Determination of organophosphate esters in water samples by mixed‐mode liquid chromatography and tandem mass spectrometry

A simple and sensitive method for the determination of organophosphate esters in water samples by mixed-mode liquid chromatography with electrospray ionization tandem mass spectrometry coupled with solid-phase extraction is developed. Using seven alkyl phosphates, three chlorinated alkyl phosphates, and four aryl phosphates as the targets, the developed method was systematically evaluated on the basis of the influence of the solid-phase extraction cartridge, eluting solvent, sample-loading volume, mobile phase condition, and the separation of reversed-phase chromatography and mixed-mode liquid chromatography. Under the optimal conditions, these organophosphate esters can be extracted by ENVI-18 cartridge, eluted by 6 mL of 25% dichloromethane in acetonitrile, and then qualified and quantified by mixed-mode liquid chromatography with tandem mass spectrometry in the multiple reaction-monitoring mode. The application of mixed-mode liquid chromatography endows the separation with reasonable retention for both hydrophilic and hydrophobic organophosphate esters regardless of their polarity, which is hardly achieved by reversed-phase chromatography. Good linearity (from 0.9877 to 0.9969), low quantification limits (1-35 ng/L after extraction of 100 mL of river water), and acceptable recovery rates (58.6-116.2%, with the relative standard deviation <18.0%) were obtained. Finally, the established method was used for analyzing surface water samples, and the good applicability of this method was demonstrated.