Patent ID: 6852547

Claim:
A method for measuring low levels of a substance in a sample, said method comprising the steps of: dispersing a plurality of paramagnetic particles in a colloidal solution; introducing an amount of said colloidal solution into a sample chamber; subjecting said sample chamber to a substantially uniform magnetic field; forming at least one substantially linear structure from said plurality of paramagnetic particles with said substantially uniform magnetic field; introducing a sample having low levels of said substance to be measured into said sample chamber for analysis; rotating said substantially uniform magnetic field at a constant rate to cause said substantially linear structure to rotate therewith; binding a portion of said substance to be measured in said sample to said substantially linear structure; detecting signals having a time-varying component originating from said sample chamber; and processing said signals with a lock-in amplifier to measure said time-varying component by improving the signal-to-noise ratio thereof using a reference signal having a frequency twice that of the rotation of said substantially uniform magnetic field as a multiplier to measure said low levels of said substance to be measured.