This article reports the findings and methodology of an initial study that applied a new sampling interface for mass spectrometers called Direct Analysis in Real Time (DART) to the “in situ” analysis of writing inks on paper without visible alteration.
The analysis of ink used in writing is often conducted to establish document authenticity, as well as the sources and relative ages of written entries. Most current analytical methods require removing samples or visibly altering the document. The DART procedure tested in the current study anticipated being able to analyze writing inks on paper without visible alteration. DART uses a heated stream of excited and ionized helium or nitrogen to evaporate or ablate material from a sample and then ionizes that material so that it can be analyzed by the mass spectrometer. The study examined 43 black and blue ballpoint, black fluid, and black gel inks. Both dyes and persistent but thermally labile components of the inks contributed to the mass spectra, primarily as protonated molecules [M+H]+. Numerous ink components were identified from the spectra. The spectra were placed in a searchable library, which was then challenged with two spectra from each of the 43 inks. The best match for each of the challenge spectra was correct for all but one ink, which matched with a familiar ink by the same manufacturer. The ink lines were not visibly altered by the DART analysis. Inspection with a low-power microscope failed to show any difference between the sampled and unsampled regions of individual ink lines. The study concludes that mass spectrometry using DART sampling is a quick, nondestructive method for analyzing ink in situ on questioned documents, except for the most similar of inks. 4 figures and 17 references