Experimental 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 13 C 6- 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).