A Comparison of Neutron Activation Analysis and Atomic Absorption

11 A Comparison of Neutron Activation Analysis and Atomic Absorption Spectroscopy on Gunshot Residue

Downloaded by UNIV OF PITTSBURGH on June 22, 2013 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0013.ch011

W. D. KINARD and D. R. LUNDY Bureau of Alcohol, Tobacco, and Firearms, Forensic Branch, Department of the Treasury, Washington, D. C. 20226

Several techniques have been utilized f o r the d e t e c t i o n of firearms discharge residue on the hands of an i n d i v i d u a l who has r e c e n t l y handled o r discharged a weapon. T e s t i n g for the presence of n i t r a t e s proved u n r e l i a b l e and was d i s c a r d e d . Colorimetric t e s t s for barium and antimony from primer composition were found to l a c k s u f f i c i e n t sensitivity f o r general a p p l i c a t i o n . Neutron a c t i v a t i o n a n a l y s i s (NAA) with a r a p i d radiochemical s e p a r a t i o n has been the method g e n e r a l l y used in recent years, but r e q u i r e s s u b s t a n t i a l investment, has h i g h o p e r a t i n g cost and limited availability. Modern flameless atomic absorption (AAS) instruments provide s e n s i t i v i t y approaching that of NAA and o f f e r a v i a b l e a l t e r n a t i v e for the d e t e c t i o n o f firearms discharge residue. Measurements made by both NAA and AAS on samples taken from a c t u a l firings will be compared in t h i s paper, and the advantages and l i m i t a t i o n s of each methodology will be d i s c u s s e d . Introduction Faced with the widespread use of firearms i n c r i m i n a l activity, law enforcement o f f i c e r s have l o n g sought an e f f e c t i v e method to determine i f an i n d i v i d u a l has r e c e n t l y handled o r f i r e d a weapon. A t e s t o f t h i s type i s o b v i o u s l y v a l u a b l e i n the i n v e s t i g a t i o n o f a l l e g e d s u i c i d e s , homicides, armed a s s a u l t s and other v i o l a t i o n s i n v o l v i n g the use of f i r e a r m s . The diphenylamine-sulfuric a c i d dermal n i t r a t e t e s t , i n t r o duced i n the 1 9 3 0 s, was a method to detect the presence o f n i t r i t e s and n i t r a t e s from gunpowder discharge r e s i d u e s . Res t r i c t i o n s on the use o f t h i s procedure were suggested i n 1 9 3 5 and again i n 19^0 by the F e d e r a l Bureau of I n v e s t i g a t i o n (l,2). Although the l i m i t a t i o n s of t h i s t e s t f o r d e t e c t i n g the presence of gunpowder residues were known, i t s use continued, due mainly to the l a c k of a s u i t a b l e a l t e r n a t i v e method. 1

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In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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In 1 9 5 9 j H a r r i s o n and G i l r o y ( 3 ) demonstrated the d e t e c t a b i l i t y o f barium, antimony and l e a d i n f i r e a r m discharge residue u s i n g a s p e c i f i c "spot t e s t f o r each element. Inadequate c o l o r i m e t r i c s e n s i t i v i t y f o r barium and antimony (1+) has s e v e r e l y r e s t r i c t e d the use o f the method as a f i e l d t o o l . 11

