producing a distinct electropherogram for each type of ink. The cationic method was able to separate the cationic dyes by using CTAB to suppress the ...
Forensic Gel Ink Analysis Using Capillary Electrophoresis Scott B.
1 Minchenberg ,
Keith D.
1 Zauderer , Andrew
2 O'Connell ,
Ling
1 Huang
1Department
of Chemistry, Hofstra University, Hempstead, NY 11549-1510 2Massapequa High School , Massapequa, NY 11758
ABSTRACT The recent popularity of gel ink pen usage demands rapid and reliable forensic chemical analysis techniques for the separation and identification of ink components. The complex nature of gel polymer and dye mixture makes capillary electrophoresis (CE) the instrument of choice because of its high speed and small sample size requirement. The ink was extracted directly from the pens into water, triton, ethanol or pyridine for UV-Vis analyses to determine the maximum absorbance wavelengths for optimal CE detection. Three different CE methods were developed to investigate gel ink of different brands and colors: cationic dye method (Capillary Zone Electrophoresis, CZE), anionic dye method (CZE), and micellar electrokinetic chromatography (MEKC). Gel polymer and dye molecules were effectively separated using MEKC method as the surfactant coated the polymer gel with negative charges, producing a distinct electropherogram for each type of ink. The cationic method was able to separate the cationic dyes by using CTAB to suppress the electroosmotic flow. The anionic method was able to separate the gel and anionic dyes and used cyclodextrin as a complexing reagent. Pyridine, ethanol, water and triton are being explored as solvents to extract ink directly off paper. A high sensitivity cell is being tested to detect low amount of dye from paper extracts. A library of gel pen ink electropherogram is being compiled to serve as a database for unknown ink identification.
INTRODUCTION
Figure 3 UV-VIS SPECTRA of Ink Extracts
Figure 1 Electropherograms of Blue Ink Extracts Standard s
Pilot-G2TM
Matched Unknowns Unknown #2
A. PapermateTM gel inks extracted from paper samples using 1% w/v SDS in water
PapermateTM
Unknown #1
Gel ink pens are gaining popularity because of the ease of use, the writing comfort and the diverse utility. Gel ink pens from different manufactures contains different compositions as a mixture of solvent (usually water), gel polymer matrix (i.e. polysaccharides, Figure 2C) and smaller organic dye molecules. The identification of the type of pen is crucial for forensic purposes, which can provide more evidence in crime cases. Various solvents (methanol, ethanol, pyridine, surfactants in water) are tested to provide a universal extraction technique for the gel ink samples. When the gel matrix dissolves, the dye is released allowing it to flow more freely through the capillary for analysis. CZE separates molecules based on mass to charge ratio causing the dye and gel polymer to migrate at different speeds. Based on the unique peak patterns, the dyes and essentially the ink-gel can be recognized from a database of known patterns for forensic evidence matching. Compared to HPLC, CE is cheaper to run, faster, and uses less solvent per run, which is essential as high-throughput runs are necessary to develop an extensive database for gel inks. CE is able to separate gel polymer from smaller dye molecules in one run.
METHODS
B. Pilot G-2TM gel inks extracted from paper samples using 0.5% Triton X-100
Conclusions Extraction: 0.5 % Triton (Figure 2) in deionized water proved to be the most effective and most green solution to extract gel ink off paper samples. The gel matrix appears to be a neutral polymer causing the neutral surfactant triton to bind to the ink effectively removing it from the paper. Extracted samples kept in room temperature tend to produce degraded patterns with possible gel polymer aggregations (data not shown here).
StaplesTM
Unknown #3
Extraction: A mock sample is prepared by writing on white computer paper with a gel ink pen. The ink sample is allowed to dry for 5 minutes before a round piece is punched out with a hole puncher and placed it into 1.0 mL of 0.5 % (w/v) Triton X-100 (Figure 2A) solution and sonicated in water bath for 30 minutes. The sample is then taken out to prevent paper fibers from dissolving. The sample is injected into the capillary without further dilution (anionic) or is diluted at 1:1 v/v ratio with run buffer (MEKC).
Capillary Gel Electrophoresis: CZE for anionic dyes yields consistent “fingerprints” for gel ink extracts from paper. More than one DAD wavelength must be used to make a positive identification on a specific gel ink sample due to slight variability’s between runs. Three mock written blue ink samples were successfully matched with standards to yield positive identification (Figure 1). The extraction pattern recognition and UV-Vis spectra from the extract and from CE detection also improve the identification (Figure 3).
CE Method: Micellar Electrokinetic Chromatography (MEKC) Capillary – 50 μm ID x 25 cm (to the detector) uncoated fused silica capillary Running solution – 50mM Triton in 50 mM borate buffer pH 8.5 (used Na+ salts only) Applied Voltage – 10kV keep max current at 50 μA Temperature – Kept cassette at 40 oC and tray above 35 oC Sample Solvent – Water for running buffer and triton for ink extractions Sample Conc. – 0.1-1 mg/mL Injection method – Pressure injection at 20.7 mbar for 5.00 seconds Injection volume – Less than 1% of the capillary length Detection – Absorbance at 214, BW 20 nm 421, BW 30 nm and 590, BW 20 nm Capillary Zone Electrophoresis (CZE) – for Anionic Dyes Capillary – 50 μm ID x 25 cm (to the detector) uncoated fused silica capillary Running solution – 25mM CHES with 5 mM β-cyclodextrin pH 8.80 (used Na+ salts only) Applied Voltage – 15kV keep max current at 30 μA Temperature – Kept cassette at 25 oC Sample Solvent – Water for running buffer and triton for ink extractions Sample Conc. – 0.1-1 mg/mL Injection method – Pressure injection at 25.0 mbar for 10.00 seconds Injection volume – Less than 1% of the capillary length Detection – Absorbance at 214, 20 nm 421, 30 nm and 590, 20 nm
Future Work Optimization o f the CE methods to explore other CZE and MEKC methods Building the database of gel inks of different colors and manufactures Using high sensitivity detection cell to analyze signature samples Comparing results with ball pen and fountain pen ink extracts
Triton Standard Blank
FIGURE 2
Molecular Structures
References Bryan, E'lyn. "Questioned Document Examination." Evidence Technology Magazine May-June 2010. LaPorte, Gerald. "An Evaluation of Matching Unknown Writing Inks with the United States International Ink Library." Journal of Forensic Science May 2006: 51. Becker, A. "Investigation of Ball Point Pen Inks by Capillary Electrophoresis (CE) with UV/Vis Absorbance and Laser Induced Fluorescence Detection and Particle Induced X-Ray Emission (PIXE) ." Journal of Forensic Sciences. 44.4 (1999). Egan, James. "Forensic Analysis of Black Ballpoint Pen Inks Using Capillary Electrophoresis." Forensic Science Communications. 7.3 (2005).
2A
Supports 2B Figure 2A. Triton x100; 2B. Sodium Dodecyl Sulfate; 2C. Gel Pen Polysacchrides Matrix
2C
The authors acknowledge the financial support from Hofstra University Chemistry Department. AO and LH thank the sponsorship from National Grid for the 2010 Hofstra University Summer Science Research Program. The consultation from Prof. Richard Picciochi, a retired document examiner from NYPD, is greatly appreciated.
