Peptide nucleic acid (PNA) was immobilized onto Au-coated surfaces via a thiol linker (DTSP, 3,3'-dithio-bis(propionic acid N-hydroxysuccinimide ester)). The immobilization process of DTSP and PNA to these surfaces was performed by either binding PNA to DTSP in a solution and then immobilizing it onto the surface or by immobilizing DTSP onto the surface and then attaching PNA to it. Deprotonated (M-H)(-) signals of the different PNA bases as well as characteristic peaks of DTSP fragments were used in TOF-SIMS measurements to study and optimize the different immobilization processes. The data show that the best result could be achieved at a concentration of 10 mM and an immobilization time of 24 It. The binding of PNA to the DTSP layer takes significantly longer than attaching DTSP to the surface. From the obtained data it can be concluded that the second attachment method described above is preferable to the first one. (C) 2002 Elsevier Science B.V. All rights reserved.