Analysis of trace organic pollutants in wastewater to assess biodegradation using wrong-way-round ionization in liquid chromatography/tandem mass spectrometry

Monitoring the concentrations of pharmaceuticals and personal care products (PPCPs) in wastewater is an integral step toward understanding the fate of these contaminants in wastewater treatment plants (WWTPs). This paper aims to develop a method that allows for the simultaneous analysis of multiple classes of PPCPs that can be used as tracers to assess the performance of WWTPs.

Five PPCP tracers - carbamazepine (CBZ), 17α-ethinylestradiol (EE2), nonylphenol (NP), salicylic acid (SA), and trimethoprim (TMP) - were analyzed by liquid chromatography/triple quadrupole mass spectrometry (LC/MS/MS) using a highly basic mobile phase (pH 10.3). Conventionally, TMP (pKa 7.12) and CBZ (pKa 13.94) are analyzed in positive ion mode using an acidic mobile phase. However, the high pH mobile phase allowed the quantification of all the tracers by polarity switching, with TMP undergoing wrong-way-round (WWR) ionization.

The instrument limits of detection for the five tracers, without solid-phase extraction, were in the range of 1.3 to 5.9 ng/mL, except for NP, which was 238 ng/mL. The signal-to-noise (S/N) ratios for TMP and CBZ with the mobile phase at pH 10.3 were higher than the S/N ratios observed at pH 2.7 under positive electrospray ionization. The mechanism of WWR ionization for TMP was investigated, and we propose that a charge transfer from solvent clusters to TMP molecules due to electrolytic reactions at the surface of the droplet leads to WWR ionization in electrospray.

A method to simultaneously analyze five representative PPCP tracers with a wide range of pKa values using WWR ionization in LC/MS/MS with polarity switching was developed. The method was successfully used to monitor the selected PPCPs in samples from full-scale WWTPs to assess their biodegradation under various treatment conditions.