Some samples need to be filtered before they can be analysed. This refers to pushing or pulling the sampled water through a membrane filter of a certain pore size (usually specified in microns). An example is the analysis for dissolved metals, where the sample needs to be filtered to remove the insoluble metals out of the sample. Field filtration refers to this process being carried out on-site rather than in the laboratory. This could be because the sample needs to be preserved by adding nitric acid before being shipped to the laboratory. If this is the case and samples also require filtration, field filtration needs to be performed as filtration has to occur before preservatives are added.
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