Nitrate ion chemical ionization mass spectrometer

Nucleating precursor gases and molecular clusters were measured by a NO₃-CIMS (50). The custom-built ion-source is based on the design by Eisele and Tanner (51) creating nitrate reagent ions (HNO₃)_n NO₃⁻ with n = 0–2 by corona discharge, which are then focussed towards the sample flow. The resulting sample ions were analysed by a commercially available high-resolution atmospheric-pressure interface time of flight mass spectrometer (APi-TOF, Tofwerk AG, Thun, Switzerland). The nitrate ionization technique is highly selective towards sulfuric acid, iodic acid, dimethylamine and multifunctional OOMs. The instrument was calibrated for sulfuric acid according to Kürten et al. (52). Because of the lack of appropriate standards and the high reactivity of OOMs, a direct quantitative calibration of these compounds is not possible. However, as shown in previous studies (53), we can assume a detection efficiency of OOMs with an O:C ratio > 0.6 by the nitrate reagent ions similar to H₂SO₄, after correcting for the (measured) instrumental mass dependent transmission efficiency (54). Because of reduced presence of functional groups, such as hydroperoxy (–OOH) or hydroxy (–OH) groups, required for clustering with the nitrate reagent ion, OOMs with O:C ratio < 0.6 can be prone to a lower charging efficiency and an increased thermal dissociation of the ion-molecule cluster in the mass analyser (55). This leads to a lower detection efficiency of OOM with O:C ratio < 0.6 compared to sulfuric acid. In terms of quantity, the concentrations of these compounds can therefore only be regarded as lower limits.