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A Matter of Ink Evidence


Cationic Dye Capillary Electrophoresis Method Electropherograms of Pens A-J Presented on the Same Axes for Comparison PurposesEvidence from handwritten notes has been a hallmark of crime detection for a long time--but forensic technology has just made the process that much more sophisticated.

That's good news for investigators of insurance fraud, currency counterfeiting, tax evasion, and insider trading violations. Just consider a recent case involving the ink of a particular entry on a stock worksheet that was tested against the rest of the document's ink. Differences in the samples suggested that the questioned entry was made on a separate occasion, possibly to cover insider trading violations.

In their articles in this month's Forensic Science Communications, FBI research chemists James Egan, Jason Brewer, and Kristin Hagan demonstrate advances in the forensic analysis of ballpoint pen inks--both for black inks and blue inks.

Why different articles on different colors? Black and blue inks contain dye formulations that have different properties, which requires different methods to separate the dye components.

What's the usual way? A process called thin-layer chromatography (TLC) is normally used. An ink sample is spotted on a silica-gel slide and separated into its components by a solvent system. The spots can then be visualized under visible or ultra-violet light then compared to a reference library (e.g. the Secret Service's TLC library consists of ~8,500 inks). It's an important technique, however the process can't be automated. In addition, the TLC plates should be stored in a controlled environment and no spectroscopic data is obtained.

What's an alternative approach? Capillary electrophoresis (CE) has recently been used for ink analysis. A minute volume of ink (nanoliters) is injected in a narrow silica capillary filled with a buffer solution. Electrical current is then applied to the capillary to separate the ink into its components. Each component passes a photodiode array detector, which records an ultraviolet-visible spectrum. The process is automated, fast, and results can be stored electronically allowing the development of a searchable reference library. This process also detects non-dye additives in the ink that potentially can be used as identifiers.

Do you speak chemistry? It helps to be a chemist when you're reading these articles. We started to bog down when we read Figure 5 in the Black Ink article... "shows the presence of two dye peaks that are close in µep values and have spectral similarities greater than 90 percent for Crystal Violet and Rhodamine Base B. The first dye peak (tm = 3.40 min, µep = –1.08 x 10–4 cm2V–1sec–1) agrees well with the µep and ultraviolet-visible spectrum of Rhodamine Base B, whereas the second peak (tm = 3.99 min, µep = –1.41 x 10–4 cm2V–1sec–1) agrees with the µep and spectrum of Crystal Violet."

But everyone can appreciate the results and the possibilities for future applications. And what might these be? "Initial experiments suggest that the combination of the anionic and cationic dye capillary electrophoresis methods can be applied to food dyes, textile dyes, and ink-jet dyes."

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