260/230 ratios as estimated by the Nanodrop can vary a lot among repeat measurements, and decline over time as the drop is left on the pedestal. In contrast, the 260/280 ratio and concentration remains stable over several minutes. I have confirmed this behaviour in multiple tests. I have no idea about the mechanism behind - if it was from evaporation the concentrations should increase. In conclusion - for better accuracy, make replicate measurements, but do so quickly!
