Expanding the chemical coverage of polar compounds in water analysis by coupling supercritical fluid with hydrophilic...

Persistent and mobile organic compounds (PMOC) are of great concern for water quality and human health. The recent improvement and availability of high-resolution mass spectrometry in combination with liquid chromatography have widely expanded the potential of analytical workflows for their detection and quantitation in water. Given their high polarity, the detection of some PMOC requires alternative techniques to reversed-phase chromatography, such as hydrophilic interaction liquid chromatography (HILIC) and supercritical fluid chromatography (SFC). Unified chromatography (UC), an SFC gradient in which the state of the mobile phase changes continuously from supercritical to liquid at 100 % polar co-solvent, has shown potential for the analysis of compounds in a broad range of polarity, including very polar compounds.

In the present study, for the first time, a UC-HILIC method coupled with high-resolution mass spectrometry was set up for PMOC analysis in water. SFC and HILIC gradients were run sequentially on the same bare-silica column, with the first separation running to 100 % modifier (UC) followed by a HILIC gradient transitioning to water. The UC and UC-HILIC gradients were previously optimized on a mix of 18 representative PMOC to assess solvent and mobile phase composition and for the instrumental system setup. The final method was employed for the analysis of water samples in comparison with a traditional reversed-phase separation, resulting in a significant increase in the number of annotated polar PMOC, including compounds listed in the Candidate List of substances of very high concern for Authorisation by the European Chemicals Agency.

The proposed approach represents a robust alternative to traditional methods for broadening the chemical space of separation and mass spectrometric detection. The introduction of the HILIC section of the gradient was necessary for the elution of strongly retained compounds on the silica phase, thus also reducing the amount of compounds that would be permanently bound onto the phase of the column, resulting in possible irreproducibility, pressure increase, and loss of efficiency in the compound separation.