It is possible to excite by a Penning's discharge the neutral atoms sputtered during the bombardment of a target by positive ions and thus to increase the sensibility of the ionoluminescence analysis method. However, the characteristic lines of the target's elements may be masked by the discharge lines. We describe in this paper an arrangement using a lock-in amplifier which permits to reduce the discharge lines. The results obtained with a duraluminium target show that it is possible to identify the characteristic target's lines even when they are weak and near the discharge lines.