Stable isotopes as valid components for identification of drugs in biological specimens

Selected ion monitoring in mass spectrometric methods (SIM-MS) is generally used to confirm the presence of a drug in biological samples. Criteria for identification of a compound by MS are based on some specific guidelines. However, some disparities exist between the guidelines as to how many and what type of ions to monitor. Although European guidelines allow the monitoring of isotopic ions, such monitoring is not valid by SOFT/AAFS guidelines. The feasibility of monitoring a stable isotopic ion as an alternate to the fragment ion was examined in our study. Area ratios of stable isotopic ion m/z 275 and its parent ion m/z 274 of optical isomers of methamphetamine as (R)-(-)-alpha-methoxy-alpha-(trifluoromethyl)phenylacetyl derivative were found to be within +/- 4% of theoretical value (14.969) calculated from fragment formula C(13)H(15)F(3)NO(2) and isotopic abundances (C(13) = 1.1%, H(2) = 0.015%, F = 0%, N(15) = 0.37%, and O(17) = 0.037%). In another example, the area ratios of a stable isotopic ion m/z 283 and the parent ion m/z 282 of 6-pentafluoropropionyl codeine was also within +/- 4% of theoretical value (20.542) calculated from fragment formula C(18)H(20)NO(2). This relationship between the isotopic abundance and fragment composition was also useful in assigning structures to many fragment ions of methamphetamine, LSD, morphine, and 6-acetylmorphine derivatives, whereas structural assignment to the ions based on mass alone was difficult. The predictability of the relative abundance of the examined isotopic ions has proven reliable in our studies. The use of an isotope was found to be an important additional tool for compound identification by MS.