ABSTRACT
Xanthene dyes can be regarded as derivatives of diaryl- or triarylmethanes in which two aryl rings are bridged by an oxygen atom. In hydroxyxanthenes there is an ionized phenolic hydroxy group as donor with a keto group as acceptor. In commonly used xanthene dyes the group R may be either a hydrogen atom (e.g. pyronine Y) or a substituted phenyl ring (e.g. fluorescein or rhodamine 6G). Under routine staining conditions the chromophore of aminoxanthene dyes is cationic, and that of hydroxyxanthenes anionic. Hydroxyxanthenes are all anionic under neutral or alkaline conditions, with cations only present in strongly acid solutions. Fluorescein and some of its halogenated derivatives are offered for sale in various forms. A standardized methyl green-pyronine procedure, based on pure dyes, has been described. The procedure can be carried out on resin sections, both glycolmethacrylate and epoxy. Rhodamine B hexyl ester is available commercially as the chloride and perchlorate salts.
