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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
Author(s) -
Cui Linlin,
Wang Xu,
Feng Lianjun
Publication year - 2018
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.8309
Subject(s) - chemistry , isotope ratio mass spectrometry , analytical chemistry (journal) , mass spectrometry , nitrogen , spectrum analyzer , sample preparation , gas chromatography , chromatography , organic chemistry , electrical engineering , engineering
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 δ 15 N analysis of samples containing trace amounts of nitrogen (N). Nevertheless, N blanks, which are limiting factors for the accuracy and precision of measured δ 15 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 N 2 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 N 2 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) CO 2 /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 N 2 entrained into the system through the autosampler, and the N retained in the CO 2 /water trap filled with CO 2 absorbent and Mg(ClO 4 ) 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 (δ 15 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 δ 15 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 δ 15 N measurement using conventional EA‐IRMS.