McReynolds system

Solvent selectivity is a measure of the relative capacity of a solvent to enter into specific solute-solvent interactions, characterized as dispersion, induction, orientation and coaplexation interactions, unfortunately, fundamental aiq>roaches have not advanced to the point where an exact model can be put forward to describe the principal intermolecular forces between complex molecules. Chromatograidters, therefore, have come to rely on empirical models to estimate the solvent selectivity of stationary phases. The Rohrschneider/McReynolds system of phase constants [6,15,318,327,328,380,397,401-403], solubility... 

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. 

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). 

One of the most valuable usage for the McReynolds system is to identify stationary phases. For example, SE-30 and DC-200 or OV-101 are very similar stationary phases (with different names but practically similar retention patterns). This system also allow for a rapid selection of phases with different retention properties, to separate compounds that have different functional groups, such as alcohol from ketones, aromatic fi-om aHphatics, or saturates from unsatvaates. It shoxdd be noted that the McReynolds system is not only apphed to packed columns, but also to the selection of capillary column stationary phases. 

Are there any uses for the McReynolds System Consider OV-202 and OV-210. They have identical values indicating that these two polymers are... 

For use in the McReynolds system, the A/sp = Isp — fsquaiane is calculated for five test substances benzene, 1-butanol, methyl-n-propylketone, nitropropane, and pyridine. 

The most widely used system of classifying liquid phases is the McReynolds system (28) and has been employed to characterize virtually every stationary phase. McReynolds selected 10 probe solutes of different functionality, each designated to measure a specific interaction with a liquid phase. He analyzed these probe solutes and measured their I values on over 200 phases, including squalane. 

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

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... 

Bousse, L., McReynolds, R., Micromachined flow-through measurement chambers using LAPS chemical sensors. Micro Total Analysis Systems, Proceedings pTAS 94 Workshop, University of Twente, Netherlands, 21-22 Nov. 1994, 127-138. 

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... 

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). 

The retention mechanism for porous polymers is not known exactly. At low temperatures adsorption dominates but at higher temperatures it is possible that the porous polymers behave as a highly extended liquids with solvation properties. Various attempts to characterize selectivity differences based on the system of McReynolds... 

The system of stationary phase constants introduced by Rohrschneider [282,283] and later modified by McReynolds [284] was the first widely adopted approach for the systematic organization of stationary phases based on their selectivity for specific solute interactions. Virtually all-popular stationary phases have been characterized by this method and compilations of phase constants are readily available [28,30]. Subsequent studies have demonstrated that the method is unsuitable for ranking stationary phases by their selectivity for specific interactions [29,102,285-287]. The solvation parameter model is suggested for this purpose (section 2.3.5). A brief summary of the model is presented here because of its historical significance and the fact that it provides a useful approach for the prediction of isothermal retention indices. 

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),... 

It is clear from the foregoing discussion that no convenient system has been found for guiding the selection process. Certainly one cannot rely on McReynolds constants alone. The simple maxim commonly used is the one with which we began this section— like dissolves like. Which is to say, one chooses a nonpolar column for a nonpolar mixture and a polar column for a polar mixture. 

The system based on characteristic phase constants, known as the McReynolds constants, proved to be the most successful for describing phase selectivity. A similar system was introduced simultaneously by Rohrschneider, but his characteristic phase constants have not become so popular. 

Applying these systems, the phase selectivity, defined as its relative capacity to enter into specific intermolecular interactions, such as dispersion, induction, orientation, hydrogen-bond formation, and charge-transfer complexa-tion, can be characterized by 5 or 10 constants. These constants represent differences in the Kovats retention indices " of test substances on the phase under study and on a reference column containing a non-polar phase, at the same temperature. McReynolds proposed 120°C as a standard reference temperature. As a standard non-polar phase, Rohrchneider introduced squalane. ... 

The system of McReynolds constants inspired many researchers to carry out more extensive explorations. Based on this system, Tacacs constructed a unified system for the prediction of retention data in gas—liquid chromatography. 

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 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 system of retention indices is also used for characterization of adsorbent selectivity (see Table 3-4). For example. Table 3-4 [28] provides values for McReynolds constants [29] at 200 °C using Chromasorb 106 as the reference adsorbent. The data in parentheses were obtained after aging the columns for 22 days. Polarity of polymer adsorbent changes greatly as follows from the data of Table 3-4. 

L Bousse, R J McReynolds, G Kirk, T Dawes, P Lam, W R Bemiss, and J W Parce, Micromachmed multichannel systems for the measurement of cellular metabolism, International Conference on Solid State Sensors and Actuators, Yokohama, 1993, pp 916-920... 

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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