The development o f neutron a c t i v a t i o n a n a l y s i s (NAA) as a s e n s i t i v e and s p e c i f i c method o f t r a c e elemental a n a l y s i s l e d t o i t s a p p l i c a t i o n d u r i n g the 1 9 5 0 s f o r the d e t e c t i o n o f f i r e a r m discharge r e s i d u e . I t s a b i l i t y t o detect and i d e n t i f y very low concentrations o f barium and antimony, elements a s s o c i a t e d with most primer compositions, was encouraging. Neutron a c t i v a t i o n a n a l y s i s i s based upon the production o f r a d i o i s o t o p e s by n u c l e a r r e a c t i o n s r e s u l t i n g from neutron bom bardment , followed by i d e n t i f i c a t i o n and measurement o f the d i f f e r e n t r a d i o i s o t o p e s formed. Element a c t i v a t i o n can a l s o be c a r r i e d out by bombardment with high-energy charged p a r t i c l e s , X-rays o r gamma rays ( 5 ) . Since the development o f h i g h - f l u x research r e a c t o r s , neutrons, mostly i n the thermal energy range, have been widely used as bombarding p a r t i c l e s . Most o f the s t a b l e isotopes are capable o f c a p t u r i n g "thermal" neutrons, but with widely v a r y i n g capture p r o b a b i l i t i e s . These p r o b a b i l i t i e s are determined by the elemental neutron capture c r o s s - s e c t i o n . The capture o f a neutron produces an e n e r g e t i c a l l y " e x c i t e d " r a d i o n u c l i d e which then may r e l i n q u i s h i t s excess energy by emission o f gamma r a d i a t i o n . The o v e r a l l process i s commonly known as an "n-gamma r e a c t i o n " . Elemental 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 based on n-gamma r e a c t i o n s are p o s s i b l e because the energy o f gamma r a d i a t i o n emitted by e x c i t e d n u c l e i i s c h a r a c t e r i s t i c o f the n u c l e a r species and the i n t e n s i t y o f r a d i a t i o n i s p r o p o r t i o n a l to the number o f such n u c l e i i n the sample. Most (75% t o 80%) o f the n a t u r a l l y οccuring elements are capable o f undergoing n-gamma (n,v) r e a c t i o n s .


1

The u t i l i z a t i o n o f NAA i n f o r e n s i c i n v e s t i g a t i o n s began i n the l a t e 1 9 5 0 s, when s e v e r a l p o t e n t i a l a p p l i c a t i o n s were r e p o r t ed ( 6 , 7 ) » Soon a f t e r , Ruch and co-workers ( 8 ) demonstrated the s u i t a b i l i t y o f the NAA method t o the d e t e c t i o n o f elements a s s o c i a t e d with f i r e a r m discharge r e s i d u e . A decade o f e f f o r t by t h a t group produced s e v e r a l volumes o f u s e f u l information r e g a r d i n g the a p p l i c a t i o n aspects o f t h i s method ( 9 $ 1 0 ) « Between 1966 and 1 9 7 3 · the a p p l i c a t i o n o f NAA t o the d e t e c t i o n o f f i r e arm discharge residues were examined by a number o f other i n v e s t i g a t o r s ( 1 1 , 1 2 , 1 3 ) t and encouraging r e s u l t s were obtained. A major problem c o n f r o n t i n g f o r e n s i c l a b o r a t o r i e s i n t e r e s t e d i n employing NAA i s the s c a r c i t y o f s u i t a b l e r e a c t o r f a c i l i t i e s . I f firearms discharge residue a n a l y s i s i s t o be widely employed, an a l t e r n a t e technique must be developed. Atomic absorption 1



