ToF-SIMS evaluation of markings made by the same black ballpoint pens at different times

Abstract

The differentiation and relative dating of ink lines in questioned documents are important tasks in forensic science. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a surface-sensitive mass spectrometry technique with potential to distinguish inks based on chemical differences such as those caused by differences in the age of ink markings from the same pen. In this work, both positive and negative polarity ToF-SIMS spectra were evaluated for three commercial black ballpoint pen inks over an analysis time of 4 to 671 days. Changes in the spectra included demethylation of triarylmethane dyes such as crystal violet, and loss of the anionic dye metanil yellow. ToF-SIMS spectra of the black ballpoint inks continued to change up to 6 months for all inks and up to 22 months in some cases. Samples with 1-day, 1-week and 1-month drying times were also evaluated using several data processing approaches. Markings from the same pen were most likely to be distinguished by ToF-SIMS if the age difference between the markings was 1 month or longer, although some samples were distinguished with smaller differences in writing times. Furthermore, the results indicated that ToF-SIMS imaging is best performed within 3–6 months after sample preparation as differentiation becomes more difficult after longer analysis delays. This is the first systematic ToF-SIMS study of ink aging, which provides valuable information about the characteristic ions and data analysis approaches for future forensic analyses.