A method of quantitatively determining 8-isoprostane is provided that includes fluorescently labeling 8-isoprostane with a quinoxalinone derivative in an excess amount relative to 8-isoprostane, separating a fluorescently labeled 8-isoprostane from a unreacted quinoxalinone derivative by contacting a reaction mixture containing the fluorescently labeled 8-isoprostane and the unreacted quinoxalinone derivative with a cation exchange support having a sulfonic acid or a sulfonate immobilized thereon, and quantitatively determining the fluorescently labeled 8-isoprostane that has been separated from the unreacted quinoxalinone derivative.