Ubiquitin (bovine, 8.6 kDa) and myoglobin (equine, 16.9 kDa) were each introduced using a heated electrospray ionization (HESI) source from solutions composed of 2 µM protein, water (25%), acetonitrile (25%), methanol (50%), and formic acid (0.1%). The ionization voltage was set to 3200 V at a flow rate of 2 µL/min from a syringe pump. Spectra acquired under supercharging conditions utilized 4-vinyl-1,3-dioxolan-2-one (4V) added at 1-5% (v/v) to the electrospray solution to achieve high abundances of high charge states. MS1 and MS/MS spectra of all proteins were recorded on a Thermo Scientific Orbitrap Lumos system outfitted with a 193 nm ArF excimer laser, enabling UVPD.1-2 laser pulses at 0.5 or 1.0 mJ per pulse was utilized for 193 nm UVPD. All UVPD was performed in the high-pressure linear ion trap. HCD spectra were recorded utilizing 15, 17, or 23 NCE.