## 2 Results and Discussion
### Ptr-Ms Analysis
#### 2.3.2 High-Throughput Headspace Analysis of Kaffir Lime Juice Using PTR-MS  
Dynamic headspace analysis using PTR-MS was performed to develop a high-throughput and convenient method for the assessment of terpene conversion in kaffir lime juice. The justification for the application of the PTR-MS-based method of the proposed method is (1) direct qualitative determination of multiple VOCs, (2) short analysis--in seconds, (3) limited sample preparation--headspace analysis, and (4) analytical greenness--solvent-free approach. Moreover, many terpene degradation products are structural isomers; thus, PTR-MS measurement simplifies the interpretation and depicts the total load of all products with the same chemical structure. However, problems related to the rich headspace composition of kaffir lime juice and the behavior of terpen in the PTR-MS drift tube had to be overcome. Terpenes tend to fragment in the drift tube along with the clustering with the H\({}_{5}\)O\({}_{2}\)+ (37.03 \(m/z\)) ion [20; 21]. This phenomenon is shown in Table S2 (Supplementary Material), where fragment ions of 67.06, 81.07, and 95.09 \(m/z\) were registered, as well as the ion 155.14 \(m/z\), which is the clustering product. At the same time, the ion 155.14 may represent terpinen-4-ol or \(\alpha\)-terpineol (both C\({}_{10}\)H\({}_{18}\)O), which have been shown to be the final terpene conversion products of terpen (C\({}_{10}\)H\({}_{16}\)). For PTR-MS analysis, ions 133.10, 135.12, and 155.14 \(m/z\) were selected as the representatives of terpene conversion products, where 133.10 \(m/z\) may represent \(p\)-cymenene (C\({}_{10}\)H\({}_{12}\)), and 135.12 \(m/z\) may be a product of the protonation of \(p\)-cymene (C\({}_{10}\)H\({}_{14}\)).  
The ratios of the conversion product ion and terpene ion (137.13 \(m/z\)) in different storage conditions are shown in Figure 3. The rationale for describing the conversion products as a ratio is that it overcomes several problems and simplifies measurement and analysis. First, the ratio on day 0 shows a representation of the compounds in the fresh juice, and any change in the ratio indicates chemical transformations that occur during storage. It can represent both the decomposition of C\({}_{10}\)H\({}_{12}\), C\({}_{10}\)H\({}_{14}\), and C\({}_{10}\)H\({}_{18}\)O compounds (the ratio decreases) and the conversion of terpen into these compounds. Second, the change in the 155.14 \(m/z\) signal may result in terpene concentration, as this could be the clustering product of terpen. However, using the ratio value between these ions overcomes this problem, and whenever the ratio arises, the headspace concentration of the C\({}_{10}\)H\({}_{18}\)O conversion products also occurs. Finally, there is no need to measure the concentration of these volatiles or use corrected counts per second (cps) because the ratio is dimensionless. This simplifies data processing.  
As can be seen in Figure 3, the ratio of different ions changes with storage time. However, an increase in the ratio with time was observed only for the storage at 35 \({}^{\circ}\)C. This was confirmed by GC-MS analysis, where C\({}_{10}\)H\({}_{12}\), C\({}_{10}\)H\({}_{14}\), and C\({}_{10}\)H\({}_{18}\)O were found to be good shelf-life indicators. At the same time, C\({}_{10}\)H\({}_{12}\) seemed to be the best indicator since the ratio increase is the most pronounced and the standard deviations between repetitions are small.  
To determine when the conversion of terpenes to \(p\)-cymenene occurs, the polynomial function was fitted to the obtained data (see Figure S1 in Supplementary Material). The second derivative of this function was then used to find the inflexion point. According to the obtained calculations, the conversion of terpenes to \(p\)-cymenene takes place after 5 days of storage (5.33 days).