Laser desorption laser postionization time-of-flight mass spectrometry (L2MS) was applied for unambiguous discrimination of pigment-based inks in blue, black, and red gel pens and molecular imaging of equivocal documents in a quasi-non-destructive way. In comparison to laser desorption mass spectrometry (LD-MS), additional discriminatory information on ink components is acquired uniquely, facilitating the distinct differentiation of various pigmented gel inks. More importantly, diversified images of additional characteristic ions achieved using L2MS offer reliable support to discriminate forged documents and decipher important hidden contents. Apart from minimized matrix effect and maximized ionization yield, direct and confirmatory identification of forged documents is achieved successfully without solvent or matrix involved, not only eliminating unwanted damage and contamination to the samples but significantly shortening the overall analysis time. In addition, L2MS is a minimally destructive approach with tiny analyte consumption. With these appealing qualities, L2MS imaging is poised to be a powerful tool for confirmatory surface analysis of complex pigment-based samples. Graphical Abstract Weight and see: Highly distinct and comprehensive images of counterfeit documents with blue-pigmented gel inks are achieved successfully, due to the high sensitivity and increased ion yield of laser desorption laser postionization time-of-flight mass spectrometry. The hidden important contents of the obliterated documents are visually deciphered with the help of the additional chemical information.
Keywords: Equivocal documents; Laser desorption; Laser postionization; Mass spectrometry imaging; Pigmented gel inks; Surface analysis.