A method of detecting target molecules comprising conjugating a first magnetic particle to a first ligand to form a first magnetic particle ligand conjugate adding the conjugate to a sample containing target molecules to form a mixture comprising, the free conjugate and conjugate-target molecule binding pairs measuring a first magnetization of the mixture subjecting the mixture to a first force measuring a second magnetization value of the mixture subtracting the second magnetization value from the first magnetization value to calculate a first force-induced magnetization contrast subjecting the mixture to a second force measuring a third magnetization of the mixture and subtracting the third magnetization value from the second magnetization value to calculate a second force-induced magnetization contrast.
