Retention index McReynolds’ phase 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 ... 

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). 

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. 

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. 

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. 

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. 

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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