Assay 496309 207522898 2008-04-23T03:27:52Z Escarbot 1554195 robot Adding: [[es:Ensayo químico]] {{Unreferenced|date=November 2006}} {{Otheruses}} 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 [[laboratory|laboratories]]. ===DNA=== Assays for studying [[interaction]]s 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|Viral plaque assay]]: Used to calculate the number of [[virus]]es 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=== [[Drug test|Illegal drug testing]] ==Environmental contaminants== *[[Lead]] *[[Coliform bacteria]] *[[Arsenic]] ==Methods of assay of precious metals== [[Image:Tombstone courthouse assay lab.jpg||thumb|200px|A 19th century assay laboratory in [[Tombstone Courthouse State Historic Park]], [[Arizona]].]] [[Image:Klondike Gold Rush Nat Hist Park Seattle seal 03A.jpg|thumb|200px|A late 19th century Canadian seal used to certify the quality of assayed gold.]] 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).<ref>[http://www.thegoldsmiths.co.uk/assayoffice/hallmarkingprocess.htm The Goldsmiths' Company - The Hallmarking Process<!-- Bot generated title -->]</ref>,<ref>[http://www.waarborg.nl/index_uk.php WaarborgHolland, the Masters of Europe<!-- Bot generated title -->]</ref> 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 [[hallmark]]ed, 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 (coin)|mint]] or [[assay office]] to determine and assure that all [[coin]]s 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. ==See also== *[[ELISA]] *[[MELISA]] *[[Analytical chemistry]] *[[Pharmaceutical chemistry]] *[[Titration]] ==References== <!-- ---------------------------------------------------------- See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for a discussion of different citation methods and how to generate footnotes using the <ref>, </ref> and {{reflist}} tags. ----------------------------------------------------------- --> {{reflist}} [[Category:Laboratory techniques]] [[Category:Titration]] [[Category:Coins]] [[Category:Watches]] [[Category:Jewellery]] [[Category:Silver]] [[Category:Gold]] [[de:Assay]] [[es:Ensayo químico]] [[id:Asai]]