n e the p o s s i b l e scope of t h i s new field and we illustrate the use of m a t e r i a l s science methods in c r i m i n a l i s t i c s by d i s c u s s i n g two examples: the r e covery of erased serial numbers and the i d e n t i f i c a t i o n of papers from t h e i r i n o r g a n i c components. The Scope of F o r e n s i c M a t e r i a l s Science A l l o b j e c t s of f o r e n s i c i n v e s t i g a t i o n s are m a t e r i a l s of some k i n d , ranging from t r a c e s of evidence substances to l a r g e items that must be i d e n t i f i e d . Therefore, proper c h a r a c t e r i z a t i o n f o r f o r e n s i c purposes r e q u i r e s more than a mere determination of the most obvious chemical p r o p e r t i e s , such as elemental composition or d e n s i t y ; i n s t e a d , i t must i n c l u d e a thorough understanding of these substances as m a t e r i a l s . From the standpoint of the m a t e r i a l s s c i e n t i s t such an understanding would r e s u l t from adopting an i n t e g r a t e d view of the many aspects of a m a t e r i a l ; i t would be based on the i n t e r p r e t a t i o n of data from many a d d i t i o n a l t o o l s of i n v e s t i g a t i o n , some of which w i l l be discussed below. The increase i n the number of c h a r a c t e r i z i n g methods has two consequences: 1) a d i r e c t e f f e c t i s that more d i s t i n g u i s h i n g parameters become a v a i l a b l e f o r the purpose of f o r e n s i c examination. In some cases where no other means of i d e n t i f i c a t i o n e x i s t , t h i s may provide a new, sole means of i d e n t i f i c a t i o n of items of evidence capable of e s t a b l i s h i n g r e l a -

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t i v e or absolute i d e n t i t y . In cases where t r a d i t i o n a l chemical means of i d e n t i f i c a t i o n e x i s t , a d d i t i o n a l methods may i n c r e a s e the c e r t a i n t y and hence evidence value of t h i s i d e n t i f i c a t i o n . a more long range e f f e c t of adopting the i n t e g r a t e d view of m a t e r i a l s i n f o r e n s i c examinations c o n s i s t s of producing a broader and deeper e x p e r t i s e i n concerned f o r e n sic scientists. This gives them a greater f l e x i b i l i t y i n performing current tasks and an a b i l i t y to c o n t r i b u t e to f u t u r e developments b e n e f i c i a l to the f i e l d .

Personnel and Personnel T r a i n i n g . The goals discussed here l i e i n the f u t u r e , when a c l o s e r l i a i s o n between c r i m i n a l i s t i c s and m a t e r i a l s science w i l l have been brought about by b u i l d i n g up personnel with thorough t r a i n i n g i n both f i e l d s and by establ i s h i n g appropriate research p r o j e c t s . At present, the l i s t of a c t i v e c r i m i n a l i s t s i n c l u d e s s e v e r a l m e t a l l u r g i s t s , scanning e l e c t r o n m i c r o s c o p i s t s , X-ray d i f f r a c t i o n i s t s , s o l i d s t a t e p h y s i c a l chemists and others; however, with a number of notable except i o n s , few workers i n the c r i m i n a l i s t i c s f i e l d have had an orthodox advanced t r a i n i n g as m a t e r i a l s s c i e n t i s t s . P o s s i b l e remedies to t h i s s i t u a t i o n l i e i n academic programs i n "Forensic M a t e r i a l s Science" on s e v e r a l l e v e l s , which w i l l be discussed elsewhere (1). A continued supply of p r o f e s s i o n a l s with dual t r a i n i n g (analogous to that of the f o r e n s i c chemists now being educated) i s , however, c e r t a i n l y long removed. In t h i s context we note that the c u r r i c u l u m f o r the planned M.S. program i n F o r e n s i c Chemistry at Northeastern U n i v e r s i t y which i s discussed i n d e t a i l i n Reference ^2 w i l l contain a new course e n t i t l e d "Forensic M a t e r i a l s " as a step i n the d i r e c t i o n i n d i c a t e d above. The a b s t r a c t of t h i s course i s as f o l l o w s : F o r e n s i c M a t e r i a l s (2 Quarter Hours): Fundamental types of s o l i d s , such as metals, ceramics, minerals, organic s o l i d s , i n c l u d i n g drugs, polymers, p l a s t i c s , f i b e r s ; t h e i r p r o p e r t i e s and determination by modern methods. F o r e n s i c a l l y important m a t e r i a l s such as a l l o y s , g l a s s , s o i l s , f i b e r s , wood, paper, rubber, dyes, p a i n t s , i n k , and t h e i r determination. I l l u s t r a t i o n of v a r i o u s m a t e r i a l s as a s s o c i a t i v e or d i s s o c i a t i v e items of e v i dence . T y p i c a l Areas of F o r e n s i c M a t e r i a l s Science. In the f o l l o w i n g , some types of f o r e n s i c m a t e r i a l s and tasks i n v o l v i n g them which a r i s e i n a crime l a b o r a t o r y are l i s t e d , and p o s s i b l e a p p l i c a t i o n s of the m a t e r i a l s s c i e n t i f i c approach to these substances and tasks are b r i e f l y d e s c r i b e d . 1. Metals. In f o r e n s i c p r a c t i c e , m e t a l l i c o b j e c t s are i n v e s t i g a t e d p r i m a r i l y by the f i r e a r m and toolmark examiner; t y p i c a l examples are weapons, b u l l e t s , c a r t r i d g e casings and hand t o o l s . Metals are a l s o encountered i n cases of f a i l u r e a n a l y s i s ( f r a c -



