A method was developed for the determination of the nuclide-specific concentrations of U, Pu, Nd and Gd in two types of spent nuclear fuel (UOx and Gd-enriched). High-performance ion chromatography (HPIC) was used to separate the target elements from one another while sector-field inductively coupled plasma-mass spectrometry (SF-ICP-MS) was used for their determination relying on isotope dilution for calibration. In order to obtain the best possible precision for these isotope ratios extracted from the transient HPIC-SF-ICP-MS signals, the SF-ICP-MS data acquisition parameters were optimized and the most suitable method for calculating the isotope ratios from the transient signals was identified. The point-by-point (PbP), linear regression slope (LRS) and peak area integration (PAI) approaches were compared in the latter context. It was found that data acquisition in the flat centre of the spectral flat top peak using a mass window of 25 %, a dwell time of 10 ms and 20 samples per peak, while using PAI for isotope ratio calculation, gave the best precision on the isotope ratios extracted from the HPIC-SF-ICP-MS transient signals. These parameters were used in the determination of the nuclide-specific mass fractions of Pu, Nd and Gd in two types of spent nuclear fuel using isotope dilution HPIC-SF-ICP-MS. For U, which was present at a higher concentration, the element fraction was collected and analysed off-line after dilution. For the other target elements, an online approach was used. An uncertainty budget estimation was made using the bottom-up approach for the resulting mass fractions, and the accuracy and precision obtained when using isotope dilution HPIC-SF-ICP-MS were compared with those obtained with the routinely used techniques, isotope dilution TIMS & alpha spectrometry (an ISO 17025 accredited method).