Study: Hyperspectral imaging for the discrimination of gel pen inks


HSI analysis of gel pen inks


The growing popularity of gel ink pens has, over the past 15 years, presented questioned document examiners with a unique challenge as existing techniques, developed for the analysis of traditional ink pens and the ubiquitous biro, fail to discriminate between the complex gel polymer/dye mixtures used in gel pens.


A Japanese innovation introduced in the late 1990s, the significant difference between the formulation of gel pen inks when compared with traditional writing inks is the use of pigments in place of dyes as the colourant.

To date a variety of techniques have been applied to the task of differentiating these visibly and chemically similar pigment based inks with Laser desorption ionisation mass spectrometry (LDI-MS), thin layer chromatography (TLC), Laser Induced Breakdown Spectroscopy (LIBS), capillary electrophoresis, and Raman spectroscopy all meeting with varying levels of success.

Most recently a study published by researchers at the Department of Pure and Applied Chemistry at the University of Strathclyde has identified Hyperspectral Imaging (HSI) as a rapid, non-destructive method capable of providing high levels of discrimination.

Using the Foster + Freeman VSC6000/HS, researchers studied a total of 42 gel ink pens (15 blue, 13 red and 14 black) produced by 15 different brands. In VSC’s Hyperspectral Imaging mode, images and spectra were captured of each sample across a spectral range 400-1000nm. Data was compared through the visual examination of absorption spectra and the statistical analysis of reflectance spectra and seen to demonstrate dicriminating powers of 1.00, 0.90 and 0.40 for red, blue and black gel inks respectively.

While the rate of discrimination achieved by HSI does not exceed that achieved by elemental analysis (including LIBS), it does offer several important advantages to document examiners. HSI can be performed using a VSC system, an instrument that is already commonplace in many QDE laboratories. HSI is rapid and requires little or no sample preparation. And perhaps most importantly, HSI is entirely non-destructive leaving evidence intact for further examination if required.



Hyperspectral imaging of gel pen inks: An emerging tool in document analysis, by G.Reed, K. Savage, D. Edwards and N. Nic Daeid of the Centre for Forensic Science, WestChem, Department of Pure and Applied Chemistry at the University of Strathcylde is published by Elsevier Ireland Ltd. and can be viewed on the Science Direct website.