Determination of Phenylalanine Isotope Ratio Enrichment by LC/Time-of-Flight Mass Spectrometry
Time-of-flight mass spectrometry has the advantages of high resolution, high sensitivity, and high mass accuracy but with relatively narrow dynamic range if a TDC (Time-to-Digital Converter) is used as a data acquisition system. This disadvantage of narrow dynamic range hinders the applications for isotope ratio enrichment measurement. Recently, a new LC/TOF-MS system was introduced that achieved a wide dynamic range by using an ADC (Analog-to-Digital Converter) instead of a conventional TDC. We used this system to evaluate the measurement of phenylalanine (Phe) isotope ratio enrichment. The accuracy, reproducibility, and sensitivity for the method were determined. The method is simple, rapid, and accurate and presents an attractive alternative to traditional GC/MS applications.
The system included an Agilent 1100 HPLC and a JEOL AccuTOF™ time-of-flight mass spectrometry system. Complete system control and data evaluation were carried out using a JEOL MassCenter™ workstation.
All solvents used were of HPLC grade. The standard solutions were prepared by mixing unlabelled Phe and 13C6-Phe in 0.1 % formic acid solution to make solutions with isotope ratio enrichments ranging from 0.02 % to 9.2 and a constant total concentration of 100 nmol/mL. The isotope ratio enrichment was expressed as tracer/tracee ratio (TTR).