Analysis of Writing Ink Dyestuffs by TLC and FT-IR and Its Application to Forensic Science

Transcription

1 ANALYTICAL SCIENCES APRIL 1998, VOL The Japan Society for Analytical Chemistry 269 Analysis of Writing Ink Dyestuffs by TLC and FT-IR and Its Application to Forensic Science Kazuhiro TSUTSUMI* and Kazuya OHGA** *Criminal Investigation Laboratory, Oita Prefecture Police Headquarters, Otemachi, Oita 870, Japan **Department of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu, Oita , Japan Black, blue and red writing pens were classified into various groups using the R f 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. Keywords Forensic science, ink, dyestuff, thin-layer chromatography, Fourier transform infrared spectroscopy 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 reagents In 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 Table 1 Number of pens used in this work and the number of groups into which the pens were classified by the present TLC analysis Pen Mnemonic symbol Number Number of group Oil-based ball-point pen Black BP-Bk 16 9 Red BP-R 10 8 Blue BP-Bu 9 5 marking pen Black OM-Bk Red OM-R Blue OM-Bu Aqueous roller-ball pen Black WB-Bk 6 3 Red WB-R 6 5 Blue WB-Bu 6 5 marking pen Black WM-Bk Red WM-R Blue WM-Bu 18 11

2 270 ANALYTICAL SCIENCES APRIL 1998, VOL. 14 without further purification. TLC TLC was carried out using a Merck precoated silicagel 60 F 254 backed with an aluminum sheet and a RP-18 F 254s backed with a glass sheet. Chloroform solutions were applied to the TLC plates for the analyses of oilbased ball-point pen inks and oil-soluble dyestuffs. Aqueous solutions were used for water-soluble dyestuffs and the other inks were directly spotted with their refills. All of the spots were mm in diameter, and the amounts of the inks and reference dyestuffs applied were mg. The origins were at 1.0 cm from the bases of the plates. The developing solvents used were ethyl acetate/ethanol/water (14:7:6), ethyl acetate/methanol/28% aqueous ammonia (5:2:1) and trichloroethylene/1,1,1-trichloroethane/ethyl acetate (10:1:1) for the normal-phase mode chromatography, and acetonitrile/3% aqueous KBr (7:1) for the reversed-phase mode; they were allowed to creep up to the plates a distance of 6.0 cm. The uniform origin position on the plate, proper loading at the origin and the solvent saturation in the developing tank, which had been recommended by Lewis 4, were kept during all of the TLC analyses in order to obtain a high reproducibility. Microsampling A procedure devised by Golding and Kokot 11,12 for dyestuff extraction from a filament was modified in order to accommodate to an imitative microsampling experiment in this work. A fraction of a line (ca. 3 mm long) drawn on filter paper (Toyo Roshi #2) was put in a haematocrit glass capillary (1 mmf 75 mm) together with 5 mm 3 of a given solvent. The capillary was sealed, and then sonicated in a Yamato 1210 ultrasonic bath at 60 C for 5 to 10 min. The use of filter paper was grounded on the fact that suspicious letters are usually written on paper made from wood pulp. An effective extraction solvent was searched by paper chromatography according to the criteria that it should leave no colors due to the dyestuffs at the starting point, give large R f values to the dyestuffs and have a high volatility. The solvents examined were methanol, ethanol, 2- propanol, aqueous ammonia, acetic acid, pyridine, DMF, THF, acetone, chloroform, acetonitrile, toluene, ethyl acetate and a mixture of two of these. On the normal-phase TLC plate was applied a dot