How to choose the wavelength for the spectrophotometer?
If working with unknown compounds, it is best to take an absorbance spectrum (or reflectance spectrum for opaque materials).for as large a range as possible with your instrument. This will generate a spectrum, such as the one shown below, that will allow you make a determination as to which wavelength is most useful (if that is your project goal). Of course CRAIC instruments are capable of measuring from the deep Ultraviolet to the Near Infrared so a lot of information can be collected in a single spectrum. However, for example, if you wanted to measure the fluorescence spectrum of the sample, you would look where the sample absorbs light most strongly and then use that wavelength of light to excite fluorescence. Another example would be the determination of concentration. It is best to see how the absorbance changes at the peak maxima of the correct absorbance peak shown in your spectrum to make sure that you have the most reliable results.
These last are just two examples and your project goal may use the absorbance spectrum in a different manner. But it is always a good idea to take the maximum wavelength range possible for an unknown sample.
UV-Visible-NIR Absorbance Spectra of a blue textile fiber