Revalidation and long-term stability of National Institute of Standards and Technology Standard Reference Materials 1566, 1567, 1568, and 1570

Multiple units of Standard Reference Materials (SRMs) 1566 Oyster Tissue, 1 567 Wheat Flour, 1568 Rice Flour, and 1570 Trace Elements in Spinach, produ ced by the National Institute of Standards and Technology (NIST, then the N ational Bureau of Standards), were analyzed 17-20 years after the original certification dates and 12-15 years after the certificates became invalid. Instrumental neutron activation analysis and thermal neutron prompt gamma-r ay activation analysis were used to measure mass fractions for 27 elements in these SRMs to revalidate them for use in quality assurance (QA) programs required for food analysis programs within the U.S. Food and Drug Administ ration. With the exception of Se in SRM 1567, all element mass fractions we re in agreement with certified values and literature data. Some evidence of B loss from SRM 1568 was observed. These materials were judged to be suita ble for continued use in QA programs. Findings showed that these matrixes e xhibited stability of moisture, mass fraction, and weight basis for far lon ger (greater than or equal to 15 years) than was indicated by the 5-year va lidity statement on the NIST Certificates of Analysis.