Fig. 2
The precision and repeatability of the nitrogen isotopic values determined by GC/C/IRMS and EA/IRMS. a The analytical relationship between S/N ratio (signal-to-noise ratio) and ∆15N (defined as difference between δ15NEAIRMS and δ15N GCCIRMS ) for alanine (Ala, C3 neutral amino acid), valine (Val, C5 neutral AA), glutamic acid (Glu, C5 acidic AA), and phenylalanine (Phe, C9 aromatic AA). b Comparison of nitrogen isotopic compositions of amino acid standards (as Piv/iPr ester derivatives) determined by EA/IRMS (before derivatization) and GC/C/IRMS (after derivatization). The isotopic composition of each amino acid was expressed as the per mil (‰) deviation from a standard (AIR), as defined by: δ15N = [(15N/14N)sample/(15N/14N)standard − 1] × 1000 (‰)