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Environmental
Sampling
The purpose of sampling is to obtain a fraction of some lot of material that accurately represents the characteristics of the entire lot. The "lot" is what we are sampling. It may be a landfill, a tank car, a drum, sediments under a pond, or a jar in the analytical laboratory. In practice, it is not possible to obtain a perfectly representative sample of soils or sediments. Sampling procedures should be designed to minimize the sampling error and to document an estimate of the overall error, which includes: sampling error, sample handling error, and analytical error. Some sources of sampling error cannot be eliminated, however with proper understanding of sampling theory and with clear understanding the purpose of the sampling, error can be minimized and documented.
It is important to remember that these sources of error are introduced each time sub-sampling occurs. When sample size is reduced by removing a portion of a core or by taking an aliquot from a jar, due consideration must be given to sampling procedures.
Incorrect sampling design or incorrect sampling procedures are just as damaging to the defensibility of data as are incorrect analytical methods. Planning for sampling and assessment of sampling methods should be given the same consideration as analytical data validation.
Bias can be introduced by routine sampling operations. Attempts to homogenize solids or suspended solids (such as sediments) for subsequent sample size reduction almost never succeed. A well-stirred soil sample can be well-mixed without being homogenized. Attempts to homogenize soils and sediments often fail because denser material quickly settle resulting in a stratified material.
Traditionally, sample-size reduction is achieved by riffle-splitting or by coning-and-quartering. Riffle-splitting and coning-and-quartering are not methods for homogenizing samples, they are methods for obtaining representative samples. These methods work well for dry unconsolidated materials but not as well for typical environmental samples. For non-volatile and non-reactive analytes, it is recommended ("Sampling Methodologies for Monitoring the Environment - Theory and Practice", a short course given by Francis Pitard and sponsored by the US EPA, 1993) that the sample be parceled into smaller samples and one selected at random.
For example, if a five-fold size reduction is required, small sample increments are removed from the original container and consecutively placed in five containers. This is repeated until the entire sample has been removed from the original container. One of the containers is then selected at random. No attempt is made to homogenize the sample.
This method is not applicable to environmental samples taken for analysis of volatile or reactive constituents. In these circumstances, non-tradition methods may be required. The methods selected will depend on the purpose of the sampling.
Many states and EPA Regions are adopting SW-846 Method 5035 as a protocol for field collection of volatile organic samples. In Method 5035 (SW-846 Update III), the EnCore(TM) sampling system is used to collect discrete samples. These samples are preserved by freezing them or placing them in solutions of sodium bisulfate. Both methods of preservation have drawbacks. Frozen samples have a short hold time (48 hours) and samples that are rich in carbonates can effervesce causing loss of the volatiles. Methanol preservation has fewer problems but cannot be used for low-level analysis. Samples collected using Method 5035 can give significantly higher results than samples collected by traditional methods.
Information on sampling is available on the world wide web.
EPA Region 1 - New Monitoring Well Sampling Procedures
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Last update: April 15, 2005.