Atmospheric Measurement Techniques

Abstract. We constructed a new chemical ionization time-of-flight mass spectrometer (CI-TOFMS) that measures at-mospheric 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 sensi-tive (>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 detec-tion of both bare ions and ion-molecule clusters, and char