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ture by f a t i g u e or impact). The c h a r a c t e r i z a t i o n of m e t a l l i c samples to a s c e r t a i n sample i d e n t i t y and o r i g i n i s a l s o o f t e n of importance (the f o l l o w i n g d i s c u s s i o n r e f e r s p r i n c i p a l l y to t h i s case). Depending on the type of a l l o y encountered, various chara c t e r i z a t i o n techniques could be used (see, e.g., r e f s . 3-5). Study of a m e t a l l i c object might i n v o l v e the f o l l o w i n g determinat i o n s : (a) major and t r a c e metal content by chemical a n a l y s i s ; (b) m i c r o s t r u c t u r e by q u a n t i t a t i v e o p t i c a l or e l e c t r o n microscopy, as w e l l as phase a n a l y s i s by X-ray d i f f r a c t i o n or Mossbauer spectroscopy, e.g., f o r r e t a i n e d a u s t e n i t e (y-Fe s o l i d s o l u t i o n ) i n i r o n a l l o y s ; (c) p r e f e r r e d o r i e n t a t i o n (texture) by X-ray d i f f r a c t i o n ; (d) degree of cold-working by d i s l o c a t i o n d e n s i t y measurement or d i f f e r e n t i a l scanning c a l o r i m e t r y ; (e) nature and d i s t r i bution of i m p u r i t i e s by e l e c t r o n microscopy, i n c l u d i n g s e l e c t e d area e l e c t r o n d i f f r a c t i o n , e l e c t r o n or i o n microprobe a n a l y s i s , chemical separation coupled with X-ray d i f f r a c t i o n m i c r o a n a l y s i s , and perhaps small angle s c a t t e r i n g and high p r e c i s i o n density determination; (f) fractography by scanning e l e c t r o n microscopy (SEM) or scanning i o n microscopy; (g) l a t t i c e impurity l e v e l by low-temperature e l e c t r i c a l r e s i s t a n c e and other mean-free-path dependent measurements; (h) short or long range order (e.g. f o r b r a s s ) ; ( i ) domain s i z e or magnetic p r o p e r t i e s f o r ferromagnetic a l l o y s ; and ( j ) surface s t r u c t u r e by SEM, scanning Auger spectroscopy or low energy e l e c t r o n d i f f r a c t i o n . A few of these methods are i n current f o r e n s i c use, but most are not. While a m a j o r i t y of the proposed methods, taken alone, w i l l not y i e l d unique specimen i d e n t i f i c a t i o n , some may provide a d d i t i o n a l parameters f o r determining m a t e r i a l s o r i g i n or sample i d e n t i t y , e.g., f o r wires used i n e x p l o s i v e devices. In such cases, the i n t e g r a t e d method of m a t e r i a l c h a r a c t e r i z a t i o n may turn out to be of considerable value. A f o r e n s i c a p p l i c a t i o n to a problem that occurs p r i m a r i l y w i t h m e t a l l i c o b j e c t s , namely the recovery of erased s e r i a l numbers, i s d e a l t with i n a separate s e c t i o n below. 2. Nonmetallic Inorganic S o l i d s . This category i n c l u d e s many items of f o r e n s i c importance: ceramic and g l a s s e s ; n a t u r a l l y o c c u r r i n g substances such as b u i l d i n g and i n s u l a t i o n m a t e r i a l s and s o i l components; a d d i t i v e s to papers, p a i n t s , e x p l o s i v e s , drugs and many other m a t e r i a l s . In c o n t r a s t to metals, even the task of b a s i c m a t e r i a l i d e n t i f i c a t i o n o f t e n r e q u i r e s considerably more than the o v e r a l l chemical a n a l y s i s f o r these substances. X-ray powder d i f f r a c t i o n data may be h e l p f u l but are o f t e n hard to i n t e r p r e t f o r complex mixtures; use of computer data f i l e search programs (6) and microcamera methods f o r s i n g l e p a r t i c l e a n a l y s i s (7) may be u s e f u l f o r i d e n t i f i c a t i o n . Comparative sample i d e n t i f i c a t i o n i s g e n e r a l l y l e s s o f t e n p o s s i b l e than f o r metals s i n c e the l a t t e r are manufactured while the nonmetallic i n o r g a n i c s o l i d s are o f t e n unprocessed m a t e r i a l s with l a r g e property v a r i a t i o n s . However, where a p p l i c a b l e , the f o l l o w i n g are some examples of determinations which might be made: (a) p a r t i c l e s i z e by microscopy; (b) m i c r o s t r u c t u r e and sub-microstructure c h a r a c t e r i z a t i o n



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e.g., f o r minerals, by the methods described above; (c) impurity trace a n a l y s i s by p a r t i c l e e x t r a c t i o n and a n a l y s i s (see the corresponding methods l i s t e d above f o r i m p u r i t i e s i n metals); (d) c r y s t a l p e r f e c t i o n , mosaic s i z e and m i s o r i e n t a t i o n , e.g., by X-ray microscopy such as the Berg-Barrett or other topographic techniques (3) or transmission e l e c t r o n microscopy; (e) atomic order by X-ray d i f f r a c t i o n ( e s p e c i a l l y f o r minerals such as s i l i cates) ; ( f ) i d e n t i f i c a t i o n of heteroatomic d e f e c t s by e l e c t r i c a l r e s i s t a n c e measurements, o p t i c a l property s t u d i e s or s p i n r e s o nance techniques; (g) c o n c e n t r a t i o n of p o i n t or l i n e d e f e c t s , e.g., by d e n s i t y s t u d i e s ; and .(h) thermal and, where a p p l i c a b l e , magnetic p r o p e r t i e s . Current e f f o r t s i n f o r e n s i c science have made some use of the above concepts. For example, the p o s s i b i l i t y of d e t e c t ing small l o c a l d i f f e r e n c e s i n the atomic environments i n g l a s s e s by monitoring luminescence which i s s e n s i t i v e to the atomic environment has been demonstrated by Jones (8). 3. Organic S o l i d s . M a t e r i a l s i n t h i s category are: p l a s t i c s and polymers, e s p e c i a l l y f i b e r s ; drugs and dyes; some n a t u r a l products, such as wood and n a t u r a l f i b e r s , and many others. Here a l s o , the elemental chemical a n a l y s i s i s g e n e r a l l y not s u f f i c i e n t . However, the use of i n f r a r e d a n a l y s i s , mass spectrometry, X-ray c r y s t a l l o g r a p h y , chromatography and other methods to supplement the compositional a n a l y s i s data i s w e l l known f o r organic product c h a r a c t e r i z a t i o n , e s p e c i a l l y i n the determination of the chemical compounds present. In a d d i t i o n to these chemical a n a l y t i c a l methods, t y p i c a l m a t e r i a l s science approaches could be used f o r sample i d e n t i f i c a t i o n . Thus, drugs could be f u r t h e r c h a r a c t e r i z e d by: (a) p a r t i c l e morphology (by microscopy); (b) c r y s t a l l i t e p e r f e c t i o n (by X-ray d i f f r a c t i o n or e l e c t r o n microscopy), or (c) trace impurity l e v e l (found as a second phase i n c r y s t a l l i n e m a t e r i a l s by transmission or scanning e l e c t r o n microscopy and i d e n t i f i e d by e l e c t r o n d i f f r a c t i o n and emission spectroscopy). The recent a p p l i c a t i o n of luminescence p r o p e r t i e s i s d e s c r i b e d i n Reference 8. Polymers have a number of e x p l o i t a b l e p r o p e r t i e s (thermal, mechanical, thermomechanical (9), r h e o l o g i c a l , s t r u c t u r a l (chain l e n g t h ) , NMR and ESR, o p t i c a l , e l e c t r i c a l and s u r face) that are not used or, at l e a s t , not commonly used f o r f o r e n s i c i d e n t i f i c a t i o n at present (10). 4. Organic-Inorganic Composite Products. In t h i s category we i n c l u d e here only paper, rubber and c e r t a i n b u i l d i n g m a t e r i a l s . Many methods suggested above f o r organic and i n o r g a n i c s o l i d s may be u s e f u l . Some a p p l i c a t i o n s f o r the i d e n t i f i c a t i o n of paper, one of the f o r e n s i c a l l y most important products i n t h i s category, are discussed i n d e t a i l below. The l i s t of new methods given above i s incomplete and i s i n tended only as a guide to the techniques that the f o r e n s i c m a t e r i a l s s c i e n t i s t may introduce i n t o the f i e l d . For most of the m a t e r i a l s p r o p e r t i e s we have considered here the establishment of reference l i b r a r i e s l i s t i n g c h a r a c t e r i s t i c values and t h e i r v a r i a t i o n through the p o p u l a t i o n would be necessary to make any


