Monensin

A 3% vanillin solution was used as a derivatization reagen<sup>t</sup> to react with monensin, and the product has a maximum absorption peak at 525 nm [50]. 2 mL of 3% vanillin solution and methanol were added to the pre-treated sample solution in a 50 mL volumetric flask, shaken, diluted, and then heated in a 45 ◦C water bath for 20 min. After the solution cooled to room temperature, its absorbance was measured at 525 nm. The linear regression equation was: y = 0.0329x + 0.0008, R<sup>2</sup> = 0.9973.
