collecting samples

analysts work with samples. The size of the sample needed depends on what tests need to be carried out. Samples of any items must be representative. In other words, there should be enough of the sample to give a picture of the whole item. The sampling method depends on the nature of the sample.

It is important to prevent any changes or deterioration of the sample. Some samples have to be analysed immediately. Others can be stored under suitable conditions.

accurate analytical results depends on protecting samples from contamination. Crime scene investigators wear protective clothing and put all samples into sealed bags or bottles to avoid contamination samples. They may have to appear in court to give evidence about possible routes of contamination.

It has been known for urine samples taken for drug testing to be swapped. For same samples it is important to have 'chain of custody' evidence and tamper-proof seals, so it can be shown exactly who had access to the samples.

Taking samples needs careful planning. There are many factors to be considered, such as

• where, when, and how to collect samples
• how many samples, and how much of each to take
• how to store and transport the samples to the laboratory
• how to avoid contamination
• how to safeguard the samples from tampering