Physical grab" samples

The conclusion is inescapable sensor/probe heads perfonn in a manner identical to grab sampling. There is little sense in relying on physical specimens only, or specimen-signals, to characterize the entire cross stream or product in the PAT context. Figure 3.16 is the case in point. 

Stratified random sampling, which is a variation of simple random sampling, is used for media that are stratified with respect to their chemical and physical properties. Each stratum is identified and randomly sampled. The number of grab samples and the sampling point selection depend on the nature of contaminant distribution within each stratum. Stratified random sampling is used for the characterization of multiphase liquid wastes or process waste batches that undergo stratification over time and/or space. 

The most common error in physically obtaining the sample occurs when the sampling tool cannot take the sample that has been properly defined. For example, an extraction error is produced by a retractable cross-stream sampler that goes only partway across the stream before returning to its idle position. This is in effect a grab sample since only one side of the stream is sampled. Any segregation across the stream will produce a biased sample. 

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