Capillary Electrophoretic Fingerprint Investigation of Gel Pen Inks

Journal of Chromatographic Science 2014;52:271– 276 doi:10.1093/chromsci/bmt025 Advance Access publication April 1, 2013 Article Capillary Electrophoretic Fingerprint Investigation of Gel Pen Inks Hong Zhou1,2, Xiaotian Lv1, YuYou Sun2, Yangke Quan2 and Bao-Yuan Guo1* 1 Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China, 100085, and 2Institute of Forensic Science of Ministry of Public Security, PRC, Beijing, 100038 *Author to whom correspondence should be addressed. Email: Received 22 March 2012; revised 21 February 2013 In this study, two capillary electrophoresis modes, capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC), were employed for the examination of gel ink fingerprints. The optimized CZE conditions were as follows: running buffer, 0.2M boric acid and 0.05M sodium tetraborate; pH, 6.47; detection, 214 nm; separation voltage, 20 kV MEKC conditions were as follows: 20 mM sodium tetraborate–20 mM sodium dodecyl sulphate; detection, 214 nm; separation voltage, 20 kV. The CZE and MEKC fingerprints of 18 el pen inks from Beijing markets were analyzed and the inks were classified according to the fingerprints of the two CE modes. Introduction Contemporary inks can be classified into three major categories: carbon ink, fountain pen ink and ballpoint pen ink. Gel pen inks share characteristics of both ballpoint pen inks and fountain pen inks. They are widely used for their smooth writing. Gel pen inks used in different writing instruments or printing methods are comprised of thousands of different formulations. Currently, formulations include synthetic organic and inorganic dyes, surfactants, resins and other components. Dyes, the major components of ink, contain acidic dyes and alkaline dyes such as sulfonates, chlorosulfonates or sulfonamides (1). In addition, additives like very fine pigment dispersions of carbon and Cu-phthalocyanine enhance the permanence of these inks. Almost all formulations of inks are proprietary. Ink analysis is important in forensic science; it can reveal quite useful information for questioned documents such as insurance claims, wills, contracts and tax returns. Many researchers have explored different instrumental techniques. Chromatography and capillary electrophoresis (CE) have been explored as potential methods for ink analysis. Thin-layer chromatography (TLC) (2, 3) is a popular method for its ease and its ability to rapidly generate qualitative information through nondestructive spectroscopy. However, because its resolution is limited, highperformance liquid chromatography (HPLC) has recently been employed in this area (4 –5). Fanali and Schudel (6) reported the first CE analysis of inks in 1991. CE has been widely used in forensic analyses due to its high resolution efficiency. Generally, methods based on capillary zone electrophoresis (CZE) are used for the separation of acidic dyes, whereas basic or hydrophobic dyes are analyzed with micellar electrokinetic chromatography (MEKC). Thus far, the types of pen ink that have been examined by CE include fountain pen inks (7, 8), ballpoint pen inks (9, 10), rollerball pen inks (11, 12) and sepia inks (13). Scientists usually employ one CE mode for ink analysis. Considering that the two modes, CZE and MEKC, are based on different mechanisms and may obtain different composition information according the fingerprints of the electropherograms of inks, the present investigation intended to examine 18 gel pen inks of eight different brands coupled with CZE and MEKC. Experimental Electrophoresis Electrophoretic separations were performed on a Beckman P/ACE MDQ electrophoresis system (Beckman Coulter, Indianapolis, IN) equipped with an ultraviolet (UV) visible absorbance detector. Separations were conducted in polyimide coated fused silica capillaries of 50 mm i.d. with a total length of 50 cm (40 cm from injection to detector). Before every experiment, the capillary was rinsed with 0.1M NaOH for 20 min, followed by a 5 min rinse with water, a 2 min rinse with methanol, another 2 min with water and a rinse with buffer solution in the final 20 min. In the following analysis, good repeatability achieved by changing the running buffer every three runs, rinsing with 0.1M NaOH for 2 min and flushing for 3 min with water, followed by the running buffer for 15 min before each sample injection. All solutions, buffers and samples were filtered through a 0.45 mm nylon membrane filter. Reagents and materials Methanol, boric acid and sodium tetraborate were all of HPLC grade (Beijing Chemical Plant). For the investigative studies of the separation of ink components by MEKC, sodium dodecyl sulfate (SDS) was used (Sinopharm Chemical Reagent Co.). The pH value was determined by a DELTA 320 pH meter (Mettler-Toledo, Shanghai, China). The 18 investigated gel pen inks were purchased from local markets; they are listed in Table I. Preparation of ink extracts The original inks were drawn with syringe needles from the bottoms of the gel pens. After transferring the original ink to an Eppendorf tube (500 mL total volume), 30% methanol and 70% distilled water were added. The tube was sealed and ultrasonicated for approximately 15 min. After this, the solution was filtered through a 0.45 mm nylon membrane filter and pipetted into a clean plastic tube for storage at 48C. Results and Discussion Optimization of CZE method Detection wavelength The detection wavelength was investigated. Ink 17 (Chenguang K35 black) was used as the example for wavelength selectivity, # The Author [2013]. Published by Oxford University Press. All rights reserved. For Permissions, please email: which was selected at radium. The optimization of the detection wavelength was conducted with Ink 17; three wavelengths of 214, 254 and 500 nm were investigated. As shown in Figure 1, better signal-to-noise ratio was obtained by detecting at 214 nm. Thus, the detection wavelength of 214 nm was employed in the following study. Evaluation of separation buffers in CZE The separation buffer composition may significantly influence separation and detection. Two buffers of borate ( pH 6.47) and Tris (50 mM adjusted with HCl to pH 6.34) were tested for their suitability to separate gel pen inks in this study. It was found that inks exhibit qualitatively different resolutions in the two buffers, as shown in the electropherograms of Figure 2. Only one peak Table I Investigated Gel Pen Inks Ink Color Brand 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Black Red Black Black Blue Black Black Blue Black Black Black Red Black Black Blue Blue Black Blue Tianzhuo TG3119 Tianzhuo TG3119 Baitong BT-418 Zhencai 0916C Zhencai M-158 Zhencai 0221B Zhencai CK-126 Zhencai GP-246 Baoke PC-748 Baoke 1128 Chenqi GP-593 Chenqi GP-593 JinWanNian K-1136B JinWanNian K-1000 JinWanNian K-1136A JinWanNian K-1000 Chenguang K35 Jilifa GL-155R was detected in the Tris buffer, whereas more peaks were found in (...truncated)