文摘
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 δ15Nair 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.