## 3 Advanced Multidimensional Techniques for Aerosol WSOM Analysis
### Application of Off-Line Multidimensional MS Methodologies for Water-Soluble OA Analysis  
Offline MS methodologies, especially when associated with chromatographic separation procedures, allow a high selectivity and molecular discrimination of the OA constituents [45; 52]. These hyphenated techniques, which can be considered multi-method strategies, have been widely used in target analysis, aiming at the quantitative identification of specific organic compounds and chemical markers for OA source apportionment [45]. The most widely used methodology for the chemical characterization of OA compounds is gas chromatography coupled to MS (GC-MS) [45]. This powerful, sensitive and selective hyphenated assemblage allows the separation, identification, and quantification of specific organic species within atmospheric aerosols. Several studies using GC-MS for molecular identification and quantification of target compounds in OA have been the focus of several reviews [22; 43; 45; 53; 54], and the main classes of compounds include aliphatic acids, aromatic compounds, amino acids, multifunctional oxygenated compounds, polyols, sugars, nitrogenated compounds, and polycyclic aromatic hydrocarbons. However, the GC-MS usually requires time-consuming derivatization procedures, especially for the analysis of highly polar and less volatile compounds, such as those of aerosol WSOM [43]. Liquid chromatography coupled to MS (LC-MS) has become a robust alternative to GC-MS for the analysis of individual compounds in water-soluble OA. Although it is less quantitative, LC-MS is especially suited for the analysis of high-molecular-mass, thermally unstable and/or very polar molecules without derivatization [22; 53]. The most recent examples of the application of LC-MS to molecular identification and quantification of target com  
Figure 1: Example of a 2D map showing \(t_{44}\) vs. \(t_{60}\) obtained from the online high-resolution time-of-flight (HR-ToF)-aerosol mass spectrometry (AMS) analysis of fresh and aged organic aerosols (OA), including biomass-burning OA (BBOA) and fog samples. Reprinted from the work of [PERSON] et al. [26], under the Creative Commons Attribution 4.0 License. pounds in OA have been reviewed by [PERSON] et al. [22]. In particular, LC-MS techniques have been used to investigate secondary OA (SOA) formation and aging from a range of precursors in laboratory studies, as well as to identify water-soluble organosulfates and their nitrated derivatives in ambient PM samples (see, for example, references [22] and [53] and references therein). Additional main classes of compounds studied using this technique include aliphatic, alicyclic and aromatic carboxylic acids, carbohydrate-like substances, and amino acids and other organo-nitrogen compounds. More recently, LC coupled to HR-MS has been used to study the photochemical evolution of the WSOM composition in combustion-derived aerosols (namely, wheat straw burning aerosol samples), translated by the photodegradation of low oxygenated species and the formation of highly oxygenated products [55]. However, several challenges limit the application of LC-HR-MS to aerosol WSOM analysis, such as the complex chemical composition of these samples and the consequent poor resolution of the chromatographic separations, the presence of compounds at low concentrations or compounds that are poorly ionized, and the lack of authentic standards for compounds identification/quantification. One possible way to deal with this problem is to increase the chromatographic resolution by, including a second chromatographic dimension, as in comprehensive multidimensional chromatographic systems. Comprehensive 2D gas chromatography (GC\(\times\)GC) coupled to time-of-flight mass spectrometry (ToF-MS) has been successfully applied to the analysis of ambient volatile organic compounds, OA, and analysis of reaction products in chamber experiments concerning SOA formation [22; 56].