The composition of the matrix to be sampled can often vary with time. If the rate of change is significant relative to the time needed to collect the sample, this alone makes meaningful interpretation difficult. The sampling of flowing water and gases frequently presents these kinds of challenges. Resolution of such sampling difficulties usually requires careful consideration of the use to which the data will be put. For example, if the measurements being made are for the purpose of assessing average exposures or loadings of contaminants, numerous samples can be taken over a long period of time and pooled prior to analysis to form a composite sample. However, if the release pattern of contaminants, and concentrations to which sensitive organisms are exposed is required, forming composites would not be appropriate. Instead the capture and analysis of many discrete samples, each taken over a short time interval may be more relevant. For some constituents of flowing material, continuous sampling using appropriate sensors and instrumentation may be appropriate.
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