Application of MECC-DAD and CZE-MS to examination of color stamp inks for forensic purposes

Questioned document examination in a wide variety of criminal and civil cases often focuses on the identification and comparison of stamp inks. Increasingly frequent counterfeiting of documents, especially those that have been stamped, has led to a growing need for instrumental methods that will allow an in-depth examination of stamp inks on a document. Therefore, the usefulness of two capillary electrophoresis techniques: micellar electrophoretic capillary chromatography with UV/Vis diode array detection (MECC-DAD) and capillary zone electrophoresis with electrospray mass spectrometric detection (CZE-ESI/MS) was verified for this purpose. Violet, red, blue and green stamp inks on paper surfaces were investigated. Electropherograms and mass spectra of inks purchased from 10 different producers were obtained in accordance with optimized and validated analytical procedures. It was shown that effective differentiation of individual inks was possible in terms of migration time and UV/Vis spectra provided that there was more than one characteristic peak. However, the most problematic inks with only one very similar (or identical) dye component - violet and blue ones - were additionally differentiated by CZE-MS. The obtained results showed that the proposed procedures can be useful for the objective analysis of questioned documents involving stamp inks. Consequently, the developed methods can be applied in the forensic field, including in the investigation of the authenticity of documents.