A system and related method for determining the volume of a sample solution delivered by a liquid delivery device. The system includes devices for adding and removing diluent, the sample solution under analysis and a mixture of the two in a cell having a substantially constant cross sectional area and a known horizontal optical path length. The system includes one or more spectrophotometers to measure horizontally and vertically absorbance associated with two distinct chromophores. A first chromophore of unknown concentration is in the diluent and a second chromophore of known concentration is in the sample solution. The method determines absorbance horizontally and vertically of the first chromophore prior to adding the sample solution and then determining absorbance of the first chromophore horizontally and vertically and the second chromophore horizontally after mixing the diluent and sample solution together. That information is used to determine the volume of the sample solution added to the cell.