Estimation of absorption coefficients of scattering suspensions using opal glass

Previously, the algal absorption product (absorption coefficient at wavelength times cuvette depth) was estimated as Dλ − D750, the difference in optical densities of a suspension, measured against water in opal glass cuvettes. Such estimates are subject to “cuvette error” dependent on dimensions and materials of the cuvettes and to “A error” due to inadequacy of 750 nm as a reference wavelength. Monte Carlo calculations were performed to assess cuvette error; then limits of the combined cuvette and λ errors were explored. With cuvettes and suspensions used previously, the combined error was usually ±10% (10–20% at 440 nm), not 100% as supposed in a recent publication. Cuvette error is minimal in cubical cuvettes with black sidewalls, but substantial λ error may still occur. Both errors are avoided in a method using two pairs of opal cuvettes.