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Background effects on Faraday collectors in gas‐source mass spectrometry and implications for clumped isotope measurements
Author(s) -
Bernasconi Stefano M.,
Hu Bin,
Wacker Ulrike,
Fiebig Jens,
Breitenbach Sebastian F. M.,
Rutz Tanja
Publication year - 2013
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.6490
Subject(s) - isotopologue , mass spectrometry , chemistry , isotope , natural abundance , analytical chemistry (journal) , faraday cup , spectrometer , ion , optics , physics , nuclear physics , molecule , organic chemistry , chromatography , ion beam
RATIONALE The measurement of the abundances of minor isotopologues by mass spectrometry requires correction of subtle non‐linearities in the mass spectrometer that cause deviations in the relationship between actual and measured isotope ratios. Here we show that negative backgrounds on the Faraday cups recording the minor ion beams are the cause of the observed non‐linearities in the measurement of CO 2 isotopologues, and propose a new correction procedure for clumped isotope measurements. METHODS We carefully investigated the cause of non‐linearity effects in the measurement of the abundance of 13 C 18 O 16 O, a minor isotopologue of CO 2 with m/z 47, on two different mass spectrometers. By using gases of different composition with close to stochastic and with non‐random distribution of isotopes we demonstrate that the apparent dependence of the excess abundance of the isotopologue of m/z 47 on the bulk isotopic composition of CO 2 is due to a background interference that is linearly dependent on the partial pressure of the gas in the source of the mass spectrometer. CONCLUSIONS Background determination with gas flowing into the source of the mass spectrometer is necessary for accurate clumped isotope measurements of CO 2 . Background corrections can be performed accurately if the slit width of the m/z 44 Faraday cup significantly exceeds that of the one for m/z 47, using a correlation between m/z 44 signal intensity and the corresponding minimum in m/z 47 background. We propose two new correction schemes that reduce the time‐consuming measurement of gases of different bulk isotopic compositions. These findings may also be relevant for the measurement of other rare isotopologues by mass spectrometry. Copyright © 2013 John Wiley & Sons, Ltd.