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spectroscopy (AAS) appeared to be a promising candidate. I t has, f o r many elements, s e n s i t i v i t y comparable with t h a t o f NAA, and a l l work can be done i n the f o r e n s i c l a b o r a t o r y . The e a r l y AAS work, c a r r i e d out on flame-type instruments encountered s e n s i t i v i t y l i m i t a t i o n s ; however, such problems have decreased by the i n t r o d u c t i o n of f l a m e l e s s u n i t s , with attendant improvements i n instrumentation and o p t i c s . Although d i f f i c u l t i e s have been en countered with carbon rod techniques, the tantalum s t r i p atomizer has a f f o r d e d e x c e l l e n t r e s u l t s . Atomic a b s o r p t i o n spectroscopy (AAS) i s based upon the a b s o r p t i o n of l i g h t a t s p e c i f i c and c h a r a c t e r i s t i c wavelengths by elements i n t h e i r atomic s t a t e s . A n a l y s i s f o r an element i s accomplished by p a s s i n g l i g h t of a s e l e c t e d wavelength through a "cloud" of f r e e atoms and observing the amount o f l i g h t absorbed. One method o f o b t a i n i n g the "cloud" uses a flame. The sample i s d i s s o l v e d and the s o l u t i o n i s a s p i r a t e d through a hot flame t o atomize any m e t a l l i c components. T h i s method was f i r s t r e p o r t e d i n gunshot r e s i d u e determination by Krishnan e t . a l . i n 1 9 7 1 I t s s e n s i t i v i t y was s u f f i c i e n t f o r l e a d and copper but not f o r " antimony, so antimony was determined by NAA. Green and Sauve (l5) used flame atomic a b s o r p t i o n t o analyze f o r barium, copper, l e a d and antimony i n gunshot r e s i d u e , and found the s e n s i t i v i t y f o r barium and antimony to be q u i t e low. Both s t u d i e s showed that copper l e v e l s v a r i e d too much t o be u s e f u l as an i n d i c a t o r of gunshot r e s i d u e . An a l t e r n a t i v e method f o r o b t a i n i n g the sample i n atomized form i s termed " f l a m e l e s s " and uses an e l e c t r i c a l l y heated carbon rod o r tantalum s t r i p . Despite s e v e r a l promising i n i t i a l e f f o r t s ( 1 6 , 1 7 , 1 8 ) use of the carbon rod has encountered problems i n the determination o f b a r i u m ( l ^ ) . With the tantalum s t r i p , on the other hand, good r e s u l t s have been obtained f o r both b arium and ant imony ( 20 ) . I.

Experimental

A. Sample c o l l e c t i o n . A s e r i e s o f t e s t f i r i n g s was con ducted under known c o n d i t i o n s and hand swab samples were obtained Immediately f o r examination by NAA o r AAS. F i r i n g s were con ducted indoors u s i n g a Smith and Wesson Model 1ζ, . 3 8 c a l i b e r r e v o l v e r . T h i s weapon was s e l e c t e d as b e i n g r e p r e s e n t a t i v e o f weapons encountered i n a c t u a l c r i m i n a l cases. Ammunition used f o r a l l t e s t s was Remington/Peters. Each o f t e n l a b o r a t o r y s t a f f members, engaged immediately beforehand i n h i s normal work a c t i v i t i e s , f i r e d a s i n g l e shot, u s i n g only the r i g h t hand. Both hands o f each shooter were then swabbed twice, "palm" and "back", u s i n g f o r each hand area two


