PLS-DA and infrared spectroscopy based rapid and non-destructive discrimination of black ball and gel pen inks for forensic application

Abstract

In the Questioned Document, the examination of ink is crucial scientific evidence to discern the difference between genuine and forged documents. The document examination which is examined by physical and chemical analysis of ink involves the matching of the source of writing material used to fabricate the document of ink. In this study, fifty black ink samples (25 ball-point and 25 gel-point pens) are used. These ink samples are analyzed with Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy conjoined with chemometric method i.e. Principal Component Analysis (PCA) and PLS-Discriminant Analysis (PLS-DA). The spectroscopic discrimination is determined by the absence or presence of particular peaks or bands in the fingerprint region of black ink samples. The first-order derivative graphs are used for spectral characterization of ink samples. PCA reduces the dimensionality of large datasets into small number of independent Principal Components and increases interpretability but at the same time minimizes the data information loss. The utilization of the PLS-DA algorithm gives 100 % accuracy to differentiate two classes of black ink samples. The ROC curve value of the PLS-Discrimination Analysis model is 1 which means 100 % sensitivity and specificity. This non-destructive study is an analytical approach that helped to maintain the spectral library based on spectroscopic results of all fifty samples. The adequacy of the PLS-Discrimination Analysis model is checked by running a blind test of ten unknown samples.

Keywords

• Black ink
• ATR-FTIR spectroscopy
• Chemometric
• PLS-DA
• Analytical chemistry
• Forensic chemistry