13C labelled internal standards—A solution to minimize ion suppression effects in liquid chromatography–tandem mass spectrometry analyses of drugs in biological samples?

Abstract Liquid chromatography–tandem mass spectrometry (LC–MS/MS) is frequently used to identify and quantify drugs in human biological samples due to the high selectivity and sensitivity of this technique. However, ion suppression effects caused by co-eluting compounds: drugs, metabolites, matrix components, impurities and degradation products, are a major concern. Stable isotope labelled internal standards (SIL ISs), usually deuterium ( 2 H) labelled, are often used to compensate for these effects. In many LC separations the retention times of 2 H labelled ISs and their analogues will differ. Ultra performance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS) is increasingly being used for bio-analysis. With the better chromatographic resolution provided with sub 2 μm particles, larger separation between analytes and their 2 H labelled analogues can be expected, which might reduce the benefits of the SIL IS. There is a greater difference in physico-chemical properties between hydrogen isotopes than between isotopes of other elements. 13 C, 15 N and 18 O labelled ISs are more similar to their analytes than 2 H labelled ISs and thereby expected to behave more similarly in chromatographic separations. In this study we have investigated the use of 13 C and 2 H labelled ISs for the determination of amphetamine and methamphetamine by UPLC–MS/MS. The 13 C labelled ISs were co eluting with their analytes under different chromatographic conditions while the 2 H labelled ISs and their analytes were slightly separated. An improved ability to compensate for ion suppression effects were observed when the 13 C labelled ISs were used. Furthermore, an UPLC–MS/MS method for determination of amphetamine and methamphetamine in urine using 13 C labelled ISs has been developed and validated. Unfortunately, there are few 13 C labelled ISs commercial available today. If more 13 C labelled ISs become commercial available they may well be the coming solution to minimize ion suppression/enhancement effects in LC–MS/MS analyses of drugs in biological samples.