This test method is a widely used technique to accurately quantify these sulfur-containing amino acids in food, feed, and biological samples. Because cysteine and methionine are sensitive to oxidation and do not naturally absorb UV light strongly, they require chemical modification (derivatization) prior to chromatographic analysis. Samples are first oxidized to stabilize these amino acids, then hydrolyzed to release them from proteins. The free amino acids are then derivatized with a reagent (e.g., AccQ•Tag™) to make them detectable by UV. The derivatized sample is separated by reversed-phase UPLC and quantified using the detector, allowing accurate measurement of cysteine and methionine in complex samples.