I am trying to use the ninhydrin assay method to determine hydrolysis degree of whey proteins without nitrogen atmosphere. I used ninhydrin reagent in the ethylene glycol - sodium acetate solvent system and in the methoxyethanol - sodium citrate solvent system but I can't get a good calibration curve. It seems that the reagent isn't stable for more than 20 minutes. Do you have any advice about this?

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