An instrumental comparison of mobility and mass measurements of atmospheric small ions

Ambient, naturally charged small ions (<2000 Da) were measured in Hyytiälä, Finland, with amass spectrometer (atmospheric pressure interface time-of-flight, APi-TOF) and two mobility spectrometers (air ion spectrometer, AIS, and balanced scanning mobility analyzer, BSMA). To compare these different instrument types, a mass/mobility conversion and instrumental transfer functions are required to convert high-resolution mass spectra measured by the APi-TOF into low-resolution mobility spectra measured by the AIS and BSMA. A modified version of the Stokes-Millikan equation was used to convert between mass andmobility. Comparison of APi-TOF andBSMAresults showed good agreement, especially for sizes above 200 Da (Pearson's R = 0.7-0.9). Below this size, agreement was fair, and broadening BSMA transfer functions improved the correlation. To achieve equally good agreement between APi- TOF and AIS, AIS results needed to be shifted by 1-1.5 mobility channels. The most likely cause was incorrect sizing in the AIS. In summary, the mass and mobility spectrometers complement each other, with the APi-TOF giving superior chemical information, limited to relatively small ions (<2.5 nm diameter), whereas the mobility spectrometers are better suited for quantitative concentration measurements up to 40 nm. The BSMA and AIS were used to infer a transmission function for the APi-TOF, making it possible to give quantitative estimates of the concentrations of detected chemical ions.