A field-deployable, chemical ionization time-of-flight mass spectrometer: application to the measurement of gas-phase organic and inorganic acids

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We report a new field-deployable chemical ionization time-of-flight mass spectrometer (CI-TOFMS) for the direct measurement of trace gases in the atmosphere. We apply the technique to the measurement of gas-phase inorganic and organic acids via negative-ion proton transfer, using acetate as the reagent ion. A novel high pressure interface, incorporating two RF-only quadrupoles is used to efficiently focus ions through four stages of differential pumping before analysis with a compact TOFMS. The high ion-duty cycle (>20%) of the TOFMS, coupled to efficient production and transmission of ions in the high pressure interface results in a highly sensitive (>300 ions s−1 pptv−1) instrument capable of the fast measurement of atmospheric gases at trace levels. We demonstrate the efficient transfer and detection of both bare ions and ion-molecule clusters, and characterize the instrument during field measurements aboard the R/V Atlantis as part of the CalNex campaign during the spring of 2010. The in-field short-term precision is better than 5% at 1 pptv (pL/L), for 1-second averages. The detection limit (3σ, 1-second averages) of the current version of the CI-TOFMS, as applied to the in situ detection of gas-phase acids, is limited by the magnitude and variability in the background determination and was determined to be 4 pptv.