Gas Chromatography/Mass Spectrometry of Polyethylene Glycol Oligomers

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.