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Matrix effects have been shown to be very pronounced and highly variable in the analysis of mobile chemicals, which may severely exacerbate accurate quantification. These matrix effects, however, are still scarcely studied in combination with hydrophilic interaction liquid chromatography (HILIC) and for very polar chemicals. In this study, the matrix effects of 26 polar model analytes were investigated in enriched drinking water, wastewater treatment plant effluent and solutions of inorganic salts, utilizing post-column infusion of the analytes into a HILIC-electrospray ionisation (ESI)-high-resolution mass spectrometry system. These experiments revealed the occurrence of structure-specific and unspecific matrix effects. The unspecific matrix effects were mainly observed in positive ESI polarity and predominantly coincided with a high ion count, resulting in ion suppression of all analytes. Thus, the excess charge is hypothesized to be the limiting factor in ion formation. Structurespecific matrix effects were more pronounced in negative ESI polarity and even structurally similar compounds were observed to react entirely differently: perfluoroalkyl carboxylic acids were suppressed, while perfluoroalkane sulfonic acids were simultaneously enhanced. These matrix effects were traced back to inorganic anions and cations, which eluted over a significant fraction of the chromatographic run time with this setup. Hence, it was concluded that inorganic ions are a main cause for matrix effects in the analysis of mobile chemicals utilizing HILIC.

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