# Analysis of Potent Odour-Active Volatile Thioids in Foods and Beverages with a Focus on Wine
## 3 Thiol Isolation--Extraction and Derivatisation
### Selective Extraction with Metal Ions
\begin{tabular}{c c c c c c c c} \hline \hline
**Entry** & **Reference** & **No. of** ** **Analytes1 ** & **Matrix** & **Sample** **Amount** & **Isolation Overview2 [FOOTNOTE:2]Footnote 2: In the following, we present the results of the experiments. \begin{tabular}{c c c c c c c c} \hline \hline
**Entry** & **Reference** & **No. of** ** **Analytes1 ** & **Matrix** & **Sample** ** **Amount** & **Isolation Overview2 [FOOTNOTE:2]Footnote 2: In the following, we present the results of the experiments. \begin{tabular}{c c c c c c c c} \hline \hline \hline
**Entry** & **Reference** & **No. of ** **Analytes1 ** & **Matrix** & **Sample** **Amount** & **Isolation Overview2 [FOOTNOTE:2]Footnote 2: In the following, we present the results of the experiments. \(p\)-HMB extraction procedures are quite time consuming due to the need for pH adjustment and clean-up, and, more importantly, thiols are prone to oxidation during the laborious sample preparation steps, which could influence their quantification. Affinity chromatography with Affi-Gel 501 (formed from Affi-Gel 10 by treatment with 4-aminophenylmercuric acetate) was developed for thiol extraction from extracts prepared by LLE to simplify the selective extraction step (Table 1, Entry 3) [25]. Affinity chromatography with Affi-Gel appears to be less time-consuming when compared to the previously developed \(p\)-HMB protocol, but it demonstrates lower recovery rates (e.g., for 4-MSP, 38% vs. 75%-80%). This approach is possibly more suited to thiol discovery, with a similar affinity chromatography approach having been used to screen for volatile thiols in various fruits and wines [23]. Another slight limitation is that Affi-Gel 501 needs to be prepared in-house, and these approaches still involve intense extraction and concentration steps (more problematic for routine quantitation than for thiol screening). Another selective extraction method is based on the high affinity between thiols and Ag\({}^{+}\) (Table 1, Entry 5) [28]. Using commercially available Ag\({}^{+}\) based SPE cartridges (Meta-Sep IC-Ag), volatile thiols in the organic extracts of beer and hops were retained and then eluted with thioglycerol in CH\({}_{2}\)Cl\({}_{2}\)[28]. Ag\({}^{+}\) extraction is advantageous when compared to Hg\({}^{+}\), because it avoids the use of toxic mercury and the SPE cartridges can be commercially obtained. However, the Ag\({}^{+}\) extraction procedure is still somewhat complicated, with the LLE extraction of volatiles, and multiple clean-up and concentrating steps after SPE. However, from purely an extraction viewpoint, selective extraction methods have high reaction efficiency, selectivity, and permit the recovery of thiols for analysis in their unmodified form. This is particularly useful for GC-Olfactometry (GC-O) screening for new thiol odorants, and it has enabled the discovery of many important volatile thiols in wine [21; 22; 24], tea [46], hop extracts [12], and beer [12; 47]. On the other hand, the drawbacks of these extraction approaches are obvious: large amounts of sample and solvent are required for the preparation of volatile extracts; procedures are lengthy and time-consuming; final concentrated thiol distillate/extracts are in their original sulfhydryl form, which can cause reaction, separation, and detection issues; in the case of \(p\)-HMB, handling highly toxic organomercurial compounds poses significant health and environmental risks.