Browsing by Author "Ntlhokwe, Gaalebalwe Elizabeth"
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- ItemApplication of comprehensive two-dimensional gas chromatography for the characterisation of the volatile composition of Honeybush tea(Stellenbosch : Stellenbosch University, 2016-03) Ntlhokwe, Gaalebalwe Elizabeth; De Villiers, Andre J.; Stellenbosch University. Faculty of Science. Department of Chemistry and Polymer ScienceENGLISH ABSTRACT : Honeybush tea (Cyclopia spp.), like many other natural products, contains a large number of volatile compounds with different physiochemical properties and present at widely varying concentrations. This presents a significant analytical challenge for conventional one-dimensional gas chromatography (GC). The aim of this study was to explore the potential of comprehensive two-dimensional gas chromatography (GC×GC) for the improved analysis of honeybush tea volatile compounds. The first part of the study focused on the evaluation of a GC×GC system equipped with a novel single-stage thermal modulator. A chromatographic method well suited for the analysis of honeybush volatiles was successfully developed by systematic optimisation of instrumental parameters. Solid phase micro-extraction (SPME) was used for extraction of volatile compounds prior to GC×GC separation. Three Cyclopia species were analysed: C. genistoides, C. subternata and C. maculata. Chromatographic separation of honeybush volatiles was significantly improved on this system. 69 compounds were identified based on the retention data obtained by injection of reference standards in combination with flame ionisation detection (FID). Hyphenation to quadrupole mass spectrometry (qMS) was also explored, although the slow acquisition rate of the detector provided limited scope for identification of additional compounds. The highly reproducible performance of this modulator proved beneficial in the use of multivariate data analysis of the GC×GC-FID data. Significant differences in the volatile composition between the three Cyclopia species were observed, in general agreement with sensory studies for the same set of samples. In the second part of the study, a commercial GC×GC instrument equipped with a cryogenic modulator and hyphenated to time-of-flight MS (TOF-MS) was used for the detailed qualitative analysis of the same set of samples. This configuration provided new insight into the volatile composition of honeybush tea: the combination of enhanced GC×GC separation and sensitivity, coupled to the high-speed acquisition of TOF-MS allowed the tentative identification of 158 compounds in these samples for the first time. A total of 272 compounds were identified using either reference standards or mass spectral and retention index (RI) data. Significant differences in the volatile composition of C. genistoides, C. subternata and C. maculata were also observed in the GC×GC-TOF-MS data. (E)-cinnamaldehyde was identified in honeybush tea for the first time, only in C. maculata samples. This compound