Refinement and evaluation of an automated mass spectrometer for nitrogen isotope analysis by the Rittenberg technique

R L Mulvaney,Y P Liu

doi:10.1155/s1463924691000433

R L Mulvaney, Y P Liu

Open Access

https://doi.org/10.1155/s1463924691000433

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Abstract

An apparatus designed to automatically perform hypobromite oxidations of ammonium salt samples for nitrogen isotope analyses with a mass spectrometer was modified to improve performance and reduce analysis time. As modified, reference N2 is admitted to the mass spectrometer between samples from a dedicated inlet manifold, for calibration at the same pressure as that of the preceding sample. Analyses can be performed on samples containing 10 μg to 1 mg of N (or more), at a rate of up to 350 samples/day. When operated with a double-collector mass spectrometer, the standard deviation at the natural abundance level (10 analyses, 50-150 μg N) was <0.0001 atom % 15N. Very little memory was observed when natural abundance samples (0.366 atom % 15N) were analysed. following samples containing 40 atom % 15N. Analyses in the range, 0.2 to 1 atom % 15N (50-150 μg N), were in good agreement with manual Rittenberg analyses (1 mg N) using a dual-inlet system, and precision was comparable. For enrichments of 2 to 20 atom % 15N, automated analyses were slightly lower than manual analyses, which was attributed to outgassing of N2 from the plastic microplate used to contain samples.

Highlights

Use of nitrogen-15 as an isotopic tracer has been stimulated by the automation of mass spectrometers for nitrogen isotope analysis
Automation may be accomplished by interfacing an automatic N/C analyser (ANCA) to an isotope ratio mass spectrometer [1,2,3,4]; the combination is commonly referred to as ANCA-MS
The N is purified by gas chromatography, and a small fraction (- 1%) of the effluent is admitted to the mass spectrometer for measurement of the ion currents at rn/z

Summary

Introduction

Use of nitrogen-15 as an isotopic tracer has been stimulated by the automation of mass spectrometers for nitrogen isotope analysis. As originally designed [8], the ARA was equipped with a single inlet manifold, from which either sample or reference N2 was admitted to the mass spectrometer. An important consideration in the design of the ARA is the internal volume of the inlet manifold, which must be minimized to obtain sufficient pressure from microgram quantities of N2 for analyses with the mass spectrometer [6,7].

Results

Conclusion