LAUSR

Direct infusion analysis using soft ionisation techniques coupled to ultrahigh resolution mass spectrometers (UHRMS) allows screening of thousands of organic species in complex samples. Despite the high analytical throughput of direct infusion, this technique is known to be prone to matrix effects caused by changes in the ionisation efficiency of an analyte, ion suppression or enhancement due to the presence of certain compounds and inorganic salts in the sample. In this study we compared two soft ionisation sources i.e., Heated Electrospray Ionisation (HESI) and nano-ESI for the analysis of atmospheric aerosol samples in the negative ionisation mode. In-source fragmentation tests were conducted and experiments involving sample desalting through solid-phase extraction (SPE) with a reversed phase functionalised polymeric sorbent and spiking samples with inorganic salt were performed. Both ionisation sources showed specific advantages and disadvantages for the direct infusion analysis of atmospheric aerosol extracts. The mass spectra of aerosol samples analysed using HESI contained a large number of high molecular weight homologues containing sulfur and nitrogen, suggesting that this source is prone to formation of salt adducts and non-covalent compounds in samples enriched with inorganic salts. Data from the same aerosol sample extracts analysed using nano-ESI show less adduct formation, however, a decrease in the number of homologues was observed, as well as loss of molecules at higher mass range, indicating that the nano-ESI source is more prone to ion suppression. Irrespective of ionisation source, SPE pre-treatment significantly improved ion recoveries for organic species with nonpolar and moderately polar functional groups, but lower recoveries were obtained for highly oxygenated molecules. Therefore, while SPE reduced in-source adduct formation, it also limited the range of compounds identified through a single analysis.