Comparison Microscopy

Q10. What is a photomicrograph and did you take one in respect to this case?

A10. A photomicrograph is simply a photograph taken under the magnification of a microscope. It could be a simple photomicrograph or a comparison photomicrograph. The answer should be 'Yes, I took a/several representative photographs for my own reference, but not specifically for court purposes.

Q11. Why not? Was this an attempt to deny some important knowledge to the court?

A11. Simply put, it takes years of experience to become a competent comparison microscopist. It is thus totally unrealistic to expect members of the court to become instant experts and to be able to interpret the significance of a comparison photomicrograph from a single print. At best, a photograph of a match will be illustrative, at worst, totally misleading.

In addition, a photomicrograph only shows a small portion of any match obtained. To obtain a photographic representation of the whole circumference of a bullet, thus illustrating the concordance between the two, would require hundreds of photographs.

Despite this, some jurisdictions do require the production of photomicrographs. In these instances, the examiner should make clear to the court the limitation of this type of evidence.

Most, if not all, comparison microscopes are now fitted with a video camera and video recorder which can simplify the matter considerably. If the court demands this type of photographic evidence, a video recording of the match is the only real way of demonstrating how the positive comparison was made.

As an alternative, the examiner could offer the court access to a comparison microscope. In this way, it will be possible for the judge and jury to see the match at first hand and to have a clearer idea as to the problems involved. Under no circumstances should the witness tell the court or give it the impression that ' I am the expert, believe me.'

Q12. A question as to the expert's experience with either a pantoscopic camera or a peripheral camera could follow this.

A12. These merely take low-magnification photographs of the circumference of a bullet and are totally unsuitable for comparing the micro stria. It is also unlikely that a modern laboratory would have one of these cameras.

Q13. Can you see the marks that you are using to prove that the bullet came from the gun in question?

A13. Only gross marks will be visible to the naked eye and it would be impossible to even contemplate making a comparison from these. Having said that, I have observed ' expert witnesses' demonstrating to a court how a comparison was made using a simple hand lens.

Q14. How much magnification do you require?

A14. Between 25 and 80 times as a general rule.

Q15. If you don't use enough magnification, you cannot see all the detail. Is that correct?

A15. Yes, but further qualification is required as per the following question.

Q16. But if you use too much, you lose sight of the small details?

A16. A nonsense question but one which can easily trip the unwary. Basically, you require enough magnification to see the fine detail produced on the bullet by manufacturing defects in the barrel. This is generally accepted as being about 40x magnification. Once the magnification rises above 100x, stria made by dirt dust and general debris in the bore becomes visible. This is obviously of no significance, but at this magnification, this very fine stria becomes readily visible and interferes with the overall picture.

Q17. Is it not true that even on a positive match there are many non-matching stria?

A17. This is true, and it is by experience alone that the examiner is able to determine which are relevant and which are non-relevant stria (see A19 and A20). Non-relevant stria would include those made by debris in the bore, microscopic traces of corrosion and fragments of the bullet being torn off by the rifling and becoming trapped between the barrel and the bore of the weapon. The variation in these micro stria could be illustrated by taking photographs of consecutively fired bullets. This could help to demystify the concepts of comparison microscopy by reducing the subjectivity of the process and increasing the objectivity, that is, scientific aspects, as much as possible. Reference should be made to the PhD thesis by Dr J. Hamby on matching and non-matching stria.

Q18. When you are comparing the rifling on a bullet, how much agreement do you require before you can identify a bullet having come from a particular weapon?

A18. An amount that exceeds the best known non-match. See Section 4.6.

Q19. How much agreement is required?

A19. A non - quantifiable amount and one that must be determined by the individual examiner based on his experience. This is not to say 'I am the expert believe me' and qualification (Section 4.6 and A20 below) is required.

Q20. What is the standard amount of agreement required by other firearms examiners?

A20. No real standard, but experience of other firearms examiners' work has shown that the 'mind's eye' criteria used is fairly consistent. Every forensic firearms laboratory should be part of an external proficiency review programme American Society of Crime Laboratory Directors Laboratory Accreditation Board (ASCLD LAB) or similar) for stria matching. There should also be an internal proficiency programme. The results of these should be readily available should they be required by the court.

Q21. Would you expect to find some matching stria between bullets known to have been made by different weapons?

A21. The answer is yes, but with the proviso that with the thousands of stria in any bullet comparison, there are bound to be a number of accidentally matching stria. It is the experience of the examiner that enables him to determine which matching stria are relevant and which are accidental.

Q22. Have you ever deliberately compared bullets from different weapons to determine the best known non-match?

A22. The answer here must be a resounding 'yes'; otherwise, it would not have been possible for the examiner to formulate a criteria for a ' best known non-match'.

Q23. If a barrel is rusty, doesn't each bullet fired through it change its characteristics?

A23. Depends upon the degree of rusting. Light rusting will have little effect on the characteristic stria, whilst heavy rusting could make it impossible to match successive bullets.

Q24. Could you compare and match the first and the one hundredth bullet fired through the same barrel?

A24. As long as the barrel had not been damaged by rusting or some other external influence, for example, cleaning with a steel rod, heavy use of steel wool, the answer to this must be 'most definitely yes'. This type of comparison should form part of every firearms examiner's training.

Q25. Is it not true that two guns of the same make and model will impart the same characteristics on bullets fired through them?

A25. Class characteristics will be the same, that is, calibre, number, direction and angle of twist, groove profile, groove depth will be the same. The individual characteristics will not. See Section 4.4.

Q26. Would you agree that the matching of bullets is not an ' exact science ' such as fingerprint examination, which requires 16 points of similarity?

