Polarity of the stationary phase

Two gas chromatograms showing the effect of polarity of the stationary phase on the separation efficiency for three substances of increasing polarity toluene, pyridine, and benzaldehyde. (a) Separation on silicone SE-30, a nonpolar phase, and (b) separation on elastomer OV-351, a more polar phase. Note the greatly changed absolute and relative retention times the more polar pyridine and benzaldehyde are affected most by the move to a more polar stationary phase. 

The eluting power of a solvent is determined by its overall polarity, the polarity of the stationary phase and the nature of the sample components. Table 4.9 lists some widely used solvents in order of their eluting power, this being known as an eluotropic series. In practice, better separations are achieved with the least polar solvent possible and mixtures of solvents are often used to achieve optimum separation conditions. 

Answer the following with normal phase or reverse phase. For which type of liquid chromatography is a C18 column used Which is similar to adsorption chromatography in terms of the polarity of the stationary phase ... 

To complete the determination of all constants, a value of 1 is assigned to each of the test solutes. In the case of benzene, X, its coefficient from Equation 4.1 would have the value of a = 1, whereas the remaining coefficients would be set equal to zero (i.e., b = 0, c = 0, d = 0, and e = 0). This process is repeated for the remaining solutes. The magnitude of each phase constant indicates the importance of the interaction in solute retention. Additionally, the overall polarity of the stationary phase can be determined by taking the average of all five phase constants. 

The degree of interaction between the mobile phase and the stationary phase, whether the latter be normal or reversed phase, affects the retention times of the analytes. In principle, the polarity of the stationary phase can lead to two situations ... 

For an ideal solution, the total standard partial molar excess Gibbs free energy, AGT, equals the ideal contribution, AG, and AGp = O. In other words, there is no interaction occurring between the solute and the stationary phase. When a nonpolar mole-, cule such as an alkane is chosen as the solute, AG will increase as the polarity of the stationary phase increases. There-—c... 

Of course AG° will not be the same value for every alkane, especially as the polarity of the stationary phase increases. So it becomes necessary to define a unit of the alkane, the methylene group, on which to base comparisons. Thus, the partial molar excess Gibbs free energy of the methylene group, AGg(cH2>, is defined by... 

Table 2.6 shows a series of silicone polymer stationary phases from the OV series. These are dimethylsiloxane polymers, with a varying percentage of the methyl groups replaced by phenyl groups. It turns out that the Rohrschneider constants closely follow this increase in phenyl group percentage. The constants therefore appear to be a reliable indication of the polarity of the stationary phase. 

In figure 3.7 two lines have been drawn, which represent two examples of possible phase systems for the elution of a solute with <5,= 12.5 (cal,/2.cm -3/2). The line with a positive slope connects a moderately polar mobile phase (<5m = 9) with a polar stationary phase (<5S= 16). Because the polarity of the stationary phase exceeds that of the mobile phase (Ss > 8m), this is by definition a normal (or straight) phase system. 

De Beer et al. [501] reported an extensive comparative study of the chromatographic behaviour of methyl esters and pentafluorobenzyl esters of these substances. They correlated retention data (Kovats retention indices) on nine stationary phases with the structure of the derivatives and with the polarity of the stationary phases with the aim of utilizing these dependences for identification purposes. However, much more significant is the better separation obtained with pentafluorobenzyl esters and the possibility of increasing the sensitivity of the analysis. 

Except for group 5, the polarity of the stationary phases increases with increasing group number... 

The eluting power of a solvent is determined by its overall polarity, the polarity of the stationary phase and the nature of the sample components. 

Various deactivation procedures are applied to support materials. These include acid- or basewashing to remove impurities and fine particles, and treatment with a silanising agent which reacts with surface hydroxyl groups and reduces adsorptive effects. A very light coating of a polar stationary phase may also be used to increase deactivation. Commercial support materials which have been treated by these procedures are available and are usually designated by a suffix to the name, e.g. AW (acid washed) and AW HMDS (acid washed, hexamethyldisilazane treated). Deactivated supports are nearly always to be preferred, but it should be noted that the deactivation procedure may impose an upper temperature limit and may modify the polarity of the stationary phase. 

It has been shown that the retention behaviour of benzene, butanol, pentan-2-one, nitropropane, and pyridine can be used to classify stationary phases in terms of their polarity (W.O.McReynolds, J. chromatogr. Set., 1970,5,685-691). The retention indices of each of these five reference compounds are measured, first on the stationary phase being tested and then on a standard phase (squalane). The differences in retention index between the two phases (AI) for the five reference compounds are added together to give a constant which is a measure of the polarity of the stationary phase. This constant is known as iheMcReynolds Constant and can be used to compare the ability of stationary phases to separate different classes of compounds (see below). However, this constant gives no information about peak shape, temperature limits, or the suitability for use in capillary colimms. 

