Forensic Analysis of Laser Printed Ink by X-ray Fluorescence and Laser-Excited Plume Fluorescence

Abstract

We demonstrated a minimally destructive two-tier approach for multielement forensic analysis of laser-printed ink. The printed document was first screened using a portable-X-ray fluorescence (XRF) probe. If the results were not conclusive, a laser microprobe was then deployed. The laser probe was based on a two-pulse scheme: the first laser pulse ablated a thin layer of the printed ink; the second laser pulse at 193 nm induced multianalytes in the desorbed ink to fluoresce. We analyzed four brands of black toners. The toners were printed on paper in the form of patches or letters or overprinted on another ink. The XRF probe could sort the four brands if the printed letters were larger than font 20. It could not tell the printing sequence in the case of overprints. The laser probe was more discriminatory; it could sort the toner brands and reveal the overprint sequence regardless of font size while the sampled area was not visibly different from neighboring areas even under the microscope. In terms of general analytical performance, the laser probe featured tens of micrometer lateral resolution and tens to hundreds of nm depth resolution and atto-mole mass detection limits. It could handle samples of arbitrary size and shape and was air compatible, and no sample pretreatment was necessary. It will prove useful whenever high-resolution and high sensitivity 3D elemental mapping is required.

Atomic spectrometry update. Review of advances in the analysis of metals, chemicals and functional materials

“…The forensic analysis of laser printed ink by XRF and plume laser excited atomic uorescence (plume-LEAF) for the determination of ink origin and illicit overprinting was presented by Chu et al 96 Four branded black toner samples were selected that are widely used and similar in composition. Text of various font sizes was printed onto standard copier paper for analysis.…”

Portable X-Ray Fluorescence Spectrometry: Principles and Applications for Analysis of Mineralogical and Environmental Materials

“…Due to its ability to provide reliable qualitative and quantitative information about the composition of materials allied with its nondestructive character, XRF has been used in many different fields, such as geology [1], petrology [2], mineralogy [3], archaeology [4], agriculture [5], environmental remediation [6], forensics [7], art conservation [8].…”

Elemental analysis of Chinese red seal inks on xuan paper by ArF laser-induced plume fluorescence

“…We have used pXRF to analyze laser printed black toners on photocopier paper and Chinese black ink on xuan paper [14,15]. We showed that it is a good screening tool when the analyte elements are abundant (percent range) and when the specimen is optically thick.…”

“…However, for the elemental analysis of seal imprints on artwork the inked area is likely to be sparse; and for authentication purposes, the signature elements are probably present at low concentrations. In such cases, we have shown that pXRF is best supplemented by an analytical technique that we named plume-LIF, or PLIF for short [14,15]. PLIF stands for laser-induced-fluorescence of plumes.…”