Procuring a Representative Sample

The sample must be truly representative of the bulk to be analyzed.

The results of a chromatographic analysis (or any analysis for that matter) can only be as reliable as the integrity of the sample that is taken.

The same care should be taken over the sampling procedure as that taken over the actual analysis. Often, the analyst does not take the sample and, thus, has no control over the sampling procedure. Nevertheless, the analyst must try to ascertain the sampling method and other pertinent sampling details in order to access the validity and precision of the analytical results. It is a fairly easy to obtain a sample that is representative from a container of a homogeneous liquid. However, if a sample of water is taken from a flowing river, a silo of corn or even a sample from a drum of aspirin tablets, the way to obtain a representative sample is not immediately obvious and each will require special sampling techniques.

There are instances where a representative sample is impossible to obtain and, indeed, may not even be necessary. For example. if the sample consists of a deposit scraped from the surface of a incinerated container, then the significance or even the meaning of the term representative becomes lost. In this case the analysis will probably only have a qualitative significance for forensic purposes. Any quantitative pertinence will lie in the relative proportions of the constituents within the sample, not in their absolute values.

Transportation and Storage

The method of transportation and storage must be chosen to suit its physical form and its sensitivity to the environment. Sealing the sample in an airtight container will probably not be adequate. Thermally labile samples or those of a biological origin may need to be refrigerated until ready for preparation. Samples susceptible to atmospheric oxidation will need to be stored under nitrogen or helium. If the sample contains only traces of the material of interest (e.g. pesticides in a water sample) then the container must not adsorb the pesticide. Ideally, sample containers must be chemically and physically inert to both the materials of interest and the sample matrix. Glass, polythene, Teflon or stainless steel may be appropriate materials for construction. Thus, each sample container must be made of a specific material appropriate for the sample. The individual sample containers must then be well protected from physical shock and thermally insulated or refrigerated. On arrival the samples should be stored in a (preferably locked) enclosure and be readily available to the analyst