The interferences caused by reactions with fast neutrons may be corrected with the help of substances containing the interfering element, which are irradiated together with samples and standards. For the correction of the apparent sodium content caused by the28Si(n,p)-reaction we need silicon with an extremely low sodium content. Irradiation without such a standard needs calculation for interference with the help of flux monitors. Because of the highly effective threshold energy of the reaction an additional correction factor taking into account various neutron spectra in different irradiation positions is necessary.