Sample Collection

The methodology for sample collection for GSR analysis is simplicity itself. The most commonly used technique uses a 1 x l cm strip of double-sided adhesive tape stuck onto a thin acetate strip. The acetate strip allows the adhesive surface to be conveniently manipulated without any fear of contact with the sampler's hands. One of these tapes is used to take samples from each of the four areas as illustrated in Figure 6.3. During the taking of the samples, the skin must be stretched as much as possible to ensure that any GSR particles which may be hidden within the folds of the skin or inside the hair follicles are removed.

It is important to cover the sampled area at least three times even if the adhesive has lost its tackiness. The adhesive is quite soft and particles can still be pressed into the surface even if there is no discernable stickiness left. It is also important to be consistent in the number of times the area is covered to ensure consistency, for interpretation of the results.

6.6.1 Alternative sampling technique

Whilst a 1 x 1 cm square of double-sided adhesive tape on a strip of acetate is an effective and cheap way of collecting samples for GSR examination, it does have a number of disadvantages. Of these, the most serious is the requirement for carbon coating and the concomitant possibility of contamination.

This carbon coating must be applied to prevent the sample charging whilst it is being scanned by the SEM electron beam. It is an essential stage in the sample preparation when using this type of sampling technique and cannot be skipped. For carbon coating, the sample must go through the following procedures:

1. removal of acetate strip containing sample from protective tube;

2. removing the 1 x 1 cm sample from the acetate strip;

3. sticking the sample onto a SEM stub with double-sided tape;

Gunshot Residue Retention The Hands

Figure 6.3 Diagram of areas of the hand to be taped for gunshot residue recovery.

Figure 6.3 Diagram of areas of the hand to be taped for gunshot residue recovery.

4. placing the stub in a multiple stub holder for coating;

5. sputter coating the sample in a near vacuum;

6. re-pressurizing the coater to ambient conditions;

7. transfer of the sample from the coater to the SEM.

Each and every step involves the possible contamination of the sample with the two most serious being:

1. placing the sample alongside other samples and, more seriously, control GSR samples in the sputter coater whilst air is drawn across the sample as the coating chamber is evacuated;

2. re-pressurizing the coater chamber where a large volume of outside, potentially contaminated, air is drawn across the sample.

This whole process can, however, be simplified and the number of procedures where the sample is exposed to the outside air reduced to an absolute minimum. This involves the use of pre-carbon-coated adhesive discs.

These adhesive discs are similar to double-sided tape but the adhesive material, which is specially formulated for SEM use, is pre-impregnated with carbon dust. This completely eliminates the requirement for carbon coating of the sample.

These discs are available from SEM supply companies who can pre - apply these to SEM stubs. These stubs are then individually placed in clean sealed SEM stub tubes under ultra-clean conditions. These tubes have special stub holders in the cap which enable the tube to be simply removed from the cap for sampling. Once the tube is replaced over the cap, it self-seals, preventing any chance of the sample being tampered with.

With this sampling technique, the tube is simply taken off the cap, the adhesive disc dabbed over the relevant part of the hand and then replaced back in the tube.

When it comes to the SEM examination, the SEM stub is removed from the cap with a pair of SEM stub tweezers and transferred to the SEM stub holder and into the SEM chamber. It can then be directly examined in the SEM without any further treatment.

If the hands are wet, they should be allowed to dry naturally. Blow-drying the hands must not be used since it will remove all GSR particles. Sampling from areas of the hand covered with blood should be avoided at all costs; the imaging technique used during the search for the particles (backscattered imaging) is completely overloaded by the iron content of haemoglobin in blood.

It is extremely important that any chance of contamination is avoided. There will, in all probability, be only a few particles of GSR deposited on the hands after firing a round of ammunition. Contamination by a single particle of stray GSR from the sampler would be extremely difficult to detect and could easily be construed as a false positive result.

If the sampler has any contact at all with firearms, he should, before taking the samples, change his clothes, shower and wash his hair thoroughly. During the taking of samples, disposable gloves, boiler suit and hair cover should also be worn. This should be done even if the sampler has had no contact with a weapon. These disposable clothes and gloves should be changed for each suspect examined. It should also be stressed that if the suspect has to be handcuffed, disposable nylon restraints should be used rather than the police issue handcuffs, which may well have been contaminated either from range courses or the gun the officer might be wearing. A control blank taping should also be submitted with the samples. The tapings should be placed in individual bottles and sealed. The individual bottles should be placed in sealed bags and the sealed bags placed in another sealed bag.

To give a rather extreme example of how easy it is to contaminate GSR samples, during conversations with the firearms examiners at a particular laboratory, it was discovered that all their shooting incidents were with 0.22" weapons. The officers were also proud to point out that they had never had one negative case with respect to GSR and that they always found lead, barium, antimony and aluminium on their hand tapings. This was a little surprising as the vast majority of 0.22" priming compounds contain only lead and barium. It transpired that the officers, who did all the sampling themselves, were also firearms instructors firing anything up to 200 rounds a day. The ammunition they fired, surprisingly enough, contained lead, barium, antimony and aluminium in the primer.

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