McReynolds number

The McReynolds number gives an approximate indication of polarity on a scale of 0 (nonpolar) to about 1000 (extremely polar). 

Sum of McReynolds numbers used for stationary-phase characterization Initial photon flux Infrared infrared detector Internal standard technique... 

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. 

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.

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. 

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. 

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. 

Knowing that the retention time for butanol is reduced we can calculate the number of equivalent carbons in an alkane which would have the same retention time. We find 6.45. The Kovats factor of the butanol is therefore 645. The corresponding McReynold s constant will be... 

Table 5.2 GC stationary phases, retention indices and McReynold s numbers ...

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... 

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. 

Rohrschneider constant for methyl ethyl ketone number of carbon atoms of a n-paraffin eluting before the peak of interest McReynolds constant for methyl n-propyl ketone Number of carbon atoms of a n-paraffin eluting after the peak of interest... 

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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