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Physical Characteristics

Examination of evidence for physical characteristics
© Image: USAPHC (2002) USAPHC lab technician assessing TOX samples.

Chain of Custody (CoC)

The CoC is designed to prove sample integrity by chronologically documenting custody from seizure/collection through to control, storage, transfer, analysis and destruction, to standards that can withstand a forensic audit. This should be sufficient to prove that the samples analysed are the same as those collected at seizure. The CoC gives an account of who had the sample at any given time. Only authorised personnel should come into contact with the sample after seizure. It is essential that these personnel (who include forensic scientists) are properly trained to manage samples and ensure the integrity of the CoC. Post-training periodic check of protocol awareness and adherence should be regularly conducted.

Visual examination

Upon receipt of samples in the forensic lab, it is important to note the physical appearance of sample and packaging. This documentation is necessary for CoC as well as helping to characterise the unknown sample (eg through a drug database search). For this, a record of the sample’s physical appearance (ie colour, shape, size as measured with ruler/caliper, weight using an analytical balance and any other characteristic features) supported by photographic evidence is required.

Where a sample is determined to be an illicit drug, the weight and volume seized affect the severity of the assigned charge (ie possession vs intent to supply).


By examining the packaging, a forensic scientist may be able to determine the presence of striation marks and manufacturing features on plastic bags or cling film that may have been used to wrap drugs. Analysis by visual comparison of cut or tear lines can provide a physical fit between sample and reference. Visualisation of otherwise invisible patterns with polarised lights can highlight otherwise transparent Schlieren patterns (optical inhomogeneities) due to the different densities in polymer sheets that originate from manufacture. Like the physical drug, the quantity of bags can help in deciding what a charge should be. Linking the unique patterns between bags found at different locations may indicate a common origin. What impact can this have on a case?

© Image: USAPHC (2002) USAPHC lab technician assessing TOX samples.
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The Science Behind Forensic Science

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