Investigating Polymerization of Ethyl Cyanoacrylate During Fuming of Latent Fingerprints

Despite being widely used in forensic laboratories to reveal latent fingerprints, the polymerization mechanism occurring during cyanoacrylate fuming is still not clearly established. Notably, mass spectra of the so-produced polymers have never been reported. In order to gain new insights into the initiation step of the fuming process, an analytical approach was developed based on positive-mode electrospray ionization (ESI), accurate mass measurements by high-resolution mass spectrometry, and tandem mass spectrometry (MS/MS) for end-group characterization. Experiments performed on model polymers prepared by fuming ethyl cyanoacrylate (ECA) monomers onto water droplets first showed that solvents used for ESI need to be carefully selected to prevent chain-end modification during analysis. Using safe conditions consisting of a binary mixture of acetonitrile and chloroform supplemented with sodium chloride, detection of HO/H-terminated poly(ethyl cyanoacrylate) (PECA) chains provides clear evidence that ECA chain growth can be induced by H₂O. While the high reactivity of the terminal OH group favors specific rearrangements in collision-induced dissociation, MS/MS study of a retro Knoevenagel side product permits us to establish robust fragmentation rules for PECA. These rules have been applied to MS/MS data recorded for PECA films generated upon ECA fuming of fingerprints enriched with a variety of fatty acids (FAs). Production of FA/H-terminated chains is systematically demonstrated for fatty acids of low melting points, showing that their carboxylate form acts as a nucleophilic agent toward ECA monomers. Fatty acids that remain solid at room temperature are not incorporated in polymer chains that, instead, exhibit a cyano termination.