You can suspend the powder in deionized water

You can mesuare solid sample directly using an intregrating sphere.

You can measure the powdered sample using a praying mantis set-up or integrating sphere on the UV-VIS spectrometer.

An integrating sphere is the way to go. It eliminates the addition of other materials to cloud the issue.

Thank you so much for your answers.Could you please tell me what is the sample preparation and how i should place the sample in the sample holder. Thank you.