A Semi-Quantitative\nApproach to Nontarget Compositional\nAnalysis of Complex Samples

Nontarget analysis (NTA) by liquid chromatography coupled\nto high-resolution\nmass spectrometry improves the capacity to comprehend the molecular\ncomposition of complex mixtures compared to targeted analysis techniques.\nHowever, the detection of unknown compounds means that quantification\nin NTA is challenging. This study proposes a new semi-quantitative\nmethodology for use in the NTA of organic aerosol. Quantification\nof unknowns is achieved using the average ionization efficiency of\nmultiple quantification standards which elute within the same retention\ntime window as the unknown analytes. In total, 110 authentic standards\nconstructed 25 retention time windows for the quantification of oxygenated\n(CHO) and organonitrogen (CHON) species. The method was validated\non extracts of biomass burning organic aerosol (BBOA) and compared\nto quantification with authentic standards and had an average prediction\nerror of 1.52 times. Furthermore, 70% of concentrations were estimated\nwithin a factor of 2 (prediction errors between 0.5 and 2 times) from\nthe authentic standard quantification. The semi-quantification method\nalso showed good agreement for the quantification of CHO compounds\ncompared to predictive ionization efficiency-based methods, whereas\nfor CHON species, the prediction error of the semi-quantification\nmethod (1.63) was significantly lower than the predictive ionization\nefficiency approach (14.94). Application to BBOA for the derivation\nof relative abundances of CHO and CHON species showed that using \npeak area underestimated the relative abundance of CHO by 19% and\noverestimated that of CHON by 11% compared to the semi-quantification\nmethod. These differences could lead to significant misinterpretations\nof source apportionment in complex samples, highlighting the need\nto account for ionization differences in NTA approaches.