Development of a novel mass-selected internal positive chemical ionization quadrupole ion trap mass spectrometry technique for the quantitative analysis of isotopic polyunsaturated fatty acids
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Analytical instrumentation for quantitative in vivo stable isotope metabolic studies has included gas chromatography-mass spectrometry (GC-MS) and gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Limitations of existing MS techniques include excessive parent ion fragmentation, time-consuming sample preparation, and complex instrument operating parameters. In this thesis, procedures for set up and implementation of four GC-MS techniques are described. The first three replicate existing GC-MS single quadrupole methods on an ion trap, and are electron ionization (EI), external methane positive chemical ionization (PCI), and methane negative chemical ionization (NCI). The fourth method is a novel GC-MS mass-selected ion trap internal isobutane positive chemical ionization technique. Four groups of rats were administered isotopic linoleic acid, and liver tissue was collected for labelled linoleic and n-6 polyunsaturated fatty acids (PUFA) metabolites analyses. Qualitative utility of EI was confirmed, and its quantitative limitations exposed. Labelled 18:2n-6 and n-6 PUFA metabolites were detected using external methane PCI, though limited due to significant fragmentation. Methane NCI also detected labelled 18:2n-6, as well as minimizing fragmentation. However, time-consuming sample preparation and non-linear responses were major limitations. Internal isobutane PCI was useful in detecting isotopic 18:2n-6 and n-6 PUFA metabolites. Fragmentation was reduced compared to EI and external methane PCI. Limitations include fragmentation of HUFAs such as EPA and DHA. The novel internal isobutane PCI is more sensitive than external methane PCI and NCI, produces highly linear responses, is simpler and less expensive to operate than C-IRMS, offers reliable instrument operation, and sample preparation time is minimal. Regular quantitative analyses of HUFAs such as EPA and DHA may require further refinements such as using lower energy reagents than isobutane, including acetonitrile and ammonia.
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Hamid Izadi (2009). Development of a novel mass-selected internal positive chemical ionization quadrupole ion trap mass spectrometry technique for the quantitative analysis of isotopic polyunsaturated fatty acids. UWSpace. http://hdl.handle.net/10012/4520