Fast and Versatile Forensic Analysis of Ink and Paper with an Easily Operated Laser

The information from the forensic examination of inks and paper can be critical to investigations involving financial crimes, counterfeiting, terrorism, and anonymous letters used for threatening correspondences, ransom notes and kidnappings. Soil is another trace material that can be used to link suspects to a crime scene, although that was not a major focus of this research.

The purpose of this NIJ-supported project was to demonstrate the utility of an analytical chemistry tool that can conduct elemental analysis of ink, paper, and soil evidence quickly and with little or no sample preparation. The researchers, with Florida International University, compared four Laser Induced Breakdown Spectroscopy (LIBS) systems – three commercially available and one built by the researchers – to the more complex and costly Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) system that is used in forensic analysis.

Overall, the researchers said, the LA-ICP-MS systems have advantages in high sensitivity, minimum consumption of the sample, and the ability to directly characterize solids, but they also have very high costs and significant complexity. Although LIBS systems are “less mature” than the more complex systems, the researchers said, LIBS has the advantage of improved speed, versatility, ease of operation, affordability, and portability.

All of the systems tested work by laser ablation, which is a progressive and superficial destruction of a material by melting, fusion, sublimation, erosion, and explosion, the researchers said. The ablation produces a cloud of very small particles that are removed from the sampling cell for analysis.

A major focus of the project was to examine the effectiveness of LIBS in analyzing ink and paper. “Document related crimes are considered the most prevalent form of crime in society and the examination of ink and paper has been the focus of many criminal investigations,” the researchers said in their summary report. The traditional non-destructive methods for ink and paper examination, such as microscopic and optical techniques, are often insufficient to identify the inks used to prepare a document or to determine whether questioned pages originate from the same source, or if pages have be fraudulently replaced.

The researchers also noted that as papers and inks are constantly being changed in the marketplace, document examiners have an increased interest in finding alternative and complementary methods of analysis. For instance, they said, gel pen inks have become a prominent type of ink in forensic document examinations but present a challenge because gel inks are difficult to analyze by conventional techniques. Laser sampling works well on gel inks, they noted.

A total of 400 different types of ink were analyzed and more than 97 percent of writing inks and up to 100 percent of printing inks were correctly “discriminated” by the LIBS system.

About This Article

The research described in this article was funded by NIJ cooperative agreement number 2010-DN-BX-K179, awarded to Florida International University. The article is based on the grantee report "LA-ICP_MS and LIBS analysis of paper, inks, and soils" (pdf, 15 pages) by Jose Almirall, Florida International University.

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