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new methods acceptable i n the f i e l d . The question of c o s t - e f f e c t i v e n e s s would a l s o have to be considered f o r each proposed innov a t i o n ; the use of r e g i o n a l or other s e r v i c e l a b o r a t o r i e s would be advantageous from equipment and manpower c o n s i d e r a t i o n s .


The Recovery of Erased

S e r i a l Numbers

A common c r i m i n a l i s t i c s problem to which m a t e r i a l s science technology i s a p p l i c a b l e i s the recovery of s e r i a l numbers which have been o b l i t e r a t e d from metal items. We d i s c u s s here the metall u r g i c a l background of s e r i a l number o b l i t e r a t i o n and recovery, the theory and p r a c t i c e of chemical or e l e c t r o - c h e m i c a l methods which form the bulk of the p r e s e n t l y employed methods, some techniques based on a l t e r n a t e approaches that are mostly experimental or have been proposed and, l a s t l y , a r e c e n t l y developed s e r i a l number marking technique capable of producing more permanent markings. M e t a l l u r g i c a l Background. G e n e r a l l y , the o b l i t e r a t e d numbers d e a l t with i n the crime l a b o r a t o r y have been produced by stamping, i . e . , s t r i k i n g the item with a d i e with a f o r c e s u f f i c i e n t to deform the metal so as to leave behind an impression of the t i p of the d i e . The metals of i n t e r e s t are p o l y c r y s t a l l i n e ; the atoms have a three-dimensionally p e r i o d i c arrangement w i t h i n l o c a l regions of 0.01-0.1 mm s i z e which are c a l l e d g r a i n s by m e t a l l u r g i s t s . Permanent deformation, or p l a s t i c flow, occurs i n these m a t e r i a l s by the motion of l i n e d e f e c t s , c a l l e d d i s l o c a t i o n s , through the c r y s t a l l i n e array. The movement of d i s l o c a t i o n s through the p e r i o d i c a l l y arranged atoms i n a g r a i n causes one p a r t of the g r a i n to move r e l a t i v e to the other p a r t so as to give a macroscopic change of shape. This i s represented schematically i n Figure 1. As a f o r c e i s a p p l i e d to the item through the d i e , the metal f i r s t becomes e l a s t i c a l l y s t r a i n e d and would r e t u r n to i t s i n i t i a l shape i f the f o r c e were removed at t h i s p o i n t . As the f o r c e i n c r e a s e s , the metal's e l a s t i c l i m i t i s exceeded and p l a s t i c flow occurs v i a the motion of d i s l o c a t i o n s . Many of these d i s l o c a t i o n s become entangled and trapped w i t h i n the p l a s t i c a l l y deformed m a t e r i a l ; thus, p l a s t i c deformation produces c r y s t a l s which are l e s s p e r f e c t and contain i n t e r n a l s t r e s s e s . These c r y s t a l s are designated as cold-worked and have p h y s i c a l p r o p e r t i e s which d i f f e r from those of the undeformed metal. As shown s c h e m a t i c a l l y i n Figure 2, each stamped number thus c o n s i s t s of a v i s i b l e i n d e n t a t i o n , a p l a s t i c a l l y deformed r e g i o n surrounding and d e f i n i n g the i n d e n t a t i o n , and an e l a s t i c a l l y s t r a i n e d r e g i o n bordering the p l a s t i c a l l y deformed area. T y p i c a l l y , s e r i a l numbers are removed i l l e g a l l y by f i l i n g or g r i n d i n g u n t i l the v i s i b l e i n d e n t a t i o n has been removed, o f t e n l e a v i n g behind the p l a s t i c a l l y deformed metal which was present beneath the i n d e n t a t i o n (See Figure 2). A l l s e r i a l number recovery tech-


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(b) Figure 1. Two-dimensional schematic of the motion of a dislocation through a crystalline array of atoms which causes a change in the shape of the crystal and contributes to change in the macroscopic shape of a metal item. The dislocation, visible at the left center of (a), is centered at the trapezoid connecting five atoms and is due to an extra vertical line of atoms in the upper half of the crystal. A shearing force which pushes the top half of the crystal to the right relative to the bottom half causes the dislocation to move from its position in (a) across the crystal, as shown in (b) and (c). This leads to the change in shape apparent by comparing (a) and (c).

