## 2 Materials and Methods
### Data Acquisition  
The data in this study was acquired using a Bruker high-field 600 MHz NMR spectrometer (Bruker, London, UK) with a 5 mm broadband inverse diameter probe. The Icon NMR software (Version 3.0) was used to set the NMR experiments and control the data acquisition. The NMR experiment was a simple single-pulse sequence (zg30 from the Bruker library) for (1) neat petrol, and (2) a second dataset was acquired in cyclohexane with a solvent suppression pulse sequence (NOESY) for the evaporated (due to limited volume) and burnt petrol samples. A pulse sequence program (selfdigpzs from the Bruker library) was used for the acquisition of \({}^{1}\)H sel (selective) TOCSY. Data were collected with 64 k points as the size of the free induction decay (fid), with a spectral width of 20.0 ppm, a mixing time of 0.06 s, an acquisition time of 2.7 s, a pre-scan delay of 6.5 s and a minimum of 16 scans for the neat petrol samples. The acquisition parameters were based on the default pulse sequences in the Bruker library. \({}^{1}\)H selTOCSY was performed on the following bands of chemical shifts: 4.65-4.72 ppm (olefin set 1), 4.73-4.85 ppm (olefin set 2), 4.95-5.10 ppm (olefin set 3) and 5.10-5.35 ppm (olefin set 4). The couplings were resolved and used for the assignment of the chemical species. The four discriminative sets of olefins were identified as 3-methyl-1-buttene by irradiating the signal at 4.64-4.72 ppm, a mixture of 3-methyl-1-buttene and 1-pentene by irradiating the signal at 4.73-4.85 ppm, 2-methyl-2-buttene by irradiating the signal at 4.95-5.10 ppm and a mixture of cis- and trans-2-pentene by irradiating the signal at 5.10-5.35 ppm. For the double-blind study, the exhibits were analyzed using headspace-ATD-GC-MS using a Tenax TA sorbent sampling  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{(1) Summary table of the neat petrol samples used for the double-blind study.} \\ \hline
**Blind Sample Name** & **Class** \\ \hline BLIND A & Jet \\ \hline BLIND B & Esso I (from regions) & different \\ \hline BLIND C & Esso II \\ \hline BLIND D & Esso III \\ \hline BLIND E & Texaco I \\ \hline BLIND F & Texaco II \\ \hline BLIND G & Shell I \\ \hline BLIND H & BP M \\ \hline BLIND I & Shell II \\ \hline BLIND J & BP S \\ \hline \hline \multicolumn{3}{c}{(2) Summary table of the evaporated, burnt, and burned on substrate petrol samples used for double-blind study.} \\ \hline
**Blind Exhibit Name** & **CLASS** & **Weathered Status** \\ \hline BLIND EXHIBIT A & BPM & Evaporated 50\% \\ \hline BLIND EXHIBIT B & BPM & Cardboard Substrate \\ \hline BLIND EXHIBIT C & JET & Burnt \\ \hline BLIND EXHIBIT D & JET & Evaporated 25\% \\ \hline BLIND EXHIBIT E & ESSO & Evaporated 25\% \\ \hline BLIND EXHIBIT F & SHELL & Cardboard Substrate \\ \hline BLIND EXHIBIT G & SHELL & Burnt \\ \hline BLIND EXHIBIT H & TexACO & Burnt \\ \hline BLIND EXHIBIT I & TexACO & Evaporated 25\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary table of the blind sets of (1) neat petrol samples and (2) combinations of evaporated, burnt and fire debris petrol samples.  
tube. A 1 mL headspace was taken from within the packaging after a period of incubation at around 100 \({}^{\circ}\)C. The interpretation of the results was based on pattern recognition and comparing the chromatography results obtained from evidential items with the standard references. Where possible, comparison against a reference of the relevant liquid was preferable, but if not possible, the sample was compared to the laboratory reference database or the published literature.