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p l a s t i c - s h a f t c o t t o n swabs moistened with f o u r drops of n i t r i c a c i d ( 5 % ) · Immediately a f t e r use, each p a i r o f swabs was sealed i n t o an a p p r o p r i a t e l y l a b e l l e d 3" x U" Zip-Lok p l a s t i c bag. A f i f t h p a i r o f swabs (per shooter), moistened with the same amount of n i t r i c a c i d but otherwise i n "new" c o n d i t i o n , was s i m i l a r l y packaged f o r use as a " c o n t r o l " sample. The f o r e g o i n g sample c o l l e c t i o n procedure i s e s s e n t i a l l y that d e s c r i b e d a few years ago by Hoffman (2l)> but m o d i f i e d t o e l i m i n a t e the h i g h "back ground" o f barium contamination p o t e n t i a l l y a s s o c i a t e d with the use o f e i t h e r wooden swab s t i c k s o r g l a s s containers (22). B. Neutron a c t i v a t i o n a n a l y s i s . Each p a i r o f t e s t swabs i s removed from i t s c o n t a i n e r and i n s e r t e d i n t o a ρ re-numbered polyethylene envelope, t i p - e n d f i r s t . The p l a s t i c s h a f t s are cut o f f a few m i l l i m e t e r s above the c o t t o n and d i s c a r d e d . The p l a s t i c bag i s h e a t - s e a l e d and p l a c e d i n t o a c o n t a i n e r f o r i r r a d i a t i o n . Samples are i r r a d i a t e d , simultaneously with s u i t a b l e standards, i n the N a t i o n a l Bureau o f Standards r e a c t o r f o r 15 minutes a t a thermal neutron f l u x o f 5 χ 10^3 η cm^ s e c " . F o l l o w i n g i r r a d i a t i o n , the samples are allowed t o decay f o r approx. 30 min. t o reduce background r a d i a t i o n from 38qi. A f t e r t r a n s f e r from t h e i r p l a s t i c bags t o p r e - l a b e l l e d beakers, the samples are ready f o r the chemical s e p a r a t i o n o f barium and antimony from i n t e r f e r i n g r a d i o n u c l i d e s . A number o f s e p a r a t i o n procedures have been proposed (23,21+); however, the one developed and used i n our l a b o r a t o r y (25) has been designed t o process e f f i c i e n t l y l a r g e numbers of"samples. T h i s procedure i n v o l v e s a c i d l e a c h i n g o f the a c t i v a t e d barium and antimony from the swab i n t o 10ml o f a n i t r i c a c i d " c a r r i e r " s o l u t i o n c o n t a i n i n g 1000 ppm each o f non-radio a c t i v e barium and antimony. By s e q u e n t i a l a d d i t i o n o f s u l f u r i c a c i d (98%) and thioacetamide ( s a t d . aqueous s o l u t i o n ) , barium and antimony are p r e c i p i t a t e d as barium s u l f a t e and antimony t r i s u l f i d e . The p r e c i p i t a t e i s f i l t e r e d , washed and d r i e d . A standard s o l u t i o n c o n t a i n i n g 5 pë each o f barium and antimony ( p r e v i o u s l y i r r a d i a t e d together with the t e s t samples) i s s i m i l a r l y processed. The samples and standards (which have e s s e n t i a l l y i d e n t i c a l geometries) are counted on an 80cc G e ( L i ) d e t e c t o r i n conjunction with a 1^.096-channel pulse-height analyzer. Gamma ray emission a t 0.166 Mev. f o r 3 9 Ba and 0.561+ Mev f o r 1

1

1

1

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^ Sb i s measured. The emission data are f e d t o a Nova 1200 computer c o n t a i n i n g a program (developed i n t h i s l a b o r a t o r y ) f o r t h e i r r e d u c t i o n t o weights o f barium and antimony. χ

C. Flameless Atomic Absorption Spectroscopy. A Jarre11-Ash Model 810 Dual Monochromator Atomic Absorption Spectrophotometer was used f o r t h i s work. The instrument was equipped with the Barnes Instrument s tantalum ribbon flameless atomizer and a two1


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pen r e c o r d e r . T h i s instrument allows a choice between the deter mination o f two elements simultaneously and the a n a l y s i s o f one element with simultaneous monitoring o f a s e l e c t e d l i n e c l o s e t o the a n a l y t i c a l l i n e . The l a t t e r choice permits improved accuracy o f determination even i n the presence o f s i g n i f i c a n t background. With h i g h backgrounds o c c a s i o n a l l y b e i n g observed i n the d e t e r mination o f antimony, the background c o r r e c t i o n f e a t u r e was u t i l i z e d . The s e n s i t i v e 2176A l i n e f o r Sb was s e l e c t e d with background monitored a t 2179A · F o r barium, the 5536A l i n e was used with the 51+00A Neon l i n e as background monitor; however, i n t h i s s p e c t r a l r e g i o n , l i t t l e background was encountered. F o r ηίφ. l e v e l s o f barium, the l e s s s e n s i t i v e 3 0 7 I A l i n e was s a t i s f a c t o r y with 3057A used as r e f e r e n c e . The t i p s o f the t e s t swabs were cut o f f and p l a c e d i n l a b e l e d plastic vials. One ml o f 1 M HNO^ was added, the samples were a g i t a t e d and allowed t o l e a c h f o r 15 min. A 10 pi a l i q u o t was p l a c e d on the tantalum s t r i p and the purge gas f l o w was s t a r t e d (Ar alone f o r Sb and A r & H 2 f o r barium). The atomizer u n i t was a u t o m a t i c a l l y c y c l e d through p r e s e t time f o r d r y i n g , ashing and atomization (at 2500°C). Absorbance values were recorded on the c h a r t r e c o r d e r and r e s u l t s were obtained by comparison with a standard curve prepared f o r each tantalum s t r i p . 0

