assay

In chemistry, the determination of the quantity of a given substance present in a sample. Usually it refers to determining the purity of precious metals. In biochemistry, it refers to quantitative measurements, for example of enzyme activity.

The assay may be carried out by ‘wet’ methods, when the sample is wholly or partially dissolved in some reagent (often an acid), or by ‘dry’ or ‘fire’ methods, in which the compounds present in the sample are combined with other substances.