Patent ID: 8663990

Claim:
A measurement method for an analyte by a capillary electrophoresis method, comprising: identifying a peak of the analyte with an internal standard substance which is a combination of a compound I, and a compound II, wherein the compound I is a compound represented by the general formula [1] or a salt thereof: wherein R 1 to R 6 each independently represent an alkyl group having —COO − or —SO 3 − as a substituent or being unsubstituted, and optionally having an amide bond; wherein R 7 to R 10 each independently represent an alkyl group, an alkynyl group, an aryl group, an alkoxy group, an aryloxy group, an alkylsulfonyl group, an arylsulfonyl group, a substituted amino group, —COO − , —SO 3 − , a halogen atom, a hydrogen atom, a hydroxyl group, a cyano group or a nitro group; wherein R 11 represents a hydrogen atom, an alkyl group, an alkynyl group or an aryl group, and n represents an integer from 0 to 3, and wherein at least 3 groups of R 1 to R 10 are —COO − , —SO 3 − , or groups having —COO − or —SO 3 as a substituent; and wherein the compound II is a compound II′ represented by the general formula [1′] or a salt thereof: wherein compound II has a same skeleton as compound I, except that 1 to 3 anion groups are substituted by cation groups represented by the following general formula [103]: wherein R 1′ to R 6′ each independently represent either (i) an alkyl group having —COO − or —SO 3 − as a substituent or being unsubstituted, the alkyl group optionally having an amide bond, or (ii) an alkyl group having a group represented by the general formula [101] as a substituent: wherein R 101 to R 104 each independently represent a hydrogen atom or an alkyl group of C 1 to C 3 ; wherein m represents an integer from 2 to 6; and wherein 2 to 3 groups of R 102 to R 104 together with a nitrogen atom to which they are bound may form a heterocyclic ammonium cation, wherein R 7′ to R 10′ each independently represent an alkyl group, an alkynyl group, an aryl group, an alkoxy group, an aryloxy group, an alkylsulfonyl group, an arylsulfonyl group, a substituted amino group, —COO − , —SO 3 − , a halogen atom, a hydroxyl group, a cyano group or a nitro group; wherein R 11′ represents a hydrogen atom, an alkyl group, an alkynyl group or an aryl group; and n represents an integer from 0 to 3, and wherein at least one of R 1′ to R 6′ is an alkyl group having a group represented by the general formula [101] as a substituent.