Abstract: A method and solid phase matrix is disclosed which has special utility for use in conjunction with automatic pipetting apparatus for conducting reactions at a liquid-solid interface. A reaction component is fixed on the surface of the solid phase matrix which is wetted by a liquid phase containing a freely diffusing, mobile component. The matrix is comprised of a support surface to which are attached a plurality of inwardly projecting fins. The innermost extension of the fins defines a centrally located well into which an automatic pipetting apparatus may be inserted. The method may be used for qualitative or quantitative determinations of the mobile component contained in a liquid which wets the solid phase matrix.