Challenges Determining the Correct Deposition Order of Different Intersecting Black Inks by Time-of-Flight Secondary Ion Mass Spectrometry.

The distinction of different inks and determination of their deposition order are important forensic tasks when evaluating questioned documents. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has received attention for these tasks due to the technique's nondestructive nature, rich mass spectral information, ability to provide chemical images, and excellent surface sensitivity. Prior literature results demonstrate the ability of ToF-SIMS to differentiate between many varieties of blue ballpoint pens and to determine the correct deposition order for selected ink intersections. The current work therefore sought to further study the intersections of different black inks. Three black pens were initially tested and were successfully distinguished. However, interesting phenomena were observed where certain inks dominated over others, causing distinctly incorrect conclusions to be made regarding ink deposition order. To explore whether these incorrect results could be corrected, different primary ions (Bi, Bi, and Ar) were used and positive and negative secondary ions were evaluated. In data analysis, restriction of the mass range was considered and different multivariate analysis techniques, principal component analysis (PCA) and multivariate curve resolution (MCR), were performed on the secondary ion images and spectra from selected regions of interest. Issues regarding incorrect apparent order of deposition persisted through the variation of all these parameters. In addition to ink differentiation, and deposition order, evidence of ink aging was also observed. These results raise several questions to be answered regarding the widespread use of ToF-SIMS for forensic ink analysis.