A for traceably determining an unknown optical path length of a sample in an optical measuring device comprises the steps of: providing a drop analyzer connected to a standard spectrophotometer; providing a certified reference material contained in first and second closed high accuracy cuvettes; measuring absorbance of the certified reference material to obtain a first absorbance measurement for the first specified path length; measuring absorbance of the certified reference material for a second path length to obtain a second absorbance measurement; using a dropping device to drop a specified volume of the solvent on an optical surface so that the path length of the specified volume can be determined by reference to the first and second absorbance measurement; and using the dropping device to drop the same volume of sample as the specified volume of solvent on the optical measuring device.