High precision nitrogen isotope measurements in oceanic basalts using a static triple collection noble gas mass spectrometer

We describe a new system for the simultaneous static triple-collection of nitrogen isotopes at the < 10 mu cm(3) STP [N-2] (< 1 x 10(-5) cm(3) STP; < 0.5 nmol) level using a modified VG-5440 noble gas mass spectrometer. The system consists of an internal N-2-STD with a delta N-15 value of -0.11 +/- 0.22 parts per thousand (1 sigma) calibrated against an air-standard (Air-STD). The N-2-STD was measured repeatedly with an average uncertainty on an individual delta N-15 measurement being 0.03 parts per thousand (1 sigma) versus an average single day reproducibility of 0.38 parts per thousand (1 sigma). Additional refinements include (1) monitoring of interfering CO contributions at mass 30, allowing a comprehensive CO correction to be applied to all samples, (2) quantification of procedural N-2 blanks (n = 22) in both size (4.2 +/- 0.5 mu cm(3) STP) and isotopic composition (delta N-15 = 12.64 +/- 2.04 parts per thousand), allowing consistent blank corrections to all samples, and (3) independent measurement of N-2/Ar ratios using a quadrupole mass spectrometer (QMS). The new system was tested by measuring nitrogen isotopes (delta N-15), concentrations and N-2/Ar ratios on 11 submarine basalt glasses. Results show that the uncertainty on the delta N-15 data is improved as a consequence of multiple standards being run per day. Reduced analytical times, afforded by triple collection, also minimize sample depletion and memory effects, thus improving measurement statistics. Additionally, we show that CO corrections can be accomplished using mass 30 to monitor CO interferences, leading to substantial improvements in reproducibility and the overall accuracy of results when the contribution of CO is significant.