Polarity, of stationary phases

M c Polarity of mobile phase S c Polarity of stationary phase C c Phase ratio k vs. VJ Vm S ftt linear 1.10... 

A method to select the appropriate stationary phase for analysis of a sample mixture is to consider the polar characteristics of the analytes and select a stationary phase of similar polarity. An analyte with similar polar character to the stationary phase will be well retained, the principle of like attracts like applies, and useful retention is then likely to occur leading to adequate selectivity and separation of the analytes primarily on the basis of volatility. Conversely, if the solute is immiscible with the stationary phase then little or no retention difference will be obtained. Further useful indication of retention characteristics may be obtained by analysing a sample on a non-polar and polar column, for example, a dual column GC fitted with Apiezon/OVlOl and Carbowax 20M columns temperature programmed 50-220°C at 10°Cmin with a final hold of 10 min. The chromatogram will indicate the polarity of stationary phase required for the components and the analysis can be repeated with columns of differing polarity, e.g. OV17, OV1701. Tables of RIs for various classes of compounds have been published, mainly for squalane, the reference non-polar stationary phase and Apiezon L, with RI values for Carbowax 20M as a reference polar stationary phase [10]. 

Figure 5.9 Chromatograms illustrating GC analyses using a range of column types and detectors. Note the polarity of the stationary phase, analysis times of the components and sequence in which they are eluted. Polarity of stationary phases are given in Table 5.5. (Chromatograms reproduced by permission of Restek Corporation.)...

Let us return to our discussion regarding the determination of the polarity of stationary phases by beginning with an example using Kovats retention indexes. From McReynolds [3] we find that toluene has a Kovats retention index of 773 on the nonpolar phase squalane and 860 on the more polar dioctylphthalate. The difference in these indexes, 87, provides a measure of the increased relative polarity of dioctylphthalate relative to squalane. The difference can be designated as A/. 

Eluting power of solvents as function of polarity of stationary phases. 

A proper SEC experiment has to be balanced with respect to polarities. In order to obtain a true and pure SEC separation, the polarity of stationary phase (column material), the polarity of eluent, and the polarity of sample have to be matched. This is visualized by the magic triangle (Fig. 9.6). Dominance of size separation is only maintained in the center of the triangle (bright area), where the overall systan is balanced. Otherwise, specific interactions will occur, which will overlay with the normal SEC elution behavior. 

Stationary phases is used to characterize the separation. However, the polarity is not a unique, well-defined characteristic. Different authors define it differently. Eight polarity parameters are used to characterize the polarity of stationary phases [5]. The information they carry is redundant. Which polarity parameter is similar to others can be seen in the figure. Proximity of points means similarity The closer a point, the more similar its polarity parameter is. The four polarity parameters form a dense cluster the points for MR and RP are identical and XN is an outlier. 

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. 

The main criteria for selecting a stationary phase are that it should be chemically inert, thermally stable, of low volatility, and of an appropriate polarity for the solutes being separated. Although hundreds of stationary phases have been developed, many of which are commercially available, the majority of GLC separations are accomplished with perhaps five to ten common stationary phases. Several of... 

The stationary phase in LC is a fine granular solid such as silica gel. It can be used as such (mainly for nonpolar compounds), or the granules can be modified by a surface-bonded coating that changes (reverses) the polarity of the gel. A very small selection of stationary phases is listed in Table 35.2. 

Figure 8.17 depicts MD-PC performed on three different types of stationary phase (6). The three grafted chromatographic plates (Figure 8.17(a)) are clamped in lap-joint fashion with the edges of their stationary phases in close contact. The manner in which the three plates are prepared and the separation which can theoretically be achieved are also apparent from the schematic diagrams in Figures 8.17(b-d), in which the most polar stationary phase is phase A and the least polar is stationary phase C . 

Excellent open tubular columns may now be purchased, providing a number of stationary phases of differing polarity on WCOT and SCOT columns, and whose efficiency, greatly improved sample detectability, and thermal stability surpass those exhibited by packed columns their chief disadvantage is that they have a lower sample capacity than packed columns.65,66... 

Returning to the molecular force concept, in any particular distribution system it is rare that only one type of interaction is present and if this occurs, it will certainly be dispersive in nature. Polar interactions are always accompanied by dispersive interactions and ionic interactions will, in all probability, be accompanied by both polar and dispersive interactions. However, as shown by equation (11), it is not merely the magnitude of the interacting forces between the solute and the stationary phase that will control the extent of retention, but also the amount of stationary phase present in the system and its accessibility to the solutes. This leads to the next method of retention control, and that is the volume of stationary phase available to the solute. 

A large range of stationary phases is available, and according to their polarity they can be divided into normal phase and reversed phase types. Silica gel, aluminium oxide, and a nitrile-bonded-phase are normal adsorbents used to separate carotenoids... 

The problems of stationary phase erosion can be largely overcome by solvent-generated LLC, where the stationary liquid phase is generated dynamically by the mobile phase, in this approach, one of the phases of an equilibrated liquid-liquid system is applied as a mobile phase to a solid support which is better wetted by the other phase of the liquid-liquid system. The support is usually silica when the stationary phase is aqueous or a polar solvent and a reversed-phase chemically bonded support when the stationary phase is a nonpolar solvent. Under these Conditions a multimolecular layer is formed on the surface of the solid support which has the properties of the liquid phase in... 

The mechanism of reversed phase chromatography can be understood by contrast with normal phase chromatography. Normal phase liquid chromatography (NPLC) is usually performed on a polar silica stationary phase with a nonpolar mobile phase, while reversed phase chromatography is performed on a nonpolar stationary phase with a polar mobile phase. In RPLC, solute retention is mainly due to hydrophobic interactions between the solutes and the nonpolar hydrocarbon stationary surface. The nonpolar... 

Polarity is a term that is used in chromatography as an index of the ability of compounds to interact with one another in these various ways. It is applied very freely to solutes, stationary phases and mobile phases. The more polar a molecule, the more strongly it can interact with other molecules through the mechanisms above. If the polarities of stationary and mobile phases are similar, then... 

Multidimensional GC with time of flight MS (ToF-MS) is increasingly being employed to examine complex mixtures. In this technique non-polar and polar GC stationary phases connected by a thermal modulator enable increased resolution of GC peaks. The fast scanning made possible by the ToF measuring system leads to many data points across peaks and the possibility of deconvolution of complex overlapping peaks. 

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