Application of comprehensive two-dimensional gas chromatography for the characterisation of the volatile composition of Honeybush tea

Ntlhokwe, Gaalebalwe Elizabeth (2016-03)

Thesis (MSc)--Stellenbosch University, 2016

Thesis

ENGLISH 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

AFRIKAANSE OPSOMMING : Heuningbostee (Cyclopia spp.), soos wat ook die geval is vir talle ander natuurprodukte, bevat ‘n goot aantal vlugtige komponente met baie diverse fisiochemiese eienskappe en wat voorkom in ‘n groot verskeidenheid van verskillende konsentrasies. Dit bied ‘n groot analitiese uitdaging in terme van die gebruik van konvensionele een-dimensionele gaschromatografie (GC). Die doel van hierdie ondersoek was om die potensiaal van twee-dimensionele gaschromatografie (GC×GC) vir die verbeterde analise van heuningbostee te ontgin. Die eerste deel van hierdie studie fokus op die evaluasie van ‘n GC×GC sisteem wat toegerus is met ‘n nuut-ontwikkelde enkelfase termiese modulator. ‘n Chromatografiese metode, geskik vir die analise van vlugtige komponente in heuningbostee, is met sukses ontwikkel deur die sistematiese optimalisering van instrumentele instellings. ‘Solid phase micro-extration’ (SPME) is aangewend vir die ekstraksie van die vlugtige komponente alvorens die GC×GC analise uitgevoer is. Drie Cyclopia spesies is geanaliseer: C. genistoides, C. subternata en C. maculata. Die chromatografiese skeiding van vlugtige verbindings in heuningbostee is beduidend verbeter deur gebruik te maak van hierdie sisteem. 69 komponente is geïdentifiseer, gebaseer op die retensie data wat verkry is met die inspuit van verwysing standaarde en in kombinasie met vlam ionisasie deteksie (flame ionisation detection, FID). Koppeling met ‘n kwadrupool massaspektrometer (qMS) is ook ondersoek maar die stadige data-opneemspoed het die identifikasie van nuwe komponente belemmer. Die baie goeie vlak van herhaalbaarheid wat hierdie modulator bied was veral voordelig m.b.t. die gebruik van multi-veranderlike dataprosessering. Beduidende verskille in die samestelling van vlugtige komponente tussen die drie Cyclopia spesies is gevind en stem in die algemeen ooreen met sensoriese studies wat op dieselfde monsters uitgevoer is. In die tweede deel van hierdie studie is ‘n kommersieel beskikbare GC×GC sisteem, toegerus met ‘n ‘kriogeniese’ modulator en gekoppel aan ‘n ‘vlugtyd’ MS (TOF-MS) gebruik vir die in-diepte, kwalitatiewe analise van dieselfde stel monsters. Hierdie opstelling het tot nuwe insig met betrekking tot die samestelling van vlugtige verbindings in heuningbostee gelei: die kombinasie van die verbeterde skeiding en sensitiwiteit, tesame met die vinnige data-opneemspoed het die voorlopige identifikasie van 158 nuwe komponente moontlik gemaak. ‘n Totaal van 272 is geïdentifiseer met behulp van óf standaarde, óf massaspektrometrie en retensie indeks (RI) data. Beduidende verskille is ook hier in die vlugtige komponente se samestelling van C. genistoides, C. subternata en C. maculate waargeneem in die GC×GC-TOF-MS inligting. (E)-kaneelaldehied is vir eerste keer in heuningbostee geïdentifiseer, en ook slégs in C. maculata. Hierdie verbinding speel moontlik ‘n rol in die unieke aroma van hierdie tee, soos wat deur ‘n sensoriese ondersoek aangetoon is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98875
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