I believe absorbance will provide you more bulk information as the it considered the light passing through the sample. However the reflectance in a surface phenomenon which depends on the surface of the analyze. if you want to measure the characteristic properties of the analyte fluid, absorbance may give you more information in the spectrum.

Chathura Abeywickrama I would argue that there is not only reflectance from the first interface(s), but part of the light is also reflected from the backside, which means that it has passed (at least) two times through the bulk of the fluid. However, in the case of strong absorption or very weak absorption not much information of the bulk will be provided by reflectance, either because no or not much light reflected from the backside will reach the frontside and the detector, or because the indices of refraction of the cuvette and the liquid will not be very different to cause large (multiple) reflection(s). So yes, measuring transmittance ratios (solution vs. pure solvent as blank) and calculate absorbance from this ratio would be the way to go.

Hi Thomas, It is a nice explanation. But instrumentally what is detect is the transmit range and then cover it to optical absorbance. Please correct me if I’m wrong here. The question is to choose either absorbable or reflectance?. So yes the answer is absorbace which is based on the transmittance ( Absorbance = 2 - (log %T).

Thank you.

Shure, transmittance is the standard for liquids! But a lot of instruments can also be used to detect reflectance, so you in principle you have both options...

It depends on the restrictions of your sample/measurement set up. If you cannot measure transmission through your sample, then the reflection from the surface can be analysed in terms of the complex refractive index of the liquid.