Mass spectrometry and gas chromatography in chemical analysis and identification : experimental factors and case studies
Creators: name: Davies, Noel William
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis is based on developments in mass spectrometry (MS), gas chromatography (GC) and combined gas chromatography-mass spectrometry (GC-MS) as analytical techniques, and specific applications of these developments and traditional MS and GC-MS to the analysis of organic samples. The majority of this work has been published in the refereed scientific literature. A detailed theoretical study was undertaken into the relationships between chromatographic resolution in combined GC-MS and carrier gas flow rate. The influence that the type of GC-MS interface has on optimum flow rate requirements was quantified. A simple equation relating column length, column 'hold-up' time and volumetric flow rates in directly coupled capillary GC-MS was derived, facilitating the setting of the appropriate optimum gas flow in this mode. Average linear gas velocities and flow rates required to deliver optimum performance for a variety of column diameters, carrier gases and types of GC-MS interface were reported, together with the 'hold-up' times and inlet pressures necessary to deliver these values. The dependence between carrier gas flow rates, average linear velocities and temperature programming was determined for a number of GC-MS interface and flow control combinations. Discrimination caused by the 'open-split' interface in GC-MS was examined, and a numerical relationship was established between GC oven temperature and effluent yield in temperature programmed analyses. A GC-MS screening procedure for a large group of target analytes was created using a 'bench-top' quadrupole mass spectrometer, enabling automatic detection at trace levels of genuine or potential 'positives' from crude extracts. Two representative ions of the target substance had to occur in the correct ratio and at a specific and automatically adjusted retention time for a 'positive' report. For maximum sensitivity a number of time windows were created, and selected ion monitoring of the two characteristic ions for each target compound was carried out. Although the protocol was developed for screening extracts of biological fluids for drugs, it could be applied to screening for target compounds from any matrix. The technique of 'Selected Reaction Monitoring' of fragmentations within the first field-free region of a double-focussing instrument of Nier-Johnson geometry was developed, and applied to the quantitative determination of drugs and drug metabolites from body fluid extracts using stable isotope labelled internal standards. This method, an aspect of the now widely adopted 'Mass Spectrometry/Mass Spectrometry' (MS/MS) techniques, greatly increased the specificity of the detection of target compounds. Warfarin from human blood and N-hydroxy phenacetin from human urine were quantified by this technique. These two examples were among the first to use a conventional geometry mass spectrometer in this way for the simultaneous monitoring of daughter ions from different parents from the first field-free region of the spectrometer. Conventional low and high resolution MS and GC-MS were applied to the analysis of essential oils, wasp secretions and plant gums and waxes. The essential oils of six hop cultivars were analysed in detail, particularly with respect to the levels of selinene isomers. Commercial extracts of Boronia megastigma flowers were analysed by GC-MS; (Z)-heptadec-8-ene and dodecyl acetate were identified as major components, together with the new oxygenated sesquiterpene 'sesquicineole' in some clones, and some fifty other volatile components. A library of several hundred reference mass spectra was compiled to enable analyses on a range of essential oils to be undertaken. This was made available to the MS community via the U.S. National Institutes of Standards and Technology (NIST) database. A collection of 'Kovats' GC retention indices, important corroborative evidence for the MS identification of essential oil related compounds, was obtained from a diverse range of literature sources for 400 mono- and sesquiterpenes. This collection of data was published as a review article. Based on MS evidence in conjunction with microchemical reactions and subsequent comparison with authentic standards, (E,E)-2,8-dimethy1-1,7-dioxaspiro[5.5]- undecane, (E,E)-2-ethy1-7-methy1-1,6-dioxaspiro[4.5]- decane, (Z,E)-2,8-dimethy1-1,7-dioxaspiro[5.5]undecane and 2-methyl-6-penty1-3,4-dihydro-2H-pyran were identified in the mandibular gland secretions of parasitic wasps. The structures of three new cycloartenone related triterpenes from Gardenia species were determined primarily by MS from first principles; these were 9,19-cyclolanostan-3,24-dione, 9,19- cyclolanost-24-en-3,23-dione, and 9,19-cyclolanostan- 3,23-dione.