Assay

Overview
An assay is a procedure where a property or concentration of an analyte is measured.

There are numerous types of assays, such as an antigen capture assay, bioassay, competitive protein binding assay, crude oil assay, four-point assay, immunoassay, microbiological assay, stem cell assay, and many others, including concentration assays.

Molecular biology assays
Assays are regularly utilized in molecular biology scientific research laboratories.

DNA
Assays for studying interactions of proteins with DNA include:
 * DNase footprinting assay
 * filter binding assay
 * gel shift assay

RNA

 * nuclear run-on

Protein

 * Bicinchoninic acid assay (better known as the BCA assay)
 * Bradford protein assay
 * Lowry protein assay
 * Secretion assay

Cytotoxicity
Assays for studying how toxic a compound is to cells:
 * MTT assay
 * SRB (Sulforhodamine B) assay

Viruses

 * Viral plaque assay: Used to calculate the number of viruses present in a sample. This technique requires counting the number of plaques formed by a virus sample, from which the actual virus concentration can be determined.
 * Trofile assay: Used to determine HIV tropism.

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

Methods of assay of precious metals
There are methods of assay suitable for use on raw materials and other methods which are more properly suited for finished goods. Raw precious metals (bullion) are assayed by an assay office. Silver is assayed by titration, gold by cupellation and platinum by inductively coupled plasma optical emission spectrometry (ICP OES). ,

Precious metal items of art or jewelry are frequently hallmarked (depending upon the requirements of the laws of either the place of manufacture or the place of import). Where required to be hallmarked, semi-finished precious metal items of art or jewelry pass through the official testing channels where they are analyzed or assayed for precious metal content. While different nations permit a variety of legally acceptable finenesses, the assayer is actually testing to determine that the fineness of the product conforms with the statement or claim of fineness that the maker has claimed (usually by stamping a number such as 750 for 18k gold) on the item. In the past the assay was conducted by using the touchstone method but currently (most often) it is done using X-ray Fluorescence (XRF). XRF is used because this method is more exacting than the touchstone test. The most exact method of assay is known as fire assay or cupellation. This method is better suited for the assay of bullion and gold stocks rather than works or art or jewelry because it is a completely destructive method.

The touchstone
The age-old touchstone method is particularly suited to the testing of very valuable pieces, for which sampling by destructive means, such as scrapping, cutting or drilling is unacceptable. A rubbing of the item is made on a special stone, treated with acids and the resulting color compared to references. Differences in precious metal content as small as 10 to 20 parts per thousand can often be established with confidence by the test. It is not indicated for use with white gold, for example, since the color variation among white gold alloys is almost unperceivable.

X-ray fluorescence
The modern X-ray fluorescence is also a non-destructive technique that is suitable for normal assaying requirements. It typically has an accuracy of 2 to 5 parts per thousand and is well-suited to the relatively flat and large surfaces. It is a quick technique taking about three minutes, and the results can be automatically printed out by computer. It also measures the content of the other alloying metals present. It is not indicated, however, for articles with chemical surface treatment or electroplating.

Fire assay/cupellation
The most elaborate but totally destructive assay method is fire-assay,also called cupellation, with an accuracy of 1 part in 10,000. In this process the article is melted, the alloys separated and constituents weighed.

The assay of coins
An assayer is often assigned to each mint or assay office to determine and assure that all coins produced at the mint have the correct content or purity of each metal specified, usually by law, to be contained in them. This was particularly important when gold and silver coins were produced for circulation and used in daily commerce. Few nations, however, persist in minting silver or gold coins for general circulation. For example the U.S. discontinued the use of gold in coinage in 1933. The U.S. was perhaps the last nation to discontinue the use of silver in circulating coins in its 1969 half dollar coin, although the amount of silver used in smaller denomination coins was ended after 1964. Even with the half dollar, the amount of silver used in the coins was reduced from 90% in 1964 and earlier to 40% between 1965 and 1969. Copper, nickel, cupro-nickel and brass alloys now predominate in coin making. Notwithstanding, several national mints, including the Australian Mint at Perth, the Austrian Mint, the British Royal Mint, the Royal Canadian Mint, the South African Mint and the U.S. Mint continue to produce precious metal bullion coins for collectors and investors. The precious metal purity and content of these coins is guaranteed by the respective mint or government and therefore the assay of the raw materials and finished coins is an important quality control.

In the UK the Trial of the Pyx is a ceremonial procedure for ensuring that newly-minted coins conform to required standards.