In a method of detecting target analytes, the target analytes are labeled with first magnetic beads and the contaminants are labeled with second magnetic beads, wherein the first magnetic beads have a first magnetic relaxation characteristic and the second magnetic beads have a second magnetic relaxation characteristic. A labeled target analyte or a labeled contaminant passes through a detection region one at a time. A magnetic field is applied to the detection region, thereby magnetizing the first magnetic beads in the labeled target analyte or the second magnetic beads in the labeled contaminant in the detection region. The magnetic field is then removed. The magnetic relaxation characteristics of the magnetized first magnetic beads or of the magnetized second magnetic beads are detected after the magnetic field is removed. A target analyte or a contaminant can be differentiated based on the detected magnetic relaxation characteristics.