Stationary phases classification

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. 

Figure 5.6 Chiral stationary phases classification according to chiral recognition mechanisms and chemical structures. (From Reference 256.)...

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... 

Various scales of solvent strength (polarity) and selectivity have been used to classify stationary phases. Classification based on polarity had to be abandoned because of the lack of a working definition. There is no substance that is uniquely polar and suitable to probe the polarity of other substances. Selectivity is defined as the relative capacity of a stationary phase for specific intermolecular interactions, such as dispersion, induction, orientation, and hydrogen-bond formation. Early attempts at a systematic definition of selectivity scales were based on the system of phase constants introduced by Rohrschneider and... 

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. 

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. 

Seleetion of basic parameters of chromatographic process (stationary phase, proper pure solvents according to Snyder s classification, and vapor phase)... 

The mobile phase can be either a gas or a liquid, while the stationary phase can be either a liquid or solid. One classification scheme is based on the nature of the two phases. All techniques which utilize a... 

Adsorption HPLC is the classification in which the highly polar silica particles are exposed (no adsorbed or bonded liquid phase). Aluminum oxide particles fit this description too and are also readily available as the stationary phase. As mentioned earlier, this classification can also be thought of as normal phase... 

If a given HPLC system is using a methanol-water mixture for the mobile phase and a C18 column for the stationary phase, what classification of chromatography would be in use Explain. 

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

The two obvious ways for classification are based on the iilica gel support and the chemical nature of the hydrocarbonaceous liga. The names of commercial stationary phases usually refiect such a d tinction and contain the designations of both the silica and the ligate, e.. Spherisorb ODS HtuI I.K hrosiirb RP8, where Spherisorb and LK hroj rb are trade... 

In a chromatographic separation procedure the parameters of the chromatographic system (stationary phase, flow, temperature, etc.) have to be selected respectively optimized with respect to some criterion (resolution, time, etc.). In gas chromatography retention data series are published and used for the sttidy of solvent/solute interaction, prediction of the retention behaviour, activity coefficients, and other relevant information usable for optimization and classification. Several clKmometrk techniques of data anal s have been employed, e.g. PCA, numerical taxonomic methods, information theory, and j ttern recognition. 

Pattern recognition can be applied for the determination of structural features of unknown (monofunctional) compounds (Huber and Reich ). The information about the chemical structure is contained in a multidimensional gas-liquid retention data/stationary liquid phases set. The linear learning machine method is applied in a two step classification procedure. After the determination of a correction term, the skeleton number, a classification step for the determination of the functional group is executed. It is remarkable that 10 stationary phases are sufficient for the classification. 

Certain phases combine distribution coefficients of very different natures. The K values are much higher (e.g. up to 1000) if the mobile phase is a gas and the stationary phase is a condensed phase. The classifications that follow give priority to the nature of the phases present. 

Figure 1.2—Classification of some chromatographic techniques as a function of the polaritv of the stationary phase. The existence of a wide range of techniques permits the most suitable one to be selected for a specific analytical problem.

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