# Analysis of Potent Odour-Active Volatile Thioids in Foods and Beverages with a Focus on Wine
## 4 Analytical Instrumentation
### Analysis by GC
These detectors record signal responses of sulfur atoms in the compounds of interest, which provides preliminary chromatographic information. For instance, the retention indices that were calculated upon analysing linear thiol compounds with PFPD have been used for identification purposes (Table 2, Entry 10) [26]. Similar to FPD, SCD has also been applied for screening volatile thiols due to the demonstrated high selectivity and sensitivity (absolute limit of detection below 1 ng, Table 2, Entry 31) [23]. The chromatographic conditions (e.g., GC column and oven program) that are obtained from using detectors such as FPD can then be applied for GC-MS quantitation, as in the case of thiols in soy sauce [66]. Being less expensive to purchase and less complicated to operate and maintain are other practical reasons for using these detectors for routine thiol screening. For thiol quantitation, AED has been evaluated for three volatile thiols in wines (Table 2, Entry 3) and showed good detection performance for 4-MSP, but was not suitable for 3-SH and 3-SHA due to co-elution problems with the IS used [25]. Even so, the LOD of 5 ng/L with AED detection was a few times higher than the ODT of 4-MSP (0.8 ng/L) [25]. MS detectors are still required because of their indispensable ability of obtaining mass spectra for the identification of analytes of interest and the superior quantitation capacity, despite the highly specific coverage of FPD, PFPD, and SCD towards sulfur-containing volatiles. Other than retention index, MS spectra of peaks of interest can be compared to those in commercial databases (e.g., NIST [67] or Wiley [68]), or an in-house thiol database [23]. Accordingly, identity confirmation of new thiols then necessitates the synthesis of reference standards if they are not readily available from commercial suppliers [23; 64; 69]. The majority of GC-based quantitative analyses are conducted with MS detectors, with electron ionisation (EI) (Table 2, multiple entries) or less frequently used chemical ionisation (CI) (Entries 3, 5, 7, 8, 12) [18; 25; 29; 30; 31] being applied for volatile thiol detection. Selected ion monitoring (SIM) has been frequently used over full scan mode, with one quantifier ion and desirably at least two other qualifier ions when single stage MS is applied for quantitative analysis. MS/MS is used in multiple reaction monitoring (MRM), selected reaction monitoring (SRM), or consecutive reaction monitoring (CRM) mode, depending on the detector (QqQ and ITMS), which provides better selectivity and sensitivity than single stage MS. Taking 3-SH, which is probably the most evaluated thiol in wine and beer, as an example, many methods have used MS for its detection. When detected in the native form, SIM ions at _m/z_ 134, 100, 82, and 67 [19; 60] were chosen as the qualifiers and quantifier for single stage MS, whereas transitions _m/z_ 134\(\rightarrow\)82 and 100\(\rightarrow\)82 were the pairs used for MRM with MS/MS [28]. If 3-SH has been derivatised, ions that were selected in EI-MS have higher _m/z_, for instance, 314, 181, and 133 for the PFBBr derivative [32] and 232, 187, and 132 for the ETP derivative [19]. The reported LOD values for 3-SH when using single stage MS were 30 ng/L [32], 69 ng/L [60], 7 ng/L [29], and 2 ng/L [30], which were higher than those values that were obtained with MS/MS (1.9 ng/L [28] and 0.7 ng/L [25]).