High-precision measurement of phenylalanine and glutamic acid δ N by coupling ion-exchange chromatography and purge-and-trap continuous-flow isotope ratio mass spectrometry.

Rationale: Nitrogen isotopic compositions (δ N) of source and trophic amino acids (AAs) are crucial tracers of N sources and trophic enrichments in diverse fields, including archeology, astrobiochemistry, ecology, oceanography, and paleo-sciences. The current analytical technique using gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) requires derivatization, which is not compatible with some key AAs. Another approach using high-performance liquid chromatography-elemental analyzer-IRMS (HPLC/EA/IRMS) may experience coelution issues with other compounds in certain types of samples, and the highly sensitive nano-EA/IRMS instrumentations are not widely available.