The lateral variation of the energy distributions of sputtered Ti atoms has been investigated by energy resolved mass spectrometry during reactive dc magnetron sputter deposition of TiN from a Ti target in an argon/nitrogen gas mixture. The mass spectrometer was placed at the substrate position and scanned across the target surface of a 2 inch planar circular magnetron. The energy distribution of sputtered particles is influenced by their origin, showing significant differences between the center and the erosion zone of the target. The results are interpreted in terms of laterally different states of target poisoning, which results in a variation of the surface binding energy. The reactive gas target coverage as derived from the sputtered energy distributions is in reasonable agreement with predictions from model calculations.