Non-destructive identification of different types and brands of blue pen inks in cursive handwriting by visible spectroscopy and PLS-DA for forensic analysis

The analysis of inks in paper represents an important area in forensic science, since it can be used to identify possible falsifications and changes in a document. However, most of the instrumental methods proposed in the literature do not allow a nondestructive analysis of cursive writing texts, which represent a serious limitation in document examination. This paper proposes the application partial least squares for discriminant analysis (PLS-DA) and visible spectrometer measurements obtained by the Video Spectral Comparator (VSC6000®) for the nondestructive identification of blue pen inks of different types and brands in cursive handwriting texts for forensic applications. The method was developed based on twenty-five brands of inks from different blue pens. Standard samples were prepared to simulate the characteristics of the cursive handwriting. Reflectance spectra were recorded at different positions along a written line for each type and pen brand. The calibration set was optimized by the elimination of outliers and bias corrections. Two independent models were developed, the first one for identification of the pen type and the second one for brand identification. PLS-DA models presented mean prediction errors ranging from 0.03 to 0.11, which allowed the correct classification of all pen types and brands studied. The method showed to be robust in relation to different paper types and batches of pens. The analysis of a blind test indicates that the method is also free of biased judgments of the analyst and robust regarding the handwriting of different individuals. The application in a real forensic case enabled one to conclude that two ink strokes of different pages of a questioned document were written with the same pen type and brand.