## 3 Results and Discussion
### Non-Volatile Compounds (Glucosinolates)
#### 3.2.3 Watercress  
Fourteen GSLs were found in watercress leaves, amounting to 31.6 \(\pm\) 1.4 \(\upmu\)mol g\({}^{-1}\) dw (Table 4). In most previous studies of this species, GNT (1.5 \(\pm\) \(<\)0.1 \(\upmu\)mol g\({}^{-1}\) dw) has been found to have the greatest abundance [4]; however, our analysis revealed that 7 MSH had the highest total concentration (21.5 \(\pm\) 1.2 \(\upmu\)mol g\({}^{-1}\) dw), dominating the overall profile in these samples. There were also relatively high concentrations of 7 MTH (2.7 \(\pm\) 0.1 \(\upmu\)mol g\({}^{-1}\) dw), and the indidic GSLs 4 MGB (2 \(\pm\) 0.1 \(\upmu\)mol g\({}^{-1}\) dw), and NGB (1.6 \(\pm\) \(<\)0.1 \(\upmu\)mol g\({}^{-1}\) dw). Minor amounts of SIN, GRA, and GTP were also observed, and they are not frequently reported in this species.  
Discrepancies between Identified Glucosinolate Hydrolysis Products and Glucosinolate Profile Precursors  
There is often an 'elephant in the room' regarding volatile GSL hydrolysis products and reported GSL profiles in Brassicales crops: there are often GSLs found with no corresponding hydrolysis products, or more troublingly, hydrolysis products observed but no GSL precursor. Table 5 presents a list of the GSL-derived compounds identified within the headspace of'salad' rocket, wasabi, and watercress, alongside their expected GSL precursors. It is apparent from our data that the present study is no exception when it comes to discrepancies of this nature, and there is a need to find a robust solution to prevent the inaccurate reporting of both GSLs and their volatile hydrolysis products.  
As discussed in previous sections, compounds such as methyl thiocyanate (**2**) may be produced from degradation of other hydrolysis products. Others such as the presence of _sec_-butyl ITC (**9**) in rocket, butyl ITC (**12**) in wasabi, and octyl ITC (**21**) in watercress cannot be so easily explained. There are several explanations with varying levels of likelihood: firstly, the most likely is that the ITCs and other GSL hydrolysis products have been identified incorrectly, and that they belong to other parent GSLs present within the analysed tissues. Despite researchers' best efforts to obtain authentic standards to test and match MS profiles, this is not always possible or affordable, and so there is a heavy reliance upon reference libraries. These are often incomplete and not always accurate. It is also possible that the high temperatures utilised in GC-MS cause thermolytic reactions to occur in GHPs, thus yielding compounds not 'naturally' produced by tissues. The next possibility is that the GSLs responsible for producing hydrolysis products are below the LOD of the MS/MS method. In this case, this is unlikely, as the LOD for each standard GSL was low, and it is likely much more sensitive and accurate than volatile compound measurements by GC-MS. Thirdly and perhaps least likely is that there is some as-yet-unknown mechanism(s) by which GSL hydrolysis products are modified post-hydrolysis. This is speculation, but there have been recent reports of previously unknown tautomeric rearrangements [82] and enzymatic actions [28] upon hydrolysis products that do not preclude this as an impossibility. Indeed, the dynamics of reactions occurring within the headspace of Brassicales is virtually unstudied, and so some may be produced through the degradation or rearrangement of structurally similar compounds. This is an area that requires much more detailed scrutiny.