Taking the mass of a sample is a routine operation in the laboratory, which is often repeated during the collection of data. In chemical industry this operation is often done manually. In this work a procedure for fully automatic weighing of liquid substances with a laboratory robot is presented. To demonstrate the functionality of the laboratory robot and to show a possible application, the laboratory robot was used to create test samples for the calibration of a gas chromatograph. The achieved accuracy is greater than that of careful manual weighing. The presented method allows handling of liquids commonly used in the laboratory.