Positive chemical ionization triple-quadrupole mass spectrometry and ab initio computational studies of the multi-pathway fragmentation of phthalates

• Published in: Journal of mass spectrometry. 2010-06, Vol.45 (6), p.678-685
• Main Authors: Jeilani, Yassin A, Cardelino, Beatriz H, Ibeanusi, Victor M
• Format: Article
• Language: English
• Subjects: Chemistry; collision induced dissociation; energetics of phthalate fragmentation; Esters; Exact sciences and technology; Fragmentation; Mass spectrometry; Mathematical analysis; Organic chemistry; Pathways; Phthalates; Phthalic anhydride; positive chemical ionization; precursor ion scanning; product ion scanning; Reactivity and mechanisms; Spectra; triple-quadrupole mass spectrometry
• ISSN: 1076-5174; 1096-9888; 1096-9888
• DOI: 10.1002/jms.1761

We report the first positive chemical ionization (PCI) fragmentation mechanisms of phthalates using triple-quadrupole mass spectrometry and ab initio computational studies using density functional theories (DFT). Methane PCI spectra showed abundant [M + H]⁺, together with [M + C₂H₅]⁺ and [M + C₃H₅]⁺. Fragmentation of [M + H]⁺, [M + C₂H₅]⁺ and [M + C₃H₅]⁺ involved characteristic ions at m/z 149, 177 and 189, assigned as protonated phthalic anhydride and an adduct of phthalic anhydride with C₂H₅ ⁺ and C₃H₅ ⁺, respectively. Fragmentation of these ions provided more structural information from the PCI spectra. A multi-pathway fragmentation was proposed for these ions leading to the protonated phthalic anhydride. DFT methods were used to calculate relative free energies and to determine structures of intermediate ions for these pathways. The first step of the fragmentation of [M + C₂H₅]⁺ and [M + C₃H₅]⁺ is the elimination of [R---H] from an ester group. The second ester group undergoes either a McLafferty rearrangement route or a neutral loss elimination of ROH. DFT calculations (B3LYP, B3PW91 and BPW91) using 6-311G(d,p) basis sets showed that McLafferty rearrangement of dibutyl, di(-n-octyl) and di(2-ethyl-n-hexyl) phthalates is an energetically more favorable pathway than loss of an alcohol moiety. Prominent ions in these pathways were confirmed with deuterium labeled phthalates. Copyright © 2010 John Wiley & Sons, Ltd.