When you have a limited sample size, it is critical to have a plan for the sequence of events of analysis.
Limits of detection and limits of quantitation, as well as other physical limitations can affect testing procedures and the reliability of results.
Examiners are typically trained to preserve as much of an evidence sample as possible, for retesting by defense counsel and for independent validation testing. In some jurisdictions, such preservation is mandatory.
Even so, during many types of analyses a sample can be completely destroyed or used up.
A laboratory supervisor is in the best position to choose the most appropriate test sequence to maximize specimen availability for analysis. Or perhaps the decision will be made to employ another strategy, such as splitting the sample before testing.
Many samples need to be tested immediately because the targeted substrate in the specimen dissipates over time. Substances such as alcohol or other drugs are unstable, and break down. If the specimen is to be tested for these unstable substances, it needs to be processed in a more immediate time frame than other types of samples in evidence, or be properly preserved using special containers with chemical preservatives.
The level of a substance detected in a sample is frequently brought into question in the court room. One reason for this scrutiny is that analysts are forced to operate under strict time limitations on certain samples, to ensure reliability. This can be one more pressure point for an examiner with a burgeoning case-load, and only so many hours in a day.