δ¹⁵N Analyses of Ammonium-Rich Silicate Minerals by Sealed-Tube Extractions and Dual Inlet, Viscous-Flow Mass Spectrometry

This chapter discusses the development of techniques for the routine, quantitative extraction and isotopic analysis of trace amounts of nitrogen in micas and whole-rock silicate samples. In order to obtain highly accurate and precise δ¹⁵N and N concentration data for such materials relatively straight-forward sealed-tube extraction techniques and standard cryogenic purification methods and dynamic-vacuum, dual inlet, viscous-flow mass spectrometry are used. For δ¹⁵N values of samples containing > 100 ppm N, 1σ for 2-10 duplicate analyses typically approaches 0.1%, and N concentration data nearly always have < 5% uncertainty. Blanks of these extraction techniques are < 0.025 μmol N2, affording analyses of small N₂ samples to 1.0 μmole. The precision of these analyses is typically higher than that obtainable by static mass spectrometry techniques, with the latter methods commonly producing data with 1σ nearer 0.5%. The requirement of viscous-flow conditions and the geometry of the sealed-tube experiments impose a practical limit of ~ 10 ppm as the minimum N concentration that can reliably be analyzed by these technique. In many crustal igneous and metamorphic rocks, white-micas and biotite, both of which strongly partition N as NH_{4 +} into their interlayer sites, contain the majority of the whole-rock N and commonly have N concentrations in the range of 10 - 1000 ppm.