A process for (1) separating a target biological ligand known to be, or suspected of being, present in dilute concentration in an aqueous fluid and (2) determining the amount of said target ligand so separated, which process comprising the steps of
- (a) coating with a first biological binding partner for said target biological ligand a group of superparamagnetic particles, which particles have an average mean diameter of at least about 100 nm and are each composed of discrete subunits of superparamagnetic material, which subunits have an average mean diameter of 1–30 nm and are separately spaced apart from one another within a covering matrix of non-metallic, non-magnetic material that is compatible with, but non-reactive with, said biological ligand and its first biological binding partner,
- (b) immersing the coated superparamagnetic particles from step (a) in a sample of said aqueous fluid which is known to contain, or is suspected of containing, said target biological ligand and allowing said particles and said fluid to incubate for a time sufficient to enable the target biological ligand, if present, to react with its first biological binding partner coated on said particles, thereby forming complexes,
- (c) exposing said complexes to a gradient of a magnetic field whereby the complexes acquire a magnetic charge and are attracted toward one another and away from the bulk of said fluid,
- (d) removing said fluid from said complexes by any suitable means,
- (e) washing said complexes and adding them to a small volume of an aqueous buffer to form a dispersion of said complexes in said buffer,
- (f) applying said dispersion to the sample receiving end of an immunochromatographic (“ICT”) device configured as a dipstick and comprising a strip of bibulous material having at least one immovable stripe of a second binding partner for said target biological ligand which has been previously permanently affixed thereto at the end thereof remote from said sample receiving end,
- (g) allowing said dispersion to migrate along said strip of bibulous material and contact said at least one immovable stripe of a second binding partner for said biological ligand, whereby said target biological ligand on the surface of said complexes binds to its second binding partner on said at least one immovable stripe,
- h) measuring the magnetic charge intensity of said at least one immovable stripe, and
- i) determining from a previously established standard curve, obtained by constructing a plot of measurements of magnetic charge intensity against amount of biological ligand present, obtained when a series of standardized samples each containing a different known amount of the target biological ligand were tested in the foregoing process, the amount of biological ligand present in the fluid sample.