The Recovery of Trace Materials for Spectrographic Examination using a Microbrush Abrasion Technique

T h e Recovery of Trace Materials for 3pectrographic Examination using a n Technique J. A. EVANS and W. N. WALLER Home Ofice, Northern Forensic Science Laboratory, Newcastle-upon-Tyne, England Dificulty has been exPerienced in the past in obtaining satisfactory emission spectra of very small amounts of material, both as particles alone, and as smears on clothing, instruments, etc. T w o methods of overcoming this problem are presently being used at this laboratory, namely the point and micro-brush methods using preformed Pointed graphite electrodes and standard spectrographic procedure.

Point method A small hole approximately lmm deep and lmm diameter is made in the tip of a pointed electrode (See Figure 1). The particle to be analysed is placed in the hole and held in position with a spot of "Durofix". I t has been found that by using this method particles of 25-30 pg give consistently good spectra. The advantages of this method over that using graphite pellets (Ellen, 1965) are:-the simplicity of operation, and the avoidance of extra apparatus. Micro-brush method A small step 1/16" deep is cut in the tip of a pointed electrode (See Figure 2) and a small plug of cotton wool is cemented to the step with "Durofix", and later trimmed. I t has been found that arcing is made easier by using a step rather than by central tip plugging. This method has successfully been used for the analysis of:(a) Powdery smearing on clothing. (b) Paint smears on instruments. (c) Metal smears on instruments. (d) Paint smears on paint. (e) Powder smearing around a bullet hole. With the combination of micro-brush abrasion and a suitable solvent various trace materials can be easily transferred to the electrode. The transference of the trace materials shown in Fig. 3 was facilitated by the use of solvents as follows:Abrasion/solvent technique. Powder smears on clothing--dilute HCI (For MgO, copper corrosion, plaster, etc.). Metal smears on instruments--dilute HCI (if no P b present) or dilute aquaregia (with P b present). Paint on instruments-Acetone (for cellulose paints). Chloroform (other paints). Paint on paint-Choose solvent which least affects the top layer of the control paint. The authors wish to thank Detective Sergeant W. R. Hopper, for producing the photograph and figures.

ReferenceELLEN, D. M. (1965), J. Forens. Sci. Soc., 5, 196. 189

I

A-y0L E

CUT

Imm. X lmm.

POINT METHOD

1K

PAINT HELD IN POSITION WlTH DUROFIX

I

C O T T O N WOOL-

BRUSH M

m

W T T O N WOOL ATTACHED WlTH D U L O F I X AND. TRIMMED.

Fig. 1 (top) Point Method ; and Fig. 2 (bottom) Brush Method.