An ultra-sensitive detection method for small molecules such as antibiotics was developed using ICP-MS with magnetic and $TiO_2$ nanoparticles. Since most of the antibiotics are too small to employ a sandwich-type extraction through an immunoreaction, a non-specific platform was employed, in which the target was extracted by magnetic separation, followed by tagging with $TiO_2$ nanoparticles of 11.2 nm for ICP-MS measurement. The detection limit for salinomycin obtained from spiked serum samples was $0.4ag;mL^{-1}$ (${pm}10.3%$), which was about $1.5{ imes}10^6$ times lower than that of LC-MS/MS and about $1.2{ imes}10^{11}$ times better than that of ELISA. Such an excellent sensitivity enabled us to study the toxicity of antibiotics exposed to human beings by determining them in serum.