Chapter 28
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Describe three general methods for improving resolution in partition chromatography.
Three methods for improving resolution include: (1) Adjustment of kA and kB by employing a multicomponent mobile phase and varying the rate of the solvents to find an optimal mixture. (2) Variation in the chemical composition of the solvent system in such a way as to make α larger. (3) Employing a different packing in which α is greater.
Describe a way to manipulate the retention factor of a solute in partition chromatography.
In partition chromatography,k is conveniently varied by using a two (or more) component solvent system and varying the ratio of the solvents.
What is meant by the linear-response range of a detector?
The linear response range of a detector is the range of analyte concentration or mass over which the detector responds linearly.
Define isocratic elution.
In an isocratic elution, the solvent composition is held constant throughout the elution.
Define gradient elution.
In a gradient elution, two or more solvents are used and the composition of the mobile phase is changed continuously or in steps as the separation proceeds.
Define stop-flow injection.
In a stop-flow injection, the flow of solvent is stopped, a fitting at the head of the column is removed, and the sample is injected directly onto the head of the column. The fitting is then replaced and pumping is resumed.
Define reversed-phase packing.
A reversed-phase packing is a nonpolar packing that is used in partition chromatography with a relatively polar mobile phase.
Define normal-phase packing.
In a normal-phase packing, the stationary phase is polar and the mobile phase is relatively nonpolar.
Define ion-pairing chromatography.
In ion-pair chromatography a large organic counter-ion is added to the mobile phase as an ion-pairing reagent. Separation is achieved either through partitioning of the neutral ion-pair or as a result of electrostatic interactions between the ions in solution and charges on the stationary phase resulting from adsorption of the organic counter-ion.
Define ion chromatography.
In ion chromatography, the stationary phase is an ion-exchange resin, and detection is ordinarily accomplished by a conductivity detector.
Define bulk property detector.
A bulk property detector responds to some property of the mobile phase (such as thermal or electrical conductivity) that is altered by the presence of analytes.
Define solute property detector.
A solute property detector responds to some property of analytes, such as absorption or fluorescence.
Define sparging
Sparging is a process for removing dissolved gases from a solution by sweeping the liquid with a stream of fine bubbles of an inert gas of low solubility.
Describe the fundamental difference between ion-exchange and size-exclusion chromatography.
In size-exclusion chromatography separations are based upon the size, and to some extent the shape, of molecules with little interactions between the stationary phase and the sample components occurring. In ion-exchange chromatography, in contrast, separations are based upon ion-exchange reactions between the stationary phase and the components of the sample in the mobile phase.
What types of species can be separated by HPLC but not by GC?
Nonvolatile and thermally unstable compounds can be separated by HPLC by not GC