Isotopic analogs as internal standards for quantitative analyses by GUMS -evaluation of cross-contribution to ions designated for the analyte and the isotopic internal standard

Isotopic analogs of the analytes are currently preferred internal standards (IS) for quantitative analyses of drugs and their metabolites in biologica l matrices by GC/MS procedures. Contributions of the analyte and the IS to the intensities of ions designated for the IS and the analyte, respectively - an undesirable phenomenon termed "cross-contribution" - greatly weakens the effectiveness of this approach. The cross-contribution phenomenon has b een, in the past, evaluated by a "direct measurement" approach, in which in tensities of interested ions were measured in two separate experiments usin g equal quantities of the analyte and the IS. Alternate procedures that may generate improved results are hereby studied. For the "improved direct mea surement" approach, ion intensity data derived from the previously reported direct measurement procedure are first normalized before being used to cal culate the extent or cross-contribution. An "internal standard" approach is also developed, in which a set amount of a third compound is incorporated into these two separate experiments, thus allowing corrections of ion inten sity data that are imbedded with variations inherent to separate experiment s. Finally, a "standard addition" approach, involving a series "addition" o f "standards", generates multiple data points, thus, providing a mechanism to validate the resulting cross-contribution data. Secobarbital/H-2(5)-seco barbital and secobarbital/C-13(4)-secobarbital pairs are adapted as the exe mplar systems for this study. (C) 2001 Elsevier Science Ireland Ltd. All ri ghts reserved.