Review: Forensic Analysis of Laser Printed Ink by X-Ray Fluorescence and Laser-Excited Plume Fluorescence
Emily C. Lennert
laser, print, ink, questioned, document, forgery, x-ray fluorescence, XRF, fluorescence, plume laser-excited atomic fluorescence, PLEAF, LEAF
- Chu, P.C.; Cai, B. Y.; Tsoi, Y. K.; Yuen, R.; Leung, K. S.Y.; Cheung, N.H. Forensic analysis of laser printed ink by X-ray fluorescence and laser-excited plume fluorescence. Analytical Chemistry. 2013, 85, 4311-4315.
The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.
In cases of suspected document forgery, laser printed ink analysis may serve to uncover instances of likely tampering. Chemical analysis of the ink may reveal differences in ink composition between a suspected forged area and an area known to be authentic, indicating re-printing or over-printing on the suspected area of the document. However, analysis methods must be non-destructive to preserve the integrity of the evidence. In this study, the authors present a method for laser printed ink analysis by a two-step optical approach: screening by x-ray fluorescence (XRF) followed by plume laser-excited atomic fluorescence (PLEAF). This method provides a means of examining ink composition while producing minimal damage to the document.
Four toners were selected, due to popularity and similar compositions, for use in this study: two black Hewlett-Packard toners (H1 and H2), Panasonic black toner (P) and FujiXerox black toner (F). Samples were printed on white A4 copier paper. Non-printed toner samples were analyzed via inductively coupled plasma – mass spectrometry (ICP-MS) to determine elemental concentrations for reference. Concentrations were determined using calibration curves, which were prepared using a series of dilutions prepared from stock solutions.
Samples were screened using XRF, then analyzed using PLEAF. PLEAF uses a laser to create a plume from the sample, and then another laser pulse vaporizes the sample to produce analyte atoms, which then fluoresce. The fluorescence is detected and used to identify elements present in the sample; each element will produce fluorescence at a distinct wavelength.
By XRF screening, H1 and H2 were distinguished from P and F. H1 and H2 were high in iron and easily distinguished from the other samples. However, H1 and H2 were not as easily distinguished from one another. XRF was able to detect higher levels of zinc in H2, but only at font sizes greater than 20. Similarly, F and P were not easily distinguished from one another. Higher levels of copper were detected in the F toner only at font sizes greater than 20.
PLEAF was used for highly sensitive multi-element analysis and provided more discriminatory results. The high sensitivity of the method allowed for fewer laser shots to be required for analysis. While the laser created a small crater, the ablated crater was not visible to the eye. The crater was also not visible under a microscope, indicating that the technique is minimally destructive. All four samples were easily distinguished from one another by PLEAF analysis, as seen in the results displayed in Figure 4 within the study. H1 contained high levels of titanium but lower levels of strontium, while H2 contained high levels of both titanium and strontium. F contained high levels of copper and low levels of iron. P contained low levels of copper and high levels of iron. Results of both XRF and PLEAF analysis corresponded with ICP-MS results. Principal component analysis (PCA) was performed, and a PCA plot shows clear distinction between the toners, as seen in Figure 5 within the study.
Finally, the authors sought to determine whether the method could be applied to over-printed areas, such as a period that was turned into a comma. Samples of F toner printed over an area of P toner, and vice versa, were examined by XRF and PLEAF. XRF was incapable of discriminating over-printed toners. However, PLEAF was successful in identifying both toners. PLEAF may be used to analyze multiple layers of the ink, and was therefore capable of resolving the over-printed areas.
- XRF showed limited capabilities in resolving the four toners examined in the study.
- PLEAF was capable of highly sensitive analysis with no visible damage, allowing for discrimination between all four samples in the study.
- Over-printed areas were successfully determined by PLEAF, but not by XRF.
In cases of document forgery, it is necessary to determine which areas of the document are authentic and which areas have been tampered with. PLEAF provides a highly sensitive means of discriminating between printed toner inks. If a tampered area was printed with a different toner than the authentic area, PLEAF can be used to determine the differences in toner and identify the forgery with no destruction of the document.
- XRF may be used for screening to determine toner differences, but not for over-printed areas of a document.
- PLEAF may be used to determine differences in toners on any area of a questioned document. Over-printed areas are easily resolved by PLEAF.