Non-polar stationary phases were also found to show a structure-dependent elution order of toxaphene congeners [147]. For example, the substitution pattern at the bridge can be deducted. 8,8-substituted polychlorinated bornanes eluted much earlier than 8,9-substituted ones [147,149,150], The less polar the stationary phase, the more this rule comes to fruition. However, such structure-dependent selectivities do not necessarily improve the separation performance. For 8,8,9- or 8,9,9-substituted chlorobornanes having otherwise the same substitution pattern (e.g., B8-806 (P-42a) and B8-809 (P-42b)), the difference in retention decreases with decreasing polarity of the stationary phase. These components can only be separated on more polar stationary columns. Resolution of B8-806 (P-42a) and B8-809 (P-42b) was obtained on Optima 17 and Optima 3 stationary phases [151,152]. When using ECD it has to ensured in addition that no other organochlorine compounds interfere with the toxaphene compounds. 

If these small silica particles are used, then the chromatography is called normal phase, and the polarity of the stationary phase is higher than that of the mobile phase this is what happens, for example, when silica is used in adsorption chromatography. However, almost all the work in analytical HPLC is now carried out with chemically modified silica, which is the bonded phase. In a bonded phase, the highly polar surface of silica is modified by the chemical attachment of various functional groups. Bonded-phase chromatography is experimentally much easier, more versatile, and quicker it also has better reproducibility than the older modes. When a nonpolar-bonded phase is used, the operation is performed in an RP mode, which means that the polarity of the stationary phase is less than that of the mobile phase. These columns, contrary to normal silica columns, elute polar compounds more rapidly than nonpolar compounds. 

To have a reasonable residence time in the column, an analyte must show some degree of compatibility (solubility) with the stationary phase. Here, the principle of like dissolves like applies, where like refers to the polarities of the analyte and the immobilized liquid. Polarity is the electrical field effect in the immediate vicinity of a molecule and is measured by the dipole moment of the species. Polar stationary phases contain functional groups such as —CN, —CO, and —OH. Hydrocarbon-type stationary phases and dialkyl siloxanes are nonpolar, whereas polyester phases are highly polar. Polar analytes include alcohols, acids, and amines solutes of medium polarity include ethers, ketones, and aldehydes. Samrated hydrocarbons are nonpolar. Generally, the polarity of the stationary phase should match that of the sample components. When the match is good, the order of elution is determined by the boiling point of the eluents. 

In adsorption chromatography, the only variable that affects the distribution coefficient of analytes is the composition of the mobile phase (in contrast to partition chromatography, where the polarity of the stationary phase can also be varied). Fortunately, enormous variations in retention and thus resolution accompany variations in the solvent system, and only rarely is a suitable mobile phase not available. 

Systematic method development guidelines akin to those available for HPLC have not been developed. However, details beyond the scope of this chapter are available.Separation conditions should be evaluated based on the polarity of the solute and the polarity of the stationary phase. Stationary-phase polarity increases in the order C18phenylnonpolar solutes on nonpolar stationary phases, separation may be achieved using pure carbon dioxide. As solute or stationary-phase polarity increases, carbon dioxide modified with methanol (or isopropanol, ethanol, or acetonitrile) or carbon dioxide modified with solvent and an additive such as TFA, acetic acid, triethylamine, or isopropylamine (0.5% or less) is required. 

Relative retention volumes and retention indices have been reported399 for silicon tetrachloride, methyltrichloro- and dimethyldichloro-silane, phosphorus trichloride and phosphorus oxychloride at 50 °C on stationary phases containing vaseline oil, poly-siloxane liquids VKZh-94, PFMS-2 and DS-701, fluorosilicone oil 169, dinonyl phthalate and dibutyl phthalate. The retention index for phosphorus oxychloride increased with increase in the polarity of the stationary phase that for silicon tetrachloride remained practically constant. 

A few materials in order of decreasing polarity and adsorption strength are Alumina > magnesium oxide > charcoal > silica gel > calcium oxide > magnesium carbonate > calcium carbonate > potassium carbonate > sodium carbonate > sucrose > starch > cellulose. Increasing the polarity of the stationary phase increases the retention time i.e. the time for which components of the mixture are retained on the column while these are being washed down the column. 

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