If your drug shows multiple peaks in UV-Vis, the absorbance should generally be proportional to concentration unless interactions like dimerization or aggregation occur. These phenomena can shift peaks, alter their intensities, or result in the appearance of additional peaks. At high concentrations, saturation might also occur, leading to flattened peaks and deviations from linearity. Diluting the solution can help determine if these effects are concentration-dependent, and checking the absorbance across a range of concentrations can confirm proportionality. If the issue persists, consider the impact of the solution’s chemistry, such as pH or solvent effects, on the observed spectra.

You can use consistent absorbance peaks at wavelengths other than lambda max to construct a standard curve in UV-Vis spectrometry, provided they show reliable linearity with concentration. It's crucial to validate that these alternative peaks yield reproducible and accurate results.

In addition and continuation of the reply Abdelhak Maghchiche . You can use the approach used in analytical methodology validation to select analytical wavelengths and thus working absorption peaks. In either case, you will need to measure a series of dilutions of your sample and statistically process the results to determine the most accurate and informative peaks.