## 4 Concluding Remarks  
This study provides fundamental information on the ion-molecule chemistry associated with the development of soft chemical ionisation mass spectrometric analytical techniques for the real-time monitoring of propofol and its major metabolite in exhaled breath. This is of potential benefit to developing user-friendly clinical analytical instrumentation for use in surgery and for examining metabolic processes. Detailed knowledge of the ion-molecule chemistry occurring under different operational conditions is crucial for the development of such instrumentation.  
In this paper, we have presented results from the first detailed PTR/SRI-ToF-MS investigations of propofol and 2,6-disopropyl-1,4-benzoquinone. The analyses of the reactions of the reagent ions H\({}_{3}\)O\({}^{+}\) and H\({}_{3}\)O\({}^{+}\).H\({}_{2}\)O with propofol and 2,6-disopropyl-1,4-benzoquinone have been aided by density function calculations used to determine the proton affinities and gas-phase basicities of both compounds.  
A key outcome of this study is that we have demonstrated that soft chemical ionisation mass spectrometric techniques can be used to detect propofol and its major metabolite with a high level of confidence and in real-time through the monitoring of specific distinct product ions. However, to measure propofol and 2,6-disopropyl-1,4-benzoquinone simultaneously on breath, only H\({}_{3}\)O\({}^{+}\) can be used as the reagent ion, the reaction of which leadsto the protonated parent species, with the maximum analytical sensitivity for these product ions being found at the lowest \(E/N\) value investigated, namely 60 Td.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13074623/s1](https://www.mdpi.com/article/10.3390/app13074623/s1)]([https://www.mdpi.com/article/10.3390/app13074623/s1](https://www.mdpi.com/article/10.3390/app13074623/s1)), Figure S1: NMR spectra of 2,6-disopropyl-1,4-benzoquinone, \({}^{1}\)H-NMR (400 MHz, DMSO-d6, 25 \({}^{\circ}\)C): 8 6.54 (singlet, 2H, 2\(\times\) CH (atom.)); 2.97 (septet, 2H, 2\(\times\) CH (iPr); 1.08 (duplet, 12H, 2\(\times\) CH) (iPr).  
[PERSON] developed the experimental methods, undertook all measurements, analysed the data and was involved in writing all drafts of the paper. [PERSON] assisted with the experimental measurements and in the interpretation of the data. [PERSON] assisted with the PTR-ToF-MS measurements and the analysis of the data. [PERSON] aided [PERSON] in synthesising 2,6-disopropyl-1,4-benzoquinone and the NMR analysis for determining its purity. [PERSON] proposed the PTR-ToF-MS/SRI-ToF-MS propofol and 2,6-disopropyl-1,4-benzoquinone study. [PERSON] also supervised [PERSON] and [PERSON] and undertook the lead in writing all draft versions of the paper. All authors have read and agreed to the published version of the manuscript.  
We wish to acknowledge the FFG IKT der Zukunft, IKT der Zukunft, IKT der Zukunft-Resilienz und Distancing for the support of this study through the \"Screening of Infection by Chemical Evaluation of Breath Volatiles for Rapid Deployment during Viral Outbreaks\" DEVICE for the funding of [PERSON]'s Ph.D. programme and the EU HORIZON Innovation Actions HORI-ZON-CL3-2021-DRS-01-05, Project Number 101073924 (ONELAB) for funding [PERSON]'s Ph.D. programme.  
Not applicable.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.