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Rapid chromatographic and high resolution mass spectrometry techniques for drugs screening and identification
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posted on 26.02.2017by Mokhtar, Siti Umairah
This thesis involved a range of research studies employing analytical screening methods using advanced chromatography separation with mass spectrometry (MS) detection on a broad range of drugs in a forensic toxicology environment. The research applied chromatography and mass spectrometry to the assessment of designer drugs and their analogues in nutritional supplements available on the market. The general focus was on rapid chromatographic and high resolution MS approaches to drug identification and quantification.
Gas chromatography hyphenated with triple quadrupole mass spectrometry (GC-QQQMS) was used to screen a wide range of drugs of relevance to toxicological analysis, in a single sample (referred to here as simultaneous analysis) using both the full scan and multiple reaction monitoring (MRM) modes. Both modes were compared in respect to their capability to generate acceptable results with high selectivity and sensitivity. Without the use of derivatisation methods, the MRM mode offered sufficient sensitivity with low background noise compared to the full scan mode. Adequate selectivity was found for both methods.
This same method was then applied to screen sildenafil and five of its analogues in five dietary supplements. Two different ionisation techniques, electron ionisation (EI) and chemical ionisation (CI), were contrasted by applying their respective approach for the identification of each analogue. As a triple quadrupole MS method was used, collisional induced ion decomposition was applied to provide detection specificity, especially for the CI. This study showed that the CI method plays an important role as a complementary tool. Together with EI it can provide additional information for each analogue. These methods also proved that the dietary supplement samples were adulterated with either sildenafil or their thioketone analogues.
In the next phase of the investigation, liquid chromatography coupled with the Orbitrap quadrupole exactive mass spectrometer (LC-Q-Exactive MS) was used as an approach for the identification and quantitation of a broad range of drugs – again in a single analysis. An initial study established operating conditions and analytical linearity for a large number of standards. Compounds were detected with high resolution MS operating in full scan mode, resulting in high selectivity based on the full scan exact mass measurement and a high mass accuracy (<5 ppm). In this study, nominally isobaric compounds were easily distinguished and identified by their different retention times and exact mass measurement. Extracted ion chromatograms (XICs) with a narrow mass tolerance window (5 ppm) provided analysis with acceptable linearity (r²), low limits of detection (LOD) and low limits of quantification (LOQ). The results were compared with results obtained from liquid chromatography – tandem mass spectrometry (LC-MS/MS). Subsequently, the study was continued with the application of the parallel reaction monitoring mode (PRM), using the same Orbitrap instrument, to provide accurate mass fragment ions for further confirmation. This study was successfully applied to identify and differentiate structurally isomeric drugs. The developed method was applied to analyse drugs in 26 biological samples from positive forensic cases, and proved that this technique was able to detect such target analytes at trace level.
As a conclusion, a green sampling approach using direct injection of ionic liquid (IL) solvent, containing a variety of analytes, using programmable temperature vaporisation (PTV) injection with gas chromatography (GC), was presented. Several PTV injection parameters were investigated to ensure both that IL did not enter the GC column, and that the analyte was effectively transferred to the column. The method was applied to extract essential oil compounds from eucalyptus leaves as a test sample using IL in a headspace single drop microextraction (HS-SDME) experiment. This research paves the way for a range of further studies using the IL extraction method for drug extraction in complex matrices.