Abstract
A 5-g sample of commercial 4,5,6,7-tetrachlorofluorescein (TCF) was subjected to pH-zone-refining countercurrent chromatography to separate an unidentified contaminant present in all TCF batches obtained from four different suppliers. The separated contaminant (44 mg) was characterized by 1H- and 13C nuclear magnetic resonance spectrometry and negative ion chemical ionization mass spectrometry as a new seven-membered lactone TCF isomer with the structure 11,12,13, 14-tetrachloro-3, 14b-dihydroxy-[2]-benzoxepino- [3,4,5-kl]-xanthene-10(14bH)-one.
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