Monovalent nickel ion as ionization probe in matrix-assisted laser desorption/ionization mass spectrometry

We have developed NiO/HM20, a matrix composed of NiO nanoparticles (NiO) loaded on zeolite surface, for matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). By photoexcitation, monovalent nickel ion (Ni⁺) was dominantly observed with high intensity. The mechanism of Ni⁺ production was studied by picosecond time-resolved emission spectroscopy. By loading NiO on the zeolite surface, a new relaxation pathway to other electronic excited states was observed, which increased the lifetime of electron–hole pairs and promoted the reduction of nickel. The long-lived charge-separated electronic excited state corresponding to the large polarization of NiO was confirmed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction spectroscopy (XRD). The correlation between the Auger parameter related to the polarization energy and the Ni⁺ peak intensity in the mass spectrum was confirmed. By applying the developed NiO/HM20 matrix to the MS measurement of small molecules, we found that molecules were ionized through complex formation with Ni⁺ using the coordination of lone electron pairs or π-electrons in the analyte molecules.