The Rohrschneider characterization scheme

Rohrschneider [205,210] has developed a scheme for the characterization of stationary phases for gas chromatography. The scheme is based on the retention index (/). The retention index is a dimensionless retention parameter, designed to be independent of flow rate, column dimensions and phase ratio. The retention index of a solute is defined as 100 times the number of carbon atoms in a hypothetical n-alkane, which shows the same net retention time as that solute. This definition is illustrated in figure 2.2. By plotting the logarithm of the net retention time against the number of carbon atoms in n-alkanes, a straight line is obtained. The net retention time for a solute may then be located on the vertical axis, and the retention index found on a horizontal scale, which represents 100 times the scale for na 

The definition of the retention index does not necessarily require that the calibration line in figure 2.2 is linear. However, the observed linear relationship allows us to express the retention index in a simple mathematical formula  

The retention index can be obtained experimentally in a straightforward manner, can be measured accurately and is relatively robust with regards to small deviations in the temperature [210]. In the Rohrschneider scheme, I values are measured at 100 °C. 

Rohrschneider assumed that the polar interaction of stationary phases can be characterized by substracting the retention indices observed on a completely non-polar phase from those obtained on a polar phase. As a completely non-polar stationary phase, squalane was selected. For the polar interactions of a solute on a polar stationary phase we may now write  

Because squalane is not entirely non-polar, (small) negative values for 4/ are sometimes obtained for very non-polar stationary phases [211]. 

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Table 2.6 shows that some stationary phases show exactly the same Rohschneider constants. The fact that these phases also show identical chromatographic selectivity in practice forms an indication for the validity of the Rohrschneider approach. Any of these phases can be selected, but it would be a waste of time to investigate the selective effect of more than one of them. The choice will now be based on secondary considerations, such as stability, temperature range, availability and cost. Indeed, one of the first consequences of the Rohrschneider characterization scheme was for some manufacturers to reconsider the program of available stationary phases and to remove obsolete ones (see e.g. ref. [212], p.62). It is seen that not all non-polar stationary phases are identical, and that minor differences in selectivity may be anticipated from the use of Apiezon L instead of one of the silicone polymers. 

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