Abstract: TA method for the content determination of methionine sulfoxide and methionine sulfone in compound amino acid injection (18AA-II) was established in order to investigate their level in 155 batches of this product, and to explore the reason for the generation of these two impurities.The determination was performed on an Agilent Poroshell 120 EC-C₁₈ column with mobile phases of sodium acetate/tetrahydrofuran solution (A) and sodium acetate solution -acetonitrile-methanol (B, 200∶400∶400) (gradient elution) at the flow rate of 0.5 mL/min.The excitation wavelength and the emission wavelength of the fluorescence detector were 233 nm and 441 nm, respectively.The column temperature was 40 °C, and the injection volume was 8 μL.The contents of methionine sulfoxide and methionine sulfone from 155 batches of compound amino acid injection (18AA-II) was determined using this method, and the residual oxygen content was detected by headspace gas analyzer.The results showed that the linear range of methionine sulfoxide and methionine sulfone were 0.128 1-10.250 0 μg/mL (r = 0.999 9) and 0.261 0-10.440 0 μg/mL (r = 0.999 8), respectively.The limits of quantitation were 0.13 μg/mL and 0.26 μg/mL, respectively; the limits of detection were 0.04 μg/mL and 0.09 μg/mL, respectively.RSDs of precision, stability and repetitive test were all lower than 1.3%.The recoveries ranged 98.00%-100.79% (RSD = 1.15%, n = 9) and 98.19%-102.31% (RSD = 1.33%, n = 9).The content level of oxidized related substances from different manufacturers showed significant difference, showing relevance with the residual oxygen content to some extent, yet no significant correlation with the added amount of antioxygen (sodium pyrosulfite).The method is validated to be useful for the content control of methionine sulfoxide and methionine sulfone in compound amino acid injection (18AA-II).It is quite necessary to include the determination of oxidized related substance into the quality specification.Manufacturers should strengthen the control of remaining oxygen in their products.