Development and use of Negative Ion Proton Transfer Reaction Chemical Ionization Mass Spectrometry (NI-PTR-CIMS) for the Measurement of Trace Organic Acids in the Atmosphere

Negative-Ion Proton-Transfer Reaction Chemical Ionization Mass Spectrometry (NI-PTR-CIMS) has been developed for real-time measurement of gas-phase organic acids in the atmosphere. The method is based on the non-dissociative proton transfer reactions of CH3COO- with most of the common organic acids. The potential to detect organic acids (monocarboxylic, dicarboxylic, inorganic, and rare acids) has been shown by a number of laboratory investigations. The sensitivity of the instrument to several organic acids has been observed to be on the order of several ion counts per pptv. Detection limits well below the ppbv level are expected for a 1-second integration time. Various instrumental features including zeroing, time response, calibration and validation of the ion chemistry have been investigated and will be discussed. Intercomparison of formic acid measurements with Proton Transfer Reaction Mass Spectrometry (PTR-MS) and Quantum Cascade Laser IR absorption (QCL) serve to validate this measurement technique. Further development of this instrument will provide a fast, sensitive, and selective measurement technique for gas-phase acids. NI-PTR-CIMS has great potential for use in atmospheric chemistry field studies.