of an extract in the above-mentioned capillary tube, whose tip had been previously stretched with a small flame and cut off. FT-IR FT-IR spectra were obtained with a Nicolet System 710 spectrometer. Among the dyestuff bands obtained from aqueous pen inks through the above-mentioned microsampling/tlc procedure, the desired one was scraped up from the normal-phase TLC plate onto a filter of 0.45 mm pore size (Millipore Sumplep HV4), and then extracted with ca. 40 mm 3 of water. The extract was dried up on a 0.1-mm hydrophobic perfluorinatedpolymer film, with which a stainless-steel mirror was coated. This type of drying procedure has been named pin-point condensation by Ikeda and Uchihara. 10 The residue was subjected to microscope/ft-ir in the reflectance mode. The analysis was performed while receiving the aid of a Spectra-Tech IR-PLAN microspectrometer equipped with a mercury-cadmiumtelluride detector. The resolution was 4 cm 1 and the scanning was performed 256 times. The spectra of reference samples were taken without the attachment in the transmission mode using the KBr tablet. HPLC HPLC was performed with a Hitachi L-6000 equipped with an Ohtsuka Denshi MCPD-3600 photodiode array detector. The column was a Kagakuhin- Kensa Kyokai L-column ODS (4.6mmf 250 mm) and the eluent was acetonitrile/aqueous 3% KBr (7:1). Results and Discussion TLC There are many reference books 13 and reports 1 4 on the TLC of inks and dyes, in which proper developing solvents have been noted, principally for normal-phase silica-gel plates. As a result of a thorough examination made for most of those solvents with the Merck normal phase plate, the one reported by Brunelle and Pro 2, ethyl acetate/ethanol/water (14:7:6, abbreviated as the B.P. solvent hereafter), was found to provide high degrees of dye separation to most of the present inks. The results obtained for oil-based and aqueous red marking pens with the B.P. solvent are typically summarized in Tables 2 and 3, respectively. The relative standard deviations for the R f values were below 3.1% for 5-times repeated analyses of the same samples. Frequently observed overlapping of dyestuff bands was confirmed through their separation with ethyl acetate/methanol/28% aqueous ammonia (5:2:1) or a less polar solvent, trichloroethylene/1,1,1-trichloroethane/ethyl acetate (10:1:1). Less-resolved bands having high R f s were separated into more bands with the less-polar solvent; for example, the R1 dye band having R f of 0.96 in Table 2 yielded four bands having R f s of 0.53, 0.47, 0.39 and Also, the use of aqueous 3% KBr instead of the water appreciably improved the tailing bands, such as the Y24 and Bw4 bands in Table 3, which were probably due to ionic dyes. This modified solvent system only slightly affected the R f s and color tones of the other bands. Classification The R f values and color tones of the bands separated by the normal-phase TLC analysis using the B.P. solvent permitted us to classify the writing pens into various groups, as listed in Table 1. Thus, standard TLC tables, such as Tables 2 and 3, appear to be available

