An assay is a procedure where the concentration of a component part of a mixture is determined.

There are numerous applications of an assay, such as an antigen capture assay, bioassay, competitive protein binding assay, four-point assay, immunoassay, microbiological assay, stem cell assay, and many others.

In the laboratory

Assays are regularly utilized in scientific research laboratories.

DNA

Assays for studying interactions of protein with DNA include:

• DNase footprinting assay
• electron microscopy
• filter binding

Protein

• UV spectroscopy
• Bradford protein assay
• Lowry protein assay
• Bicinchoninic acid assay (better known as the BCA assay)

Virii

The number of viruses present in a sample can be determined using a viral plaque assay. This technique requires counting the number of plaques formed by a virus sample, from which the actual virus concentration can be determined.

Cellular secretions

A wide range of cellular secretions (say, a specific antibody or cytokine) can be detected using the ELISA technique. The number of cells which secrete those particular substances can be determined using a related technique, the ELISPOT assay.

Drugs

Illegal drug testing

Environmental contaminants

• Lead
• Coliform bacteria
• Arsenic

Coins

An assayer is often assigned to each mint to determine and assure that all coins produced there have the correct content or purity of each metal in them. This is particularly important with gold and silver coins.

Precious metals

Precious metals, platinum, gold and silver, used in jewellery, silverware and other items, are assayed to test the purity of the metal.

This is done in the UK by an Assay Office. Once an item has been assayed, it is usually hallmarked to the relevant standard.