Articles | Volume 4, issue 7
Atmos. Meas. Tech., 4, 1471–1479, 2011
Atmos. Meas. Tech., 4, 1471–1479, 2011

Research article 20 Jul 2011

Research article | 20 Jul 2011

A field-deployable, chemical ionization time-of-flight mass spectrometer

T. H. Bertram1, J. R. Kimmel4,3,2, T. A. Crisp1, O. S. Ryder1, R. L. N. Yatavelli5, J. A. Thornton5, M. J. Cubison4,3, M. Gonin3, and D. R. Worsnop2,6 T. H. Bertram et al.
  • 1Department of Chemistry, University of California San Diego, La Jolla, CA, USA
  • 2Aerodyne Research Incorporated, Billerica, MA, USA
  • 3Tofwerk AG, Thun, Switzerland
  • 4Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 5Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA
  • 6Department of Physics, University of Helsinki, Helsinki, Finland

Abstract. We constructed a new chemical ionization time-of-flight mass spectrometer (CI-TOFMS) that measures atmospheric trace gases in real time with high sensitivity. We apply the technique to the measurement of formic acid 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 combined with the efficient production and transmission of ions in the high pressure interface results in a highly sensitive (>300 ions s−1 pptv−1 formic acid) instrument capable of measuring and saving complete mass spectra at rates faster than 10 Hz. 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-s averages. The detection limit (3 σ, 1-s averages) of the current version of the CI-TOFMS, as applied to the in situ detection of formic acid, is limited by the magnitude and variability in the background determination and was determined to be 4 pptv. Application of the CI-TOFMS to the detection of other inorganic and organic acids, as well as the use of different reagent ion molecules (e.g. I, CF3O, CO3) is promising, as we have demonstrated efficient transmission and detection of both bare ions and their associated ion-molecule clusters.