Classification of commercial inorganic acids used in vitriolage from fibres analysis via FTIR-PCA

Vitriolage (often referred to as 'acid attack') is a heinous crime with reported instances becoming more common, yet, literature precedence for elucidation of information pertinent to assisting vitriolage investigation is still scarce. In fact, approaches for acid type elucidation rely solely on observations on the extent of human tissue degradation with very limited research on fibres analysis for acid type elucidation. Therefore, this study presents a novel process for acid type elucidation through fibres (cotton, wool, polyester and a wool-polyamide blend) analysis which were briefly exposed to the most commonly used commercially available acids in vitriolage (sulphuric, nitric and hydrochloric) via readily accessible FTIR-PCA. Visually, samples treated with sulphuric acid could be determined from the others, however, PCA allowed for classification of all acid types with varying accuracy. Clothing comprised of wool (pure wool and wool-polyamide) allowed for 100 % classification accuracy, precision and recall across all acid types, as did all clothing types when treated with sulphuric acid. Whereas clothing comprised of cotton treated with hydrochloric acid and polyester treated with nitric or hydrochloric acid showed a decrease in accuracy values (77.5 - 85 %) due to their higher chemical resistance. The findings here present a method which may allow vitriolage investigators to refine suspected perpetrators based on their capacity in accessing specific acid types as well as a perpetrator's own unintended clothing acid exposure.