Figure 2. Schematic of the cross section through a number stamped into metal. Removal of metal down to level (a) results in incomplete obliteration although the number may no longer be readily visible because metal has been smeared into the groove forming the number; recovery is easiest in this case. Removal of metal to level (b) leaves behind plastically deformed material; this is the situation for which recovery techniques, e.g., etching, can bring out the obliterated numbers. Removal of metal down to level (c) removes all metal plastically deformed during the stamping of the number; in this case, recovery is impossible.



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niques aim at d e t e c t i n g the l o c a t i o n of t h i s remaining p l a s t i c a l l y deformed metal. As a consequence, no recovery technique can p o s s i b l y be succ e s s f u l i f the number had o r i g i n a l l y been produced i n the item by c a s t i n g ( i . e . , without p l a s t i c deformation of the underlying region) and the i n d e n t a t i o n s were l a t e r f u l l y removed. In a d d i t i o n , recovery i s at l e a s t d i f f i c u l t and probably a l s o impossible f o r items which have been heated to a temperature high enough to cause recovery of the metal by the annealing out of d e f e c t s or r e c r y s t a l l i z a t i o n ( i . e . , atomic rearrangement forming new g r a i n s ) a f t e r the numbers were stamped or a f t e r s e r i a l number o b l i t e r a t i o n . (In the case of r e c r y s t a l l i z a t i o n , however, abnormal g r a i n growth might be observed near h i g h l y deformed regions.) There are many commonly measured p r o p e r t i e s of metals which are known to change upon cold-working. The best known e f f e c t i s an increase i n hardness. A d d i t i o n a l l y , the r e s i s t i v i t y increases and the thermal c o n d u c t i v i t y decreases; the e l e c t r o n i c work funct i o n i s changed; the X-ray d i f f r a c t i o n p a t t e r n i s broadened. Each of these property changes can be considered as the b a s i s of a method to detect p l a s t i c a l l y deformed regions l e f t behind a f t e r the v i s i b l e indentations of s e r i a l numbers have been removed. Chemical or E l e c t r o c h e m i c a l S e r i a l Number Recovery Methods. These methods form the bulk of the procedures i n current p r a c t i c e i n crime l a b o r a t o r i e s ; they are t h e r e f o r e discussed here i n more d e t a i l than other, s t i l l experimental techniques. 1. E t c h i n g . I t i s w e l l known to m e t a l l u r g i s t s that metal i n the v i c i n i t y of a g r a i n boundary or d i s l o c a t i o n or from a r e g i o n of l o c a l i z e d e l a s t i c s t r e s s i s more e l e c t r o c h e m i c a l l y a c t i v e , i . e . , i t can be made to d i s s o l v e p r e f e r e n t i a l l y i n a s u i t a b l e a c i d i c s o l u t i o n . This occurs because the metal at these l o c a t i o n s has a higher chemical p o t e n t i a l than i n the r e s t of the substance due to the stored energy of cold-working, i . e . , the r e g i o n c o n t a i n i n g the d e f e c t i s more negative i n the EMF s e r i e s . T h i s e f f e c t i s the b a s i s f o r the v i s u a l i z a t i o n of metal d e f e c t s by e t c h i n g . Many years of e m p i r i c a l t e s t i n g have r e s u l t e d i n l i s t s of etchants s u i t a b l e f o r p a r t i c u l a r metal s t u d i e s (11,12); the d e s i r e d r e s u l t of etching g e n e r a l l y c o n s i s t s of a l o c a l change i n the l i g h t r e flectivity. This change may be due to p r e f e r r e d a t t a c k at c r y s t a l d e f e c t s , a change of g r a i n surface o r i e n t a t i o n to expose c r y s t a l planes with a lower r a t e of a t t a c k or v a r i a t i o n s i n the r a t e of a t t a c k f o r grains with d i f f e r e n t c r y s t a l o r i e n t a t i o n s . Various etchants s u i t a b l e f o r s e r i a l number recovery and the procedures to be followed have been discussed i n the l i t e r a t u r e (13-15). Here, we give a b r i e f review of the process used f o r s t e e l s and add some observations made i n our l a b o r a t o r y . An important f i r s t step i n v o l v e s proper surface p r e p a r a t i o n . The area to be t r e a t e d must be smooth f o r optimal a p p l i c a t i o n of t h i s recovery process. A smooth f i n i s h to remove a l l g r i n d i n g and f i l i n g scratches i s r e q u i r e d as the shallow c o l d worked



