In situ profiling of pen ink degradation on paper using Raman spectroscopy and ToF-SIMS

The relative ageing of inks is a service of high demand for forensic document examiners, as it may prove critical in disputes over potential document forgery. Conventional approaches to the dating of inks typically require extraction of ink from the paper via plugging (resulting in the partial destruction of document samples) and lack the time-specificity desired. This research instead utilised confocal Raman microscopy and Time-of-Flight Secondary Ion Mass Spectrometry for in-situ surface analysis to study the degradation of inks over a short-term period. Currently, there is limited research into how ink degradation processes may be impacted by the type of paper substrate, thus, multiple paper types were considered for this study. Analysis was carried out on six pen inks (2 pens each of rollerball, ballpoint, and gel ink formulations) deposited on three different paper types with a 4-month ageing period. Relative dating curves were developed tracking the degradation of triarylmethane dyes. ToF-SIMS analysis of sputtered samples indicated a potential reduction in degradation below the first few atomic layers of the surface. Developing in-situ and more spatially accurate (laterally and depth) analysis methods will improve the ability to identify forged documents.