Why are results for sulfite standards ~ 20% lower than expected when using the Total Sulfite Assay Kit (Enzymatic) (K-ETSULPH)?

Due to the characteristic instability of aqueous sulphite samples, recoveries of less than 100% should be expected.  This is consistent with values obtained with other commercially available enzymatic sulphite test kits.  

Also, Sometimes the reaction can take a little longer than expected especially if the reaction is performed lower than the stated temperature.  A suggestion is to extend the incubation time beyond the 30 min to ensure that the action has finished (indicated when the absorbance stops decreasing). The K-TSULPH assay kit also measures total sulfite and employs a chemical reaction with Ellman’s reagent.  This may be a suitable alternative to K-ETSULPH for some sample matrices.

Product Page (K-ETSULPH)