3 ANALYTICAL SCIENCES APRIL 1998, VOL Table 2 TLC of oil-based red marking pens Dyestuff band a Marking pen e Number b Color c R f Y1 yellow 0.96 R1 yellowish red 0.96 R2 red 0.91 R3 red 0.91 Y3 yellow 0.89 Y4 yellow F 0.89 Y9 orange T (0.79) d R9 yellowish red 0.68 R12 yellowish red 0.64 Y15 yellow 0.62 R15 yellowish red F 0.61 R21 yellowish red F 0.46 R22 pink F 0.46 Y24 yellow T (0.11) d a. Obtained on a Merck silica-gel 60 F 254 plate using ethyl acetate/ethanol/water (14:7:6) as the developing solvent. b. Serial numbers assigned to dyestuff bands in each color series in order of magnitude in R f. Yellow and red series are abbreviated as Y and R. For typical color series, see Tables 4 and 5. c. Colors observed on the TLC plates. Superscripts F and T represent fluorescent and tailing bands, respectively. d. R f of the tailing band obtained with ethyl acetate/ethanol/aqueous 3% KBr (14:7:6). e. Pens are, in numerical order, Pentel N50, Mitsubishi A-50, PIN-10 and PA-121T, Sakura PK, PGK and YK, Pilot M-10EF, Tombow OS-FF1, Konishi and , Lion , Zebra 150-MC-R, Kokuyo PM-41, and Shachihata POM-2A and K-36T. Marks and represent dark and light colors, respectively, and blank spaces express the lack of bands. Table 3 TLC of aqueous red marking pens Dyestuff band a Marking pen e Number b Color c R f Y7 reddish yellow 0.85 R5 yellowish red F 0.77 R7 purplish red 0.71 R8 yellowish red F 0.69 Y13 yellow 0.69 R15 yellowish red F 0.61 Y17 orange F 0.58 R16 purplish red F 0.58 Y19 yellow 0.52 R20 yellowish red 0.50 Y22 orange 0.47 Y23 yellow 0.46 R21 yellowish red F 0.46 R25 pink 0.26 Bw4 brown T (0.19) d Y24 yellow T (0.11) d a, b, c and d. See the footnotes a, b, c and d in Table 2, respectively. Bw is the abbreviation of the brown series. e. Pens are, in numerical order, Pentel S520, Mitsubishi OFD-20BOXY and MyT-7, Sakura AK and WK, Pilot S-10PP, Tombow WS-100FA, WP-FN and WM-480P, Konishi , Lion P-55, Zebra MWS-101-R, MWS-102-R and MWS-100F-R, Kokuyo PM-3, Shachihata PM-20B, K-803 and K-210, and Platinum SPM-150N and FE Marks and represent dark and light colors, respectively, and blank spaces express the lack of bands. for a preliminary examination of pens used in crime scenes. The incomplete pen identification was due to the presence of pens whose chromatograms were the same as one another in the separation pattern: for example, #1, #5, #10 and #12 of the oil-based red marking pens listed in Table 2. It should be noted here that indistinguishable pens were not differentiated, even if they were analyzed with different developing solvents and/or in reversed phase mode. A complete identification thus requires an additional analysis, such as

4 272 ANALYTICAL SCIENCES APRIL 1998, VOL. 14 Table 4 Red TLC band series of ink dyestuffs Red band Pen Number a Color b R f Type c Manufacturer d R1 yellowish red 0.96 OM-R Pe, Sa, Uc, Li R2 red 0.91 OM-R Pe, Sa, Uc, Li, Ko R3 red 0.91 OM-R Mi R4 red 0.90 OM-Bk Pe, Mi, Sa, Pi, To, Uc, Li, Ze, Sh R5 yellowish red F 0.77 WB-R Pe, Mi, Sa, Pi, Ze, Ko WM-R R6 pink 0.71 OM-Bu Uc R7 purplish red 0.71 WM-R Mi R8 yellowish red F 0.69 WB-Bk Ko Pe, Mi, Sa, Pi, To, Uc, Li, Ze, Sh, Pl WB-R Pe, Mi, Sa, Pi, Ze, Ko WM-Bk Pl WM-R Pe, Mi, Sa, Pi, To, Uc, Li, Ze, Sh, Pl R9 yellowish red 0.68 OM-R Sa R10 red 0.65 WM-Bk Mi R11 pink F 0.64 BP-R Pi, Sh, Pl R12 yellowish red 0.64 OM-R Sa R13 yellowish red F 0.63 BP-R To R14 purplish red 0.61 WM-Bk To, Sh R15 yellowish red F 0.61 BP-R Pe, Mi, Pi, Sh, Oh, Pl, To, Ze, Ko OM-R Mi, Sa, Pi, To, Uc, Ze, Ko, Sh WM-R Ko R16 purplish red F 0.58 BP-R Mi, Oh WB-R Pe, Mi, Sa, Pi, Ko WM-R Pe, Mi, Sa, Pi, To, Sh, Pl WM-Bu Sh R17 red 0.57 OM-Bk Sa, Li R18 yellowish red 0.56 WB-R Ze R19 red 0.52 WB-Bk Pe, Mi, Sa, Pi, Ze WM-Bk Uc, Ze R20 yellowish red 0.50 WM-R Ze, Sh R21 yellowish red F 0.46 BP-R Ze OM-R Mi, Sa, Pi, To, Ze, Sh WM-R Ko R22 pink F 0.46 BP-R Pe, Mi, Pi, To, Ko, Sh, Oh, Pl BP-Bu Pe OM-R Ko R23 purplish red F 0.44 WB-R Pi WB-Bu Mi WM-Bk To, Mi WM-Bu Pi, To R24 red 0.40 WM-R Li, Pl R25 pink 0.26 WM-R Ze, Sh a. Red bands