Various writing implements have often been used at crime scenes in Japan. One of the purposes for the examination of writing ink is to specify writing implements. A number of papers have been published regarding techniques for forensic examinations, including visible spectrophotometry 1,2 , thin-layer chromatography (TLC) 1-4 , X-ray microanalysis 1 , microspectrophotometry 3,5 , high-performance liquid chromatography (HPLC) 6 , gel electrophoresis 7 and capillary zone electrophoresis. 8,9 TLC is the simplest of those methods, and is effective for separating dyestuff components. A systematic TLC study, however, has been pursued only slightly concerning the analysis of Japanese writing inks. In the present paper we describe a preliminary preparation of a standard TLC library for inks, and also discuss the effectiveness and limitation of TLC for the identification of writing implements. We also report on a microsampling technique, that is often requested in the analysis of inks on questioned documents, and the application of a pin-point condensation technique 10 to the identification of dye TLC bands by microscope/Fourier transform infrared spectroscopy (FT-IR).
Experimental
Pens and reagentsIn this work were used 161 kinds of black, red and blue pens, which were classified for convenience into four groups, as shown in Table 1: oil-based ball-point and marking pens as well as aqueous roller-ball and marking pens. They are now on the market, and some of them were kindly supplied by their manufacturers. Reference dyestuffs were obtained from manufacturers and the Identification Reference Data Center, National Police Agency, Tokyo, Japan. Water was purified by reverse osmosis (Millipore Milli RO15) and deionization. Developing organic solvents for TLC were of reagent grade, and were distilled before use. All other chemicals were also of reagent grade and were used 269 ANALYTICAL SCIENCES APRIL 1998, VOL. 14 1998 © The Japan Society for Analytical Chemistry Black, blue and red writing pens were classified into various groups using the Rf values and color tones of dyestuff bands separated by normal-phase thin-layer chromatography (TLC) of their inks. The classification is effective for the preliminary identification of pens used in crime scenes. A microsampling technique was proposed for the TLC analysis of minute quantities of inks on questioned documents. Furthermore, a combination of reflectance-mode microscope/Fourier transform infrared spectroscopy and the pin-point condensation technique was proved to be useful for the precise discrimination of trace amounts of analogous water-soluble dyestuffs on TLC plates.
Analysis of Writing Ink Dyestuffs by TLC and FT-IR and Its
