[en] In situ surface analysis in TFR 400 and TFR 600 have been performed by QWAASS (Quantitative Wall Analysis by Auger and Secondary Ion mass Spectroscopy). New results were obtained for the scrape-off region of the limiter by varying the sample position. Two processes, deposition and erosion, determined the molybdenum deposited on a carbon sample after a few discharges. A simple model based on these two processes is developed which explains the main features of the experimental results. Impurity fluxes in this region can be determined. It is further assumed that both deposition and erosion decrease exponentially with increasing distance between plasma and sample. Erosion decreases faster than deposition. The characteristic decay length for Mo-deposition is about 0.5 cm. Further elucidation of the results presented is obtained from a simple model of plasma-limiter interaction which is based on thermal flow parallel and Bohm diffusion perpendicular to the magnetic field. Assuming an edge temperature of about 10 eV the calculated decay length is of the same order of magnitude as that found experimentally. The results are discussed together with other processes that may contribute to the deposition of metal impurities on samples in the limiter shadow region. (Auth.)