On the desorption of electrosprayed organic compounds from supporting metal foils by laser induced pressure waves

Abstract

The desorption of organic compounds by indirect effects of laser impact, as pressure waves, was examined in a transmission type laser microprobe mass analyzer. For this, the samples were electrosprayed on one side of a supporting metal foil a few microns thick. The sample layer was directed towards the mass analyzer and the uncovered side of the foil was exposed to the laser pulse. The metal foil served to absorb the laser energy, to transfer it to the sample layer and, at the same time, to shield the mass analyzer against the ions generated by direct laser-sample interaction. Mass spectra of the ions produced on the sample-layer side were registered and, afterwards, both sides of the foil were examined in a scanning electron microscope. At laser irradiances exceeding 5 × 10 8W cm −2, but not enough to perforate the foil, the electrosprayed material was, independent of the type of compound, split off from a circular region comparable to the spot size of the direct interaction zone. From compounds which are known to be desorbed directly as preformed ions, the spectra showed only the molecular cation peak. However, from compounds like oligosaccharides, which normally require cationization with alkali ions, neither quasi-molecular nor fragment ions were detected.