Document ink dye age estimation by direct injection-mass spectrometry and correlation analysis

Abstract

Unsupervised Linkage Threshold (ULT) method was applied for the analysis of ballpoint pen inks with the purpose of estimating the date on which manuscripts were launched on documents. In order to verify ink degradation patterns over time, manuscript samples were removed from forensic expert reports produced along the years ranging from 1962 to 2014, and which had been stored in the archives of the National Institute of Criminalistics (NIC) of the Brazilian Federal Police. A total of 505 ink samples were analyzed by Electron Spray Ionization (ESI) mass spectrometry, and these showed a significant variation in the ink formulation over time. From the normalized mass spectrometry peak areas, the ULT method was employed in the correlation analysis of ink manuscripts using the cosine squared function as a similarity measure. The method allowed a simple, fast and reliable determination of threshold correlation values as well as ink dating confidence intervals for each kind of formulation studied. It has been observed that dye stability may change drastically depending upon the chemical group (methyl or ethyl) lost in the degradation process. Due to its statistical nature, the developed method does not rely upon prior knowledge of factors such as storage conditions or initial dye formulation. This feature can be an advantage in real forensic applications since this type of information is not usually available to forensic experts.