In the process of examining fraudulent documents ink analysis is a small but important part of the operation of forensic laboratories. Systematic approaches to ink comparison and identification have been performed by optical methods and various chromatographic techniques.
Capillary electrophoresis (CE), a relatively new separation technique with very high resolution power, and Particle Induced X-Ray Emission (PIXE) were used for the analysis of ball point pen inks. In comparison to water soluble fountain pen inks, ball point inks are less soluble or insoluble in water and these inks contain only few components. The study focused on the optimization of the separation of ink extracts from paper material of commercially available inks with respect to resolution and analysis time. During the method development process different buffers, organic modifiers, and surfactants were tested. Good results were obtained with a 50 mM borate buffer pH 9.0 containing 50% acetonitrile. Reproducible extraction procedures as well as separations enables one to perform the quantification of the ink peaks within 1–8% standard deviation for parallel extractions of the same ink. Electropherograms of 20 inks of various origin showed patterns which were in most cases distinctly different from each other.
PIXE measurements with an external proton beam were used to determine the metal composition. The ratio of the peak areas for copper and zinc as well as differences in the elemental composition could be used to distinguished between the samples.
No coincidence was observed between samples hardly distinguishable by electrophoretic separations and by PIXE-measurements. Samples with nearly identical metal composition showed different peak pattern in the electropherograms, and nearly identical electrophoretic behavior of two or more samples was accompanied by quite different copper/zinc-ratios or supplementary metals identified by PIXE.