[en] The determination of burn-up is one of the essential components of post-irradiation examination of nuclear fuel samples. In the framework of the ARIANE programme (Actinides Research in a Nuclear Element), at PSI the analysis of the isotopic vectors of uranium, plutonium, neodymium and some other fission and spallation products was carried out for the first time in the Hot Lab using high-performance liquid chromatography (HPLC), coupled on-line with an inductively coupled plasma mass spectrometer (ICP-MS). The results of these investigations, a comparison with the classical technique for burn-up determination, the advantages and limitations of the new method, the accuracy and precision of these types of analyses, the applicability of the technique to other samples and separation problems, and conclusions for further analyses of nuclear fuel samples, are here presented. (authors)