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Figure 5.2 Schematic representation of the final column-switching system: (a) forward-
flush position; (b) back-flush position (further details are given in the text). Reprinted from
Journal of Chromatography, A 828, A. K. Sakhi et al. ‘Quantitative determination of endoge-
nous retinoids in mouse embryos by high-performance liquid chromatography with on-line
solid-phase extraction, column switching and electrochemical detection’, pp. 451–460, copy-
right 1998, with permission from Elsevier Science.

strongly hepatotoxic cyclic heptapeptides produced by some species of freshwater
cyanobacteria (blue–green algae) (28). These microcystins represent a health risk to
humans through drinking water, since they have been found to act as tumor promot-
ers (29). Several chromatographic analytical procedures for microcystins have been