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Oxygen interference with membrane inlet mass spectrometry may overestimate denitrification rates calculated with the isotope pairing technique
Author(s) -
Lunstrum Abby,
Aoki Lillian R.
Publication year - 2016
Publication title -
limnology and oceanography: methods
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.898
H-Index - 72
ISSN - 1541-5856
DOI - 10.1002/lom3.10101
Subject(s) - denitrification , chemistry , analytical chemistry (journal) , isotope , oxygen , aqueous solution , inlet , environmental chemistry , nitrogen , organic chemistry , geology , physics , quantum mechanics , geomorphology
Membrane inlet mass spectrometry (MIMS) is used to measure gas concentrations in aqueous samples, and is often employed to measure N 2 in denitrification studies. While most denitrification studies using MIMS have employed the N 2 :Ar method, MIMS is increasingly used with the isotope pairing technique (IPT), an alternative method that calculates denitrification based on 29 N 2 and 30 N 2 concentrations, measured as mass to charge ratios ( m / z ) 29 and 30. Dissolved O 2 is known to interfere with m / z 28 and 40 ( 28 N 2 and Ar, respectively) in the MIMS, and in this study, we show that the effect on m / z 29 and 30 is also significant. This O 2 effect may result in artificially high denitrification rates with IPT if O 2 varies between samples. In lab‐based experiments, m / z 29 and 30 were impacted immediately as O 2 was depleted. The resulting effect on simulated denitrification rates was minor if final O 2 concentrations remained above 50% initial O 2 , but increased by more than 100 μ mol m −2 h −1 as O 2 fell below 50%. Similarly, denitrification rates from sediment incubations were an order of magnitude higher when analyzed in the presence of O 2 , compared with analysis without O 2 using an inline furnace with reduction column. We further show that this effect is variable over the life of the MIMS, and/or with different ion sources. Given these uncertainties, we recommend using an inline furnace to remove O 2 for all IPT experiments. Alternatively, the magnitude of the O 2 effect should be assessed often to determine the range of minimal O 2 interference.