Measuring system for the rapid determination of the concentration of coated micrometer-sized paramagnetic particles suspended in aqueous buffer solutions

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Abstract
Coated paramagnetic particles (PMPs) with diameters typically 1–10 μm are widely used in biomedical investigations, including immunoassays and molecular biology. In this article we describe an investigation into a measuring system to determine rapidly the concentration of PMPs in suspension in a buffer solution in a pot. The measuring system is based on a coil in a parallel resonant circuit with a capacitor. The sample pot is placed in the coil, causing the inductance of the coil to increase and the resonant frequency to decrease. The decrease in resonant frequency is directly related to the concentration of PMPs in the pot. Using a circuit based on a Colpitts oscillator it is possible to determine the concentration of PMPs in the pot with an error of about 10% over the range 0–9 mg g−1. Much more accurate measurements (error <1%) are possible using a circuit design based on a phase-locked loop.