A rapid and direct method for dating blue pen ink in documents using multiset modeling of infrared spectroscopy and mass spectrometry data

The dating of documents is crucial in forensic chemistry, particularly for verifying their authenticity. This study aimed to develop a rapid and direct method for the dating of pen ink in documents, using a combination of Fourier transform infrared spectroscopy in attenuated reflectance mode (FTIR-ATR), desorption electrospray ionization mass spectrometry (DESI-MS) and multiple ensemble data modeling. Two sets of paper document samples containing writing in blue pen ink were investigated: (I) artificially aged documents and (II) real documents dating from 1960 to 2022. The FTIR-ATR spectra of both sets of samples showed a decrease in absorbance at ∼1584 cm−1, related to the chemical modification of the CN bond in the molecular structure of Basic Violet 3 (BV3), one of the main dyes used in blue pen ink. DESI-MS confirmed the presence of BV3 and its degradation by-products in all the samples, indicating its widespread utilization in blue pen ink production. Moreover, DESI-MS detected combinations of dyes within the ink composition. The models were built using the DESI-MS and FTIR-ATR data separately, but the error and trend were significantly reduced when both sets of data were used. The combination of DESI-MS and FTIR-ATR spectral information resulted in a final predictive model with low error for pen inks from real documents in writing from the years 1960 to 2022. These analyses proved to be effective for the dating of pen inks and are suitable for use in routine forensic analysis, providing a direct and rapid method that allows for accurate prediction.