Development of negative-ion proton-transfer chemical-ionization mass spectrometry (NI-PT-CIMS) for the measurement of gas-phase organic acids in the atmosphere

We have developed a negative-ion proton-transfer chemical-ionization mass spectrometry (NI-PT-CIMS) technique for on-line analysis of gaseous organic and inorganic acids. In this detection scheme, acetate ions (CH₃C(O)O^-) react very selectively with atmospheric trace acids, by proton transfer, to produce unique product ion species. We tested this ion chemistry for 11 species of which only four showed measurable fragmentation. We investigated both the time response of the inlet and humidity dependence for both formic acid and pyruvic acid measurements. A formic acid calibration was performed and found a sensitivity of 21 ± 4.3 counts per second per pptv. Formic acid measurements made during two separate informal ambient air intercomparisons: (1) with a quantum cascade IR laser absorption system (QCL) and (2) a proton-transfer reaction mass spectrometer (PTR-MS) show good agreement validating this measurement technique. The measurements of the NI-PT-CIMS and PTR-MS agree to within 5% with a high degree of correlation (r² > 0.93). We have found the NI-PT-CIMS detection limit for formic acid is approximately 80-90 pptv for a 1 s integration period, and is currently limited by the formate background in the instrument. The fast time response and high sensitivity of the NI-PT-CIMS method make it a promising technique for the measurement of organic acids in ambient conditions.