# Analysis of Potent Odour-Active Volatile Thioids in Foods and Beverages with a Focus on Wine
## 4 Analytical Instrumentation
### Analysis by GC  
GC separations in the gas phase are applicable for volatile analytes, such as thiols and some of their derivatised forms (e.g., PFBBr or ETP derivatives). For injection, a purge and trap injector (PTI) [21] and cool-on-column injection (20 \({}^{\circ}\)C [25], 35 \({}^{\circ}\)C [25], 40 \({}^{\circ}\)C [23]) have been reported for the GC separation of native forms of thiol analytes (e.g., Table 2, Entries 3 and 31). PTI has great extracting and concentrating ability for relatively large volumes of sample (8 mL) [21; 56], and cool-on-column injection is preferred for labile analytes [56], hence they are both suitable for the analysis of trace to ultra-trace volatile thiols. The enhancement of thermal stability of analytes after derivatisation allows for the use of conventional injectors and injection modes for thiols, including splitless [18; 19; 32], large volume (20 \(\mu\)L) [29; 30], splitless to split [33; 35], TDU in splitless mode [26], and pulsed splitless [28] injection programs. After injection, most of the separations are performed in a one-dimensional GC system installed with fused silica capillary columns, either with non-polar (e.g., BPX-5 [22], DB-5 ms [32]) or polar (e.g., HP-Innowax [19], DB-Wax [35]) stationary phases. Selecting the right column for thiol analysis still requires practical trial-and-error approaches, even though multiple options for GC capillary columns are available (Table 2). For example, large volume injection of PFBBr derivatives followed by separation on a VF-5 ms column showed problematic chromatographic behaviour (dirty, distorted, broadened, and delayed peaks) and switching to a column with a more polar phase did not resolve this issue [29]. In another instance, peak interferences and tailing when separating PFBBr derivatives on a DB-5 column were overcome by using a DB-FFAP column [33]. In the case of large injection volumes (10 \(\mu\)L), a column with larger internal diameter (0.53 mm i.d.) was preferred [25]. Apart from one-dimensional GC, two-dimensional separations of volatile thiols have also been explored with heart cut GC or GC\(\times\)GC systems (Table 2, Entry 24, 27, 31) [23; 27; 57]. However, it was worth noting that, even with the enhanced resolving power of GC\(\times\)GC, conventional sample preparation procedures without specific chemical derivatisation failed to detect a targeted thiol (4-MSP) due to the high background noise [27]. The detectors of choice in GC applications are normally associated with the analytical aims of the methods; that is, whether for identification or quantitation purposes. O, FID, FPD, PFPD, SCD, and Q-TOF-MS appear to serve as detectors for identification purposes given the sensitivity and selectivity of detectors towards ultra-trace volatile thiols, whereas Q, QqQ, and ITMS are regularly used for quantitation (and can be coupled to GC-O as well). From some of the early work that was focused on screening/discovering volatile thiols in foods and beverages (e.g., Table 2, Entry 1) to as recent as 2017 (e.g., Table 2, Entry 30), GC-O has frequently been utilised to locate odour zones of interest and provide the odour quality of the analytes being isolated [12; 21; 23]. GC-O also serves as an important criterion for the identification of aroma compounds, and quite remarkably, the human olfactory organ has demonstrated greater sensitivity for certain thiols during GC-O analysis than PFPD or MS [12]. Such ultra-sensitivity towards volatile thiols has been related to specific thiol olfactory receptors (e.g., OR2T11, OR2W1, and OR2C1) in humans [58; 59]. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Entry** & **No. of** & \multirow{2}{*}{**Year**} & **Matrix** & **Analytic** & **Analyytic** & **Ain 3** & **Major Separation Parameters 4** & **ME 5** & **LOO * & **RSD 7 (5x)** & **Recovery 8** \\
**No.