Atmospheric pressure chemical ionization mass spectrometry for the detection of tropospheric trace gases: the influence of clustering on sensitivity and precision

摘要

An atmospheric pressure chemical ionization mass spectrometer (AP-CIMS) was set up for the detection of the atmospheric trace gases acetonitrile, acetone, and sulfur dioxide. This instrument, which was successfully employed in several, mainly airborne, field campaigns, is described in detail. The ion source makes use of a corona discharge at near ambient pressure. Acetonitrile and acetone are detected as protonated species after undergoing light-switching reactions with ions which have an H_3O~+ core. Sulfur dioxide is detected as SO_3~- produced in a reaction with CO_3~-. The instrument is calibrated using permeation tubes. The instrument response was found to be highly sensitive to the sample humidity. This is accounted for by applying a humidity dependent calibration; the AP-CIMS itself is used to measure the humidity. The detection limits are 10, 20, and 80 ppt for acetonitrile, sulfur dioxide, and acetone, respectively. The analytical precision for all three measured compounds is 18% or better for typical conditions and for an integration time of 3 s. Dimethylsulfide and methyl mercaptan were found to be potential interferents, probably due to radical chemistry in the ion source region. This restrains the applicability of this technique for sulfur dioxide measurements to regions with low dimethylsulfide and methyl mercaptan concentrations, i.e., outside the marine boundary layer.