This invention relates to a fiber optic apparatus for simultaneously measuring the scattering and concentration signals of macromolecules in a flow cell 3. The apparatus is based on the delivery/focusing of both a laser and ultraviolet light source to the same physical position in a low volume flow cell 4, via a bifurcated optical fiber 3. This configuration allows the light scattering and concentration signal changes associated with a macromolecular solution passing through the flow channel to be measured simultaneously. This invention also relates to a method that uses the optical apparatus 10 to determine properties of a macromolecular solution such as the ideal crystallization and/or formulation conditions (via B22) for a given protein solution.