That depends on the spectrophotometer. The dynamic ranges of instruments vary. Consult the instrument's user manual. Meanwhile, you can either simply dilute your sample or use a narrower cuvette (shorter pathlength) to reduce the absorbance.

sir, you mean that novel instruments effects on extinction coafficent beacuse i am working on gold nanorods . i don't know why this phenomena happen.

I meant that spectrophotometers vary in their ability to accurately measure high absorbance values. An absorbance of 2 corresponds to a 99% reduction of the incident light, an absorbance of 3 to 99.9% and an absorbance of 4 to 99.99%. High-end spectrophotometers can accurately measure absorbances of 3-4, whereas less expensive ones may be limited to 2. The dynamic range of the instrument may also depend on the wavelength.

In general, I try to keep my absorbance measurements between 0.1 and 1, unless I have experience to show that I can still get a linear relationship between absorbance and concentration for higher absorbances.

In the case of nanoparticles, the influence of multiple scattering becomes more significant as the concentration increases, leading to a change in the relationship between absorbance and concentration. Therefore, it is better to keep to the lower end of the absorbance range, i.e.,