Writing instruments inks: microspectrophotometry forensic analysis and characterisation

Abstract

An important aspect in the analysis of written documents is the type of materials used in questionable documents. The present study aims to characterise and create a database of the absorption spectra in the visible region, obtained by microspectrophotometry (in reflectance mode), of inks from blue and black writing instruments, such as ballpoint pens and liquid ink pens (rollerball pens, gel pens, felttip pens and fountain pens). The study was performed with 167 ink samples of 36 different brands commonly used in national and international markets. To validate the possible use of the database a preliminary blind test with 22 samples yielding a consistent and accurate match of 13 samples revealed that this technique has a good potential to obtain a list of inks with the same spectral characteristics. To evaluate the differentiation level of this method the samples were grouped based on the overlap of the 1st derivative spectra. As this grouping systematisation was found to present some limitations when we have a large number of samples, a multivariate analysis of the data was made. For this, a hierarchical cluster analysis (HCA) was performed. The discrimination power was calculated and compared with another works.