IMMUNOASSAY FOR PARATHION WITHOUT ITS PRIOR REMOVAL FROM SOLUTION IN HEXANE

Immunoaffinity methods are presently finding limited applications in e nvironmental analysis because many target compounds are best handled i n non-polar solvents such as hexane. These solvents can be detrimental to protein molecules, particularly when the solvating water which is necessary for bioactivity is lost. Consequently, a generic immunoassay principle which would allow the provision of samples dissolved in hex ane was sought. A competitive enzyme-linked immunosorbent assay (ELISA ) method for the measurement of aqueous parathion was developed, optim ized and then adapted to the measurement of the analyte without its pr ior removal from hexane. The parathion-specific antiserum was encapsul ated in reverse micelles composed of Aerosol OT and having aqueous cen tres, and a solution of parathion in hexane was then allowed to compet e for antibody binding sites with the antigen-coated surface of a micr otitre plate. The calibration graph for the hexane-compatible ELISA me thod is displaced from its aqueous counterpart in correspondence with the partition coefficient of parathion between water and hexane, this method being about 10(4)-fold more sensitive. It is probable that the thermodynamic barrier to the transfer of parathion from an organic bul k phase to the aqueous interior of a reverse micelle balances the anti gen-antibody binding strength and thus reduces the potency of parathio n as an antigenic competitor. Means are suggested for tuning the calib ration graph of the hexane-compatible ELISA method to the analyte conc entration range that is of interest.