Experimental factors controlling analyte ion generation in laser desorption/ionization mass spectrometry on porous silicon

Desorption/ionization on porous silicon (DIOS) is a relatively new laser de sorption/ionization technique for the direct mass spectrometric analysis of a wide variety of samples without the requirement of a matrix. Porous sili con substrates were fabricated using the recently developed nonelectrochemi cal H2O2-metal-HF etching as a versatile platform for investigating the eff ects of morphology and physical properties of porous silicon on DIOS-MS per formance. In addition, laser wavelength, mode of ion detection, pH, and sol vent contributions to the desorption/ionization process were studied. Other porous substrates such as GaAs and GaN, with similar surface characteristi cs but differing in thermal and optical properties from porous silicon, all owed the roles of surface area, optical absorption, and thermal conductivit ies in the desorption/ionization process to be investigated. Among the poro us semiconductors studied, only porous silicon has the combination of large surface area, optical absorption, and thermal conductivity required for ef ficient analyte ion generation under the conditions studied. In addition to these substrate-related factors, surface wetting, determined by the intera ction of deposition solvent with the surface, and charge state of the pepti de were found to be important in determining ion generation efficiency.