Gas chromatography/mass spectrometry of polyethylene glycol oligomers

Eur. J. Mass Spectrom. 7, 279–291 (2001)
DOI: 10.1255/ejms.438

Gas chromatography/mass spectrometry of polyethylene glycol oligomers

Adebayo Onigbinde, Gordon Nicol and Burnaby Munson^*
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716-2522, USA. E-mail: bmunson@udel.edu

ABSTRACT:

Polyethylene glycol (PEG) oligomers, H(OC₂H₄O)_nH, were analyzed by gas chromatography/mass spectrometry (GC/MS) for oligomers, n = 2–11, without indications of pyrolysis of the higher oligomers at oven temperatures of 330°C. The electron ionization (EI) mass spectra (70 eV and 12–14 eV) of these PEG oligomers contain only low molecular weight fragment ions and, essentially, no M⁺ ions. The low-voltage EI spectra contain more abundant high mass ions and allow easier identification of the PEG oligomers. The major fragment ions in all EI spectra are (C₂H₄O)_xH⁺ at m/z 45, 89, 133 and 177. The base peak in the low-voltage spectra varies with molecular weight and suggests preferential cleavage at the second and third ether linkages. The relative molar sensitivities (70 eV EI) of PEG oligomers (n = 1–6) are linear functions of molecular weight or polarizability ratios. The CH₄ chemical ionization mass spectra of the PEG oligomers contain, predominantly, (C₂H₄O)_xH⁺ ions at m/z 45, 89, 133 and 177. The relative abundances of MH⁺ ions vary markedly across the chromatographic peaks because these ions are formed predominantly by sample ion / sample molecule reactions. Proton transfer reactions from CH₅⁺ and C₂H₅⁺ to the PEG oligomers are almost entirely dissociative. The i– C₄H₁₀ CI mass spectra of PEG oligomers contain MH⁺ as the base peaks. The relative abundances of MH⁺ ions do not vary significantly across the chromatographic peaks but do increase with increasing pressure of i-C₄H₁₀. The extent of decomposition of MH⁺ ions in i-C₄H₁₀ CI mass spectra increases with increasing molecular weight of the PEG oligomers. Relative molar sensitivities (CH₄ CI and i-C₄H₁₀ CI) of PEG oligomers (n = 1–6) are essentially the same linear functions of molecular weight ratio.

Keywords: GC/EIMS, GC/CIMS, PEG oligomers, ion molecule reactions, relative molar sensitivities, low voltage EIMS

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