(1999; 168 pages) [Spanish]
5. Assignment of content
If a content is to be assigned to a chemical reference substance, it should be borne in mind that the value is based on the results of a collaborative interlaboratory programme using different analytical methods. This experimentally obtained value represents the best estimate of the true value. In general, the assignment of content for a chemical reference substance is 100% minus the content of water and volatiles, and when a substance is intended for use as an assay standard based on a separation technique the impurity content, as determined by that method, must also be subtracted. Sometimes the chemical reference substances must be dried before use, in which case the content is expressed on the basis of the dried material.