Iron concentration imaging has been proven to be a very valuable analysis technique for silicon material characterization. We applied this method to determine the influence of a low temperature annealing after surface passivation on the interstitial iron concentration. The influence of hydrogen passivation induced by silicon nitride passivation is estimated by comparison of silicon nitride and aluminum oxide passivation. The second part of this work deals with systematic errors inherent to the iron concentration technique. Simulations show under which conditions errors occur due to the non-uniformity of carrier profiles.