Isotope-ratio-monitoring gas chromatography methods for high-precision isotopic analysis of nanomole quantities of silicate nitrogen

We have developed a system for analyzing nanomole-sized quantities of silicate-derived N₂ by carrier-gas methods, through combination of a metal high-vacuum extraction line fabricated at Lehigh University and a commercially available continuous-flow, gas chromatography interface (the Finnigan Gas Bench II). This work involves heating of samples to 950-1050 °C (depending on the material being analyzed), with Cu metal and Cu oxide reagents, in evacuated and sealed 6 mm (o.d.) quartz tubes. Uncertainties (expressed as 1σ for ≥ 3 replicate analyses of both internal silicate standards and unknowns) are generally less than 5% for N concentrations and on the order of 0.15‰ δ¹⁵N for samples with > 5 ppm N. At current blank levels (minimum overall system blank of 3.8 ± 0.2 nmol N₂ with a δ¹⁵N_air value of - 7.3 ± 0.4‰, mean ± 1σ), uncertainty in δ¹⁵N increases to ∼ 0.6‰ for samples with 1-2 ppm N. Practical minimum sample size, taking into account blanks and other factors affecting N₂ transfer, is now ∼ 10 nmol, two orders of magnitude smaller than that previously possible in our laboratory using dual-inlet microvolume methods (∼ 1 μmol). These methods, which can be employed in any laboratory able to undertake continuous flow techniques (with a dynamic-vacuum, isotope ratio mass spectrometer), afford increased spatial resolution in some studies and open up many new avenues of investigation previously impeded by the absence of sufficiently N-rich materials.