Low N archean rocks

 

Summary

We have developed an in-tube combustion / continuous flow hybrid approach for the δ15N analysis of hundred nanomolar
quantities of nitrogen in ancient sedimentary rock samples. Nitrogen concentrations are as low as 5 ppm. This
quantity precludes the use of traditional elemental analysis isotope ratio mass spectrometry (EA-IRMS) because
flash-combustion of large quantities of rock powder may not quantitatively oxidize all silicate- and organic-bound nitrogen
phases.

Exhaustive description of analysis

We assembled a vacuum preparation line to prepare these low N samples for in-tube combustion. This
approach has the advantage that larger powder aliquots (400 mg or more) can be combusted efficiently. Such in-tube
combustion preparations have traditionally been analyzed by dual-inlet IRMS which can be extended down to about
220 nmole through the use of a cryogenic microvolume. To achieve better sensitivity, we developed a front-end continuous
flow line with a tube-cracker so samples could be released into a helium stream, collected on a molecular
sieve cryogenic trap, purified away from any CO with a molecular sieve GC column and analyzed as a nitrogen pulse,
much like traditional EA-IRMS. Our current blank is 11 nmole nitrogen (vacuum line plus continuous flow line).
Typical sample reproducibility (1 σ) is 0.5 ‰. Blank corrected USGS41 δ15N relative to our reference gas is +47.5 (σ
= 0.05 ‰, n = 2). Our average δ15N for SGR-1 is +17.8 ‰ (σ = 0.48 ‰, n = 5). This in-tube combustion / continuous
flow IRMS approach will allow us to measure δ15N from sedimentary and possibly igneous rock samples with comparable
precision and accuracy.

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