McReynolds retention indices

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]. 

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. 

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). 

If the retention index for pyridine on squalane is 695, what is the McReynolds constant of this compound on the column studied, if it is known that under the conditions of the experiment, the retention time is 346 s ... 

The McReynolds constants listed are differences in retention index units between die reference compound run on squalane and on die other phases listed. The last entry in the table shows die absolute retention indices for the reference compounds on squalane. Reference compounds are (1) benzene, (2) 1-butanol, (3) 2-pentanone, (4) 1 nitropropane, and (5) pyridine. (Note that Rohrschneider s constants are based on these reference compounds and may differ slightly from the McReynolds constants. The reference compounds for Rohrschneider s constants are (1) benzene, (2) ethanol, (3) 2-butanone, (4) nitromethane, and (5) pyridine.) The minimum temperature is that at which normal gas-liquid chromatography (GLC) behavior is expected. Below that temperature, die phase will be a solid or an extremely viscous gum. The maximum temperature is that above which die bleed rate will be excessive. 

Reduced parameters, 66-69 Refractive index (RI) detector, 206-207 Regular solution, 49 Relative retention, 20-21, 22, 77 Repeatability, see Precision Reproducibility, see Precision Resolution, 17-19, 55 Response factors (detector), 104, 125 Response time, 94 Retardation factor, Rf, 71 Retention index of Kovats, 78 Retention ratio, 11, 12, 71 Retention time, 6, 9 Retention volume, 9, 75 adjusted, 10, 75 corrected, 62-63, 75 net, 63, 75 specific, 110 Reverse phase LC, 158 Rohrschneider/McReynolds constants, 137-140... 

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. 

An index whose value is high, suggests that the stationary phase strongly retains the compounds that contain the corresponding organic functions. This leads to an improved selectivity for this type of compound. In the same way, to separate an aromatic hydrocarbon from a mixture of ketones, a stationary phase would be selected for which the McReynolds constant for benzene is rather different to that of pentanone. These differences in retention indexes are provide by suppliers. 

In a GC experiment a mixture of n-alkanes (up to n carbon atoms, where n represents a variable number) and butanol (CHj CH2 CH2 CH2 OH) were injected onto a column maintained at a constant temperature and whose stationary phase was of silicone-type material. The equation of the Kovats straight line derived from the chromatogram is log t = 0.39n — 0.29 (where the adjusted retention time is in seconds). The adjusted retention time of butanol is 168 s. If it is known that the retention index for butanol on a column of squalane is 590 s then deduce its corresponding McReynolds constant upon this column. 

Several attempts have been made to simplify the comparisons of retention times from column to column and compound to compound. The first and simplest is by E. Kovats Helv. Chim. Acta, 41, 1915, 1958) involving the use of hydrocarbons. The next was by L.J. Rohrschneider (J. Chromatog., 22, 6, 1966), who used benzene, ethanol, methylethyl ketone, nitromethane, and pyridine to characterize liquid phases. W.O. McReynolds J. Chromatog. Sci., 8, 685, 1970) improved upon this by using 10 compounds to relate over 200 liquid phases to squalene. The results indicated that many of the liquid phases behaved nearly the same and that really just a few were needed to separate all of the compounds. Kovats introduced a retention index, I, which is defined as ... 

The retention index system has the advantage of being based on readily available reference materials that cover a wide boiling range. In addition, the temperature dependence of retention indexes is relatively small. In 1984 Sadtier Research Laboratories introduced a library of retention indexes measured on four types of fused-silica open tubular columns. The computerized format of the database allows retention index searching and possible identity recall with a desktop computer. Measurement of retention indexes is the basis of the Rohrschneider-McReynolds scheme for classification of stationary phases in GC (see Section 27C-4),... 

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/. 

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). 

Such conditions were chosen by McReynolds after measuring and listing the retention data of more than 300 compounds on 74 liquid stationary phases at 100°C, 120 C, 140 C, and 160°C. The solid support used in all columns was Celite 545. The retention data were given as specific retention volumes (Vg) and retention index values (/). For determination of retention data of the studied... 

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. 

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. 

The stationary phases requirements of selectivity and higher thermal stability then became more clearly defined the process of stationary-phase selection and classification became logical after the studies of McReynolds (28) and Rohrschneider (29,30) were pubUshed, both of which were based on the retention index (31). The Kovats retention index procedure and McReynolds constants are discussed in detail in the following section. Kovats retention indices today remain a widely used technique for reporting retention data, while every stationary phase developed for packed and capillary GC has been characterized by generation of its McReynolds constants. 

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. 

McReynold s constant for 2-melhylpentanol-2 Effective plate height (height equivalent to one effective plate) Isoteric heat of adsorption Retention index... 

Retention index obtained in programmed-temperature analysis Compressibility correction factor McReynold s constant for iodobutane Retention factor (capacity factor)... 

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. 

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 ... 

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. 

The Kovats index has become a popular method for reporting GC data, replacing the absolute retention parameters. McReynolds [3] has published a reference book of self-consistent indices for 350 solutes on 77 stationary phases at two temperatures. From these data it can be seen that the Kovats index is not very temperature dependent and that adjacent members of any homologus series will have index values dMering by about 100 units. Using this approximation, one can estimate the index for any chemical if the index for one member of its homologous series is known. 

The polarity of the stationary-phase liquid can be characterized by a number of parameters. For this purpose, Rohrschneider in 1966 and subsequently McRey-nolds [10] in 1970 proposed a number of test components, representing specific interactions between groups of analytes and the stationary phases. The Kovats retention indices (see section 2.4) of the model compounds benzene, 1-butanol, 2-pentanone, nitropropane, and pyridine on different stationary phases are used to determine the McReynolds constants on these stationary phases. Based on the McReynolds constants, the GC column manufacturer Chrompack introduced the CP index in order to characterize the polarity of stationary phases. The CP index has a value of zero for the highly nonpolar phase squalane and a value of 100 for the very polar phase OV 275. The CP index facilitates the comparison of stationary phases from different manufacturers. A number of general-purpose stationary phases are given in Table 2. Other classification systems for GC sta tionary phases have recently been reviewed by Abraham et al. [llj. 

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