McReynolds’ phase constants

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]. 

McReynolds phase constants Developed to establish a systematic ordering of GC stationary phases with respect to specific solute interactions. This can then be used to predict the change in the retention index, AI, for the tested phase with respect to squalene ... 

Solvent selectivity is a measure of the relative capacity of a solvent to enter into specific solute-solvent interactions, characterized as dispersion, induction, orientation and coaplexation interactions, unfortunately, fundamental aiq>roaches have not advanced to the point where an exact model can be put forward to describe the principal intermolecular forces between complex molecules. Chromatograidters, therefore, have come to rely on empirical models to estimate the solvent selectivity of stationary phases. The Rohrschneider/McReynolds system of phase constants [6,15,318,327,328,380,397,401-403], solubility... 

GC phase increases. A measure of the polarity of a stationary phase is given by its McReynold s constant (Table 11.1), which is based on the retention times of benzene, n-butanol, pentan-2-one, nitropropane and pyridine on a particular phase. The higher the McReynold s constant the more polar the phase. Many stationary phases are described by an OV-number. The higher the number after the OV the more polar the phase. 

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 use of two or more columns improves the probability that the identity of an unknown compound is the same as that of a compound with identical retention times however, these data alone are not conclusive proof. The reliability of the identification depends on the efficiency and polarity of the columns used. With efficient columns, the probability of having two or more components under one peak diminishes and the peaks are generally well resolved. Care must be taken in selecting columns to be certain the columns have different selectivities and not just different names. Thus, the McReynold s constants (Chapter 3) must be compared and should be quite different for each column. For example, in the analysis of pesticides, four different liquid phases might be chosen arbitrarily (e.g., OV-1, UCW-98, SE-30, and DC-200). However, the relative retention in this case will be the same for all four columns since they are all methyl silicones and have essentially the same McReynold s constants (2). 

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 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 system of stationary phase constants introduced by Rohrschneider [282,283] and later modified by McReynolds [284] was the first widely adopted approach for the systematic organization of stationary phases based on their selectivity for specific solute interactions. Virtually all-popular stationary phases have been characterized by this method and compilations of phase constants are readily available [28,30]. Subsequent studies have demonstrated that the method is unsuitable for ranking stationary phases by their selectivity for specific interactions [29,102,285-287]. The solvation parameter model is suggested for this purpose (section 2.3.5). A brief summary of the model is presented here because of its historical significance and the fact that it provides a useful approach for the prediction of isothermal retention indices. 

The system based on characteristic phase constants, known as the McReynolds constants, proved to be the most successful for describing phase selectivity. A similar system was introduced simultaneously by Rohrschneider, but his characteristic phase constants have not become so popular. 

To choose proper test substances, McReynolds studied the behavior of a large number of substances of the following compound classes alcohols, glycols, aldehydes, ketones, esters, acetals, ethers, oxides, hydrocarbons, chloro compounds, difunctional and polyfunctional compounds, and other miscellaneous substances. " For the stationary phase classification, he eventually proposed benzene, n-butanol, 2-pentanone, 1-nitropropane, and pyridine to represent compounds of different chemical interactions. " McReynolds characteristic phase constants for these five compounds describe the selectivity of the phase. 

McReynolds characteristic phase constants are usually found in the contemporary scientific and commercial literature for characterization of liquid stationary phase selectivity. This method allows for direct comparison of the GC columns (both packed and capillary), provided the data for standard test compounds are obtained at the same temperature. 

Kersten, Poole, and Furton ° found that many ambiguities in the determination of the polarity with the McReynolds stationary phase constants are due to the use of n-aUtanes as the reference series, and estimated on 15 columns spanning a wide polarity range the 2-alkanones as the universal retention index markers to replace the w-alkanes which do not partition with polar phases. Ketones were also suggested by many authors as a good alternative series to -alkanes however, Mathiasson et al. found that, because of variation in retention volume with the amount injected, alkanols and 2-alkanones are unsuitable on both polar and non-polar columns. He suggested the use of alkylbenzenes as reference compounds, as these compounds seem to behave almost ideally on liquid phases of different polarities. 

Rohrschneider, L. Explanatory coefficients for stationary phases in gas chromatography from McReynolds phase 22. constants. Chromatographia 1994, 38 (11/12), 679-688. 

Distribution constant in which concentration in stationary phase is expressed as weight of substance per surface area of solid phase Column length McReynold s constant for 1,4-dioxane Molecular weight... 

Table 11.13 McReynolds Constants for Stationary Phases in Gas Chromatography...

Similar stationary phases Temp., °C McReynolds constants USP code... 

---