Using vibrational spectroscopy to analyse the effect of environmental aging on gel pen inks

Abstract

Document examinations are vitally important for fraudulent investigations. During a forensic document examination, one of the major questions which arises for the examiner is: when was an ink entry made on a document? During the past half century there has been a large development in the analysis in pen inks, including the introduction of new techniques and development of methods such as the ink library approach and hyphenated chromatography. Attenuated Total Reflectance – Fourier Transform Infrared Spectroscopy (ATR-FTIR) is a rapid and non-destructive technique that can objectively profile molecular changes within ink applied to document.
The aim of this study is to determine whether infrared spectroscopy can be used to calculate the age of a gel pen ink and therefore how much time has passed between an ink sample being written on a piece of paper and the sample being analysed. ATR-FTIR was used during this study as a rapid and non-destructive technique allowing multiple spectra to be produced in a short space of time, while not causing destruction to the sample.
The paper used to write ink samples on for the aging study was analysed to show consistent spectra between sheets of a single ream. This analysis was vital to determine whether all of the ink samples were to be produced onto paper which would produce the same or different spectra. As the paper provides the same spectra these differences do not need to be considered when conducting analysis between aging inks. The ink samples analysed through the drying study took 10 minutes to dry after being produced. This time frame was used as knowledge for how long the ink sample should be allowed to dry prior to the spectrum for the ink sample being first taken as a reference point on day 0.
Through examinations, spectral peaks at 929.51cm-1 and 1348cm-1 from second derivatised data were established as the most discriminative wavenumbers for ink spectra and used as the major point of referencing ink aging. Evidence for gel pen ink aging is demonstrated from spectral data and group scatter plot correlations. Spectral data at the highly discriminating wavenumbers demonstrates differences between spectra for different lengths of aging. Correlations of ink aging are present in group scatter plots. Despite a non-linear correlation between the specific time periods, correlation are detectable over ranges of time with the major occurrence of aging present between the time periods of 4 weeks and 7 weeks.
Overall, this study presents evidence that aging of a gel pen ink can be demonstrated by the use of Attenuated Total Reflectance – Fourier Transform Infrared Spectroscopy.