Analysts work with samples. The size of the sample needed depends on what tests need to be carried out. Samples of any item must be representative. In other words, there should be enough of the sample to give a pictures of the whole item. The sampling method depends on the nature of the sample.
The composition of a homogeneous item is the same throughout, like dairy milk.The composition of a heterogeneousitem is not all the same, like a kitkat which is made in layers.
Preservation Of Samples
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 depend on protecting samples from contamination. Crime scene investigators wear protective clothing and put all samples into sealed bags or bottles to avoid contaminating 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 some samples it is important to have "chain of custody" evidence and tamper proof seals, so it can be shown exactly who has access to samples.