Alkane effect in the Arizona liquid systems used in countercurrent chromatography |
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Authors: | Email author" target="_blank">A?BerthodEmail author M?Hassoun M?J?Ruiz-Angel |
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Institution: | (1) Laboratoire des Sciences Analytiques, Université Claude-Bernard, Lyon 1, 69622 Villeurbanne, France |
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Abstract: | Countercurrent chromatography (CCC) is a separation technique that uses a biphasic liquid system; one liquid phase is the
mobile phase, the other liquid phase is the stationary phase. Selection of the appropriate liquid system can be a problem
in CCC, since it is necessary to select both the “column” and the mobile phase at the same time as the first is completely
dependent on the second. A range of systems with various proportions of solvents were developed to ease this choice; 23 variations
of the heptane/ethyl acetate/methanol/water biphasic liquid system were labeled A to Z. This range proved to be extremely
useful and became the popular Arizona (AZ) liquid system. However, authors often replace the heptane with hexane. In this
work, the chemical compositions of the upper phases and the lower phases of 55 Arizona systems made with various alkanes (pentane,
hexane, heptane, isooctane and cyclohexane) were determined by gas chromatography and Karl Fischer titration. The test mixture
separated consisted of five steroid compounds. The lower phases were found to have similar compositions when different alkanes
were used, but the upper phases were found to change. Exchanging heptane for hexane or isooctane produced minimal changes
in the CCC chromatogram, while changing the proportions of the solvents resulted in an exponential change in the retention
volumes. The high density of cyclohexane made liquid stationary phase retention difficult. All Arizona systems equilibrated
within 30 min, but were not stable: water slowly hydrolyzed the ethyl acetate (as shown by a continuous decrease in the pH
of the lower aqueous phase), especially in the water-rich systems (early alphabet letters). |
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Keywords: | Countercurrent chromatography Biphasic liquid system Phase composition Elution-extrusion |
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