Purity of Analytical Standards

It is good practice to conduct additional qualification tests with these materials before beginning method development. Unless the amount of standard provided is insufficient to permit additional testing to confirm the identity and purity, some tests should be conducted to provide this confirmation prior to routine use of the standard. Typical checks 

There is also usually a recommended period of time during which the purity of an analytical standard is considered to be assured if the standard is properly stored, typically from 2 to 5 years, and this will normally be given on the certificate of analysis provided with the standard. If no expiry date is provided with the standard, then it is the responsibility of the laboratory to establish the optimum storage conditions and a maximum time period for which the standard may be used. At the end of this period, the standard should be replaced. However, the usage period may be extended if the laboratory establishes a procedure to recertify the standard with tests that show that no changes in the purity and composition of the standard have occurred. Procedures used to establish or extend expiry dates for standards should be documented. 

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Here Xj-x represent measured quantities including physical ones (e.g., mass of analytical standard weighed out, volumes of standard flasks etc.) and chemical properties (e.g., chemical purity of analytical standard, isotopic purity of an internal standard, ratios of analytical signals for analyte and internal standard etc.). Specific examples of such functions are those discussed in Section 8.4. The experimental variables (x(m+])-x ) that do not appear directly in the functional relationship could include temperature of various components of all forms of apparatus used, mobile phase composition and flow rate, operating parameters of the mass spectrometer, etc. 

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