Application of laser induced breakdown spectroscopy to examination of writing inks for forensic purposes

The research was focused on the analysis of writing inks using the laser induced breakdown spectroscopy (LIBS) technique. 34 samples of blue, 30 of black, and 21 of red writing inks were analyzed under optimized conditions to determine the variation of chemical composition of inks between different colours, brands and types. Nine elements (Ba, Cr, Cu, Fe, Li, Mo, Mn, Ni and W) were taken into account during comparative analysis of inks. Because of the strong effect of the paper spectrum, elements often found in inks (Ca, Al, Mg, Na, Ti, and Si) were eliminated from LIBS analysis of inks. It was determined that the LIBS method is capable of revealing qualitative elemental differences between ink samples. The discrimination power of this method was found to be 83, 82 and 61% for blue, black and red inks, respectively. Inks produced by the same producer were able to be differentiated in some cases. The results showed the potential of LIBS for forensic purposes as an effective and robust technique, requiring a small amount of sample and giving analytical information in a very short time.