Abstract:
A method of quantifying a target non-peptidic compound having an amino group contained in one or more biological samples, which comprises a step of producing a difference in the mass of the target non-peptidic compound between samples, by using a combination of two or more kinds of stable isotopes of a compound represented by the formula (I): wherein R1, R2 and R3 are the same or different and each is hydrogen, halogen or alkyl, or a salt thereof, as a labeling compound; and a kit and the like usable for such method.