Your blank is not matrix matched with the sample. What you are detecting are the differences of both the refractive index and absorbance between them.

I agree with Bruce, check your blank, it has to have a similar composition to your sample. By the way, in spectrophotometry it's absorption, not adsorption.

thanks alot dear Bruce and Eliza.

but the blank is just the thing that Hach Co. determined. i.e the sample without reagent is blank and the sample with reagent is the main sample.

my tests are nitrate adsorption, and by hach 2010 I try to measure the adsorption capacity by measuring before and after adsorption nitrate concentration. but at some of my results the after concentration is more than the before. and it is really unexpected.

hi! may I know if you have learned the reason behind the negative result?

se kh Did you solved the issue regarding negative test result? If yes, can you please share your views.