The AES was observed as RO-(EO)n-SO3Na+Na+ in the positive ion mode. If the sample containing AES and POEAE, the mass spectrum will be complicated because both of them can be ionized in the positive ion mode. On the other hand, AES was selectively ionized in the negative ion mode. Furthermore, end-groups were estimated by TOF-TOF negative ion mode. It was found that if an anionic surfactant such as AES is contained in the sample, it is worth trying to confirm their existence and perform structural analysis using negative ion mode.