Determination of phenylalanine isotope ratio enrichment by liquid chromatography/time-of-flight mass spectrometry

The application of time-of-flight mass spectrometry to isotope ratio measurements has been limited by the relatively low dynamic range of the time-to-digital converter detectors available on commercial liquid chromatography/time-of-flight mass spectrometry (LC/ToF-MS) systems. Here, we report the measurement of phenylalanine isotope ratio enrichment by using a new LC/ToF-MS system with wide dynamic range. Underivatized phenylalanine was injected onto a C-18 column directly with 0.1% formic acid/ acetonitrile as the mobile phase. The optimal instrument parameters for the time-of-flight mass spectrometer were determined by tuning the instrument with a phenylalanine standard. The accuracy of the isotope enrichment measurement was determined by the injection of standard solutions with known isotope ratios ranging from 0.02% to 9.2%. A plot of the results against the theoretical values gave a linear curve with R-2 of 0.9999. The coefficient of variation for the isotope ratio measurement was below 2%. The method is simple, rapid and accurate and presents an attractive alternative to traditional gas chromatography/mass spectrometry applications.