## 4 Conclusions  
This study has explored the capability of portable NIR spectroscopy for the identification of inorganic acids and bases through plastic containers. Although designed as an early feasibility study, the work outlined here has shown that, within laboratory conditions, near infrared is able to identify inorganic corrosive substances through closed containers with accuracy. The results have also proven that a chemometric model, such as Model 1, designed with the tools available and the data gathered with this commercial instrument, can discriminate a safe substance, such as water, from a corrosive solution, whilst not being affected by the container itself.  
On the other hand, the results have demonstrated how creating a varied and large dataset is important in the design of a chemometric model. Future work in this line of research could include the design of a spectral library that covers a range of solutions and concentrations resembling nearly all potential real case scenarios. This could then be tested against a new set of samples to determine whether a single model can be used to discriminate all potential concentrations. Furthermore, the samples prepared in this study were aqueous solutions of analytical grade inorganic acids and bases that did not include any additional component that may be present in the household products, which are commonly used in incidents involving corrosive substances. At the present, the current literature lacks detailed information on the analysis of inorganic acid and bases using Near Infrared spectroscopy. The source of misclassification observed in this study may be related to the signals that these different molecules, or their interaction with water molecules, may or may not present within the detected region. A better understanding of the spectral signature of these corrosive molecules would provide great insights on their influence in the NIR spectral, which in turn would aid in the improvement of the fitness of this method. Future method validation could include a new testing set comprised of commercially available solutions to assess whether these supplementary compounds can affect the analysis in a real-life case scenario. Once these aspects have been studied and the method is fully validated, it is expected that this technique could prove useful in the fight against crimes involving concealed corrosive substances and constitute a great advantage for police forces when analysing hazardous substances on the field.  
Conceptualization, methodology, resources, investigation, validation, visualization and writing--original draft by [PERSON]. Supervision and writing--review and editing by [PERSON] All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The data presented in this study is openly available in [[https://kcl.figshare.com/](https://kcl.figshare.com/)]([https://kcl.figshare.com/](https://kcl.figshare.com/)) at 10.18742/19316639.  
The authors would like to thank [PERSON] and [PERSON] from the Threat Mitigation Technologies team at the Metropolitan Police Service for their guidance as well as for supporting this research through the donation of relevant materials and reagents used in the study.  
Figure 4: Percentage of total True Positives (%TP), False Negatives (%FN), Misclassifications (%M) and ‘Invalid’ output, for (**a**) Model 3, using single scan data, and (**b**) Model 4, using triple scan data.