Coupling ATR-FTIR Spectroscopy and Chemometric Analysis for Rapid and Non-Destructive Ink Discrimination of Forensic Documents

The large increase of global counterfeit documents urges the need to explore novel methods for forensic examination toward efficient discrimination of forged and authentic documents. The attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy holds great promise for reliable chemical analysis. The chemometric analysis such as the principal component analysis (PCA) provides a very powerful analytical tool, especially when dealing with complicated multi-component spectral data sets. In this paper, we demonstrate a promising approach for the rapid and non-destructive discrimination of writing-pen inks using the ATR-FTIR spectroscopic technique coupled with the PCA. A variety of writing pens of different sorts were collected and the ink samples were applied onto one type of photocopier paper. The ATR-FTIR spectra of ink samples were measured triply over two spots yielding six spectra for each studied ink. The obtained FTIR spectra were compared to confirm the repeatable spectral profile. The ATR-FTIR spectral results obtained for the different inks indicate the presence of triarylmethane dye as a basic colorant in the studied writing-pen inks. Adopting the visual analysis of the FTIR spectra of the different inks, a relatively low discrimination power of only 72 % could be achieved which limits routine FTIR analysis in forensic documents examination. Interestingly, the coupling of the PCA as a multivariate chemometric technique with FTIR resulted in much improved spectral discrimination. The PCA score plot of the first two principal components (PC1 and PC2) using the spectral data of the 33 pen ink samples resulted in a significantly higher discrimination power of 100 % with all data points showing separately, indicating successful discrimination of all ink samples. Such high discrimination power is highly desirable in forensic examination of question documents to eliminate errors leading to poor ink identification due to human factors. The inks discrimination was achieved using samples of inks directly applied onto paper without any sophisticated sample preparation or processing. This ATR-FTIR/PCA analytical approach may serve as a potential method for reliable, and non-destructive forensic documents examination aimed at rapidly identifying a forgery.