IR fingerprint spectra (600-1900 cm -1 ) of the reference compounds were measured using an ion trap mass spectrometer (Bruker AmaZon Speed ETD, Bremen, Germany) coupled to the beamline of the Free-Electron Laser for Infrared eXperiments, FELIX . Solutions of the reference compounds (approx. 10 -5 M in 1:1 methanol:water) were directly infused into the electrospray ionization source, after which adducts of interest ([M+H] + , [M-H] -, and/or [M+Na] + ) were mass-isolated and subjected to IR measurements. LC-IRIS measurements were performed in an online fashion using a single 80 μL sample loop installed between a 6-port switching valve and the ion source . The isolated ions were irradiated with 1 to 10 FELIX macropulses of 10 µs; the pulse energy ranges from 20 to 180 mJ depending on wavelength and the laser bandwidth amounts to 0.5% of the center frequency. The internal energy of the ions increases by frequency-dependent absorption of multiple IR photons until the ions undergo fragmentation. Each IR spectrum was reconstructed from a series of mass spectra by plotting the fragmentation yield, -ln[ 𝐼 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟 / ∑(𝐼 𝑎𝑙𝑙 )], as a function of IR frequency (scanned in 3-5 cm -1 steps). When no IR-induced fragment ions were observed, the depletion of the precursor ion was plotted instead, -ln[ 𝐼 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟 /𝐼 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟 𝑛𝑜 𝐼𝑅 ] . The yield is linearly corrected for frequencydependent variations in laser pulse energy. A grating spectrometer was used to calibrate the laser frequency.