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APPLICATION OF PREFILTRATION TO THE ANALYSIS OF ACID-DEGRADED GUNSHOT RESIDUE SWABS

NCJ Number
54299
Journal
Journal of Forensic Sciences Volume: 24 Issue: 1 Dated: (JANUARY 1979) Pages: 92-95
Author(s)
R J KOPEC
Date Published
1979
Length
4 pages
Annotation
THIS STUDY WAS INITIATED TO DETERMINE IF A SIMPLE PREFILTRATION TECHNIQUE COULD MINIMIZE PROBLEMS OF ACID-DEGRADED GUNSHOT RESIDUES OR RESIDUES CONTAMINATED WITH GREASE, DIRT, OR BLOOD.
Abstract
ROUTINE ANALYSIS OF GUNSHOT RESIDUE SWABS BY FLAMELESS ATOMIC ABSORPTION SPECTROPHOTOMETRY IS STRAIGHTFORWARD WITH CLEAN SWABS. SEVERE PROBLEMS ARE ENCOUNTERED WITH SWABS THAT ARE ACID-DEGRADED OR CONTAMINATED BY GREASE, DIRT, OR BLOOD. IN SUCH SITUATIONS, ANALYSIS IS COMPLICATED BY INCREASED INSTRUMENTAL BACKGROUNDS, DECREASED LEACHING EFFECTIVEENESS, AND 'MEMORY' BETWEEN SUBSEQUENTLY ANALYZED SAMPLES. THE MOST COMMONLY ENCOUNTERED PROBLEM INVOLVES SWABS THAT HAVE BEEN DEGRADED BECAUSE OF THE ATTACK OF NITRIC ACID ON THE COTTON TIP OF SWABS. THE FILTRATION APPARATUS IN THE STUDY CONSISTED OF A POLYCARBONATE MEMBRANE FILTER DISK HELD IN A FILTER HOLDER AND A STANDARD 5-MILLILITER PLASTIC SYRINGE. ITEMS WERE RINSED IN NITRIC ACID TO REMOVE ANY RESIDUAL ANTIMONY. A DUAL-CHANNEL DOUBLE-BEAM ATOMIC ABSORPTION SPECTROPHOTOMETER, EQUIPPED WITH A TANTALUM STRIP ATOMIZER, WAS EMPLOYED FOR ANTIMONY DETERMINATIONS. SINGLE-ENDED PLASTIC-SHAFTED COTTON SWABS WERE SPIKED WITH 5 MICROGRAMS OF ANTIMONY IN NITRIC ACID. SPIKED SWABS WERE STORED IN INDIVIDUAL ANTIMONY-FREE POLYSTYRENE VIALS AND ALLOWED TO DEGRADE FOR 2 WEEKS AT ROOM TEMPERATURE. NEW SINGLE-ENDED PLASTIC-SHAFTED COTTON SWABS SPIKED WITH 0.5 MICROGRAMS OF ANTIMONY WERE USED AS CONTROLS, AND SIMILAR UNSPIKED SWABS WERE USED AS BLANKS. HIGH INSTRUMENTAL BACKGROUNDS OBSERVED IN THE ANALYSIS OF UNFILTERED LEACHATE FROM ACID-DEGRADED SWABS APPEARED TO ORIGINATE FROM A SMALL AMOUNT OF COTTON FIBERS SUSPENDED IN SOLUTION AND TRANSFERRED TO THE TANTALUM STRIP OF THE ATOMIZER. THESE COTTON FIBERS ONLY CHARRED AND DID NOT COMPLETELY ASH DURING THE ASHING CYCLE WHEN THE TEMPERATURE OF THE TANTALUM STRIP WAS MAINTAINED AT 450 DEGREES CENTIGRADE FOR 30 SECONDS. DURING THE ATOMIZATION CYCLE, WHEN THE TANTALUM STRIP REACHED A TEMPERATURE OF 2,500 DEGREES CENTIGRADE, THE REMAINING UNASHED COTTON FIBERS WERE ASHED. THE RESULTING SMOKE CAUSED SCATTERING OF BOTH ANALYTICAL AND BACKGROUND BEAMS. TRACES OF PARTIALLY CHARRED COTTON FIBERS OCCASIONALLY REMAINED ON THE TANTALUM STRIP OF THE ATOMIZER AND APPARENTLY CAUSED THE MEMORY OF APPROXIMATELY 0.02 TO 0.04 MICROGRAMS OF ANTIMONY BETWEEN SUBSEQUENTLY ANALYZED SAMPLES. WHILE THE TECHNIQUE OF PREFILTERING LEACHATE FROM ACID-DEGRADED GUNSHOT RESIDUE SWABS IS A VIABLE METHOD FOR REDUCING BACKGROUND SCATTERING EFFECTS AND MEMORY BETWEEN SAMPLES, IT INCREASES SAMPLE HANDLING PROCEDURES AND LENGTHENS ANALYSIS TIME. ILLUSTRATIONS AND REFERENCES ARE INCLUDED. (DEP)

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