## 3 Results
### Model Testing  
Model 1 was tested against a new set of samples that included samples of corrosive substances contained in three types of bottles (refer to Section 2.4) as well as negative samples of soft-drinks, water and empty bottles (i.e., safe substances).  
The results obtained are summarised in Tables 2 and 3.  
Data gathered at this stage from the positive samples showed that Model 1 had a promising performance, with a high proportion of TP when testing the high concentration positive samples. The performance of the model appeared to slightly decrease as the concentration of the sample was reduced resulting in a small increase in FNs; however, no incidences of corrosive substances classified as a different corrosive were found. The performance was observed to remain unchanged across the three types of containers, indicating that the container material has minimal or no influence on the performance of the method. The only exceptions were hydrochloric acid and sodium hypochlorite. While the performance of Model 1 with concentrated samples of hydrochloric acid (37%) was similar to that observed for sulfuric acid, sodium hydroxide and ammonium hydroxide, with diluted samples (10%), the proportion of FN was somewhat higher than those observed with  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Solution** & **TP** & **FN** & **M** \\ \hline HCl 37\% & 261 & 38 & 1 \\ HCl 10\% & 198 & 95 & 7 \\ NaOH 50\% & 286 & 14 & 0 \\ NaOH 12\% & 229 & 71 & 0 \\ NH\({}_{4}\)OH 30\% & 278 & 22 & 0 \\ NH\({}_{4}\)OH 10\% & 195 & 103 & 2 \\ H\({}_{2}\)SO\({}_{4}\) 30\% & 268 & 32 & 0 \\ H\({}_{2}\)SO\({}_{4}\) 15\% & 226 & 74 & 0 \\ NaHCO\({}_{3}\) 4\% & 149 & 148 & 3 \\ NaHCO\({}_{3}\) 1\% & 200 & 100 & 0 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Classification of corrosive solutions scanned through the walls of transparent and coloured PET and HDPE bottles using Model 1. TN = True Positives, FN = False Negatives, M = Misclassification.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Solution** & **TN** & **FP** \\ \hline Empty & 300 & 0 \\ Water & 287 & 13 \\ Lucozade & 296 & 4 \\ Fruit shoot & 292 & 8 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Classification of safe substances scanned through the walls of PET and HDPE bottles using Model 1. TN = True Negatives and FP = False Positives.  
the concentrated solution. Moreover, samples of sodium hypochlorite proved challenging, with the proportion of FN being greater in the case of the concentrated samples than for the diluted ones, a behaviour that was not observed with the other corrosive substances at this stage. When studying the overall performance of this model, the sensitivity reached a value of 0.73, in other words 73% of the positive samples were correctly identified using this model.  
Samples of water, soft drinks, as well as empty plastic bottles (i.e.,'safe' samples), were scanned in a similar fashion as the positive samples to serve as negative controls and testing of Model 1 was carried out. Empty bottles gave no FP rather giving a 'null result' in all scans. This evidence showed that the plastic containers used in this study have a negligible effect on the overall spectra of the sample and confirmed one of the main benefits of NIR spectroscopy. In addition to this, the samples of soft drinks also proved successful, with only a small fraction of the samples--the highest incidence was 8 out of 100 samples-giving a FP result. These results provide further support to the capability of the developed model to correctly discriminate a safe substance from any of the five corrosives defined within the data library.