0

0


0

0

0

Operating parameters, e. g., temperatures, flow r a t e s , e t c . were p r e v i o u s l y optimized i n our l a b o r a t o r y and r e p o r t e d ( 2 0 ) . R e s u l t s and D i s c u s s i o n Summarized i n Tables I and I I are l e v e l s o f barium and antimony found by ( l ) Flameless Atomic Absorption and (2) Neutron A c t i v a t i o n A n a l y s i s i n swab samples taken from both hands o f twenty (20) s u b j e c t s a f t e r each had f i r e d a s i n g l e shot from a . 3 8 c a l i b e r r e v o l v e r . I t i s evident t h a t the recovered amounts o f barium and antimony are much h i g h e r f o r the f i r i n g hand than f o r the nonf i r i n g hand. I t i s a l s o evident ( i n the same t a b l e s ) t h a t the ranges o f barium and antimony recovery values are broad. S i m i l a r y broad ranges have been observed by others ( 8 , l l , 2 0 , 2 l | . ) . Table I I I shows t h a t two q u i t e d i f f e r e n t a n a l y t i c a l techniques, Flameless Atomic Absorption Spectroscopy and Neutron A c t i v a t i o n Analysis, y i e l d equivalent frequencies o f detection of f i r e a r m s discharge r e s i d u e . R e f e r r i n g f u r t h e r t o Table I I I , i t i s c l e a r t h a t not every f i r i n g o f the t e s t weapon l e d t o a p o s i t i v e i n d i c a t i o n o f gunshot r e s i d u e . Any o r a l l o f s e v e r a l f a c t o r s may account f o r the 15% i n c i d e n c e o f " n o n - p o s i t i v e s i n t h i s study. One i s v a r i a b i l i t y i n the s p a t i a l d i s t r i b u t i o n o f gunshot r e s i d u e from one f i r i n g t o another. A second i n v o l v e s n o n - r e p r o d u c i b i l i t y ,f


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Table I


Amounts of Barium recovered by swabbing the f i r i n g ( r i g h t ) and n o n - f i r i n g ( l e f t ) hands of 20 s u b j e c t s , f i r i n g a s i n g l e shot as measured by (1) Flameless Atomic Absorption (FAA) and (2) Neutron A c t i v a t i o n A n a l y s i s (NAA).

Sampling Area

A.

FAA Method

Range ( y g ) *

Mean (yg)

B. NAA Method Range (yg)

Mean (yg)

Control

0.01-0.15

0.05

0.01-0.03

0.01

Right Back ( f i r i n g hand)

0.07-3.35

0.76

0.13-3.86

1.13

Right Palm ( f i r i n g hand)

0.07-2.15

0.49

0.08-2.61

0.66

L e f t Back (non-firing) (hand)

0.01-0.38

0.11

0.01-0.11

0.05

L e f t Palm (non-firing) (hand)

0.01-0.30

0.12

0.01-0.36

0.11

^Micrograms


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Table I I


Amounts of Antimony recovered by swabbing the f i r i n g ( r i g h t ) and n o n - f i r i n g ( l e f t ) hands of 20 s u b j e c t s , f i r i n g a s i n g l e shot as measured by (1) Flameless Atomic Absorption and (2) Neutron A c t i v a t i o n A n a l y s i s .

Sampling Area

A. FAA Method Range (yg)*

Mean (yg)

B. NAA Method Range

(yg)

Mean (yg)

Control

0.01-0.01

0.01

0.01-0.01

0.01

Right Back ( f i r i n g hand)

0.06-1.20

0.43

0.04-1.13

0.50

R i g h t Palm ( f i r i n g hand)

0.01-0.44

0.19

0.01-0.83

0.26

L e f t Back (non-firing) (hand)

0.01-0.12

0.03

0.01-0.07

0.02

L e f t Palm (non-firing) (hand)

