Supporting Information: Simplified Approach for Infrared Photodissociation Spectroscopy of Water-Tagged Ions with a Widely Tunable Quantum Cascade Laser

This work presents a benchtop method for collecting the room temperature, gas phase infrared (IR) action spectra of protonated amino acids and their isomers. The adopted setup uses a minimally modified commercial electrospray ionization ion trap mass spectrometer (ESI-MS) coupled to a broadband, continuous wave (cw) quantum cascade laser (QCL) source. This approach leverages messenger assisted action spec- troscopic techniques using water-tagged molecular ions with complex formation, irra- diation, and subsequent analysis all taking place within a single linear ion trap stage. 1 This configuration thus circumvents the use of multiple mass selection and analysis stages, cryogenic buffer cells, and complex high-power lasers systems typically required to execute these techniques. The benchtop action spectrometer is used to collect the 935–1600 cm−1 (6.2–10.7 μm) IR action spectrum of a collection of amino acids and a dipeptide with results cross referenced against literature examples obtained with a free electron laser source. Recorded IR spectra are used for the analysis of binary mixture samples composed of the constitutional isomers α-alanine and β-alanine with ratios determined to ∼4% measurement uncertainty without the aid of a front-end separa- tion stage. This turn-key QCL-based approach is a major step in showing the viability of deploying tag-based action spectroscopic techniques for potential in situ planetary science sensor platforms and general analytical applications