Abstract (eng)
Generating mobility standards has been of great importance for characterization and calibration of instruments in aerosol research. A common method of generating sub-2nm standards is the use of unipolar electrospray sources, which has been limited to the first few clusters. In this study a bipolar electrospray source is introduced to significantly extend the range of available singly charged clusters through charge reduction. The use of a high resolution differential mobility analyzer (UDMA) in combination with an Atmospheric Pressure interface Time-of-Flight Mass Spectrometer (ioniAPi-ToF MS, Ionicon Analytik GmbH, Austria) enabled detailed characterization of the electrical mobility and chemical composition (mass- to-charge ratio) of different tetra-alkyl ammonium halide clusters, generated by the bipolar electrospray source. Different operating settings were tested and characterized. The unipolar and bipolar mobility spectra for both positive and negative salt clusters are presented. The individual peaks are analyzed to confirm the successful charge reduction by the bipolar electrospray source and to reassure that dominantly singly charged salt clusters are produced which is required for subsequent instrument characterization. The obtained mobility standards were used to characterize an ultrafine condensation particle counter (TSI Model 3776 UCPC). Furthermore, a detailed operating manual for the bipolar electrospray source was established.