Hi Hanna, you could try Chelex.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3564657/

You could also add EDTA to chelate the metal ions.

Dear Steingrimur, yes, thank you very much, the case described in the reference you gave me is almost exactly the case - I can observe much too fast bleaching of nitrosothiol solutions when I use purchased reagents. Thus Chelex is of course the reagent of choice here:).

Dear Adam, I cannot use any chelator that isn't easily separable, as it interferes with my furter experiment, yet your answer inspired me to simply have an additional control in the presence of EDTA, for the samples where it is impossible to eliminate the metal traces.

Thank you for your kind help,

H.