A novel measuring system for the determination of paramagnetic particle labels for use in magneto-immunoassays

Coated micrometer-sized paramagnetic particles (PMPs) are readily available and widely used in immunoassays, mainly for separation and as a solid phas e. We have described in a separate paper a model sandwich assay in which ap proximate to 1 x 10(5) to 1 x 10(6) PMPs (22.8 mum diameter) are immobilise d on a plastic strip at the end of the assay. In this paper, we describe th e design of an instrument that is capable of determining the number of PMPs on the plastic strip. The paper also describes a method of making standard plastic strips with known numbers of PMPs on them. A strip, when placed in a coil of wire in parallel with a capacitor, causes the resonant frequency of the coil to decrease because of the presence of the PMPs. The decrease in frequency relates directly to the number of PMPs on the strip. A circuit based on a voltage-con trolled oscillator and a phase-locked loop is used to accurately measure the resonant frequency of the coil. The instrument is capable of detecting at least 1 x 10(5) PMPS immobilised on a plastic stri p and has a linear response (r = 0.99) for up to at least 3.33 x 10(6) PMPs . In terms of the iron content of the PMPS, the detection limit is approxim ate to 1.2 mug Fe in the paramagnetic particles and the sensitivity is appr oximate to 3 Hz per tg of Fe. The instrument is small and compact and toget her with a suitable magneto-immunoassay will have many applications, includ ing near-patient monitoring. (C) 2001 Elsevier Science B.V. All rights rese rved.