Abstract: The present application relates to a method for operating a dosing device with a control unit and a dosing unit. The dosing unit has a cannula with a first volume and a tip, and also a sampling container fluidically connected to the cannula. In a first step, the dosing unit is moved linearly at a predetermined speed in a first direction along an axis, such that the cannula is moved into a vessel containing at least one liquid. At the same time, fluid is aspirated constantly through the cannula with a predetermined volumetric flow by a pump device. At least one optical parameter of the aspirated fluid is measured by at least one optical sensor, which is arranged between the cannula and the sampling container. When a change of the at least one optical parameter is detected, a first position of the dosing unit on the axis is stored by the control unit and the movement of the dosing unit is interrupted.