Probe Rohrschneider

Symbol McReynolds Probe Rohrschneider Probe Measured Interaction... 

To fully characterize and categorize the solute selectivities of GC stationary phases, Rohrschneider and McRe5molds pioneered one of the earliest characterization methods [5,6]. The Rohrschneider-McReynolds system is the oldest and widely accepted stationary phase classification systems that is based on the retention of five probe molecules namely, benzene, bufanol, 2-penfanone, nifropropane, and pyridine. Each probe molecule is used to represenf a disfincf or a combination of interactions with the stationary phase. Benzene measures dispersive interactions with weak proton acceptor properties butanol measures dipolar interactions with both proton donor and proton acceptor capabilities 2-pentanone measures dipolar interactions with proton acceptor but not proton donor capabilities nitropropane measures weak dipolar interactions and pyridine measures weak dipolar interactions with strong proton acceptor but not proton donor capabilities. 

Equation 4.1 describes the Rohrschneider-McReynolds system in terms of the five probes and their corresponding phase constants namely, benzene (X ), butanol (Y ), 2-pentanone (Z ), nitropropane (U ), and pyridine (S ) with the overall difference in the Kovats retention index (AI). 

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

The Rohrschneider scheme has no significance, until the parameters for five different solutes (or five different stationary phases) are known. With fewer parameters, the system is undefined, while more than 25 known parameters create a problem of consistency. Realizing that the characterization scheme is completely empirical, Rohrschneider made a very convenient choice for the characterization of stationary phases. The probe solutes and their parameters are listed in table 2.5. 

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 probes and their parameters. Retention indices on squalane taken from ref. [212]. Temperature 100 °C. 

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. 

A convenient way to classify solvents of chromatographic interest in terms of their polarity and the specific chemical interactions is the empirical scheme proposed by Snyder [214,215]. This scheme is based on experimental (gas chromatographic) distribution coefficients for three test solutes ( probes ) on a large number of stationary phases, which were published by Rohrschneider [216]. The probe compounds are ethanol (e), 1,4-dioxane (d) and nitromethane (n). The experimental values for the distribution coefficients undergo several empirical modifications ... 

Table 2.8 shows the P and x values for a number of solvents of chromatographic interest. The table first shows the values for the three probe solutes. It is clear that the definitions applied for the x values do not imply that the probes show a value of unity (or 100) for one of the parameters, as was true in the scheme of Rohrschneider (see section 2.3.2). It is therefore wrong to conclude that a compound with a high value for xd closely resembles dioxane, because in that case dioxane would not resemble itself more closely than 24% ... 

In 1970 McReynolds40 went one step further. He reasoned that ten probes would be better than five and that some of the original five should be replaced by higher homologs. His probes are also listed in Table 13. (Also, he did not divide his values by 100, so his constants are 100 times those of Rohrschneider these larger values are the ones that have become popular). The first 5 of his 10 constants are the ones listed in Table 12, where their designations are distinguished by the use of a prime—x instead of x. McReynolds published constants for over 200 liquid phases, and this procedure has been followed by others to provide Rohrschneider/ McReynolds constants for all the liquid phases in use. 

Rohrschneider used five probes and McReynolds ten, but Snyder s... 

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. 

Intermolecular forces were discussed in Chapter 3 and extended to GC stationary phases in Chapter 8. Rohrschneider, followed by McReynolds, investigated the nature of GC stationary phases by using a few common chemicals as probes. Their retention on a given liquid reflected the extent of their interaction with the stationary phase. By choosing probes with selective interactions, they could determine a set of numbers that characterize the liquids under study. 

Various scales of solvent strength (polarity) and selectivity have been used to classify stationary phases. Classification based on polarity had to be abandoned because of the lack of a working definition. There is no substance that is uniquely polar and suitable to probe the polarity of other substances. Selectivity is defined as the relative capacity of a stationary phase for specific intermolecular interactions, such as dispersion, induction, orientation, and hydrogen-bond formation. Early attempts at a systematic definition of selectivity scales were based on the system of phase constants introduced by Rohrschneider and... 

Rohrschneider [6] proposed a list of five chemicals that could be used as test probes (like the solute toluene) to compare retention indexes on squalane (the universal nonpolar standard) and any other liquid phase. His choices are listed below (McReynolds probes are also listed). 

TABLE 3.7 Probes Used in McReynolds and Rohrschneider Classifications of Liquid Phases... 

The most widely used system for classifying liquid phases, the McReynolds system (55), has been employed to characterize virtually every stationary phase. McReynolds selected 10 probe solutes of different functionality, each designated to measure a specific interaction with a liquid phase. He analyzed these probe solutes and measured their 1 values for over 200 phases, including squalane, which served as a reference liquid phase under the same chromatographic conditions. A similar approach had been implemented previously by Rohrschneider (56, 57) with five probes. In Table 2.9 the probes used in both approaches and their functions are... 

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