Skilled scientists and instruments

Forensic chemistry usually relates to the identification (and sometimes quantification) of unknown substances linked to crime, and must be performed to a legally defensible standard. To achieve this, a wide variety of highly sophisticated analytical techniques and instruments may be used (discussed later in the course).

When confronted with a piece of evidence, a forensic chemist must make a decision about which analytical techniques are most appropriate, considering the type of evidence available (eg drug seizure, biological sample) and the legal questions that need to be answered (eg what is it? How much is present?). They must also be aware of practical considerations, such as how much material is available, and whether the analytical techniques to be used are non-destructive (meaning the sample can subsequently be submitted for further tests) or destructive (the process of analysis uses up the sample).

To ensure the quality of the data reported, forensic chemists must check that any instruments used are working to the required standard. This is typically done by analysing control samples (which are similar to the compounds being evaluated), along with the evidence being tested. In addition to testing the functionality of the instrument, control samples can be used to assess the entire experimental approach. Usually a positive control (indicating the presence of a compound) and a negative control (indicating the absence of a compound) are analysed along with the unknown sample. The unknown sample can be compared to the two control samples to aid interpretation.

Forensic chemists work to the evidential requirements of the courts. For example, the Scientific Working Group on the Analysis of Seized Drugs (SWGDRUG) categorises analytical techniques as A, B and C, depending on the quality of evidence they produce. Category A techniques are considered the best and produce the most reliable results, followed by category B and then C. To eliminate the possibility of a false result, it is recommended that more than a single type of test be performed. Most commonly, this involves one technique from category A and one from B or C – although depending on the circumstances, two from category B with another from B or C may also be used.

When giving evidence, the forensic chemist must communicate their scientific interpretation in such a way that it can be understood by non-experts. Also, as experts, forensic chemists may give their professional opinion as well as presenting facts, though they must always present their testimony objectively.

Discussion prompt

Why do you think it is important to use more than one type of test for identification? What might happen if a forensic chemist relied on a single test or did not use both a negative and positive control?

AND/OR:

Read Part 1 of the SWGDRUG recommendations in the attached link. In your opinion, which is the most important element of the Code of professional practice for drug analysts? Why?