Abstract: An apparatus for the conversion of substrates both for analytical and preparative purposes in a flow-through and cross-flow reactor having compact porous discs and/or compact porous tubes as carrier for conversion reaction or preparative separations which compact porous discs and/or compact porous tubes are modified with substances being capable of specifically binding and/or converting said substrates by distributing the samples to be converted and/or preparatively separated and collecting the converted and/or separated substrates preferably in a low volume, wherein the distribution of the sample does not affect the carrier adversely.