First of all you need to have the pure thymoquinone and the appropriate solvents (chloroform and ethanol 1:1, for ex ). You should use these substances to record the uv vis spectra between 750 and 190 nm and to determine the characteristic peaks for thymoq. Using different amounts of thymoq you could draw the standard curve at 271 nm. You have to take into account that in your sample may be other substances with absorbance at the same wavelength. For details please see