Detection ofphthalates in water using ultra performance liquid chromatography-electrospray ionizationtandem mass spectrometry MRM mode– ‘ghost peaks’ and measurement methodology

Abstract

In the analysis of phthalates, specifically diethyl phthalate (DEP), dibutyl phthalate (DBP), butylbenzyl phthalate (BBP), and diethylhexyl phthalate (DEHP) using ultra performance liquid chromatography-electrospray ionization tandem mass spectrometry (UPLC-ESI-MS/MS), ‘ghost peaks’ appeared when a gradient liquid chromatography elution mode was employed. A systematic diagnostic analytical protocol was designed to show that the source of the persistent ‘ghost peaks’, which jeopardized a quantitative analysis of these endocrine disruptors was the mobile phase. A trace amount of DEHP in the mobile phase, either from ultrapure water or organic solvent, was responsible for the observed phenomenon. In contrast to gradient elution, isocratic chromatographic elution mode (ICEM) was found to be free of the problem at the expense of less effective separation of DBP and BBP. Thus, a detection method for analyzing the phthalates is proposed based on ICEM and the multiple reaction monitoring (MRM) mode with multi-channel detection in the UPLC-MS/MS. In the method development, tap water matrix effects were examined and a solution for detection intensity suppression in the analysis of tap water samples was proposed. An addition of methanol into tap water samples could basically relieve the signal detection suppression by the real water matrix. This provides a detection method of these trace environmental pollutants with a linear range between 0.5 to 50 ng mL⁻¹ and detection limits (LODs) (0.32–0.54 ng mL⁻¹) without need of sample pre-concentration. This is a much simpler and faster method than gas chromatography-electron capture detection (GC-ECD) and gas chromatography-mass spectrometry (GC-MS), with comparable detection sensitivity.