Abstract: An apparatus and method are described for defining a small interrogation volume in a liquid sample suspected of containing an analyte. The apparatus and method utilize a dual optical fiber probe constructed with the ends of the optical fibers at the liquid-fiber interface spatially oriented such that the intersection of their longitudinal axes forms an included angle ranging from about 40.degree. to 140.degree.. By appropriate choice of the numerical aperture and core diameter of the optical fibers and the separation distance between the longitudinal axes of the optical fibers at the liquid-fiber interface, a finite interrogation volume of predetermined size and independent of the total liquid sample volume can be defined. The apparatus and method substantially reduce background interference with corresponding increase in assay sensitivity.

Abstract: An apparatus and method are described for defining a small interrogation volume in a liquid sample suspected of containing an analyte. The apparatus and method utilize a dual optical fiber probe constructed with the ends of the optical fibers at the liquid/fiber interface spatially oriented such that the intersection of their longitudinal axes forms an included angle ranging from about 40.degree. to 140.degree.. By appropriate choice of the numerical aperture and core diameter of the optical fibers and the separation distance between the longitudinal axes of the optical fibers at the liquid/fiber interface, a finite interrogation volume of predetermined size and independent of the total liquid sample volume can be defined. The apparatus and method substantially reduce background interference with corresponding increase in assay sensitivity.