Apparatuses and methods for separating, immobilizing, and quantifying biological substances from within a fluid medium. Biological substances are observed by employing a vessel (6) having a chamber therein, the vessel comprising a transparent collection wall (5). A high internal gradient magnetic capture structure may be on the transparent collection wall (5), magnets (3) create an externally-applied force for transporting magnetically responsive material toward the transparent collection wall (5). V-shaped grooves on the inner surface of the viewing face of the chamber provide uniform. The invention is also useful in conducting quantitative analysis and sample preparation in conjunction with automated cell enumeration techniques.