The determination of trace organic compounds in aqueous samples usually involves isolation of the fraction of interest followed by subsequent separation by means of a chromatographic technique. When high resolution is needed, the main chromatographic techniques usually employed are capillary gas chromatography (GC) or supercritical fluid chromatography (SFC), both as analytical tools in themselves and as inlet methods for mass spectrometry (MS) (1 ). SFC (see Chapter 1) has features overlapping gas and liquid chromatography, and may use numerous detectors under mild conditions, including the universal flame ionization detector (FID) and improved chromatographic-mass spectrometric interfaces, that opens additional possibilities for the study of retinoids, carotenoids, other vitamins, and related compounds (2 ). One drawback of the SFC techniques when using carbon dioxide as mobile phase is that the direct introduction of water samples poses a series of problems. Water must therefore be eliminated before it reaches the analytical column. A sample preparation step is thus essential to both concentrate the sample and eliminate the water.