[mkbh10]

I am doing photocatalysis of methyl orange dye using TiO2 as a catalyst in the presence of UV light. After the experiment when I take absorption spectrum , the baseline of spectra (sample 0 min, 30 min , 1 hr, 2 hr exposure to UV light) is shifted vertically upwards from the reference Methyl orange base line.

Why does it get shifted ?

Also what will the reference be same for 0 hr, 30 min , 1 hr spectra as when I take absorption spectra of methyl orange, i have water as reference but when i take measurements of (0hr, 30 min etc) sample, shudn't the 0hr sample (methyl orange +TiO2) be the reference one ?

I think the reference should be methyl orange as 0 hr sample is not exposed to UV light and no reaction occurs but the absorption curve is different for plain methyl orange and 0hr solution . Why is that so ?

[fledarmus]

These may be obvious questions, but bear with me...

1) What is in your reaction besides methyl orange?

2) Have you tried running the absorption spectra of your reaction mixture without methyl orange?