The surrounding sheath focuses the charged particles to distinct parts of the probe surface resulting in nonuniform physical and/or chemical properties. Then, after a time interval dependent of the degree of plasma contamination, this process results in well-defined regions with different work functions that shape the probe characteristic leading to erroneous measurement of plasma temperature and ion density. For Ar/O-2, and Ar/SF6 plasmas produced within a multipolar magnetically confined device we investigated bombardment by positive or negative charges and indirect heating to modify the surface of a planar probe.