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 ink dating typically require extraction of ink from the paper via plugging (resulting in the partial destruction of documents) 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 of rollerball, ballpoint, and gel inks deposited on three different paper types over a 4-month period. To our knowledge, this is the first study to combine these techniques in a ‘multi-modal’ approach for a common set of samples and to compare trends across multiple ink and paper substrate types. Degradation curves constructed for triarylmethane dyes showed rapid initial changes, with blue ballpoint ink giving the most linear trend, although no differences were observed between paper types. Analysis of pre-sputtered samples indicated a potential reduction in degradation below the first few atomic layers of the surface, which has not been demonstrated in published literature to date. Developing in-situ and more spatially accurate (laterally and depth) analysis methods will improve the ability for forensic document examiners to identify potentially forged documents.