EXPERIMENT 2 NORMAL-PHASE CHROMATOGRAPHY

Objective Background Additional materials Safety and disposal Experiments 

Discussion Self-help questions Additional experiments 

To demonstrate the approach used in developing a normal-phase separation on silica by observing the effect upon the solutes of changing the mobile-phase composition. 

The term normal phase refers to a chromatographic system that utilizes a polar stationary phase and a nonpolar mobile phase. Normal-phase chromatography usually succeeds in the separation of compounds that differ in the 

As a general rule, a mobile phase should be chosen which is slightly less polar than the sample (solute). Since retention occurs as a result of competition between the mobile phase and the sample for the silica surface, the mobile phase for a given separation may have to be a mixture of two solvents. There are other secondary interactions that occur in the separation process (1) between the mobile phase and the sample and (2) between the mobile phase and the adsorbent. In cases where mobile phase-sample interactions are encountered, changing from one solvent to another with similar polarity but different structure dramatically changes the separation. 

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The results confirmed that the chloroheptane/n-heptane mixture behaves in an identical manner to carbon tetrachloride and all the points were on the same straight line as that produced using a mixture of carbon tetrachloride and toluene. These experiments are similar to normal phase chromatography using pure water instead of... 

The ELS detector was previously also referred to as a mass detector, pointing to the fact that the response is (mainly) determined by the mass of the sample rather than by its chemical structure. Van der Meeren et al., though, demonstrated that the ELSD calibration curves of phospholipid classes were also dependent on the fatty acid composition (52). The dependence on the fatty acid composition is, however, completely different in nature and much less pronounced than for UV detection. The reason for this behavior is to be found in the partial resolution of molecular species, even during normal-phase chromatography. Thus, the peak shape depends not only on the chromatographic system but also on the fatty acid composition and molecular species distribution of the PL sample (47). Because it was shown before, based on both theoretical considerations and practical experiments, that the ELS detector response is generally inversely proportional to peak width (62,104), it follows that the molecular species distribution of the PL standards used should be similar to the sample components to be quantified. It was shown that up to 20% error may be induced if an inappropriate standard is used (52). 

The following is a suggested experiment which will further increase your understanding of the use of normal-phase chromatography ... 

Normal-phase, bonded-phase columns are likely underutilized for separations where they should be the method of choice. This is due both to the ease of use of reversed-phase, bonded-phase columns, discussed next, and also to the many problems inherent in the use of bare silica and alumina. Very straightforward method development in normal-phase chromatography can be performed by combining the solvent and stationary-phase selectivity triangles. The three columns, each used with the three recommended modifiers, should provide the maximum difference in selectivity available. These nine experiments, used in conjunction with chemometric optimization schemes, should then provide a ratio-... 

Practical determination of the porosities often experiences difficulties. The most common method for determining the total porosity is the injection of nonre-tained, pore-penetrating tracer substances (gray and small black circles in Figure 2.5). In normal phase chromatography, toluene or 1,3,5-tri-tert-butylbenzene is often used, while in reversed phase chromatography uracil is typically selected as the component of choice. 

Recently, Janjic et al. published some papers [33-36] on the influence of the stationary and mobile phase composition on the solvent strength parameter e° and SP, the system parameter (SP = log xjx, where and denote the mole fractions of the modiher in the stationary and the mobile phase, respectively) in normal phase and reversed-phase column chromatography. They established a linear dependence between SP and the Snyder s solvent strength parameters e° by performing experiments with binary solvent mixtures on silica and alumina layers. 

The separation scientist with experience gained from a LC background may tend to limit the modes of electrochromatography to reversed phase (RP), normal phase, ion-exchange and, maybe, size-exclusion. Analysts from an electrophoretic background typically use the term "CE" in a much broader sense to include the main modes of capillary zone electrophoresis, micellar electrokinetic chromatography, capillary gel electrophoresis, isoelectric focusing and isotachophoresis. 

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