Forensic Analysis of Commercial Inks by Laser-Induced Breakdown Spectroscopy (LIBS).

Flavio Cicconi,Ana Cristina Almeida Assis,Antonio Palucci,Francesco Saverio Romolo,Violeta Lazic

doi:10.3390/s20133744

Abstract

Laser-induced breakdown spectroscopy (LIBS) was tested for all of the relevant issues in forensic examinations of commercial inks, including classification of pen inks on one paper type and on different paper types, determination of the deposition order of layered inks, and analysis of signatures and toners on one questioned document. The scope of this work was to determine the potential of a single LIBS setup that is compatible with portable instruments for different types of ink analysis, rather than building a very large database for inks and papers. We identified up to seven metals characteristic for the examined inks, which allowed to fully discriminate all eight black inks on one type of printing paper. When the inks were tested on ten different papers, the correct classification rates for some of them were reduced for reasons thoroughly studied and explained. The replicated tests on three crossing points, each one involving a pair of blue or black inks, were successful in five cases out of six. In the test simulating documents of forensic interest (questioned documents), LIBS was able to correctly identify the differences in three inks used for signatures on one of the three pages and the use of different printing inks on each page of the document.

Highlights

The examination of inks and toners is very important in any criminal investigation involving documents
The first round of the measurements was performed on the 14 inks placed on the same sheet of printing paper, hereafter called paper #3, which was characterized at six different points
The experimental work here demonstrated that Laser-induced breakdown spectroscopy (LIBS) measurements using the portable apparatus allow the questioned documents to be analyzed without any sample preparation and with minimal damage

Summary

Introduction

The examination of inks and toners is very important in any criminal investigation involving documents. Forensic studies of inks mainly regard their discrimination from one sample to another and determining the deposition order (writing sequence) of intersecting pen lines on paper. A comprehensive review of the analytical techniques used for examination of questioned documents was published by Calcerrada and García-Ruiz [1], which refers about inks from pens and printers, papers and studies on intersecting lines. According to Denman et al, about of 80% of document analysis cases involve ballpoint pens [2], which contain organic soluble dyes, solvents, resins, and additives [3]. Dyes determine the ink’s color; solvents contribute to the homogenization of the ink; resins confer viscosity, lubricant properties, adhesion on paper, and durability to the ink; while additives improve the ink’s performance.

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Conclusion