Disclosed are photometric methods and devices for determining optical pathlength of liquid samples containing analytes dissolved or suspended in a solvent. The methods and devices rely on determining a relationship between the light absorption properties of the solvent and the optical pathlength of liquid samples containing the solvent. This relationship is used to establish the optical pathlength for samples containing an unknown concentration of analyte but having similar solvent composition. Further disclosed are methods and devices for determining the concentration of analyte in such samples where both the optical pathlength and the concentration of analyte are unknown. The methods and devices rely on separately determining, at different wavelengths of light, light absorption by the solvent and light absorption by the analyte. Light absorption by the analyte, together with the optical pathlength so determined, is used to calculate the concentration of the analyte. Devices for carrying out the methods particularly advantageously include vertical-beam photometers containing samples disposed within the wells of multi-assay plates, wherein the photometer is able to monitor light absorption of each sample at multiple wavelengths, including in the visible or UV-visible region of the spectrum, as well as in the near-infrared region of the electromagnetic spectrum. Novel photometer devices are described which automatically determine the concentration of analytes in such multi-assay plates directly without employing a standard curve.