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.
9240 Santa Fe Springs
Formerly West Coast Analytical Service (WCAS) and Bodycote Testing Group