A26. This should be answered along the lines of

'I do not understand the term ' exact science ' ; possibly you could elaborate. If you are inferring that 16 points of similarity constitutes an 'exactness', then why not 15, 17 or 63? There being no logical, rational or statistical justification to the selection of the number 16, it cannot, therefore, be inferred as endowing some magic quality of an ' exact science' to the subject.'

'With striation matches, there are often hundreds, if not thousands, of concording points which constitute a positive identification. That these matching lines are not counted or assigned an arbitrary number makes this type of examination no less of an exact science than fingerprints.'

9.4 GSRs

Q27. Who took the GSR samples in this case?

A27. Ideally, they should have been taken by the officer on the stand, and he should be able to account for any possibility of contamination. If not, then it will be necessary to call to the stand the scene of crime officer who did take the samples.

Q28. What precautions did you take to prevent any contamination of the exhibits?

A28. Preferably, the expert giving evidence has no day-to-day contact with firearms. If not, he would have to demonstrate that every precaution had been taken to prevent any contamination from himself. This would, at the very least, involve showering, washing hair, changing clothes, using disposable gloves and disposable coveralls with hair cover. Control tapings would have to be taken from himself and the disposable gloves before taping the suspect. Gloves and, preferably, coveralls must be changed for each subject taped.

Q29. How do you know that the tapings were not tampered with before being examined via the scanning electron microscope (SEM)?

A29. It should be standard procedure to examine the tapings under low power in the SEM before they are scanned. If any tampering has taken place, then the added residues will be visible as particles lying on the top of the tape. Anything picked up from the hands will be impressed into the tape's surface. Once again, this is only learnt by experience and deliberately making control false positive samples.

Q30. What steps have been taken, at the collection point and within the laboratory, to ensure that any chance of contamination has been eliminated?

A30. Disposable gloves and coveralls with hair cover must be used when taking samples from a suspect or dead body. The bags in which these items were stored must be kept in a sealed bag for future examination should questionable results occur or defence counsel requests it. It should also be laboratory practice to randomly examine used gloves and coveralls as part of contamination review procedures.

Q31. Laboratory procedure should be questioned as to the possibility for environmental contamination.

A31. Ideally, no one working with weapons should have access to the SEM preparation and examination room. The room should also be positively pressurized to minimize ingress of contamination. There should also be a vestibule in which one dons the anti- contamination suits and shoes prior to entering the SEM room. This should be at a lower pressure than the SEM room, but higher than outside.

Q32. Is there a firing range within half a mile?

A32. An irrelevant question if all of the above precautions have been taken. Having said that, the answer must be available.

Q33. Does anybody in the immediate vicinity of where the samples are examined have any connection with firearms?

A33. A very valid question. Ideally, the SEM operator should be a qualified and practicing firearms examiner as he will have the experience and background knowledge, as well as up-to-date information on ammunition developments, to recognize the relevance of any ambiguous or questionable results. He will also be able to interpret those results and, via his knowledge, be able to explain their relevance to the court. There is, however, every possibility of such an operator bringing contamination to the SEM room. If this is the case, the operator will have to demonstrate that every possible precaution has been taken to ensure that contamination has been eliminated.

As part of this daily control, samples from the SEM bench and preparation areas must be taken and scanned for contamination as a prerequisite. In addition, the following should be considered as an absolute minimum:

1. Ensure that any SEM work is carried out before entering any other part of the laboratory.

2. Wash hair prior to entering the SEM room.

3. Prior to entering the SEM room, strip off all clothes and put on disposable anti-contamination suit with hair cover, gloves and shoe covers.

4. Complete all tasks in the SEM room in one sitting to reduce the number of exits and entries to an absolute minimum. If it is necessary to leave the SEM room, dispose of the anti-contamination kit and put on a new set prior to entering the SEM room again.

Q35. Where were the bags obtained that were used to protect the hands of the suspect?

A35. Often, these are merely envelopes taken from police station supplies, or even worse, plastic bags, and are thus very susceptible to contamination. This contamination could come from either a range within the station or from officers who carry or use weapons.

The inside and outside of these bags or envelopes should be control taped before use to determine whether they have been contaminated.

Ideally, these should be paper bags obtained from an outside source. These should be randomly taped and examined in the SEM for any possible signs of contamination. The results of these examinations must be retained for court purposes.

GSR sample kits should be made up by an outside contractor. These should contain surgical gloves, plastic restraints, disposable coveralls, disposable shoe covers, five sampling tubes and an instruction leaflet.

Q36. How were the suspect's hands secured whilst he was awaiting the taking of the tapings?

A36. If they were handcuffed, there is a very real possibility of GSR particles being transferred from the cuffs to the hands of the suspect. Research has shown (B.J. Heard, unpublished paper) that during range courses, an officer's clothes, baton, handcuffs and holster will become heavily contaminated with GSR particles. The GSR particles remain in the handcuff pouch and when the handcuffs are used, these particles will be transferred to the hands of an arrested person. Only plastic cable ties (see A35) should be used as restraints. These can be supplied to police stations in sealed plastic bags.

Q37. How can you be sure that the particles found were in fact from the firing of a weapon and not environmental or other contamination?

A37. Knowledge will have to be shown of GSR/indicative GSR particle ratios as well as GSR particle/lead particle ratios and how they relate to the case statistics.

Q38. What do you consider to be a minimum number of GSR particles for a positive result and how did you decide on that number?

A38. One is the minimum number, but this would have to be backed up with the relevant GSR particle/lead particle and GSR/indicative particle ratios. As a general rule, two particles, with the aforementioned ratios, is generally considered the minimum requirement for a positive finding.

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