[en] The parameters that influence the detection limit of small concentrations of a trace element in particle induced X-ray emission analysis are considered. These are: proton-beam intensity, incident particle energy, target thickness, X-ray absorption filter thickness, irradiated target area and the solid angle of detection. The useful range of the parameters, e.g. the exciting beam intensity, is restricted by the maximum target temperature, the highest counting rate which can be handled and by radiation protection safeguards. Taking these restrictions into account the Peak-to-Square-Root-Background ratio (PSRB) has been maximised. The calculations are based on production data for characteristic as well as for bremsstrahlung background radiation reported in the literature. The procedure for parameter optimisation is described and applied for the determination of selenium concentrations in human serum. (orig.)