Hi Erel,

A standard can only be interpreted as a comparison between two quantities: one known quantity (concentration of BSA protein) and one unknown quantity (concentration of protein of interest). This kind of metric gives you an estimation on the concentration of your unknown sample. When your curve is generated and you measure your samples, you are able to extrapolate the values of the unknowns regardless of whether or not the BSA standards readings read published absorbance levels. The best course of action to make sure that your results can be rigorous in the context of standard curves is to have duplicate standards and ensure your solution in both cases and when running unknown samples are the same. This comparison is therefore relatively independent of the measured absorbance.

To actually answer your question, if you use cuvettes then these could be absorbing more light as time goes on. Perhaps there is clumping occurring in the BSA or the BSA in question is nearing the end of its shelf life. The machine could have older or dirty sensors and therefore continuously reads lower absorbance levels. There could be a film over the light emitter so transmission is lower than what the machine knows it is outputting thereby making absorbance lower. IF it seems like this could be compromising data, I would call a technician or service your machine. Otherwise, routine maintenance is likely fine.