0.01-0.15

0.04

0.01-0.13

0.03

* Micrograms


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Table I I I

Frequency o f Firearms Discharge Residue


Sampling Area

Detection

D e t e c t i o n Frequency

( f i r i n g hand) FAA

NAA

R i g h t Back

85.0

82.0

R i g h t Palm

56.0

50.0

L e f t Back

0.0

0.0

L e f t Palm

0.0

0.0

(non-firing

hand)


(%)


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o f the residue c o l l e c t i o n technique. A t h i r d i s the c r i t e r i o n used t o d e f i n e a " p o s i t i v e " r e s u l t . A " p o s i t i v e " f i r e a r m s discharge r e s i d u e r e s u l t , as r e f e r r e d to here, i s based upon occupational hand blank s t u d i e s reported by s e v e r a l i n v e s t i g a t o r s ( 2 0 , 2 6 , 2 7 , 2 8 ) . The 0 . 3 0 and 0 . 2 0 microgram q u a n t i t i e s f o r barium and antimony are conservative s t a t i s t i c a l estimates from average l e v e l s of these elements found i n hand blank determinations. I t should be noted that i n no i n s t a n c e does the non-shooting hand have a l e v e l of Sb which our l a b would c o n s i d e r p o s i t i v e . I t should be f u r t h e r emphasized that the c r i t i c a l element i n f i r e a r m s discharge r e s i d u e determination i s Sb, because of i t s uncommon environmental occurrence. However, t h i s common charact e r i s t i c has l e d some i n v e s t i g a t o r s to c o n s i d e r u s i n g Sb alone f o r the determination o f GSR ( 2 6 ) . The two methods are i n good agreement with regard to i n c i d e n c e of p o s i t i v e s as i n d i c a t e d by Table I I I . Since both methods y i e l d comparable r e s u l t s , which method should a l a b o r a t o r y use f o r f i r e a r m s discharge r e s i d u e d e t e c t i o n ? Three f a c t o r s must be kept i n mind: c o s t , turn-around-time and personnel requirements. The cost per sample of NAA a n a l y s i s i s h i g h , i n v o l v i n g a d d i t i o n a l personnel and r e a c t o r , and d e t e c t o r and p r o c e s s i n g systems. A d d i t i o n a l l y , with NAA, r e a c t o r a c c e s s i b i l i t y , sample workup and a n a l y s i s do not l e n d themselves to r a p i d throughput, e s p e c i a l l y where a heavy case l o a d i s i n v o l v e d . Reactor a c c e s s i b i l i t y i s l i m i t e d , posing s t i l l f u r t h e r d e l a y s . For the m a j o r i t y of l a b o r a t o r i e s , flameless atomic absorpt i o n i s the more p r a c t i c a l technique. I t r e q u i r e s only a modest investment and enables a l l work to be done in-house, thus e l i m i n a t i n g complex scheduling. These f a c t o r s reduce the cost per sample and speed up the a n a l y s i s . A d d i t i o n a l l y , FAAS lends i t s e l f to a wide v a r i e t y of other analyses of i n t e r e s t to the forensic laboratory. In s h o r t , NAA i s an e x c e l l e n t a n a l y t i c a l t o o l , but f o r f i r e a r m s discharge r e s i d u e , FAAS i s the more p r a c t i c a l technique. Although only a l i m i t e d number of t e s t f i r i n g s and a s i n g l e weapon were employed i n t h i s study, a much broader e f f o r t i s c u r r e n t l y i n progress. The l a t t e r study w i l l i n v o l v e as v a r i b l e s weapon c a l i b e r , b a r r e l length, brand of ammunition, f i r i n g and sample c o l l e c t i o n c o n d i t i o n s . R e s u l t s of t h i s w i l l be reported subsequently. Acknowledgment The authors wish t o acknowledge the a s s i s t a n c e i n t h i s i n v e s t i g a t i o n of Mr. Charles R. M i d k i f f and other s t a f f members o f the F o r e n s i c Branch, Bureau of A l c o h o l , Tobacco and Firearms, U. S. Treasury Department. In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

FORENSIC

106

SCIENCE

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A Comparison of Neutron Activation Analysis and Atomic Absorption

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