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regions a s s o c i a t e d with the g r i n d i n g or f i l i n g scratches a l s o produce c o n t r a s t e f f e c t s and thus i n t e r f e r e with number recovery. (Obviously, care must be taken not t o remove more metal than i s necessary so as to conserve the p l a s t i c a l l y deformed metal beneath the number.) We b e l i e v e that c a r e f u l p o l i s h i n g can be done without decreasing the p r o b a b i l i t y of s u c c e s s f u l recovery s i n c e g e n e r a l l y the c o l d worked regions beneath the s e r i a l number imp r i n t s are deeper than those under the scratches; under these c o n d i t i o n s , the e l i m i n a t i o n of i n t e r f e r e n c e from the scratches by p o l i s h i n g outweighs the advantages of r e t a i n i n g the a d d i t i o n a l metal. Thus, we have p o l i s h e d metal specimens by using a 240 g r i t paper to p o l i s h i n a d i r e c t i o n perpendicular to g r i n d i n g scratches j u s t u n t i l they disappeared. Then 320, 400 and 600 g r i t papers were used i n turn, again p o l i s h i n g p e r p e n d i c u l a r l y to the previous scratches u n t i l they disappeared. The specimen being p o l i s h e d was kept wet; the f i n a l surface had a m i r r o r - l i k e reflectivity. There may be advantages to going even f u r t h e r and producing a m i c r o s c o p i c a l l y smooth surface f i n i s h ( s c r a t c h width < 0.001mm) which can be obtained by p o l i s h i n g with alumina s l u r r y or diamond paste; t h i s area i s being explored i n our l a b o r a tory. The a c i d i s then a p p l i e d to the s u r f a c e , e i t h e r by immersion or swabbing. The a c i d i c s o l u t i o n s which are recommended f o r s t e e l s (13,15) and have been found to work w e l l f o r number recovery are aqueous s o l u t i o n s of HC1 and C u C l (which sometimes cont a i n an a l c o h o l ) . S p e c i f i c etchants of t h i s type are known as "Fry's reagent" and are known to make v i s i b l e s t r a i n l i n e s due to c o l d work (12). A p r e f e r r e d reagent (12,13) i s a mixture of 5 gm CuCl2, 40 ml HC1, 30 ml d i s t i l l e d water and 25 ml ethanol. Swabbing of the surface with t h i s s o l u t i o n has been found to r e s t o r e the number on s t e e l samples, where the i n d e n t a t i o n s have been f u l l y ground o f f and which were then p o l i s h e d , w i t h i n 5-20 minutes. T h i s etchant d i s s o l v e s away the p l a s t i c a l l y deformed regions more r a p i d l y , forming e t c h p i t s which become v i s i b l e because of d i f f e r ences i n l i g h t r e f l e c t i v i t y . The etching process depends on p r o p e r t i e s studied i n d i f f e r ent s u b d i v i s i o n s of m a t e r i a l s science and chemistry: stored energy and the nature of the d e f e c t s ( p h y s i c a l m e t a l l u r g y ) , l o c a l e l e c t r o l y t i c a c t i o n ( e l e c t r o c h e m i s t r y ) , boundary l a y e r e f f e c t s on etching (surface s c i e n c e ) , and the nature (e.g., type of complex) of the s o l u t e present i n the l i q u i d ( i n o r g a n i c s o l u t i o n chemistry). Fundamental understanding of the p a r t i c i p a t i n g processes would be required to optimize etchant compositions or f i n d new, b e t t e r etchant combinations; however, the mechanism of t h i s d i f f e r e n t i a l a t t a c k does not appear to be w e l l understood. The mechanism of attack must be dependent on the types of copperc h l o r i d e complexes which are present s i n c e a reagent s o l u t i o n without copper does not show a very pronounced d i f f e r e n t i a t i o n i n a t t a c k while a C u C ^ s o l u t i o n c o n t a i n i n g more d i l u t e HC1 r e s u l t s i n copper p r e c i p i t a t i o n on the s t e e l . I t i s not known whether 2



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the d i f f e r e n t i a l a t t a c k i s k i n e t i c a l l y as w e l l as thermodynamicall y c o n t r o l l e d , and no study of the nature of the surface s t r u c t u r e m o d i f i c a t i o n upon etching has been reported. 2. E l e c t r o e t c h i n g . The etching can be speeded up by applying an e l e c t r i c f i e l d to the specimen (13,15,16). Current p r a c t i c e i n some crime l a b o r a t o r i e s i s to use an a p p l i e d D.C. v o l tage, but i t appears that the p o t e n t i a l of t h i s approach has not yet been e x p l o i t e d f u l l y . E l e c t r o e t c h i n g has the p o t e n t i a l f o r a f i n e tuning of the a p p l i e d v o l t a g e so that the d i f f e r e n c e i n r e a c t i o n r a t e between the deformed and undeformed m a t e r i a l can be maximized, r e s u l t i n g i n f a s t e r and p o s s i b l y b e t t e r number recovery. 3. Other Chemical or E l e c t r o c h e m i c a l Methods. Of i n t e r est i s a s p e c i a l i z e d procedure f o r aluminum which has been r e p o r t ed (17). A t h i n coating of mercury i s used to c a t a l y z e the o x i d a t i o n of aluminum by a i r , p o s s i b l y by breaching the p r o t e c t i v e aluminum oxide l a y e r . The number presumably becomes v i s i b l e because the p l a s t i c a l l y deformed regions o x i d i z e f a s t e r than the surrounding m a t e r i a l , thus again making use of the electrochemic a l d i f f e r e n c e between deformed and undeformed r e g i o n s . Another e l e c t r o a n a l y t i c a l technique i s being considered. I f a heavy metal, e.g., Au, were p r e f e r e n t i a l l y p l a t e d out over the deformed r e g i o n i n the form of a t h i n l a y e r , (e.g., i n an e l e c t r o c h e m i c a l c e l l or simply by immersion i n an appropriate s o l u t i o n , the r e s u l t i n g r e p l i c a of the s e r i a l number could be made v i s i b l e i n the scanning e l e c t r o n microscope by elemental mapping of Au, where the l o c a t i o n of the Au i s d i s p l a y e d by analyzing f o r f l u o r e s c e n t Au X - r a d i a t i o n . This approach has yet to be examined experimentally. R e s t o r a t i o n Methods Based on A l t e r n a t e Approaches. Other property changes o c c u r r i n g i n the p l a s t i c a l l y or e l a s t i c a l l y deformed regions may be considered f o r u t i l i z a t i o n i n s e r i a l number r e s t o r a t i o n ; t h e i r i d e n t i f i c a t i o n and e x p l o i t a t i o n f o r f i e l d use i s a genuine challenge to the m a t e r i a l s s c i e n t i s t . 1. Hardness. The i n c r e a s e i n hardness of a metal upon c o l d working (work hardening) i s w e l l documented. D i r e c t d e t e c t i o n of the deformed regions using l o c a l micro-hardness measurements over the surface appears i m p r a c t i c a l because of the f i n e r e s o l u t i o n and, hence, time r e q u i r e d to recover a s e r i e s of numbers. Methods which would produce a surface morphology dependent on the l o c a l hardness might, however, be a p p l i c a b l e . One such experimental technique uses u l t r a s o n i c c a v i t a t i o n to detect hardness d i f f e r e n c e s (18). The sample and an u l t r a s o n i c transducer placed near the surface to be s t u d i e d are immersed i n a l i q u i d . The u l t r a s o n i c e x c i t a t i o n causes small bubbles to form i n the l i q u i d ; the c o l l a p s e of these bubbles causes abrasion of the s u r f a c e . The hardened regions are not damaged as much as the surrounding matrix ( i n c o n t r a s t to the chemical method described above I) and thus become v i s i b l e because of d i f f e r e n c e s i n l i g h t reflectivity. This method i s e s p e c i a l l y e f f e c t i v e i n removing



