Ion chemistry of phthalates in selected ion flow tube mass spectrometry: isomeric effects and secondary reactions with water vapour

Abstract

Phthalates are widely industrially used and their toxicity is of serious environmental and public health concern. Chemical ionization (CI) analytical techniques offer the potential to detect and monitor traces of phthalate vapours in air or sample headspace in real time. Promising techniques include selected ion flow tube mass spectrometry (SIFT-MS), proton transfer reaction mass spectrometry (PTR-MS) and ion mobility spectrometry (IMS). To facilitate such analyses, reactions of H₃O⁺, O₂⁺ and NO⁺ reagent ions with phthalate molecules need to be understood. Thus, the ion chemistry of dimethyl phthalate isomers (dimethyl phthalate, DMP – ortho; dimethyl isophthalate, DMIP – meta; dimethyl terephthalate, DMTP – para), diethyl phthalate (DEP), dipropyl phthalate (DPP) and dibutyl phthalate (DBP) was studied by SIFT-MS. Reactions of H₃O⁺, O₂⁺ and NO⁺ with these phthalate molecules M were found to produce the characteristic primary ion products MH⁺, M⁺ and MNO⁺, respectively. In addition, a dissociation process forming the (M–OR)⁺ fragment was observed. For phthalates with longer alkyl chains, mainly DPP and DBP, a secondary dissociation channel triggered by the McLafferty rearrangement was also observed. However, this is dominant only for the more energetic O₂⁺ reactions with phthalates, additionally resulting in a recognisable formation of the protonated phthalate anhydride. For the NO⁺ reagent ions, the McLafferty rearrangement makes only a minor contribution and for H₃O⁺, it was not observed. Experiments on the effect of water vapour on this ion chemistry have shown that protonated DMIP and DMTP efficiently associate with H₂O forming the DMIP·H⁺H₂O, DMIP·H⁺(H₂O)₂ and DMTP·H⁺H₂O cluster ions, whilst the protonated ortho DMP isomer as well as other ortho phthalates DEP, DPP and DBP does not associate with H₂O. The results indicate that the degree of hydration can be used to identify specific phthalate isomers in CI.