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summation of values relating to the carbon content of the steroid skeleton and values characteristic of the functional groups of the steroid. This rule again is a repetition of the corresponding rules discussed originally by Wehrli and Kovâts. (5) The "methylene unit" number described by Vanden Heuvel et al. (30) in this issue is physically identical to the retention index value divided by 100. WORK TO BE DONE
Investigations by Kovâts, Wehrli, and a large number of European authors (1-3, 8-10, 14, 15, 19, 21, 24-27, 34-37), proved the exceptional usefulness of the retention index concept for the generalization of the retention data. The following steps could accelerate its wide acceptance: (1) Agreement on the stationary phases to be used for retention index determination. (2) Investigations in the elimination of error sources. (3) Determination of a large number of reliable retention index values on preselected stationary phase (s). (4) Further studies on the relationship between I, Al, and the chemical characteristics of various substances. (5) Further investigations on the extension of the retention index concept to programmed temperature analysis. The G.A.M.S.—the French Society of Analytical Chemists—has spent considerable time on the standardization of the retention index concept, and in the institute of Dr. Keulemans at Eindhoven, detailed studies are now under way on these problems. The results of these activities can contribute to the establishment of a universally accepted system for the presentation of retention data in gas chromatography. LITERATURE CITED
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(1) Chovin, P., Lebbe, J., "Journées International d'Étude des Méthodes de Séparation Immédiate et de Chromatographie," J. Tranchant, éd., pp. 99103, G.A.M.S., Paris, 1961. (2) Dhont, J. H., Nature 198, 990 (1963). (3) / M . , 200, 882 (1963). (4) Evans, M. B., Smith. J. F.. ./. Chromatog. 5, 300 (1961).