An assay is a quantitative determination of the amount of a given substance in a particular sample. They can be performed by wet methods or dry methods. In the wet method, the sample is dissolved in a reagent. In the dry method, the sample is mixed with a flux (a substance such as borax or silica that helps lower the melting temperature) and then heated to the point where impurities fuse with the flux, leaving the substance as a residue.

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source: http://www.limswiki.org/index.php/Assay