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Magnetic Particle Imaging (MPI) and Magnetic Particle Spectroscopy (MPS) usually require a reference sample measurement to provide information about the non-linear dynamic magnetic behavior of a specific magnetic nanoparticle (MNP) type. This reference sample based approach presupposes that this dynamic magnetization behavior of MNP is concentration independent. We investigated Resovist® and its precursor Ferucarbotran at different concentrations to verify this assumption by means of MPS. Remarkably, for Resovist® we found a strong concentration dependence of the MPS signal. Above an iron concentration of about 150 mmol/L the shape of the moment and phase spectra changed with increasing iron concentration. In contrast, for Ferucarbotran we found no concentration dependence of the dynamic magnetic behavior even though at a two‑fold higher initial concentration. Our experimental results indicate that the dynamic magnetic behavior of MPI tracers may be altered at higher concentrations and should be studied prior to MPI by MPS experiments.