A phase contrast quadrature interferometric method and apparatus for determining the presence or absence of a target analyte in a sample. The method includes probing a substrate exposed to the sample with a laser beam. The substrate includes a reflecting surface with a first region having recognition molecules specific to the target analyte and a second region without recognition molecules. The method also includes probing the first and second region, and measuring time dependent intensity on a photodetector at one or both of a pair of quadrature angles of a reflected diffraction signal. The apparatus includes a laser source, a platform for receiving the planar array, an objective lens offset from the platform by approximately a focal length, and a split photodetector means for measuring a first quadrature and a second quadrature in a signal resulting from reflection of the laser beam.