a method for measuring low levels of a substance in a sample includes the formation of a rotor from paramagnetic particles in a substantially uniform magnetic field . the rotor is rotated by rotating the substantially uniform magnetic field . a portion of the substance in a sample is bound to the paramagnetic particles , and a signal having a time - varying component is detected . the signal is then processed using a lock - in amplifier with a reference signal having a frequency twice that of the rotation of the magnetic field . this improves the signal - to - noise ratio of the time - varying component of the signal .