1 presents a small list of solvents that can be used for various purposes in the
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1 presents a small list of solvents that can be used for various purposes in the mobile phases. For each chemistry laboratory including as liquid nstant" of the solvent is provi f seven permittivity is often used in place of dielectric modern times, the term 'relative for constant. Table 1: Values of dielectric constant some common solvents. A large value of dielectric constant indicates that the solvent is polar Solvent Dielectric and will interact strongly with an Constant external electric field. As you have learned in general chemistry, polar 1.8 entan solvents is good at dissolving polar/ chlorofor nonpolar solutes. Conversely, non 86 polar solvents are good at at t-butyl dissolving polar/nonpolar solutes. alcohol 5.1 24.6 Elution (or conversely, retention) in ethanol liquid chromatography is controlled by equilibrium partioning of solutes methan acetonitril on the column between 37.5 and phases 78.5 water In LC the stationary and mobile phases can be thought of as 'competing for solutes The solutes have intermolecular interactions with both of the phases, and will preferentially partition into the phase with which they have the strongest intermolecular interactions The general rule in chromatography (except for size exclusion chromatography) is develop the method by first choosing a stationary phase that has a polarity similar to the analytes. Then, a mobile phase is chosen that has a quite different polarity than the stationary phase. Based on this general approach, complete the following sentences: To separate a mixture of polar species, you should use normal/reverse-phase LC, along with a relatively polar/non-polar mobile phase. To separate a mixture of non- polar species, you should use normal/reverse-phase LC, along with a relatively polar/ non-polar mobile phase. CHEM 4241 Liquid Chromatography ModesExplanation / Answer
First page:
1. Polar solvents are good in dissolving polar solutes. Conversely non-polar solvents are good at dissolving non polar solutes.
2. Elution in liquid chromatography is controlled by equilibrium partitioning of solutes on the column between stationary and mobile phases.
3. To separate a mixture of polar species, you should use reverse phase LC, along with a relatively polar mobile phase. To separate a mixture of non polar species, you should use normal LC along with relatively non-polar mobile phase.
Second page:
In 'a' which is for normal LC, the first peak is 'A' as it is least polar, the second peak is 'B' with intermediate polarity and the third peak with highest elution time is the most polar 'C'. A non polar mobile phase like pentane/chloroform can be used.
In 'b' which is for reverse phase LC, the most polar component will elute first, so the first peak will be 'C'. The second peak is 'B' and the third peak is the least polar 'A'. Water/Acetonitrile can be a mobile phase solvent for this case.
Gradient Elution:
The mobile phase composition is changed such that it becomes better at dissolving solutes in the mixture as the separation proceeds.
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