Chemical Identification vs. Assay

Compendial Testing: Chemical Identification vs. Assay

Generally when one is testing raw materials, there are tests for both identification (ID) and assay. The two tests are usually quite different, and usually you need both. Most raw materials have specifications and test methods.

A raw material used in pharmaceuticals must meet the specifications given in the US Pharmacopeia/National

Formulary (USP/NF) using the test methods specified in the monograph for that raw material, so that the material can be sold as USP grade. Other industries have specifications and test methods such as ACS (American Chemical Society) for general chemicals, Food Chemical Codex (FCC) for the food grade chemicals, etc..

For materials not listed in any such collection of specifications (compendia), suppliers may have their own specifications and test methods.

An identification (ID) test can be something elaborate such as matching chromatographic retention times of a standard or matching an infrared spectrum. It can also be very simple such as a flame test. The results from the ID test generally say nothing about the concentration of chemical, especially for the more simple tests. For example, to identify sodium acetate, one confirms the presence of sodium through a flame test (the flame burns yellow) and a couple of positive and negative precipitate tests. The test does not indicate the presence or absence of other chemicals or the concentration of sodium.

An assay method, on the other hand is intended to measure the concentration of the chemical, assuming the major

ingredient has been identified. Assay specifications are usually very tight such as 99.0- 101.0%. This usually requires a very accurate and precise analytical method such as a titration, a colorimetric or gravimetric assay.

Chromatographic assays usually have larger errors. Titrations and other wet chemical assays are usually not very specific, and their use assumes the material has been correctly identified. For example the assay methods for both sodium and potassium chloride involves measuring the chloride ion concentration using a silver nitrate titration. This method does not identify the sodium or potassium.

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