HR: 0800h
AN: V51C-0590    [Abstracts]
TI: High-Precision Isotopic Analysis of Nanomole Quantities of Silicate Nitrogen
AU: * Idleman, B D
EM: bdi2@lehigh.edu
AF: Dept. Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015 United States
AU: Bebout, G E
EM: geb0@lehigh.edu
AF: Dept. Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015 United States
AU: Li, L
EM: lol2@lehigh.edu
AF: Dept. Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015 United States
AU: Hilkert, A
EM: andreas.hilkert@thermo.com
AF: Thermo Electron (Bremen) GmbH, Finnigan Advanced Mass Spectrometry, Barkhausenstr. 2, Bremen, 228197 Germany
AB: The N isotope system shows exciting potential to trace interactions among EarthAŸA›A›ƒ_sAªA›ƒ_zA›s major reservoirs, in particular providing a unique tracer of the transfer of organic components through a wide range of biogeochemical processes. However, sample size requirements of conventional dual-inlet viscous-flow mass spectrometry limit routine N isotope work to relatively high-N systems. Analyses of smaller N$_{2}$ samples ($<$ 0.5 micromoles) are possible by static mass spectrometry, but in only a small number of labs and with some compromise in precision. We have developed a system for analyzing nanomole quantities of silicate N 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 (Finnigan Gas Bench II). Testing thus far has concentrated on analyses of aliquots of air and standard tank gases, and application of sealed-tube methods in the production of small N$_{2}$ samples from silicate standards (fuchsite, buddingtonite, whole-rocks). Procedural blanks of this method, employing 6 mm (o.d.) quartz tubes heated, then cracked onto the extraction line, are presently 0.5 to 1.0 nmoles. Practical minimum sample size, taking into account blanks and factors affecting N$_{2}$ transfer, is now approximately 70 nmoles, more than one order of magnitude smaller than that previously possible in our laboratory using dual inlet methods and a cold-finger/microvolume (for our previous method, procedural blanks are about 0.025 micromoles, and minimum sample size is about 1.0 micromole). Precision of the carrier-gas method for the silicate standards (1$\sigma$ for n $>$ 3) is at present 0.1 to 0.2 per mil and mean isotope values are within 0.1 per mil of accepted values based on previous analyses by dual-inlet mass spectrometry. Our ongoing research is aimed at reducing the minimum sample size to approximately 10 nmoles, further reducing the blanks, addressing some remaining minor nonlinearities in m/z 28 beam intensity and isotopic composition with N$_{2}$ sample size, incorporating IR laser heating in extractions, and producing silicate N-isotope standards. The small sample capability of these methods will afford analyses of microgram-sized samples of mineral separates (micas, ammonium-rich K-feldspars), thus allowing higher spatial resolution and opening up many new avenues of investigation previously impeded by the absence of sufficiently N-rich materials. Particular themes in our N isotope work include tracing organic signatures into the crust and mantle and understanding modern and ancient Earth volatile cycling and atmosphere evolution.
DE: 3670 Minor and trace element composition
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting