Quaternary ammonium compounds (QACs) form a large group of surfactants with large-scale domestic and industrial applications. They are widely used as disinfectants and sanitizers in hospitals, food production, homes, and agriculture settings; as preservatives and antiseptics in health care products and pharmaceutical preparations; as antistatic, emulsifiers, and preservatives in the coating industry; and as detergents and cleaners [1]. QACs have been widely used during the Coronavirus Disease (COVID-19) pandemic, due to their recognized ability to inactivate enveloped viruses, such as SARS-CoV-2 [1], [2], [3], [4].

Although QACs may contain a wide variety of substituents around the ammonium cation, the most commonly employed QACs for sanitization and cleaning are benzalkonium compounds (BACs), dialkyldimethylammonium compounds (DADMACs) and alkyltrimethyl ammonium compounds (ATMACs) [1]. Commercial QAC formulations often consist of solutions containing a combination of homologs of the same group of QACs. It is generally accepted that alkyl chains containing 12 to 16 carbons exhibit greater antimicrobial activity, and twin-chained compounds, such as DADMACs, are more biologically active than mono-alkyl compounds (e.g., BACs or ATMACs) against Gram-positive bacteria [5,6].

Due to their high-volume use in personal care products and detergents, there is continual release of QACs into sewers and wastewater treatment plants (WWTPs). While most of these QACs (> 90 %) are removed from the liquid flow via adsorption to biosolids and biodegradation, significant amounts of QACs have been reported to occur in the environment in relation to point source pollution, land application of biosolids, or discharges of treated municipal and industrial effluents [4,[7], [8], [9], [10], [11]]. The wide occurrence of QACs in the environment raises some concern due to their documented high toxicity to aquatic organisms including algae, daphnids, and fish as well as their potential impact on the proliferation of multidrug resistant bacteria [1,10].

Liquid chromatography with mass spectrometry (LC-MS) is the most used technique for analysis of surfactants, including QACs, in environmental and biological samples. In addition, the possible interferences in environmental matrices are usually mitigated by the use of LC-MS/MS in combination with deuterated standards for unequivocal, sensitive and reproducible analysis of QACs [12], [13], [14], [15], [16], [17].

Very similarly to what is observed for per- and polyfluoroalkyl substances (PFAS) [18], the amphiphilic nature of QACs gives them the capacity to adsorb readily on solids such as soil, sediment, and sludge [19], as well as glassware [12,20,21], filters [16] and other laboratory materials and instruments [20]. This sorbing tendency can either lead to a decrease of QAC concentrations in aqueous samples due to sorption or give overestimated QAC contents due to contamination from laboratory reagents, materials, equipment and instruments. Various approaches (e.g., glassware prewash with acids, organic solvents or anionic species, glassware heating at > 250 °C, adding ion-pairing agents at sampling times, using plastic containers instead of glass ones, preconditioning glassware with QACs [7,17,18,[20], [21], [22], [23], [24]]) have been used to limit the risk of QAC loss or overestimation in defined laboratory settings, however, less effort has been dedicated to the effect of preservation, handling and storage conditions on QACs contained in field samples.

Various field campaigns have been carried out in Europe, Asia, and to a lesser extent the USA, to estimate the occurrence of QACs in anthropogenic or environmental samples [11], whereas in Canada, the occurrence and fate of QACs are yet to be documented. The objective of this study was to develop sampling, storage and sample preparation methods that minimize the loss of QACs in aqueous and solid samples prior to analysis by LC-MS. Influent and effluent samples from a Canadian WWTP were used as typical aqueous samples containing different levels of contaminants, while biosolids from the same WWTP were selected as an organic-rich challenging substrate to extract QACs from. Various approaches were compared for preservation, concentration, and extraction of QACs in order to recommend the most appropriate sampling and analysis protocols for future field campaigns.