Gas chromatography/mass spectrometry of polyethylene glycol oligomers

摘要

Polyethylene glycol (PEG) oligomers, H(OC_2H_4O)_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 ℃. 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 (C_2H_4O)_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 CH_4 chemical ionization mass spectra of the PEG oligomers contain, predominantly, (C_2H_4O)_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 CH_5~+ and C_2H_5~+ to the PEG oligomers are almost entirely dissociative. The i-C_4H_(10) 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-C_4H_(10). The extent of decomposition of MH~+ ions in i-C_4H_(10) CI mass spectra increases with increasing molecular weight of the PEG oligomers. Relative molar sensitivities (CH_4 CI and i-C_4H_(10) CI) of PEG oligomers (n = 1-6) are essentially the same linear functions of molecular weight ratio.