Analysis of PEG oligomers in black gel inks: Discrimination and ink dating

Carbon-based black gel inks are common samples in forensic practice of questioned document examination in China, but there are few analytical methods for this type of ink. In this study, a liquid chromatography-.high resolution mass spectrometry (LC-HRMS) method was established for the analysis of PEG oligomers in carbon-based black gel ink entries. The coupled instruments achieve both the identification and quantification of PEG oligomers in ink entries with reproducible results. Twenty carbon-based black gel inks, whose Raman spectra appeared identical, were analyzed using the LC-HRMS method. As a result, the twenty gel inks were classified into four groups according to the distribution of PEG oligomers. Artificially aging of PEG 400 and a gel ink showed that as PEG degraded, the relative amounts of low molecular weight PEG oligomers increased, while those of high molecular weight decreased. The degradation of PEG oligomers in a naturally aged gel ink was consistent with those in the artificially aged samples, but occurred more slowly. This study not only provided a new method for discriminating carbon-based black gel ink entries, but also offered a new approach for studying the relative ink dating of carbon-based black gel ink entries.