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metal which was smeared i n t o the grooves forming the number, i . e . i n s i t u a t i o n s i n v o l v i n g an incompletely o b l i t e r a t e d s e r i a l number. I t s e f f e c t i v e n e s s has not yet been q u a n t i t a t i v e l y compared with the chemical methods described above. 2. Magnetic Methods. The preceding methods are d e s t r u c t i v e t e s t s i n that the r e s t o r a t i o n technique permanently a l t e r s the speciman. I f improper c o n d i t i o n s are a p p l i e d i n d e s t r u c t i v e t e s t s , there i s o f t e n no second chance to recover the number. Nondestructive methods are t h e r e f o r e e s p e c i a l l y a t t r a c t i v e . Seve r a l promising, nondestructive approaches f o r s e r i a l number r e covery from ferromagnetic a l l o y s are based on the magnetization behavior of the metal. The p o t e n t i a l of t h i s method has been r e a l i z e d (15) but appears not to have been f u l l y e x p l o i t e d . Cold working of magnetic s t e e l changes i t s magnetization behavior. When the s t e e l i s magnetized by being placed i n a magnetic f i e l d , the c o l d worked regions do not magnetize as readi l y as the undeformed m a t e r i a l . T h i s i s due to the presence of the d e f e c t s described above which i n h i b i t the rearrangement of the ferromagnetic domains e x i s t i n g i n a d i s o r d e r e d arrangement p r i o r to magnetization of the metal. Conversely, the deformed regions do not demagnetize as r e a d i l y as the undeformed regions on removal of the f i e l d . The d i f f e r e n t degrees of magnetization can be d i s p l a y e d by applying a magnetic powder to the s u r f a c e . D i f f e r e n c e s i n r e s i d u a l magnetization can a l s o be detected by scanning the s u r f a c e w i t h a magnetic probe. A t h i r d approach, which i s i n the experimental stage at t h i s l a b o r a t o r y , i s based on the p o s s i b i l i t y of d i s p l a y i n g magnetic domains d i r e c t l y i n the scanning e l e c t r o n microscope. 3. E l e c t r i c a l R e s i s t i v i t y . D i f f e r e n c e s i n e l e c t r i c a l r e s i s t i v i t y and magnetic p e r m e a b i l i t y are u t i l i z e d i n another potent i a l l y u s e f u l , nondestructive technique, the eddy current method (19). In t h i s technique, the surface i s scanned a t c l o s e d i s tance w i t h a small c o i l c a r r y i n g a l t e r n a t i n g c u r r e n t . The magnet i c f i e l d of the c o i l induces eddy currents i n the nearby sample; the magnitude of the eddy currents depends on the l o c a l e l e c t r i c a l c o n d u c t i v i t y and on the p e r m e a b i l i t y of the sample. These eddy currents i n t u r n set up a magnetic f i e l d which opposes the f i e l d from the c o i l and thus changes the apparent impedance of the c o i l . Since the e l e c t r i c a l r e s i s t i v i t y and magnetic permeab i l i t y are changed by deformation, the regions underneath the i n d e n t a t i o n can be detected by scanning w i t h a s u i t a b l y small probe so as to record the apparent impedance as a f u n c t i o n of position. An Improved

S e r i a l Number Marking System

Since recovery of o b l i t e r a t e d stamped numbers cannot always be accomplished, a marking system which produces tagging more r e s i s t a n t to removal would be d e s i r a b l e . In such an improved system, the marking e f f e c t must extend w e l l i n t o the item r a t h e r



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than being a surface e f f e c t such as r e s u l t s from stamped numbers. For s p e c i f i c items, the marking should be compatible with l o c a t i o n s such that i t s removal would make the item u s e l e s s . One p o s s i b l e marking system which has such f e a t u r e s has been developed at t h i s I n s t i t u t e (20). This marking system i s based on the d r i l l i n g of an array of~~holes i n t o the item; an encoding system i s u t i l i z e d such that the s e r i a l number i s represented by the r e l a t i v e placement of the h o l e s . In each case, the holes would have to be small enough so that they would not i n t e r f e r e with the f u n c t i o n of the item. For t h i s reason and so that the number can be recorded i n a small r e g i o n , very small holes having a diameter of s e v e r a l thousandths of an inch a r e d e s i r a b l e . Such holes can be produced by using a high powered l a s e r . A pulse of l i g h t from the l a s e r i s focused to a very narrow beam which then b o i l s o f f the m a t e r i a l which i t s t r i k e s . For example, holes with a diameter of 0.005" and a depth of 1/8" can be d r i l l e d i n t o s t e e l w i t h i n a second. Various encoding systems can be envisioned; one p o s s i b l e encoding system using l a s e r holes i s shown i n F i g u r e 3. In t h i s example, the number 5488159066 i s represented by 10 holes l o c a t e d on a 10 x 10 g r i d . Each i n d i v i d u a l d i g i t of the s e r i a l number i s represented by one hole i n i t s column; the uppermost l o c a t i o n corresponds to a 1, the next lower to a 2, e t c . The three holes f l a n k i n g the g r i d a r e reference p o i n t s which d e f i n e the g r i d s i n c e the g r i d shown i n the drawing of Figure 3 would not appear on a marked item. Thus 13 holes can be used to d i s t i n g u i s h 10 , or 10 b i l l i o n , d i f f e r e n t items. Some of the d i g i t s could be used to s i g n i f y the model number of the item. Using a g r i d having p o i n t s 0.010" apart, t h i s p a t t e r n can be recorded w i t h i n a surface area of 1/8" square. A d r i l l e d array using the code described above and representing the number 5383158068 i s shown i n F i g u r e 4. Next to i t i s a 1/8" d i g i t t y p i c a l l y used f o r s e r i a l numbers on guns, shown f o r comparison of s i z e . As has been s t a t e d p r e v i o u s l y , many d i f f e r e n t encoding systems and m o d i f i c a t i o n s of t h i s method can be envisioned (20). M i n i a t u r i z a t i o n i s e s p e c i a l l y u s e f u l to record the number i n a c r i t i c a l area of a small item. The proposed marking system appears i d e a l l y s u i t e d f o r firearms where i t would be d e s i r a b l e to l o c a t e the marking i n an area such that i t s removal would make the gun inoperable. For items not having c r i t i c a l l y important regions, the hole p a t t e r n could be spread out over a l a r g e part of the s u r f a c e . D r i l l i n g of somewhat l a r g e r holes with a d i f f e r e n t encoding system could be used to produce a p a t t e r n which could be read by the unaided eye. The l a s e r d r i l l i n g based marking system i s commercially f e a s i b l e and could be f u l l y automated f o r a production procedure. I t could a l s o be used to t a g i n d i v i d u a l items. I f d e s i r e d , the holes could be f u l l y hidden by a treatment of the surface a f t e r drilling. Further work i s i n progress to evaluate the p o t e n t i a l


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Figure 3. An example of a pattern of 13 holes that can he used to represent a 10-digit serial number, The three holes flanking the grid are reference points which define it. Each digit is determined by the position of the hole in the column associated with it.

