In mass spectrometry imaging of tissues, the size of structures that can be distinguished is determined by the spatial resolution of the imaging technique. Here, the spatial resolution of IR laser ablation is markedly improved by increasing the distance between the laser and the focusing lens. As the distance between the laser and the lens is increased from 1 to 18 m, the ablation spot size decreases from 440 to 44 μm. This way, only the collimated center of the divergent laser beam is directed on the focusing lens, which results in better focusing of the beam. Part of the laser energy is lost at longer distance, but this is compensated by focusing of the radiation to a smaller area on the sample surface. The long distance can also be achieved by a set of mirrors, between which the radiation travels before it is directed to the focusing lens and the sample. This method for improving the spatial resolution can be utilized in mass spectrometry imaging of tissues by techniques that utilize IR laser ablation, such as laser ablation electrospray ionization, laser ablation atmospheric pressure photoionization, and matrix-assisted laser desorption electrospray ionization. Graphical Abstract ᅟ.
Keywords: Ambient mass spectrometry; IR laser ablation; Laser ablation atmospheric pressure photoionization; Laser ablation electrospray ionization; Laser focusing; Mass spectrometry imaging; Matrix-assisted laser desorption electrospray Ionization; Metastable-induced chemical ionization; Spatial resolution; Tissue imaging.