Sample ion/sample molecule (SISM) reactions frequently occur under chemical ionization (CI) conditions with large sample size polar compounds, and are useful in forming MH+ions for molecular weight determination. Studies about the variation of chemical ionization spectra with sample size were made in this report using gas chromatography/mass spectrometry, GC/MS, to observe the relative abundances of the major ions in the spectra of each sample as the sample size varies across the chromatographic peak. A plot of the fraction of total ionization of each major ion against the extent of conversion of the reagent ions to product ions indicated the reactivity of each major ion and those of the products of SISM reactions. The result also revealed that the relative abundances of the MH+ ion in the methyl chemical ionization (CH4 CI) mass spectra of polyethylene glycols (PEG) and polyethylene glycol dimethyl ethers (PEGDME) increased dramatically with increasing sample size because of SISM reactions. The dominant fragment ion of all PEG and PEGDME oligomers are the ion at m/z 45 (C2H5O+) and m/z 59(C3H7O+) respectively. These ions reacted to form product ions such as MH+ and (C2H4O)nH+ ions whose abundances increased with extent of conversion. Ions formed by 1,3 hydrogen rearrangements, H(OC2H4)nOH2+, were also present in these spectra and their relative abundances also increased with increasing extent of conversion.
Key words: Gas chromatography, mass spectrometry, polyethylene glycol ethers, sample ion/sample molecule.
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