  Magnetic particle spectroscopy (MPS) is one of the most versatile methods to
characterize the magnetic properties of magnetic nanoparticles (MNPs). The
excitation magnetic field is one of the most crucial factors that affects the
MPS signal of the MNPs. In this study, a multi-frequency MPS system is
developed to investigate the MPS signal of MNPs in different ac magnetic
fields. The MPS system consists of a multi-channel excitation module for the
generation of different-frequency ac magnetic fields and a detection module for
the measurement of the magnetic response of the MNPs. The MPS system allows to
generate ac magnetic fields with a frequency up to 32.6 kHz and amplitude up to
25 mT. The MPS signals of the MNPs in different ac magnetic fields are measured
to systematically evaluate the performance of the multi-frequency MPS system,
including the MNP spectra and its dynamic magnetization curve. In addition, the
signal-to-noise ratio (SNR) of the MPS system is quantitively assessed with
measured MPS signals of a given MNP sample and DI water. Furthermore, a series
of MNP samples with different iron concentrations are prepared and measured to
evaluate the limit-of-detection (LOD) in terms of iron concentration. The
influence of the excitation magnetic field, including frequency and amplitude,
is discussed based on the SNRs of the measured harmonics. Experimental results
show that the LOD is 2.3 ng in terms of iron.
