Determination of nitrogen isotopes on samples with tens of nmol of N using the combination of an elemental analyzer, a GasBench interface and an isotope ratio mass spectrometer: An evaluation of blank N contributions and blank-correction

Linlin Cui; Xu Wang; Lianjun Feng

doi:10.1002/rcm.8309

Determination of nitrogen isotopes on samples with tens of nmol of N using the combination of an elemental analyzer, a GasBench interface and an isotope ratio mass spectrometer: An evaluation of blank N contributions and blank-correction

Rapid Commun Mass Spectrom. 2019 Jan 15;33(1):74-80. doi: 10.1002/rcm.8309.

Authors

Linlin Cui^{1

2}, Xu Wang^{1

2}, Lianjun Feng¹

Affiliations

¹ Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China.
² Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China.

PMID: 30324637
DOI: 10.1002/rcm.8309

Abstract

Rationale: The combination of an Elemental Analyzer, a GasBench interface and Isotope Ratio Mass Spectrometry (EA-GasBench-IRMS system) is a promising method for the δ¹⁵ N analysis of samples containing trace amounts of nitrogen (N). Nevertheless, N blanks, which are limiting factors for the accuracy and precision of measured δ¹⁵ N values, have received little study. In this paper, a variety of N blank sources in the EA-GasBench-IRMS system were systematically evaluated in order to take effective measures to reduce the blank interference as much as possible.

Methods: N-Isotopic analysis was accomplished using an elemental analyzer coupled to an isotope ratio mass spectrometer via a sample loop and a GasBench interface. The N in the sample was converted into N₂ gas in the EA system, and then transferred and trapped in a sample loop with a deactivated stainless-steel chromatography column packed with 5 A molecular sieve polymer at liquid nitrogen temperature (-196°C). Subsequently, the N₂ gas was released by warming the sample loop up to 100°C and introduced into the isotope ratio mass spectrometer via the GasBench interface. The N blank sources in the EA-GasBench-IRMS system were investigated systematically by looking at seven parts: (a) Helium carrier gas, (b) Autosampler, (c) CO₂ /water trap, (d) Size of reactor tube, (e) Sample collection time, (f) Oxygen gas, and (g) Capsules.

Results: The N blanks are mainly derived from the helium carrier gas, atmospheric N₂ entrained into the system through the autosampler, and the N retained in the CO₂ /water trap filled with CO₂ absorbent and Mg(ClO₄ )₂ , which together can account for a total of ~507.3 nmol N. Through purifying the helium gas, modifying the autosampler and using a cryogenic trap, we reduced the N blank considerably, to ~10.7 nmol, and obtained a nearly uniform isotopic composition (δ¹⁵ N_Blank = -4.54 ± 0.36‰ AIR, n = 32, 1SD) of blank N thus guaranteeing a reliable correction.

Conclusions: Measurements on a set of IAEA-N1, IAEA-600 and collagen standards with 40 nmol-200 nmol N produced accurate δ¹⁵ N values with standard deviation of ±0.26‰, ±0.22‰ and ± 0.23‰ (1σ) after blank correction, respectively. Our findings offer clues to optimizing the analytical method for trace N isotopic determination and they are also beneficial to improving δ¹⁵ N measurement using conventional EA-IRMS.

Abstract

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