Hello,
I am trying to measure chondroitin sulfate concentrations in supernatant of cell cultures (minimal media = Phosphate buffer, MgSO4, NH4CL, and Chondroitin sulfate, pH 7.0). I am using the Dimethylmethylene Blue Assay (https://bio-protocol.org/pdf/bio-protocol1236.pdf), but have been unable to achieve a linear curve. By looking at my curves, it appears as the zero concentration reading is an outlier (below the line made by other points). The pH of my DMMB reagent is 1.5. Please suggest solutions and troubleshooting ideas.
Thanks.