# Analysis of Potent Odour-Active Volatile Thioids in Foods and Beverages with a Focus on Wine
## 2 Analytical Challenges and Requirements  
The analysis (either untargeted identification or targeted quantitation) of volatile thiols in foods and beverages has always been a challenging task due to two reasons:  
* matrix complexity;
* properties of thiols.  
Volatile thiols can be found in many foods and beverages, including wine, beer, cheese, olive oil, coffee, fruit, meat, and vegetable [14]. On one hand, these foods and beverages possess drastically different matrices (animal or vegetal, fermented or unfermented, liquid or solid, aqueous or lipid, etc.) and, on the other, the matrices are compositionally complex (containing many other volatile and non-volatile metabolites). Such complexity and diversity in matrices pose analytical challenges for developing suitable and efficient analytical methods [15]. Potent volatile thiols are highly unstable small molecules that are present at extremely low abundances with diverse chemical structures. The sulfhydryl (-SH) group in thiols is one of the most reactive functional groups found in natural organic matter [16]. As such, thiols are prone to oxidation, isomerisation, and rearrangement [17]. The highly active -SH group can cause chromatographic separation difficulties even when thiols are well preserved throughout extraction, such as peak tailing during analysis by gas chromatography (GC) [18]. Apart from the instability, volatile thiols differ in chemical structure, with the majority containing either acid (-COOH), alcohol (-OH), aldehyde (-CHO), ester (-OC(O)-), ether (-O-), and/or aromatic ring functional group(s), with only a small portion belonging to aliphatic thiols [14]. Those additional functional groups should be taken into consideration at an early stage of method development in order to minimise their modification. For instance, the analysis of thiol acetates should avoid the occurrence of acetate hydrolysis during sample preparation and analysis [19]. Besides structural diversity, some thiols are also characterised with chirality, owing to the carbon bearing the sulfur atom (Figure 1b), which gives a pair of thiol enantiomers. As with any enantiomer pairs, enantiomeric thiols are of almost identical physical and chemical properties, but they often differ in aroma quality and ODT [4]. The separation of enantiomers has always been a complex task as there is no golden rule to predict chiral separation. Lastly, thiols are generally found at trace to ultra-trace concentrations and, in many cases, at part per trillion levels. Such extremely low abundances require the careful consideration of effective sample isolation and enrichment steps, and sensitive detection techniques.  
An ideal analytical method for thiol analysis should be fast, simple, reliable, robust, green, sensitive, and cost-effective. For analytical methods that are dedicated to screening/discovering new volatile thiols, analytical information that is provided by the methods should be sufficient for identification. As for quantitative methods, limit of detection (LOD, ideally below ODT), matrix effects, repeatability, precision, and accuracy are among the important factors.