Collecting Samples

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Representitive Samples

Analysts work with samples. The size of the sample needed depends on waht test need to be carried out.

Samples of aany item must be representitive. In other words, there should be enough of teh sample to give a picture ofthe whole item. The sampling method depends on the nature of the sample.

(http://www.cadbury.co.uk/NR/rdonlyres/1722BA3A-0872-4694-9C84-F20FEBF53519/0/cdm_bar_517.jpg)The composition of a homogeneous item is the same throughout, like a dairy milk chocolate bar.

(http://food.3yen.com/wp-content/images/hokkaido_milk_white_kitkat2.jpg)The composition of a hetrogeneous item is not all the same, like a chocolate bar made in layers.

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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.

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Avoiding contamination

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.

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Avoid Tampering

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 had access to the samples.

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Planning

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 safe guard the samples from tampering
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Key Words

Sample-

A part of something taken for analysis

Representitive-

Having properties which are the same as the rest of the item or set

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