Stationary phases McReynolds classification

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

To fully characterize and categorize the solute selectivities of GC stationary phases, Rohrschneider and McRe5molds pioneered one of the earliest characterization methods [5,6]. The Rohrschneider-McReynolds system is the oldest and widely accepted stationary phase classification systems that is based on the retention of five probe molecules namely, benzene, bufanol, 2-penfanone, nifropropane, and pyridine. Each probe molecule is used to represenf a disfincf or a combination of interactions with the stationary phase. Benzene measures dispersive interactions with weak proton acceptor properties butanol measures dipolar interactions with both proton donor and proton acceptor capabilities 2-pentanone measures dipolar interactions with proton acceptor but not proton donor capabilities nitropropane measures weak dipolar interactions and pyridine measures weak dipolar interactions with strong proton acceptor but not proton donor capabilities. 

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... 

Notable attempts have been made toward a systematic classification of stationary phases in GC. The column classification system conceived by Rohrschneider [80], and further developed by McReynolds [81], does provide a valuable guide in the column selection process. Most commercial phases have now been characterized. More quantitative and elaborate approaches toward the characterization of liquid phases in GC involve solubility parameters and other thermodynamic considerations... 

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... 

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),... 

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. 

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. 

In spite of some critical opinions, connected mainly with incorrect determination of retention indices of standard substances, the McReynolds constants system gave logical basis for stationary phase classification and allowed for selection of the proper gas chromatographic column. Until now, it has been the most common approach employed for stationary phase selectivity ranking in GC. 

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. 

There were several significant consequences resulting from these classification procedures. Phases that have identical chromatographic behavior also have identical constants. In this case the selection of a stationary phase could be based on a consideration such as thermal stability, lower viscosity, cost, or availability. McReynolds constants of the more popular stationary phases for packed column GC are listed in Table 3.8. Note that the DC-200 (a silicone oil of low viscosity) and OV-101 or SE-30 (a dimethylpolysiloxane) have nearly identical... 

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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