Abstract

Ion molecule reactions usually occur between reagent ion and neutral sample molecules under chemical ionization (CI) and ion cyclotron (ICR) conditions. The set of Brønsted acids generated in methane CI plasma are all powerful protic acids and are present in the CI source in far greater concentrations than the protonated PEG and PEGDAE ions and the many fragments of these oligomers. However, sample ion/sample molecule (SISM) reactions were observed under CI conditions with large sample size of polar compounds, and are useful in forming MH+ ions for molecular weight determination. These SISM reactions will not be noticed unless studies are made about the variation of CI spectra with sample size. The CH4/CI mass spectra of polyethylene glycol and polyethylene glycol dialkyl ethers contain MH+ ions whose relative abundances are low, variable, and increase with increasing sample size across the chromatographic peaks and not with increasing value of x or n; hence, a major portion of these ions is formed by SISM reactions. The dominant fragment ion in the CH4/CI mass spectra of all PEG and PEGDAE’s are the ions at m/z 45 (C2H5O+) and m/z 59 (C3H7O+) respectively. In this paper, electronic ionization (EI), CI, and ion molecule reactions of the PEG and PEGMAE’s were used to produce C2H5O+ and C3H7O+ ions in the source of an FT/ICR mass spectrometer and were used to react with PEG’s and PEGDME’s. Reaction profiles of these two sample ions were monitored and their reaction products and profile were also obtained. The ion molecule reaction of C2H5O+ ions from PEG gave spectra similar spectra to those produced by CH4/CI. The reaction profile of isomeric C3H7O+ ions with PEGDAE’s follow the same pattern like those observed for the C2H5O+ ions. The relative abundances of the MH+ in the CH4 CI mass spectra of PEG and PEGDAE oligomers increased dramatically with increasing sample size confirming the occurrence of sample ion/sample molecule reactions under normal CI conditions. The limitation of this study include the fact that collision rate constants, the reduced mass (1/µ1/2) of the colliding pair and the proton affinities of PEG and PEGDAE’s are not known which might have helped to understand why SISM reactions preferably occurs in the presence of larger concentrations of reagent ions. Further studies needs to be done to access these parameters. ___________________________________________________________________________FTICR MS, ion-molecule reactions, polyethylene glycol, polyethylene dialkyl ethers, sample ion/sample molecule reactions