Evaluation of isotopically labeled internal standards and methods of derivatization for quantitative determination of cocaine and related compounds

Abstract

Gas chromatography-mass spectrometry (GCMS) is the preferred method for the analysis of drugs/metabolites in biological specimens with use of isotopically labeled analogs of the analytes as internal standards (ISs). An important aspect of the chemical derivatization (CD) for GC-MS analysis is that the CD products derived from the analyte and the selected IS must generate ions suitable for designating the analyte and the IS. These ions should not have significant cross contribution (CC), i.e., IS contribution to the intensities of the ions designated for the analyte, and vice versa. With this in mind, the authors have conducted a search of isotopically labeled analogs of commonly abused cocaine and related compounds (cocaine, norcocaine, benzoylecgonine, cocaethylene, ecgonine, ecgonine methyl ester, anhydroecgonine methyl ester) that are commercially available. These ISs and analytes were derivatized with various groups of reagents, and the CD products were analyzed by GC-MS. MS data are presented in two forms: (1) systematic presentation of fullscan spectra; and (2) tabulation of CC data for ions with potential for designating the ISs and analytes. Many (if not most) of these full-scan spectra are not yet available in the open literature and should be of routine reference value to forensic and clinical laboratories that are engaged in the analysis of these drugs/metabolites. Fullscan MS data were further used to select ion pairs with potential for designating the analytes and ISs in quantitative analysis protocols. The CC data of these ion pairs were evaluated using data collected under the selected ion-monitoring mode and summarized in table format. The data exhibited similar CC characteristics in each alkyl, acetyl, or TMS series. Among the potentially usable ion pairs derived from a specific CD group, there was a trend that the ion pairs with higher mass showed better CC data. The CC data derived from the use of ISs labeled with more deuterium atoms were generally more favorable. These data should save enormous amounts of time and effort for practicing laboratories in their search for optimal analytical parameters.