Nitrogen isotope analysis of free glycine in soil using isotope dilution mass spectrometry

To enable the estimation of production and consumption rates of free glycine in soils through 15N isotope dilution experiments, an isotope dilution mass spectrometric method was developed. The method, which enabled high precision N isotope ratio determination of glycine in soil extracts at δ15N levels up to 4000‰ and concentrations from approximately 2 μM, is based on the following steps: (i) addition of glycine spike to the soil extract, (ii) removal of humic substances and pre-concentration of glycine using solid phase extraction, (iii) derivatization of amino acids, (iv) separation of the derivatives using gas chromatography (GC), (v) their combustion to yield sample N2 gas, and (vi) finally the use of N isotope ratio mass spectrometry (IRMS). Judging by uncertainty budget calculations, the precision obtained (SD=0.01–0.06 at% 15N) is sufficient for detecting differences in N isotopic ratios obtained in 15N isotope dilution experiments.