Abstract: Poly( N-isopropylacrylamide methylacrylic acid)[P(NIP-co-MAA)], a linear water-soluble pHprecipitating synthetic polymer, was used as a novel separating carrier for the reactants in immunoassay. The P(NIP-co-MAA) precipitates out of water below a critical pH 5.6 at 37 ℃, and redissolved when the solution pHis above 6.1. This characteristic of the P(NIP-co-MAA) made it possible to carry out the immunochemical steps of the assay in true solution and then to quickly separate the resulting procucts from the reaction mixtures. The above approach was applied to an assay for the sandwich immunoassay of hepatitis Bsurface antigen(HBsAg). Sensitivity of this method was close to that of traditional ELISAusing same reactants. However, the assay was much faster(assay time decreased from 100—120 min to 45 min). This method has been applied to the determination of the HBsAg levels in human blood serum with satisfactory results. This general technique may also be used for a wide variety of separation processes in addition to immunoassay, in which a specific component is to be isolated for analysis, recovery, or disposal.

Key words: pH sensitive polymer, Phase separation, HBsAg, Fluorescence immunoassay