Abstract: An apparatus and method for on-column analysis in flash chromatography. The detection scheme utilizes refractive index changes as analytes move through an illuminated region of a chromatography column. The column packing material is a diffuse scattering medium when the refractive index of the solvent is significantly different than that of the packing material. The magnitude of the signal depends on the degree to which the refractive index mismatch is changed as an analyte passes through the illuminated region. This detection scheme provides a simple, inexpensive means for monitoring the end of a flash chromatography column to determine the exit time of the species of interest, thus greatly reducing the post-column analysis time. Additionally, the detector is movable along the length of the column, offering the potential to monitor separations as they occur.