The increase of detection sensitivity of micellar electrokinetic capillary chromatography method of stamp pad inks components by applying a sample stacking mode for the purpose of questioned document examination.

The study focused on comparison of on-line sample concentration techniques, i.e. sample stacking modes, applied in capillary electrophoresis in order to improve the detection of components of stamp pad inks so that destruction of analysed document could be minimized. In particular, such techniques were taken into consideration as: normal stacking mode/field amplified sample stacking (NSM/FASS), high-salt concentration, sweeping, electrokinetic injection + FASS, and field enhanced sample injection (FESI). At first, two mixtures of 4 red dyes and 4 violet/blue dyes were examined. As a result, in case of red dyes two environments: ACN/1 mg mL aqueous NaCl (95:5, v/v) and 10-fold water dilution of BGE preceded by water plug (6 s, 0.7 psi) injection, were chosen as the most promising for further examination. Regarding the blue/violet dyes, the 100-fold water dilution of BGE was selected as giving the best increase of height of peaks. Ultimately, nine stamp pad inks in three colours (red, blue and violet) produced by different manufacturers were examined under the optimal conditions. Only 5 dots cut-out from inked paper were sampled. Almost all examined samples were possible to be differentiated basing on the obtained electrophoretic profiles. In conclusion, the developed sample stacking mode of MECC-based method was found as a useful tool in the differentiation of stamp pad inks for the purposes of forensic questioned document analysis.