Abstract: A sample preparation system for an analytical system for determining a measured variable of a sample liquid, comprising: a transport unit connected with a sample taking location via a first fluid conducting line; a sample collecting unit, which serves the analytical system as a staging area for automated removal of liquid samples, based on which the measured variable is determined; a filter unit arranged between the sample taking location and the transport unit; and at least one reservoir connected via a second fluid conducting line with the filter unit for providing a cleaning medium for cleaning the filter unit; wherein the transport unit is embodied to transport sample liquid from the sample taking location through the filter unit into the sample collecting unit. The cleaning medium includes an oxidizing agent.