Abstract

The mass spectrometer provides a wealth of information for a very small quantity of sample relative to many other analytical tools and is a good technique for determining structures, molecular weights of compounds, detection and risk assessment of pollutants in the environment. Its potential as an analytical research tool is tremendous and its range covers small organic compounds (MW < 100) to large molecules (MW >10,000). Coupled techniques such as gas chromatography mass spectrometry (GC/MS), have increased the scope of quantitative and qualitative information that can be obtained from a sample. New developments in various spheres of sciences have produced many commercial products and as well increased risk to human health and environmental pollution. For example, packaging materials such as Bisphenol and paraffin’s has been found to migrate into food materials and reacts with food components such as proteins. Pesticides such as chlorpyrifos and penconazole residues have been found in wines and large biopolymers such as epoxy resins are used in art conservation and as sensors in many chemical applications. Identification and characterization of these chemicals require fast and accurate analytical techniques and instrumentation such as chemical ionization (CI). The CI technique is based on ion-molecule reactions and its usefulness depends on the types of reactions and also on the rates of the reactions. Sample ion/sample molecule (SISM) reactions frequently occur under chemical ionization (CI) conditions with large sample size polar compounds, and are useful in determining thermochemical parameters such as bimolecular rate of reactions, structure of compounds and 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 methane 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. The bimolecular rates of reaction of the isomeric C2H5O+ and C3H7O+ sample ions at m/z 45 and m/z 59 with PEG’s and PEGDMES were studied using the FT/ICR mass spectrometer. The rate constants of the isomeric C2H5O+ ions with PEG oligomers were found to increase with increasing molecular weight or polarizability of the PEG oligomers. The rate constants for reactions of C2H5O+ from PEG and ethylene oxide were very similar for all the PEG oligomers and were relatively close to the calculated ADO values. The rate constants for reaction of C2H5O+ ions from dimethyl ether with ethylene glycol and diethylene glycol were significantly lower than the rate constants from the reaction with the other isomers. Similarly, the bimolecular rate constants of the reaction of the C3H7O+ ions were measured and found to increase with increasing molecular weight of the PEGDME oligomers. The experimental rate constants are greater than their Langevin values. This research will give a new insight of analysis for professionals in the field of agriculture, toxicology, biosciences, medical, pharmaceutical, environmental science, packaging and the food industry.