## 1 Introduction  
Recent reports in the UK have shown an increasing trend in the use of corrosive substances for different criminal activities. A survey from 2021 revealed that, between 2015 and 2017, 1187 criminal offences involving corrosive substances had been reported by eight police forces in England and Wales [1]. In addition, data collected by the Home Office as part of the 'Serious Violence Strategy' showed that \"between April 2019 and March 2020, 619 violence against the person and robbery offences that involved a corrosive substance had been recorded by the police in England and Wales (excluding Greater Manchester Police)\" [2]. These reports also highlighted that violence against the person was the predominant type, making up to 77% of the total offences [1]. This type of crime is often carried out by an individual carrying the corrosive substance in an inconspicuous plastic container (e.g., a plastic bottle), and throwing the liquid on the victim with the intention to cause injury. Considering that only 8% of the victims of these offences avoid injuries and that 35% of the products used are readily available household products [1], local and national authorities have started to evaluate measures for preventing such crimes. Strategies to limit the access to these substances in retail stores, as well as additional regulations concerning these substances in the national legislation--e.g., the Offensive Weapons Act 2019 in the UK--have been introduced. Despite this, consumers can still easily access corrosive products that are sold as common household products. Therefore, there is an urgent need to develop strategies for the on-site detection and analysis of these substances through the selection and testing of analytical techniques that can be readily used by a law enforcement on the street. A method that could prove useful for this purpose is Near Infrared spectroscopy (NIR)-based analysis. NIR is a technique of great value for analytical screening, mainly due to its speed and the fact that it requires no sample preparation [3]. Furthermore, the capability of NIR to analyse samples through glass and plastic containers [4; 5; 6; 7] is a desirable feature, particularly in scenarios where the sample to be analysed can be harmful if handled in an open container. Finally, the affordability and relative simplicity of NIR instruments have allowed the design and use of portable and handheld devices for the qualitative and quantitative analysis of a wide range of substances that have already being explored, from quality analysis in the food industry, soil analysis and even for forensic purposes [8; 9; 10; 11]. Based on this, the present study focused on evaluating the applicability of portable NIR, as an on-field test of corrosive substances. In addition to this, previous research has explored the combined use of NIR with chemometric modelling as a powerful tool for the analysis and discrimination of unknown samples [8; 9; 12]. Interpretation of raw NIR spectral data is complex and cumbersome without the aid of software and mathematical processing. Methods such as mean-centering, standard normal variate (SNV) or first and second derivative have allowed a better understanding of the information provided by the broad peaks of NIR spectra [13; 14; 15]. These methods allow the user to remove unnecessary information, noise and uncontrolled variations of the baseline, or separate overlapping peaks. Additionally, these pre-treatment methods are often combined with multivariate data analysis methods such as Partial Least Square Regression (PLSR) [16], Soft Independent Modelling of Class Analogies (SIMCA) [17] and Principal Component Analysis (PCA) [18] in order to identify the main sources of variability among the classes defined by the user and, in turn, assist in the clustering of new unknown samples. These approaches have become a standard practice when using NIR for analytical purposes, particularly in the design of supervised methods, in different scientific fields [8; 9; 12; 19; 20; 21; 22].