Figure 4. A photograph of a pattern of laser drilled holes in steel representing the number 5383158068. Also shown is a Vs" stamped digit for the purpose of size comparison.


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70 of the method f o r the marking of f i r e a r m s .


Paper I d e n t i f i c a t i o n Paper i s one of the more common e v i d e n t i a l m a t e r i a l s encountered i n crimes such as f o r g e r y , conspiracy, threatening l e t t e r s and kidnapping; i t s examination i s t h e r e f o r e f r e q u e n t l y r e q u i r e d i n the f o r e n s i c l a b o r a t o r y (21). (Another aspect of document examination, v i z . , ink a n a l y s i s , i s t r e a t e d i n another chapter of t h i s volume (22).) As with other f o r e n s i c m a t e r i a l s , two quest i o n s commonly a r i s e : 1. Are two samples of paper i d e n t i c a l ? 2. What i s the o r i g i n and h i s t o r y of the paper, e s p e c i a l l y , what i s i t s date and place of manufacture? The methods c u r r e n t l y used i n f o r e n s i c science l a b o r a t o r i e s (23,24) are based, i n general, on those developed by the paper i n d u s t r y f o r i t s own uses (25). The main o b j e c t i v e s of the i n d u s t r i a l t e s t s are to monitor" and detect f a u l t s i n the manufacturing process and to improve product q u a l i t y and u n i f o r m i t y ; as the o r i g i n of the t e s t e d paper i s known, questions of i d e n t i f i c a t i o n do not a r i s e and some t e s t s are t h e r e f o r e not very h e l p f u l i n d i f f e r e n t i a t i n g papers. I n d u s t r i a l c r i t e r i a which have been used i n the f o r e n s i c s i t u a t i o n are: surface and macrostructural c h a r a c t e r i s t i c s of the papers such as wire marks and watermarks; thickness; weight per u n i t area; elemental a n a l y s i s using spectrography or chromatography; r e l a t i v e amount of d i f f e r e n t f i b e r types i n the sample. Recently, the p o s s i b l i t y of improving the elemental a n a l y s i s of paper by a q u a n t i t a t i v e determination of metals and t r a c e met a l s using neutron a c t i v a t i o n a n a l y s i s has been i n v e s t i g a t e d (26, 27). T h i s work showed the p o t e n t i a l of NAA i n attempting to answer the two questions posed above. However, the method i n volves equipment a v a i l a b l e to very few l a b o r a t o r i e s . An a l t e r n a t i v e , m a t e r i a l s - o r i e n t e d approach i n v o l v e s the q u a n t i t a t i v e i d e n t i f i c a t i o n of the c o n s t i t u e n t s of the paper. This would i n c l u d e the determination of t h e i r compositions, s t r u c tures and morphologies, as w e l l as t h e i r amounts. A study of t h i s approach i n v o l v i n g the i n o r g a n i c components of paper has been i n i t i a t e d i n this laboratory. X-ray D i f f r a c t i o n A n a l y s i s . The i n o r g a n i c components of paper are the most s u i t a b l e ones f o r q u a n t i t a t i v e X-ray d i f f r a c t i o n a n a l y s i s . Most of these compounds are minerals and are present as f i l l e r s , coatings and pigments ( o f t e n whiteners) which are added to improve the p r o p e r t i e s of the paper. Examples of compounds commonly added to paper are alumina, aluminum s i l i c a t e , barium s u l f a t e , calcium carbonate, calcium s u l f a t e , calcium s u l foaluminate, i r o n oxide, magnesium s i l i c a t e , s i l i c a , t i t a n i u m d i o x i d e , z i n c oxide, and z i n c s u l f i d e (28). Some of these, e.g., calcium carbonate and t i t a n i u m d i o x i d e , may be present i n any of



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several c r y s t a l l i n e modifications. In the paper i n d u s t r y , these i n o r g a n i c components are d e t e r mined p r i m a r i l y by microscopy, which i s time-consuming and q u a l i t a t i v e r a t h e r than q u a n t i t a t i v e . X-ray d i f f r a c t i o n has been recogn i z e d as a p o s s i b l e method of a n a l y s i s and a number of i n o r g a n i c components are described as having been e a s i l y i d e n t i f i e d at conc e n t r a t i o n s down to 0.5 to 2% of the paper weight (29). While t h i s method i s not i d e a l l y s u i t e d to the q u a l i t y c o n t r o l needs of the paper i n d u s t r y and has not been put i n t o p r a c t i c e , i t appears promising f o r the purpose of f o r e n s i c i d e n t i f i c a t i o n . An i n v e s t i g a t i o n of the p o s s i b l e v a l u e of X-ray methods i s now i n progress using both untreated and ashed paper samples. I f untreated paper samples are examined, the X-ray d i f f r a c t i o n method appears p r e s e n t l y to be l i m i t e d to the determination of those major i n o r g a n i c components c o n s t i t u t i n g more than about 0.5% of the paper. This i s caused by the swamping of the l e s s intense sharp l i n e patterns from the i n o r g a n i c c r y s t a l s by the i n t e n s e , d i f f u s e r e f l e c t i o n s from the c e l l u l o s e which c o n s t i t u t e s the bulk of the paper m a t e r i a l . Thus, s e n s i t i v i t y c o n s i d e r a t i o n s l i m i t conventional X-ray d i f f r a c t i o n to papers having s u f f i c i e n t q u a n t i t i e s of i n o r g a n i c components and, even f o r these papers, precludes the examination of minor components present with lower concentration. Two complementary approaches can t h e r e f o r e be u t i l i z e d . 1. Untreated Paper. One approach i s to use a modified X-ray d i f f T a c t o m e t e r with an increased s i g n a l - t o - n o i s e r a t i o , (e.g., employing slow scanning or step scanning, high q u a l i t y s o l i d s t a t e e l e c t r o n i c s , and s i n g l e or double-monochromated r a d i a t i o n ) to examine untreated paper samples. Advantages of t h i s method are that i t i s non-destructive and that the use of a d i f f r a c t o m e t e r makes p o s s i b l e the examination of i n c h - s i z e samp l e s ; data output from the d i f f r a c t o m e t e r may be i n d i g i t a l as w e l l as chart form and i s thus d i r e c t l y usable f o r computer treatment, e.g., i n connection with a f i l e - s e a r c h program ( 6 ) . The simultaneous input of elemental composition data (see f o l l o w ing s e c t i o n ) i n t o the computer treatment of the X-ray data subs t a n t i a l l y f a c i l i t a t e s the powder p a t t e r n i d e n t i f i c a t i o n . 2. Ashed Papers A second approach s t a r t s w i t h the ashing of the paper. T h i s i s a l s o done as a standard c h a r a c t e r i z a t i o n technique which determines ashed weight as a percentage of o r i g i n a l weight. Ashing could lead to a change i n the c r y s t a l s t r u c t u r e s of some of the i n o r g a n i c components and p o s s i b l y a l s o to t h e i r decomposition. Although the method would not be r u l e d out i f such changes were found to be r e p r o d u c i b l e , precautions should be taken to minimize these e f f e c t s ; ashing i n oxygen at a lower temperature than u s u a l has therefore been introduced. The ash i s then s t u d i e d i n a powder camera; here the m o d i f i c a t i o n s r e c e n t l y proposed (7) f o r the examination of microsamples (small camera r a d i u s , vacuum, monochromated r a d i a t i o n ) may be employed. Photometry y i e l d s r e l a t i v e i n t e n s i t i e s of the component r e f l e c -


