For laser post-ionisation (MALDI-2), a frequency quadrupled Nd:YAG laser emitting at 266 nm and operated at a frequency of 300 Hz was used (Nano L-DPSS, Litron Lasers, UK). The MALDI-2 beam was guided into the MALDI ion source to be parallel with the sample surface at an approximate height of 400 µm using a series of mirrors (NB1-K04, Thor Labs, New Jersey, USA) and then focussed via a single plano-convex lens (f = 500 mm, LA4184-U, Thor Labs, New Jersey, USA). Laser pulse energy was controlled using the internal attenuator and adjusted to be 600 µJ as measured using a calibrated energy sensor (QE12HR-H-MB-D0, Gentec-EO, Quebec, Canada) just before the beam entered the ion source. The Explorer MALDI laser provides a triggering signal recorded with an internal photodiode that can be used for synchronisation with the MALDI-2 laser, while an external photodiode was used to monitor the timing of the MALDI-2 laser. To precisely control the timing (i.e., the delay between the laser pulses), both lasers were externally triggered using a Quantum pulse generator (QC9200, Quantum Composers, Bozeman, Montana) such that the time delay between the MALDI and MALDI-2 laser pulse was 6 µs. Pulse delay was determined by monitoring the photodiode outputs using a digital oscilloscope (RIGOL, Portland, USA). A schematic of the postionisation setup is included in Fig. .