Anal. Chem. 2001, 73, 3639-3645
Experimental Factors Controlling Analyte Ion Generation in Laser Desorption/Ionization Mass Spectrometry on Porous Silicon Rebecca A. Kruse, Xiuling Li, Paul W. Bohn,* and Jonathan V. Sweedler*
Department of Chemistry, Beckman Institute for Advanced Science and Technology, and Fredrick Seitz Materials Research Laboratory, University of Illinois at UrbanasChampaign, 600 South Mathews Avenue, Urbana, Illinois 61801
Desorption/ionization on porous silicon (DIOS) is a relatively new laser desorption/ionization technique for the direct mass spectrometric analysis of a wide variety of samples without the requirement of a matrix. Porous silicon substrates were fabricated using the recently developed nonelectrochemical H2O2-metal-HF etching as a versatile platform for investigating the effects of morphology and physical properties of porous silicon on DIOS-MS performance. In addition, laser wavelength, mode of ion detection, pH, and solvent contributions to the desorption/ionization process were studied. Other porous substrates such as GaAs and GaN, with similar surface characteristics but differing in thermal and optical properties from porous silicon, allowed the roles of surface area, optical absorption, and thermal conductivities in the desorption/ionization process to be investigated. Among the porous semiconductors studied, only porous silicon has the combination of large surface area, optical absorption, and thermal conductivity required for efficient analyte ion generation under the conditions studied. In addition to these substrate-related factors, surface wetting, determined by the interaction of deposition solvent with the surface, and charge state of the peptide were found to be important in determining ion generation efficiency. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has found widespread use in the analysis of synthetic and biological macromolecules.1 In MALDI-MS, the energy-absorbing organic matrix transfers energy from a pulsed laser beam to the cocrystallized analyte molecules. This soft ionization technique affords little to no fragmentation of the analyte; however, the use of matrix has several disadvantages including an intolerance to salt, heterogeneous sample incorporation into matrix crystals (hot spots), and significant background ion intensity in the low-mass range due to matrix cluster ionization. These matrix background ions often obscure and/or suppress signals from analyte ions in the lower mass range (
