McReynolds probes

In a series of papers published throughout the 1980s, Colin Poole and his co-workers investigated the solvation properties of a wide range of alkylammonium and, to a lesser extent, phosphonium salts. Parameters such as McReynolds phase constants were calculated by using the ionic liquids as stationary phases for gas chromatography and analysis of the retention of a variety of probe compounds. However, these analyses were found to be unsatisfactory and were abandoned in favour of an analysis that used Abraham s solvation parameter model [5]. 

INTERACTIONS CHARACTERIZED BY MCREYNOLDS PROBES (ROHRSCHNEIOER PROBES IN PARENTHESES)... 

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 ... 

Compounds that are chemically similar to these probe solutes will show similar retention characteristics. Thus, benzene can be thought of as representing lower aromatic or olefinic compounds. Higher values of the McReynolds constant usually indicate a longer retention time (higher retention volume) for a compound represented by that constant, for a given liquid (stationary) phase. 

The McReynolds constants are indices with respect to the following test probe compounds ... 

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. 

McReynolds probes and their parameters. Retention indices on squalane taken from ref. [211] temperature 120 °C. 

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. 

In fact, it has turned out that McReynolds was wrong—ten probes were not better than five. Therefore, most compilations of McReynolds constants list only five, six, or seven values. In fact, later attempts to expand... 

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. 

Quasi-Theoretical Approach. Several groups of workers have taken the McReynolds constants and similar data and attempted to combine them with our present knowledge of intermolecular attractions as presented in Chapter 3. In a pair of papers, Hartkopf41 suggested that four probes (not five or ten) would be sufficient to characterize a liquid phase. He proposed to measure the following ... 

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. 

Using a different set of standard substances, i.e. substituting 1-butanol, pentan-2-one, and 1-nitropropane for the rather volatile ethanol, butan-2-one, and nitromethane, McReynolds developed an analogous approach [103]. Altogether, he characterized over 200 liquid stationary phases using a total of 10 probes. A statistical analysis of the McReynolds retention index matrix using the principal component analysis method has shown that only three components are necessary to reproduce the experimental data matrix [262]. The first component is related to the polarity of the liquid phase, the second depends almost solely on the solute, and the third is related to specific interactions with solute hydroxy groups [262]. 

From a large number of experiments (226 stationary phases were studied, and 68 compounds on 25 columns were analysed), McReynolds selected 10 most valuable compounds (the most valuable five are benzene, n-butanol, 2-pentanone, nitropropane, and pyridine) as "probes" to characterise columns. The polarity of the column as measured with benzene is termed X and is equal to ARI/100 for benzene. Similarly, y, z, u and s are the 1/100 terms for the other four probe compounds. The coefficient a, b, c, d and e for x, y, z, u and s terms are constants, which are defined for these five probe compounds. For benzene, a=100 and b, c, d and e=0. For n-butanol, b=100, and a, c,d and e=0, and so on for the other three probe compounds. Many GC manufacturers present the values of McReynolds constants for various stationary phases in their catalogues. Table 1 list McReynolds constants for some commonly used stationary phases. 

Thus, Stationary phases can have their polarities compared for a given test probe such as benzene or benzyl alcohol. In an attempt to reduce the number of liquid phases used in packed column GC, McReynolds published... 

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). 

The system of McReynolds constants is a usefiil tool for characterization of the selectivity of stationary phases in GC. The founding principle of this approach is that inter-molecular forces are additive and their individual contributions to retention can be evaluated from the difference in retention index values of selected test probes measured on a liquid phase to be characterized, and on the non-polar reference phase, i.e., squalane. To characterize the stationary phase polarity, the concept uses five special solutes (benzene, n-butanol, 2-pentanone, 1-nitropropane, and pyridine) that are considered to represent typical chemical interactions. 

Other approaches, well known in the chromatographic area, are that of Kovats [184,185] and the use of system constants of McReynolds [186]. The specific interactions are estimated by injecting polar probes and making the difference for each of them equal to that between their retention index (Ix) and the index obtained at identical conditions for a nonpolar reference material (I ef) such as solid polyethylene or Apiezon L. 

This variation on traditional IGC has been developed from the McReynolds approach [24], for characterisation of stationary phases, by McMahon and co-workers [25]. It is particularly suited to rapid comparative screening of families of solid surfaces, for example a range of carbon blacks with differing surface chemistry. However, it does not provide quantitative measures of thermodynamic parameters. The multi-probe set includes those specified by McReynolds together with several w-alkanes. Classified groups of these probes can be injected into the column as mixtures. The temperature of the column is increased at a constant rate and the temperature at which the probe elutes is recorded as the retention temperature (T ). 

The most widely used system of classifying liquid phases is the McReynolds system (28) and 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 I values on over 200 phases, including squalane. 

As the difference in the retention index for a probe on a given liquid phase and squalane increases, the degree of specific interaction associated with that probe increases. The cumulative effect, when summed for each of the 10 probes, is a measure of overall polarity of the stationary phase. In a tabulation of McReynolds constants, the first five probes usually appear and are represented by the symbols X, Y, Z, U, S. Each probe is assigned a value of zero with squalane as reference liquid phase. 

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

Symbol McReynolds Probe Rohrschneider Probe Measured Interaction... 

Another feature of the McReynolds constants is guidance in the selection of a column that will separate compounds with different functional groups, such as ketones from alcohols, ethers from olefins, and esters from nitriles. If an analyst wishes a column to elute an ester after an alcohol, the stationary phase should have a larger Z value with respect to its Y value. In the same fashion, a stationary should exhibit a larger Y value with respect to Z if an ether is to elute before an alcohol. The appendixes in Reference 12 list McReynolds constants in order of increasing A/ for each probe in successive tables that are handy and greatly facilitate the column selection process. 

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