The present study describes a new concept of magnetic detection that can be used for fast, selective measurements on magnetic nanoparticles and which is not influenced by the presence of materials with a linear magnetic susceptibility, like tissue. Using an alternating excitation field (View the MathML sourcef~5kHz) with a sequence of static offset fields, the magnetometer is selectively sensitive for the nonlinear properties of magnetic nanoparticles in samples. The offset field sequence modulates the measured inductive response of nonlinear magnetic materials, in contrast to linear magnetic materials. We demonstrate a detection limit for superparamagnetic iron oxide nanoparticles in the sub-microgram (iron) range. The mass sensitivity of the procedure increases with offset field amplitude and particle size. Compared to the sensitivity for particles in suspension, the sensitivity reduces for particles accumulated in lymph node tissue or immobilized by drying, which is attributed to a change in Brownian relaxation. The differential magnetometry concept is used as a tool to perform non-destructive analysis of magnetic nanoparticles in clinically relevant tissue samples at room temperature. In addition, the differential magnetometer can be used for fundamental quantitative research of the performance of magnetic nanoparticles in alternating fields. The method is a promising approach for in vivo measurements during clinical interventions, since it suppresses the linear contribution of the surrounding body volume and effectively picks out the nonlinear contribution of magnetic tracer.