[en] A simple additive method, based on ⁵⁵Fe and ¹⁰⁹Cd source induced fluorescence in a helium environment, is presented for sulphur and lead determination in petroleum products. Although the density of the matrix influences the X-ray yields, the similarity of photon attenuation in water and petroleum products can be exploited. It is shown that routine system calibration can be achieved by the use of inorganic chemicals dissolved in water instead of organic additives in petroleum products. The low pressure helium flow suppresses the argon X-rays provides a good background region and increases the detection sensitivity for sulphur. (author)