Stationary-phase polarity, McReynolds

Finally, the sum of the first five McReynolds values has been used to compare the polarities of silicone polymers on OT columns. It has already been stated that the polarity of the silicone polymers can be increased by increasing the percentage of phenyl groups in the polymer. Figure 8.17 shows a plot for five polymers on bonded fused silica WCOT columns (except for SP-2250, which is from packed column data). The increasing McReynolds values clearly show the validity of this method of specifying stationary phase polarity. 

Figure 8.17. Effect of number of phenyl groups on stationary phase polarity as measured by McReynolds values. Reprinted from the Supelco Reporter, Vol. IV, No. 3, May 1985 with the permission of Supelco Inc., Bellefonte, PA.

Table 2 McReynolds constants and their sum, —y + / + + m + s, used for characterization of liquid stationary phase polarity.

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. 

McReynolds constants constitute a classification system for GLC stationary phases based on their polarity ... 

McReynolds used the retention index of certain solutes to compare different stationary phases and to assess their selectivity compared with a reference liquid phase, squalane. Squalane is considered to be non-polar and any increase in the retention index of the selected solute on the test column compared to squalane may be considered to be due to the greater polarity of that solvent. McReynolds constants have been determined for all stationary phases using a range of solutes of varying polarity (Table 3.8) and may be used to assist in selecting an appropriate stationary phase. 

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. 

A well-known phase which is used as a reference because it is the only one that is well defined is squalane. Its polarity is zero by the McReynolds scale (2.13). This saturated hydrocarbon (C3oHe2) is derived from squalene, a natural terpene extracted from shark liver or skin. On this stationary phase, which can be used between 20 and 120 C (deposition or impregnation), compounds are eluted... 

The apparent polarity of the dinonylphthalate column, expressed by a Al of 84, is a measure of retardation of aromatic and olefinic substances. Since the chromatographer is interested in the selectivity of a column for a variety of functional groups, it is important to classify each of the stationary phases by their ability to retard compounds other than benzene. This has been done by Rohrschneider (22) and further developed by McReynolds (23). The system is discussed in detail with many examples by Supina (6). The Rohrschneider/McReynolds (R/M) system involves the measurement of the retention indices for several compounds on a given column to determine the degree to which each is retarded. In each case, the retention indices are compared to... 

These Al values are listed as in Table 3.2, where the x, y, z, u, and s values indicate the polarity of the phases for each type of compound. Most chromatography supply catalogs now contain these "McReynolds constants" for the stationary phases offered for sale. 

Haken has considered the applicability of "Rohrschneider/ McReynolds constants" for the classification of stationary phases for the separation of fatty esters (13). He concluded that the approach was limited since the measurements used to determine the aforementioned "constants" are made at 100°C and most fatty acid methyl ester separations are carried out at about 200°C. He had previously shown significant variation in the, what will now be called, Rohrschneider/McReynolds coefficients, with temperature (14). Polar polysiloxanes such as XF-1150 demonstrated greatest variability in the coefficients and nonpolar types such as SE-30 demonstrated least variation. Supina pointed out that the X factor in the McReynolds coefficients should be indicative of extent of interaction with olefinic substituents (15). Figure 9.5 demonstrates the utility of this approach the 18 3 and 20 0 methyl esters are used as markers for the consideration of... 

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]. 

It has been shown that the retention behaviour of benzene, butanol, pentan-2-one, nitropropane, and pyridine can be used to classify stationary phases in terms of their polarity (W.O.McReynolds, J. chromatogr. Set., 1970,5,685-691). The retention indices of each of these five reference compounds are measured, first on the stationary phase being tested and then on a standard phase (squalane). The differences in retention index between the two phases (AI) for the five reference compounds are added together to give a constant which is a measure of the polarity of the stationary phase. This constant is known as iheMcReynolds Constant and can be used to compare the ability of stationary phases to separate different classes of compounds (see below). However, this constant gives no information about peak shape, temperature limits, or the suitability for use in capillary colimms. 

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. 

