Differentiation of Black Gel Inks Using Optical and Chemical Techniques

Abstract

Gel ink pens have become a common writing instrument in the United States. Questioned document examiners often attempt to optically differentiate gel inks from each other and from other non-ballpoint ink writings (e.g., those from roller-ball pens). Since early formulations were primarily pigment-based, they do not elute when analyzed by thin-layer chromatography. However, recent gel ink formulations (i.e., within the past five years) include dye-based inks that can be easily separated. This study differentiates black gel inks using optical and chemical techniques. The techniques include: microscopy, visible and near infrared reflectance, near infrared luminescence, thin-layer chromatography (TLC), spot tests, and gas chromatography/mass spectrometry (GC/MS). As a result of this study a flow chart has been developed allowing for a systematic determination of a questioned ink. In addition, an analysis of volatile compounds found in gel inks revealed that there are some unique ingredients that may be found in gel inks that are not typically found in other non-ballpoint inks.

Evaluation of the Evidential Value of the Elemental Composition of Glass, Ink and Paper by Laser-Based Micro-Spectrochemical Methods

“…Nonetheless, the analysis of gel pen inks constitutes a challenge for the forensic ink examiner since most of the gel inks are difficult to analyze by conventional techniques such as TLC and capillary electrophoresis [Wilson et al, 2004]. As a result, other methods such as Raman spectroscopy [Zieba-Palus et al, 2006;Zieba-Palus et al, 2008], infrared spectroscopy [Zieba-Palus et al, 2008], XRF [Zieba-Palus et al, 2008], ion paring HPLC/ MS/MS and laser desorption mass spectrometry (LDI-MS) [Weyerman et al, 2010;Gallidabino et al, 2010;Weyerman et al, 2012] have been recently explored as alternative tools to cope with forensic comparisons of gel inks.…”

“…Three years later these types of pens were manufactured in the US. The inks used in fiber tip pens can be either water based or water resistant [Wilson et al, 2004].…”

“…However, some formulations manufactured after 1999 may contain both pigment and dye-based inks. [Wilson et al, 2004] 1.5.2 History of printing inks…”

“…[Wilson et al, 2004]. Some distinctive components used in gel inks are hexanol, hepthaethlyne glycol, penthaethylen glycol, 1-H benzotriazole, and triethanol amine [Wilson et al, 2004] Pseudoplasticizers such as xanthan gum, tamarind gum, gum rabic, guar gum, cellulose and water soluble acrylic synthetic polymers are used to provide the gel structure. The most common inorganic pigments found in gel pens are titanium oxide, iron oxide, carbon black and metal powder.…”

“…The solvent of choice for these formulations is ethylene glycol. [Wilson et al, 2004] Table 2 shows examples of typical writing ink formulations. These recipes are provided as an illustrative example, however there are numerous possible combinations of modifications to the components in the market ].…”