Rapid separation of phenolic flavourings by liquid chromatography and nanomolar electrochemical detection at a boron-doped diamond electrode

In recent years, there has been considerable research focus on natural plant phenolics. They are present as key contributors to the flavour of food and beverages and are also proven to function as antioxidants, providing a wide range of physiological properties including anti-cancer, anti-inflammatory and anti-microbial. Their investigation and detection in plants, food and beverages require the development of rapid, selective and sensitive analytical methods. In this thesis, an overview of plant secondary metabolites, their classification and physiological properties are given. The characteristics of the boron-doped diamond (BDD) electrode and the coupling of liquid chromatography (LC) with electrochemical detection (ECD) are also described. This work focuses on determining the phenolic analytes that are associated with the distinct flavour of whiskey. Initially, the whiskey is analysed using direct electroanalysis at a Nafion-modified BDD electrode. Inclusion complexation with cyclodextrins (CDs) and peak deconvolution reveals guaiacols, phenols and cresol isomers as the phenolic molecular fingerprint of whiskey. Pre-concentration with Nafion on the electrode surface results in a 5-fold decrease in the limit of detection (LOD) compared to the bare BDD electrode. The electro-oxidation mechanisms of the analytes are also investigated with cyclic voltammetry (CV). To specifically determine the flavour contributing analytes and to differentiate whiskey based on geographical origin, detection at the BDD electrode is preceded by chromatographic separation with a C18 core-shell column. Significantly higher concentrations of the phenolic compounds are determined in Islay whiskey, attributed to its unique production process. The detection limit (S/N = 3) of guaiacol with LC-ECD is 5 nM, 80-fold lower than that obtained with LC-ultraviolet (UV) detection (2 µM). Attention was then turned to the separation and detection of gallic acid (GA) and ellagic acid (EA) in whiskey. GA and EA are present in the oak casks used for maturation and so their presence can be used as markers of authentic whiskey. Chromatographic separation using a reversed-phase (RP) C18 column with gradient elution is followed by amperometric detection at a BDD electrode. LODs of 60 and 200 nM are obtained for GA and EA, respectively, with the highest concentrations of both acids found in a 14-year-old whiskey. Rapid and sensitive analytical methods are required for the detection of spoilage metabolites in the food and beverage manufacturing industries. In beverages, guaiacol may be present as a result of microbial metabolism, resulting in the formation of a medicinal off-odour with subsequent financial implications. LC-ECD at a BDD electrode provides LODs of 10-30 nM for guaiacol and its phenolic precursors. Notably, separation is achieved within 60 s, providing a rapid alternative to traditional microbiological culturing methods. The outcomes presented represent an advancement in the separation and selective sensing of phenolic analytes, and are of importance in the food and beverage sectors