When interpreting GSR distribution results from a SEM analysis of the four tapings taken from the hands, the following points must be considered.
(a) The gases issuing from the muzzle are projected at great velocity away from the firer. Unless there is a very strong wind blowing towards the firer, these GSR-containing gases will not be deposited on the hands.
(b) The gases issuing from the breech end of the barrel, whether it is a revolver or a self-loading pistol, are of much lower velocity. Unless the non-firing hand is held within a few inches of the gun during firing, these gases will only be deposited on the back of the firing hand.
(c) The palm of the firing hand will be protected from the deposition of any GSR particles during firing as it is wrapped round the grip.
(d) -n a weapon which has been fired, both the muzzle and breech end of the barrel will be contaminated with GSR. Handling these areas of a fired weapon will deposit GSR particles on the palms of the hands.
(e) Immediately after firing a weapon, the GSR distribution pattern on the hands will be fairly predictable. With time, however, there will be some redistribution of particles over the hands. It is the interpretations of the GSR distribution vs. time patterns which are difficult.
Bearing in mind the above, there are four basic GSR distribution patterns which may be encountered. These are as follows:
1. GSR particles found only on the taping taken from the back of the right (or left if the person is left-handed) hand. This is highly indicative of the person having fired a weapon in that hand.
2. GSR particles found only on the tapings taken from the backs of both hands. This is highly indicative that the person fired the weapon in one hand whilst supporting the firing hand with the other.
3. GSR particles found on all four tapings. This would indicate that the person was standing in front of a weapon when it was fired and was enveloped in the large cloud of gases emanating from the muzzle of the weapon. The person could have been either an innocent bystander or part of the gang carrying out the robbery.
4. GSR particles only on the tapings taken from the palms of both hands. This could indicate that the person had merely handled a weapon which had been recently fired.
When considering these interpretations, it must be kept in mind that the longer the elapsed time between the firing incident and the taking of the tapings, the greater the redistribution of particles. As the elapsed time increases, greater emphasis must be placed on the interpretation of indicative GSR particle distribution and the indicative GSR/confirmed GSR particle ratio. This interpretation is purely a matter of experience.
6.9.1 Interpretation of results
Merely looking for particles on hand tapings which match, in elemental composition, those taken from a control cartridge case is simply not enough.
Not all primer residue particles formed during the firing of a weapon will contain all the elements present in the original primer mix. If a standard primer composition of lead styphnate, barium nitrate and antimony sulphide is taken, then only particles which contain lead (Pb), barium (Ba) and antimony (Sb) can be positively identified as being GSR particles. There will, however, be a very much larger number of indicative GSR particles formed at the same time. These indicative GSR particles can contain PB/Sb, Pb/Ba or Ba/Sb. Other particles of indeterminate origin will also be present, which contain only Pb, Ba or Sb.
There will be, depending on the ammunition type and make, a very approximate relationship between the ratios of these confirmed GSR particles and indicative GSR particles. If the bullet is plain lead, there will also be a distinct GSR/Pb ratio from lead volatilized from the base of the bullet.
The ratio of plain lead particles to GSR particles (in ammunition containing a plain lead bullet) will be higher at the muzzle than at the breech end of the barrel. This can be accounted for by (i) the hot gases emerging from the muzzle having had longer to volatilize the lead from the base of the bullet than those emerging from the breech and (ii) the fact that particles of lead are torn off the sides of the bullet as it passes down the rifling.
This can be extremely useful when determining whether particles found on a suspect's hands were from actually firing a weapon or whether they were from merely being in front of the weapon, possibly as an innocent bystander, when it was fired.
As an example, at the breech end of the barrel, Winchester 0.38" Special calibre plain lead ammunition has a ratio of approximately 15 plain lead particles to every confirmed GSR particle, whilst at the muzzle, this will be in excess of 35 plain lead particles to every GSR particle.
These ratios are only very approximate and can only be obtained from controlled test firings. Likewise, they are only of use in ammunition which is plain lead, that is, non-jacketed.
The situation with fully jacketed ammunition is similar, but the Pb/GSR ratio is much lower due to the smaller area of lead which is exposed. Some interpretation of the Pb/GSR ratios at the breech can be made, but it is much more difficult. In this situation, it is probably better to search for Cu/Zn particles which may have been stripped from the bullet jacket by the rifling. These particles should only be present in the residues issuing from the muzzle.
6.9.2 Minimum requirements for a positive result
One major question when interpreting GSR results concerns the minimum number of GSR particles which constitute a positive result.
If the necessity for associated particle identification is taken into consideration, then the answer must be one. However, the fact that the above ratios for confirmed GSR to indicative GSR particles is one based on averages when a number, much larger than one, of confirmed particles are present must not be forgotten. For example, if the ratio were 1 : 5 for breech emitted particles where there are 50 positive GSR particles, then the possibility for there being only one positive GSR particle with no associated particles is high.
Several papers (DeGaetano and Siegel, 1991; ASTM E1588, 1993; Singer et al., 1996) have investigated the common laboratory practice for the threshold limit for a positive finding of GSR on hand tapings. The results are varied with most coming out in the 1-2 region.
In practice, the situation is a little more complex. If, for example, one is in an area where the police force is issued with ammunition containing Pb, Ba, Sb and Al and the criminals are using ammunition containing Hg, Sn, K and Cl, the finding of only one Hg, Sn, K and Cl particle on the hands of a suspect would have far more significance than if both the criminals and the police were using Pb-, Sb-, Ba-, Al-based ammunition.
In the author's laboratory, even though the GSR in ammunition issued to the police and that used by the criminals is radically different, the benchmark of a minimum of two confirmed GSR particles, together with associated particles, is considered the absolute lowest limit for a positive result.
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