Surface cleaning of germanium nanodot ionization substrate for surface-assisted laser desorption/ionization mass spectrometry.

In surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS), a chemical background signal, arising from organic contaminants such as plasticizers, is frequently observed mainly under m/z ca. 600, which impairs the advantages of the matrix-free approach. Silver salts, which are used for the cationization of aromatic compounds, are also difficult to remove completely after the measurements. In this study, surface cleaning techniques used in semiconductor processing were used to clean our developed silicon-based SALDI substrate on which self-assembled germanium nanodots (GeNDs) had been deposited (termed a GeND chip). An immersion cleaning method using acetone with sonication, and a sulfuric-peroxide mixture (SPM) cleaning method using a mixture of H(2)SO(4)/H(2)O(2)/deionized water, were examined for their effectiveness in removing organic compounds and residual silver salts. Removal of both types of contaminants was successfully performed by SPM cleaning. The limit of detection for glutathione was improved from ca. 5 pmol without cleaning to ca. 50 fmol after the SPM cleaning. Since GeND chips can tolerate acidic cleaning and sonication due to their chemical inertness and rigid nanodot structures, they appear to be an ideal reusable SALDI substrate.