Traps, Mistakes, and Errors - Field Sampling: Principles and Practices in Environmental Analysis - page 288

11.10. SAMPLE/TEST CONTAINER ERRORS
Sampling one type of matrix using the wrong type of container or preservative can have
dramatic and catastrophic effects on the data obtained from the sample. An example of
this is the use of plastic containers to transport and store samples to be analyzed for
organic contaminants. Plastics commonly contain phthalates, which are a class of organic
compounds used to give a plastic flexibility. Small amounts of these compounds can
readily leach out of plastics and interfere with subsequent analytical procedures.
Phthalates could also be “extracted” by the organic compounds in the sample being
analyzed.
Samples can also be contaminated by using a batch of sample containers that are not
properly cleaned and by not cleaning sampling equipment thoroughly between uses.
11.11. IMPROPER SAMPLING TECHNIQUES
Inattention to sampling methods can lead to errors in sample analysis. An example would
be leaving headspace in a sampling container intended for VOC analysis. Sample
components will volatilize into the headspace at different rates, depending on their vapor
pressures. This leads to an uncharacteristic sample and erroneous data, which can lead to
inappropriate actions being taken in the cleanup of the field.
When sampling soils, great care needs to be taken to ensure that the threads, the lid of
the container, and the seals are clean and free of particles. Such particles can prevent
proper sealing and subsequent loss of sample through vapor loss, evaporation, or leakage.
It is imperative that care be taken to make sure that a representative sample is
collected. This point is especially important for soil or other solid samples, especially
when distinct layers are present. It is equally important in situations in which liquid
samples are made up of several immiscible layers. Failure to note this and ensure that a
representative sample or samples comprising all the fractions or layers is taken will result
in erroneous data and analytical results.
One way to handle this problem is to sample each of the distinct layers or fractions
individually and separately. Each of these fractions would be analyzed using the
procedure appropriate to either the type of material or the contaminants it contains. The
resultant analytical data can be put together to produce a composite picture of the
contamination in the field; for example, if the samples are taken from a pile of excavated
soil, the outside of which may be lower in contaminant because of exposure to light and
air. In this case samples can be taken immediately after excavation or from both surface
and subsurface layers in the pile. The resulting analytical results can be combined to
describe the concentration gradients in the pile.
11.12. VARIATIONS IN METHOD
Variations in extraction techniques can change analytical results and interpretation. This