Rohrschneider and McReynold extended the RI system to predict a PI for various stationary phases measured at a column temperature of 120°C with a 20% (w/w) loading, to minimise retention contributions from the diatomite support [11,12]. A set of five reference compounds were selected to reflect a range of polar characteristics and functional groups benzene, X butanol, Y 2-pentanone, Z nitromethane, [/ and pyridine, S. Squalane, 2, 6, 10, 15, 19, 23-hexamethyltetracosane (C30H62), is used as the reference stationary phase as it is a readily available completely non-polar, non-volatile liquid, bp = 176 at 0.05 mm. The values of X, Y, Z, U and S represent the relative affinities of the reference compounds for the stationary phase, calculated as the differences, ARI, between the RI of the reference on a chosen stationary phase compared to the RI on squalane. The polarity index, PI, is the mean of the RI values (Table 5.2). 

Table 5.5 lists five commonly used stationary phases their properties and applications. Since WCOT columns have high column efficiencies over 90% of analyses can be carried out with these stationary phases using columns of varying length, internal diameter and film thickness. The equivalent liquid stationary phases are included. Reference to Table 5.2 will give their polarity index and McReynold s retention indices. 

So far we have discussed solvation properties at a reference temperature of 120°C. The choice of reference temperature arises from historical considerations. McReynolds chose this temperature to compile his extensive database of retention measurements for volatile solutes on a large number of stationary phases. His database has been widely used for exploring new approaches to stationary phase classification and has influenced others into using the same temperature to collect additional reference data to maintain compatibility with the original database. The choice of a standard reference temperature is of less concern than whether a single reference temperature is sufficient to classify solvent properties for use at temperatures distant from the reference temperature. There is only a limited amount of data for the influence of temperature on selectivity in gas-liquid chromatography [53,81,103,121,122]. In general polar interactions are... 

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... 

Retention indices find further application in characterization of stationary phases. AppHcation of the concept of McReynolds uses a series of five solutes of differing chemical class (alcohol, aromatic, ketone, nitro, pyridine) to compare their retention indices on the test phase, against their retention indices on a stationary phase accepted to have very little polarity (squalane). It can be argued that if a solute exhibits greater retention index on the test phase, then the intermolecular interactions between the solute and the phase are stronger than in the case when squalane is used, which means that the test phase should also exert a similar effect on similar solutes. Colloquially this is referred to as the phase polarity , and a scale may be set up according to the following ... 

Let us return to our discussion regarding the determination of the polarity of stationary phases by beginning with an example using Kovats retention indexes. From McReynolds [3] we find that toluene has a Kovats retention index of 773 on the nonpolar phase squalane and 860 on the more polar dioctylphthalate. The difference in these indexes, 87, provides a measure of the increased relative polarity of dioctylphthalate relative to squalane. The difference can be designated as A/. 

The studies of McReynolds and Rohrschneider had fundamental importance for chromatography. At present, the sum of five McReynolds constants is the most commonly accepted criterion for the overall polarity of the stationary phase. ... 

The McReynolds constants for liquid stationary phases, most commonly used in practice, arc given by Kenndler. These data are presented in Table 2. In his opinion, polarity of two phases can be considered as nearly equal if the sums of their indices differ by more than about 200. In general, it is not meaningful to use two such phases. This does not mean, however, that their selectivities concerning one special type of interaction are probably negligible. 

To achieve a detailed classification of the possible interaction of sohd porous non-polar polymeric phases with different functional groups of solute molecules, Gastello and D Amato used the following polarity reference substances ethanol, 2-butanone, nitromethane, benzene, pyridine, w-butanol, 2-pentanone, and 1-nitropropane. The first five represent the test substances proposed by Rohrschneider, while the last three were recommended by McReynolds. The retention indices of these substances enable evaluation of the polarity of any sohd porous polymeric stationary phase. In their studies Porapak Q, as the least polar commercially available porous polymer, was used as the reference stationary phase. 

Some authorsraise the question of experimental inaccuracy of McReynolds constants determination associated with inaccurate determination of retention times for n-alkanes on polar stationary phases. In such cases, retention depends on the amount of the analyte sample. To avoid such errors in the determination of McReynolds constants. Smith, Tameesh, and Waddington proposed the use of trace amounts of the test solutes by dipping a 1 p,l syringe needle (at the zero position of the piston) in the sample for 1 sec. 

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. 

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. 

One measnre of the polarity of a stationary phase is the cumulative value of its McReynolds constants, as discussed in Section 3.6.3. Because a variety of functional groups can be incorporated into the structure, polysiloxanes exhibit a wide range of polarities. Since many polysiloxanes are viscous gums and, as such, coat well on fused silica and can be crosslinked, they are ideally suited for capillary GC. The basic structure of 100% dimethylpolysiloxane can be illustrated as... 

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