Raman hyperspectral imaging and a novel approach for objective determination of the order of crossing ink lines.

The determination of the chronological order of crossing ink lines is a difficult problem in the examination of Questioned Documents. Modern methodologies are based on non-destructive chemical and physical analysis. However, the determination of the chronological sequence of crossing ink lines remains visual and subjective. This manuscript describes a study where Raman hyperspectral imaging is associated with different chemometric techniques and a novel method is applied for the objective determination of the chronological order of gel pen ink crossings. Eight blue and black gel pens of different brands were used to prepare 56 crossings on white paper. Raman mapping analyzed the entire crossing and the hyperspectral images were treated with k-means, Multivariate Curve Resolution - Alternating Least Squares (MCR-ALS) and Partial Least Squares - Discriminant Analysis (PLS-DA). The order of crossings was determined visually and using a novel method that consisted in calculating the relative quantity of pixels of each ink in the crossing region that were equal or higher than an established threshold. Different thresholds were tested using mean and standard deviation. The MCR-ALS and the thresholds mean minus 1,96 of the standard deviation and the mean minus 2575 of the standard deviation determined correctly more than 70% of the crossings. This method proved to be more efficient than microscopy examination and more objective since no inconclusive determinations were obtained. The methodology is promising for entirely objective and highly effective determination of the chronological sequence of crossing ink lines.