## 2 Materials and Methods  
### Samples and Samples Preparation  
Two-component mixtures consisting of gasoline and deionized and distilled water or isopropanol were prepared. The isopropanol (min. 99.7%, pure PA) was bought from the Chempur company, while deionized water was obtained directly from the laboratory supply line. The gasoline, 95-octane lead-free petrol (Orlen Eurosuper 95 lead-free), was purchased from a local petrol station. According to the specifications given by the producer, the maximum oxygenate content in 95-octane lead-free gasoline, which is comprised of isopropanol and water, is smaller than 2.7% (m/m) [15]. This clearly shows that there was a certain unknown amount of water and isopropanol in the samples during the measurements. The time-of-flight method is only capable of detecting an excess of water/isopropanol in relation to the base amount present in gasoline. However, we have paid special attention to keep this base amount unchanged and distributed equally among the samples with decreasing concentrations of admixtures: (i) pure gasoline was freshly bought and stored at low temperature in a special safe storage container to minimize any compositional changes and reduce the influence of external humidity on the water content in gasoline; (ii) the time needed to handle the solutions (weighing, mixing, pouring, etc.) was shortened to the minimum amount necessary and was comparable for each of the samples; (iii) the samples were freshly prepared before each experimental run under a fume hood with a constant flow of clean dry air, and they were measured within a maximum of 2 days after their preparation; (iv) each of the concentrations were obtained and measured at least twice to verify the self-consistence of the obtained results; (v) during the measurements, the cuvettes were tightly sealed with gasoline-resistive caps, so that their composition did not change at the time of measurement; (vi) all samples were stirred during the preparation for at least 30 min and again directly before the measurements to confirm their homogeneity based on visual observation. During all measurements, the sample containing pure gasoline served as the reference. All solutions were prepared by diluting a solution of a higher concentration with freshly added gasoline to exclude a gross error. The stock solution was weighted on a laboratory scale and mixed with water/isopropanol in the right proportions to obtain a total mass of 20 g. The samples with lower admixture concentrations were obtained by dilution of the higher admixture concentrations. Such a procedure is well-established and commonly applied in chemistry to dissolve solutes at the ppm level of their concentration [16; 17; 18]. We are, therefore, sure that the concentrations that were obtained corresponded to the nominal concentrations. The isopropanol-gasoline solutions were prepared over the concentration range of isopropanol from 75%wt. to 0.0125%wt. (detailed concentration data can be found in Sections 3.3 and 3.4). Due to the much lower solubility of water in gasoline, the samples containing water were prepared in a smaller range of water concentrations from 0.3%wt. to 0.025%wt. The solubility of water in the gasoline type used for the sample preparation was tested before the measurements. We found that it was possible to dissolve about 0.31%wt. of water in the gasoline. The upper concentration limit of water, 0.3% wt., was decided based on this result. During all measurements, the pure gasoline sample served as the reference. All solutions were prepared by diluting a solution of a higher concentration with freshly added gasoline. Each concentration was obtained at least twice and measured. All samples were stirred directly before measurements and confirmed to be homogeneous based on visual observation. The admixture solutions were measured in cuvettes.