Wavelength dependence in surface-assisted laser desorption/ionization mass spectrometry

Surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) [1,2] relies on the use of (usually inorganic) nanostructured surfaces (e.g., coatings of colloidal nanoparticles [3], nanostructured solid supports [4,5] or sputtered metal nanoclusters [6]), instead of organic matrices, as in the conventional matrix-assisted laser desorption/ionization (MALDI) MS technique. While the interplay between the laser wavelength and the absorption profile of the matrix has been widely studied and proved to be a crucial factor in MALDI-MS [7-10], very few, if any, fundamental studies have been carried out in SALDI-MS. Yet, the laser wavelength is a key parameter that needs to be tuned to correspond to the maximum of optical absorption of the assisting material, in order to provide optimal analytical results [7-10]. Indeed, the desorption in SALDI-MS has been proved to be mainly driven by thermal processes, resulting from the heating of the nanosubstrate surface upon absorption of the photon energy [1]. However, almost all SALDI-MS studies use standard laser wavelengths of 337 or 355 nm, even though the peak absorption of the SALDI nanosubstrate might completely differ from these values. Here we employed wavelength-tunable optical parametric oscillator (OPO) lasers to investigate the wavelength dependence in SALDI-MS, using citrate-coated gold nanoparticles (AuNPs) as SALDI nanosubstrates and model analytes (e.g., benzylpyridinium thermometer ions, lipids). The uniform spray deposition of the AuNPs and analyte ions on Superfrost glass slides was performed using an ultrasonic spray coater. We then recorded gold and analyte ion signals as a function of the laser wavelength and laser fluence in the UV-visible range, knowing that the maximum absorption of the colloidal AuNPs is at 529 nm. The results of this study provide new insights into the SALDI desorption/ionization processes and could assist the improvement of the analytical performance of the SALDI-MS technique, through instrumental and methodological adjustments.

References

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