High-precision isotopic analysis of palmitoylcarnitine by liquid chromatography/electrospray ionization ion-trap tandem mass spectrometry

Single quadrupole gas chromatography/mass spectrometry (GC/MS) has been widely used for isotopic analysis in metabolic investigations using stable isotopes as tracers. However, its inherent shortcomings prohibit it from broader use, including low isotopic precision and the need for chemical derivatization of the analyte. In order to improve isotopic detection power, liquid chromatography/electrospray ionization ion-trap tandem mass spectrometry (LC/ESI-itMS2) has been evaluated for its isotopic precision and chemical sensitivity for the analysis of [¹³C]palmitoylcarni-tine. Over the enrichment range of 0.4-10 MPE (molar % excess), the isotopic response of LC/ ESI-itMS2 to [¹³C]palmitoylcarnitine was linear (r=1.00) and the average isotopic precision (standard deviation, SD) was 0.11 MPE with an average coefficient of variation (CV) of 5.6%. At the lower end of isotopic enrichments (0.4-0.9 MPE), the isotopic precision was 0.05 MPE (CV=8%). Routine analysis of rat skeletal muscle [¹³C₄]palmitoylcarnitine demonstrated an isotopic precision of 0.03 MPE for gastrocnemius (n=16) and of 0.02 MPE for tibialis anterior (n=16). The high precision enabled the detection of a small (0.08 MPE) but significant (P=0.01) difference in [¹³C ₄]palmitoylcarnitine enrichments between the two muscles, 0.51 MPE (CV=5.8%) and 0.43 MPE (CV=4.6%), respectively. Therefore, the system demonstrated an isotopic lower detection limit (LDL) of ≤0.1 MPE (2 X SD) that has been impossible previously with other organic mass spectrometry instruments. LC/ESI-itMS2 systems have the potential to advance metabolic investigations using stable isotopes to a new level by significantly increasing the isotopic solving power.