Premium
The effect of N 2 O on the isotopic composition of air‐CO 2 samples
Author(s) -
Ghosh Prosenjit,
Brand Willi A.
Publication year - 2004
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.1560
Subject(s) - chemistry , analytical chemistry (journal) , mass spectrometry , ionization , isotope , ion , electron ionization , mixing ratio , chromatography , physics , organic chemistry , quantum mechanics
The ionisation efficiencies of N 2 O vs. CO 2 as well as their ratios were measured in detail introducing clean N 2 O and CO 2 into the electron impact ion source of an isotope ratio mass spectrometer. Changes in the ionisation efficiency ratio (IER) were found for different electron energy settings and compared with the ratios of literature ionisation cross‐section values for pure N 2 O and CO 2 . To establish the influence of mixtures of N 2 O and CO 2 in a mass spectrometer, artificial air mixtures were prepared by mixing different amounts of N 2 O and CO 2 from well‐calibrated spike cylinders with CO 2 ‐free air. The mixing ratios varied from 8–512 ppb for N 2 O and from 328–744 ppm for CO 2 . With these mixtures the effects of varying N 2 O concentrations on apparent CO 2 isotope ratios in air samples were determined. After applying a mass balance correction the δ 13 C results were consistent within small error margins. The data seemed almost independent from a particular choice for the IER of N 2 O vs. CO 2 in the correction algorithm. For δ 18 O a small effect of the ionisation efficiency ratio of N 2 O vs. CO 2 was found. Several sets of calculations were made varying the IER between 0.88 and 0.62. The dependence of δ 18 O was the smallest with an adopted IER of 0.68–0.72 in the mass balance correction equation for isotopic analysis of CO 2 in air. For high‐precision measurements of the CO 2 stable isotope ratios in air samples a careful assessment of the mass spectrometer performance is necessary. Different ion sources, even different ion source settings, alter the IER of N 2 O vs. CO 2 which is used in the N 2 O correction algorithm. Preferably, the specific mass spectrometric behaviour should be established with clean N 2 O/CO 2 mixtures or with air mixtures covering a larger range of N 2 O concentrations. Copyright © 2004 John Wiley & Sons, Ltd.