Rohrschneider constants

These constants, which are related to the structure of the molecules, allow an evaluation of the forces of interaction between the stationary phase and the solute for different classes of compounds. An index with an elevated value indicates that the stationary phase has a strong affinity for compounds that contain particular organic functions. This leads to a greater selectivity for this type of compound. For example, in order to separate an aromatic hydrocarbon contained in a mixture of ketones, a stationary phase for which benzenes have a very different constant than butanone will be selected. These differences in indices appear in most manufacturers catalogues of chromatographic components (Table 2.1). McReynolds constants have more or less replaced Rohrschneider constants, which are based on the same principle but use different reference compounds. 

The McReynolds constants (a modification of the Rohrschneider constant) tabulated here are based on the retention characteristics of the following test probe samples ... 

Hence, stationary phases can be characterized very quickly by measuring the retention indices of the five probe solutes. On the other hand, the characterization of solutes is not so easy, for a combination of reasons. In the first place, a set of five equations with five unknowns has to be solved. In the second place, the retention indices of the solute need to be obtained on five different stationary phases with known Rohrschneider constants, as well as on squalane. Hence, six different columns are needed. Moreover, in order to obtain reproducible data, very careful experimentation is required. It is especially difficult to maintain a squalane column. In this light, the choice of squalane as a reference phase has been unfortunate. Therefore, the Rohschneider scheme has become extremely popular for the characterization of stationary phases, and not for the characterization of both phases and solutes, allowing the prediction of retention indices through equation 2.5. 

Rohrschneider constants for some typical stationary phases for GC. Data taken from ref. [212],... 

Table 2.6 shows a series of silicone polymer stationary phases from the OV series. These are dimethylsiloxane polymers, with a varying percentage of the methyl groups replaced by phenyl groups. It turns out that the Rohrschneider constants closely follow this increase in phenyl group percentage. The constants therefore appear to be a reliable indication of the polarity of the stationary phase. 

The Rohrschneider constants obtained for a large number of stationary phases may serve as the basis for a statistical analysis, from which a limited number of stationary phases which are most likely to yield different selectivity effects can be selected. 

The Rohrschneider scheme has been modified by McReynolds [213], The modifications include the use of some more convenient test probes (for instance, ethanol often yields very low retention indices, nitromethane often yields poorly shaped peaks), the use of a somewhat higher temperature and a factor of 100 to avoid decimal points in the stationary phase parameters. The McReynolds probes are shown in table 2.7. Considering these modifications, I feel that no justice is done when McReynolds constants are tabulated. It appears to be more appropriate to refer to these constants as Modified Rohrschneider constants , or Rohrschneider constants, modified according to McReynolds . An additional set of five probes may be used according to McReynolds, but the five extra parameters are not very helpful for characterization purposes. 

Rohrschneider went on to show that one can use the liquid phase constants in reverse to get values that would represent the polarities of other solutes. The values in Table 14 were compiled by him by running a new solute on five liquid phases whose Rohrschneider constants had been previously determined. The five A/ values thus obtained are substituted into five equations [like Eq. (6)], which are solved simultaneously for the five unknowns a, b, c, d, and e. As expected, the a value for toluene is close to 100 (108), the b value for propanol is close to 100 (105 for n-propanol), and the c value for acetone is nearly 100 (95) because these three solutes have the same functional groups as the original probes. To answer the question How polar is chloroform, one can look at the five constants and tell what functional groups (or interactions) it is like and what groups it is not like. In principle, one can determine the solute constants for the components of any sample, multiply them by the Rohrschneider/McReynolds constants for a variety of common columns, sum the values, and thus find the best column for the separation. This approach is not the one analysts have taken to select the best liquid phase for a given separation. Before we take a look at the procedures that are used, let us note some related uses that can be made of the Rohrschneider/ McReynolds constants. 

The McReynolds constants have replaced Rohrschneider constants, which were based upon the same principle though using partly different reference compounds. 

WHY DONT I SEE McREYNOLDS/ROHRSCHNEIDER CONSTANTS IN CURRENT CHROMATOGRAPHY CATALOGS ... 

Fraction of sample component in stationary phase Rohrschneider constant for pyridine sound pathlength... 

Linear velocity of mobile phase interstitial velocity of mobile phase Rohrschneider constant for nitromethane Mean interstitial velocity of mobile phase i.e., average linear gas velocity... 

Rohrschneider constant for methyl ethyl ketone number of carbon atoms of a n-paraffin eluting before the peak of interest McReynolds constant for methyl n-propyl ketone Number of carbon atoms of a n-paraffin eluting after the peak of interest... 

Kovats retention index procedure and the McReynolds and Rohrschneider constants are discussed in detail in the following sections. The Kovats index remains a widely used technique for reporting retention data, and every stationary phase developed for packed and capillary gas chromatography has been characterized by its McReynolds constants. 

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