abbreviated as R are numbered in order of magnitude in the R f. b. See the footnote c in Table 2. c. See Table 1. d. Pentel, Sakura, Mitsubishi, Pilot, Tombow, Lion, Zebra, Kokuyo, Shachihata, Ohta, Platinum, Saler and Konishi are abbreviated as Pe, Sa, Mi, Pi, To, Li, Ze, Ko, Sh, Oh, Pl, Sl and Uc, respectively. quantitative HPLC, which is mentioned later. The occurrence of the same separation patterns is possibly attributed to the situation that plural products from one manufacturer contain common dyestuff components, and ink compositions depend on the limited raw dyestuffs provided by dyestuff makers, which are smaller in number compared to writing-pen makers. All of the dyestuff bands were divided into seven color band categories: 26 yellow, 25 red, 24 blue, 10 purple, 2 green, 4 black and 4 brown bands. The red and purple series are typically shown in Tables 4 and 5, respectively, where the dyestuff bands are numbered serially and the types of pens are affixed together with the manufacturers. These color series tables are useful for narrowing questioned inks down to limited candidates. In addition, all of the chromatograms were stored in the form of digital color images in library disks for a future visual collation with those from questioned documents. Identification of dyestuffs Some dyestuff components were identified by TLC analyses using the four developing solvents in the two modes described in the Experimental section. The R2 band in Table 2 was due to Color Index (C.I.) Solvent Red 18. Nigrosine was a major component of most oilbased black felt-tip marking pens and a Tombow BC- JN black ball-point pen. Most oil-based red ball-point pens contained R15 and R22 (Table 4); the R22 dyestuff was Rhodamine B Base. A yellow band of R f 0.41 (R f s mentioned in this section are the ones obtained in the normal-phase mode with the B.P. solvent) for a Kokuyo PR-P2 red ball-point pen was due to Tartrazine. A blue band dyestuff of R f 0.60 was C.I. Solvent Blue 5, which was a major dyestuff of many oil-based blue pens. The P2, P5 and P6 bands given in Table 5 were due to Tetramethyl-p-rhosaniline, Methyl Violet and Crystal Violet, respectively, major dyestuff components of the black ball-point pens, except for the above-mentioned Tombow pen. Some of the black pens contained C.I. Acid Yellow 42 or Metanil Yellow. It should be emphasized here that a determination of the content ratios of the above three purple-series dyes (P2, P5 and P6) by HPLC (not shown) permitted us to identify all of the black ball-point and aqueous marking pens. Some of the aqueous black pens contained deeply green C.I. Direct Black 154 having a R f of The fluorescent R8 and R16 bands given in Table 4 were due to Eosin and Phloxine B, respectively. These were major dyestuffs of most aqueous red pens, which enabled us to specify the red pens through a determination of their content ratios by HPLC. A major dye of many aqueous blue pens was Brilliant Blue-FCF of R f Application of microsampling The microsampling depends primarily on the selection of effective extraction solvents. Among the extrac-

