Thesis: In recent decades, human activities around the world (atmospheric nuclear tests, phosphate and uranium minings, oil and gas extraction, hydraulic fracturing) and nuclear accidents, have contributed to the release of 226Ra and 137Cs into the environment, two radionuclides (RNs) that are radiotoxic because of their similarity to Ca and K, respectively. The current challenges are related to better understanding the geochemical behavior of these two RNs and their transport mechanisms in the environment. This implies being able to precisely quantify them at ultra-trace level ( < pg L-1) in complex samples with complex matrices and sometimes of low volumes. This is challenging and requires the implementation of specific sample treatment methods in combination with sensitive analysis techniques. Various works have shown that the efficiency of extraction procedures is currently limited by the lack of specificity of the sorbents used.In this study, the potential of ion-imprinted polymers (IIPs) to specifically extract Ra2+ and Cs+ was evaluated. After identifying the best monomer candidates, porogens and complexation time from mass spectrometry experiments, solubility tests, and conductimetry experiments, different IIPs were synthesized by bulk polymerization using Cs+ and Ba2+ as template ions. Their selectivity was evaluated by confronting their extraction potential to that of non-imprinted polymers synthesized under the same conditions but in the absence of template ions. After optimizing the extraction conditions, the characterizations revealed a promising IIP for Ra2+ extraction, in terms of both selectivity and specificity. The influence of the template ions/monomers ratio on capacity, breakthrough volume, and surface area was investigated and the synthesis repeatability was demonstrated. The presence of matrix effects was observed during extraction of spiked mineral waters. In view of the conclusions drawn from these studies, several prospects for improvement of the synthesis were proposed.A similar characterization work was carried out on a resin which is being developed by the company Triskem. Its performances were compared to that of the IIPs and the AnaLig® Ra-01 resin, the only Ra-specific resin commercially available. Finally, measurements of 226Ra in 1 mL volumes of pore waters were performed using the best of these three sorbents in combination with a micro-sampler coupled to a desolvating nebulizer upstream of the inductively coupled plasma mass spectrometer (ICP-MS).