Abstract: A combination of a microcolumn packed with a suitable absorbent material and a centrifuge tube. A predetermined volume of an eluent being capable of selectively eluting a desired substance through the microcolumn is contained above the packing of the absorbent material. The microcolumn and the centrifuge tube are dimensioned in such a manner that upon centrifugation of the predetermined volume of the eluent through the microcolumn, a level of the eluent remaining in the microcolumn, and a level of the eluent in the centrifuge tube coincide substantially at or above a top surface of the packing of the absorbent material. Consequently, centrifugation of the assembly of the microcolumn and the centrifuge tube causes passage of the predetermined volume of eluent through the microcolumn. Determination or assay of the selectively eluted desired substance may be performed by spectrophotometric methods.