5 ANALYTICAL SCIENCES APRIL 1998, VOL Table 5 Purple TLC band series of ink dyestuffs Purple band Pen Number a Color b R f Type c Manufacturer d P1 reddish purple 0.81 BP-Bu Pe P2 reddish purple 0.54 BP-Bk Pe, Mi, Sa, Pi, Li, Ze, Ps, Ko, Sh, Oh, Sl, Pl BP-Bu Mi, Pi, To, Ko, Oh WM-Bk Sa, Ko, Sh P3 violet 0.53 BP-Bu Pe, Ze WB-Bk Ko P4 purple 0.51 WM-Bk Pe, Li, Ma, Pl P5 purple 0.50 BP-Bk Pe, Mi, Sa, Pi, Li, Ze, Pu, Ko, Sh, Oh, Sl, Pl, Bc BP-Bu Mi, Pi, To, Ko, Oh OM-Bu Ko WM-Bk Sh P6 violet 0.47 BP-Bk Pe, Mi, Sa, Pi, Li, Ze, Pu, Ko, Sh, Oh, Sl, Pl, Bc BP-Bu Mi, Pi, To, Ko, Oh OM-Bu Ko WM-Bk Sa, Ko, Sh WM-Bu Pe P7 violet 0.47 WM-Bu Ze P8 violet 0.43 WB-Bu Pe, Ko WM-Bk Pe, Pi, To, Li, Ma, Pl WM-Bu Mi, Sa, Pi, Li, Ze, Sh, Pl P9 violet 0.39 WB-Bk Ko P10 violet T WM-Bk Sh a. Purple bands abbreviated as P are numbered in order of magnitude in the R f. b. See the footnote c in Table 2. c. See Table 1. d. See the footnote d in Table 4. Ma, Pu and Bc in the Manufacturer is the abbreviations of Marby, Plus and Bic. tion solvents examined, DMF gave the largest paperchromatographic R f values to aqueous ink dyestuffs, but was hard to evaporate. Pyridine was judged to be the most favorable for the extraction of all oil-based inks, while it was unsuitable for some aqueous inks. In such unfavorable instances, a mixture of ethanol and 28% aqueous ammonia (1:1) was adopted as an alternate solvent, because the mixture gave comparatively good results to them. Two pens were selected at random from each of the 12 types (Table 1), and their inks were extracted with the corresponding suitable solvents. The extracts were subjected to TLC analysis according to the procedure described in the Experimental section. The chromatograms obtained with the B.P. solvent were almost the same as those in the standard library, proving validity of the present microsampling procedure. Discrimination of dyestuffs by FT-IR Figure 1C typically shows the reflectance spectrum of a trace of a P8 dyestuff (Table 5) isolated from a 3-mm line of a Pilot aqueous black marking pen. Included in Fig. 1 for a comparison are the transmission spectra of references P6 (A) and P8 (B) dyestuffs, which were isolated through TLC of aqueous black marking pens with the ethyl acetate/methanol/28% aqueous ammonia developing solvent. The P6 and P8 present in many black and blue pens had R f values close to each other, as can be seen in Table 5, and also were not distinguished from each other by a comparison of their color tones. The P8 micro-reflectance spectrum was in agreement with the reference spectrum, being distinguishable from the P6 reference spectrum. This finding Fig. 1 Transmission FT-IR spectra of purple-series dyestuffs P6 (A) and P8 (B), and the micro-reflectance spectrum of the P8 (C).

6 274 ANALYTICAL SCIENCES APRIL 1998, VOL. 14 suggests that the present combination of reflectancemode microscope/ft-ir and pin-point technique is effective for discriminating extremely small amounts of dyestuffs. References 1. H. Harada, J. Forensic Sci. Soc., 28, 167 (1988). 2. R. L. Brunelle and M. J. Pro, J. Assoc. Off. Anal. Chem., 55, 823 (1972). 3. R. N. Totty, M. R. Ordidge and L. J. Onion, Forensic Sci. Int., 28, 137 (1985). 4. J. A. Lewis, J. Forensic Sci., 41, 874 (1996). 5. A. Zeichner and B. Glattstein, J. Forensic Sci., 37, 738 (1992). 6. I. R. Tebbett, C. Chen, M. Fitzgerald and L. Olson, J. Forensic Sci., 37, 1149 (1992). 7. H. W. Moon, J. Forensic Sci., 25, 146 (1980). 8. K. Tsutsumi and K. Ohga, Anal. Sci., 12, 997 (1996). 9. S. Fanali and M. Schudel, J. Forensic Sci., 36, 1192 (1991). 10. M. Ikeda and H. Uchihara, Polyfile, 29, 32 (1992). 11. G. M. Golding and S. Kokot, J. Forensic Sci., 35, 1310 (1990). 12. S. Suzuki, Y. Higashikawa, T. Kishi and Y. Marumo, Reports of NIPS, 44, 50 (1990). 13. For example, Eisei Shikenhou/Chukai (Standard Methods of Analysis for Hygienic Chemists/Commentary, in Japanese), ed. The Pharmaceutical Society of Japan, p. 530, Kaneharashuppan, Tokyo, (Received September 1, 1997) (Accepted October 29, 1997)