Sampling and Subsampling

Developing a Sampling Plan

In the development of a sampling procedure, the first step is to reexamine the problem definition (see Part 2: Planning the Project). The analyst must review how the final results will be used and obtain an understanding of the characteristics of the sample population.

For example, the question "Has this industrial site been contaminated with Pb and Hg?" addresses the immediate concern and prompts the analyst to start thinking about other issues. This thought process would bring about additional questions, such as "Have Pb and Hg compounds been used on this site?" and "What type(s) of compounds were used?"and "Where were these compounds stored?". These questions lead the analyst to an assessment of the scope of the sample population. In some cases, the whole site would be sampled. At other times, the sampling would be limited to specific areas. The analyst can then recommend a sampling procedure using methods already published on soil sampling and tailored to answering the specific question. The analyst may determine that only 3-5 samples are needed in an area where drums that contained Pb and Hg compounds were placed. The other extreme would be to propose a random sampling over a 20 acre site, where 20 samples are pulled in a equally spaced geometric fashion. Other concerns pertain to the ‘normal' levels of Pb and Hg in the soil for that area where state and local environmental agencies would be contacted for baseline data.

By knowing the objective, the analyst is in a position to recommend sampling procedures that are designed to obtain a reliable answer.

A sampling plan should consider the following:

• The objective of the analysis.
• The liability/cost of a wrong decision.
• The accuracy and precision requirements to obtain data that will allow for a correct decision to be made.
• Whether the material has been sampled before.
• The degree of homogeneity.
• Safety risks, site hazards, and product hazards.
• Legal issues, including witnessing or permission grants.
• Whether the material is in packages, heaps, bottles, railroad cars -- i.e. - how the material is characterized.
• The contamination risks.
• The number of samples to be taken.
  
• How the samples will be collected -- by whom and in what kind of container.
• The total quantity of the aggregate sample.
• Whether specific storage conditions are required.
• Whether preservation is required.
• Whether and how the aggregate sample will be sampled (subsampling).
• Whether grinding or sieving will be used.
• The size of the final sample and whether more than one aliquot be prepared for analysis.

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