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t i o n s and hence information on the r e l a t i v e abundance of each component i s obtained. 3. Paper Separations. The s e p a r a t i o n of the i n o r g a n i c components from the remainder of the paper by d i f f e r e n t i a l c e n t r i f u g a t i o n of a suspension of the paper dispersed i n a f l u i d before X-ray examination may a l s o be u s e f u l . I n s o l u b l e components could be obtained d i r e c t l y , w h i l e s o l u b l e components would have to be extracted and i s o l a t e d by evaporation. However, t h i s d i s p e r s i o n process would probably lead to a m o d i f i c a t i o n of the components or t h e i r s t r u c t u r e s . The same i s true f o r the p o s s i b l e removal of the c e l l u l o s e by h y d r o l y s i s i n the presence of commerc i a l l y a v a i l a b l e enzymes. Scanning E l e c t r o n Microscopy of Paper. The surface morphology of papers i s a n a t u r a l area of a p p l i c a t i o n of the scanning e l e c t r o n microscope (SEM) because of i t s depth of focus. Surveys have been made, and an e x c e l l e n t a t l a s of paper s t r u c t u r e s e x i s t s (28). Attachments f o r wavelength or energy d i s p e r s i v e a n a l y s i s of f l u o r e s c e n t X-rays on the SEM allow the elemental a n a l y s i s of s e l e c t e d p a r t i c l e s ; t h i s can be done e s p e c i a l l y r a p i d l y i f a h i g h - r e s o l u t i o n energy-dispersive semiconductor detector system (30) i s used. P a r t i c l e i d e n t i f i c a t i o n i s o f t e n p o s s i b l e (29). T h i s type of a n a l y s i s would be h e l p f u l i n the i d e n t i f i c a t i o n stage of the q u a n t i t a t i v e X-ray d i f f r a c t i o n a n a l y s i s method desc r i b e d i n the previous s e c t i o n ; however, i t appears that, by i t s e l f , X-ray m i c r o a n a l y s i s only of i s o l a t e d p a r t i c l e s w i l l not i n general y i e l d a q u a n t i t a t i v e , i d e n t i f y i n g a n a l y s i s of the paper because of sampling c o n s i d e r a t i o n s r e l a t e d to the m i c r o s c o p i c a l l y inhomogeneous d i s t r i b u t i o n of these p a r t i c l e s . An a l t e r n a t i v e approach i s to do the SEM X-ray a n a l y s i s over a r e p r e s e n t a t i v e area of the paper to produce proper averaging (31) . However, the presence of a l a r g e Bremsstrahlung background i r T t h e f l u o r e s c e n c e spectrum due to the c e l l u l o s e makes the counting s t a t i s t i c s f o r the i n o r g a n i c component unfavorable, e s p e c i a l l y f o r elements w i t h low abundance i n paper. A r e p r e s e n t a t i v e elemental a n a l y s i s , however, may be obtained by s u b j e c t i n g ashed paper to SEM X-ray a n a l y s i s ; t h i s method, which i s p r e s e n t l y under study, avoids the disadvantages noted above and w i l l be reported upon s h o r t l y (32). The Fluorescence P r o p e r t i e s of Paper. Luminescence propert i e s provide h i g h l y d i s t i n c t i v e f o r e n s i c c h a r a c t e r i s t i c s , as shown by Jones ( 8 ) . In a current study i n t h i s I n s t i t u t e , the f l u o r e s c e n c e p r o p e r t i e s of s e v e r a l types of paper were determined under e x c i t a t i o n w i t h Hg r a d i a t i o n , and t h i s work w i l l be r e p o r t ed i n greater d e t a i l elsewhere (33). We note here that q u a n t i t a t i v e f l u o r e s c e n t emission spectrometry i s not, per se, s u f f i c i e n t f o r f o r e n s i c paper i d e n t i f i c a t i o n ; almost a l l papers that show any s i g n i f i c a n t f l u o r e s c e n c e emit a s i m i l a r spectrum due to a


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small number of a c t i v e organic compounds. While there are s i g n i f i c a n t i n t e n s i t y d i f f e r e n c e s i n d i f f e r e n t papers, the samples do not show s u f f i c i e n t d i s p e r s i o n of the i n t e n s i t i e s f o r the method to be u s e f u l as a prime measurement f o r f o r e n s i c i d e n t i f i c a t i o n . L i f e t i m e s t u d i e s have been made, but the very short l i f e t i m e s of

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