Proton transfer reaction mass spectrometry investigations of phthalate esters via direct headspace sampling

One of the most common environmentally relevant groups of pollutants are phthalate esters. After decades of industrial use, they have become ubiquitous in the environment and analytical methods to chemically detect them in trace amounts are required. In this study, details of Proton Transfer Reaction-Mass Spectrometry (PTR-MS) investigations for the reactions of phthalic acid and ten phthalate esters with H₃O⁺ as a function of the reduced electric field are presented. A characteristic product ion observed for several of the phthalate esters is protonated phthalic anhydride (m/z 149.02, C₈H₅O₃⁺). However, not all of the phthalates investigated in this study fragment to produce this product ion following proton transfer. For alkyl diester phthalates, loss of the corresponding alcohol results in the main product ion, but its abundance decreases with increasing alkyl chain length, whilst in comparison for the dialkyl ester phthalates, the protonated phthalic anhydride ion abundance increases with increasing alkyl chain length and with increasing reduced electric field. Collisional induced dissociation in the drift tube of the PTR-MS is shown to be useful as means to manipulate the underlying ion chemistry, leading to unique product ions distinctive to phthalates. The results reported in this work represent a wealth of new data that will be of use for developing a PTR-MS analytical method for the quick, selective and reliable identification of phthalates in the environment.