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12.10: Chapter Summary and Key Terms

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    Chapter Summary

    Chromatography and electrophoresis are powerful analytical techniques that both separate a sample into its components and provide a means for determining each component’s concentration. Chromatographic separations utilize the selective partitioning of the sample’s components between a stationary phase that is immobilized within a column and a mobile phase that passes through the column.

    The effectiveness of a chromatographic separation is described by the resolution between two chromatographic bands and is a function of each component’s retention factor, the column’s efficiency, and the column’s selectivity. A solute’s retention factor is a measure of its partitioning into the stationary phase, with larger retention factors corresponding to more strongly retained solutes. The column’s selectivity for two solutes is the ratio of their retention factors, providing a relative measure of the column’s ability to retain the two solutes. Column efficiency accounts for those factors that cause a solute’s chromatographic band to increase in width during the separation. Column efficiency is defined in terms of the number of theoretical plates and the height of a theoretical plate, the later of which is a function of a number of parameters, most notably the mobile phase’s flow rate. Chromatographic separations are optimized by increasing the number of theoretical plates, by increasing the column’s selectivity, or by increasing the solute retention factor.

    In gas chromatography the mobile phase is an inert gas and the stationary phase is a nonpolar or polar organic liquid that either is coated on a particulate material and packed into a wide-bore column, or coated on the walls of a narrow-bore capillary column. Gas chromatography is useful for the analysis of volatile components.

    In high-performance liquid chromatography the mobile phase is either a nonpolar solvent (normal phase) or a polar solvent (reversed-phase). A stationary phase of opposite polarity, which is bonded to a particulate material, is packed into a wide-bore column. HPLC is applied to a wider range of samples than GC; however, the separation efficiency for HPLC is not as good as that for capillary GC.

    Together, GC and HPLC account for the largest number of chromatographic separations. Other separation techniques, however, find special- ized applications: of particular importance are ion-exchange chromatography for separating anions and cations; size-exclusion chromatography for separating large molecules; and supercritical fluid chromatography for the analysis of samples that are not easily analyzed by GC or HPLC.

    In capillary zone electrophoresis a sample’s components are separated based on their ability to move through a conductive medium under the influence of an applied electric field. Positively charged solutes elute first, with smaller, more highly charged cations eluting before larger cations of lower charge. Neutral species elute without undergoing further separation. Finally, anions elute last, with smaller, more negatively charged anions being the last to elute. By adding a surfactant, neutral species can be separated by micellar electrokinetic capillary chromatography. Electrophoretic separations also can take advantage of the ability of polymeric gels to separate solutes by size (capillary gel electrophoresis), and the ability of solutes to partition into a stationary phase (capillary electrochromatography). In comparison to GC and HPLC, capillary electrophoresis provides faster and more efficient separations.

    Key Terms

    adjusted retention time

    baseline width

    capillary column

    capillary gel electrophoresis


    cryogenic focusing

    electroosmotic flow velocity


    exclusion limit
    gas chromatography

    general elution problem

    headspace sampling

    inclusion limit

    isocratic elution

    Kovat’s retention index

    loop injector

    mass transfer

    mobile phase

    nonretained solutes

    open tubular column

    peak capacity

    porous-layer open tubular column

    retention factor

    selectivity factor

    split injection

    stationary phase


    thermal conductivity detector

    wall-coated open-tubular column

    adsorption chromatography


    capillary electrochromatography

    capillary zone electrophoresis

    column chromatography

    electrokinetic injection

    electron capture detector

    electrophoretic mobility

    flame ionization detector

    gas–liquid chromatography

    guard column

    high-performance liquid chromatography

    ion-exchange chromatography


    liquid–solid adsorption chromatography

    mass spectrometer


    monolithic column

    normal-phase chromatography

    packed columns

    planar chromatography


    retention time

    single-column ion chromatography

    splitless injection

    supercritical fluid chromatography

    temperature programming

    van Deemter equation

    zeta potential

    band broadening

    bonded stationary phase

    capillary electrophoresis


    counter-current extraction

    electroosmotic flow


    electrophoretic velocity

    gas–solid chromatography

    gradient elution

    hydrodynamic injection

    ion suppressor column

    Joule heating

    longitudinal diffusion

    mass spectrum

    micellar electrokinetic capillary chromatography

    multiple paths

    on-column injection

    partition chromatography

    polarity index


    reversed-phase chromatography

    solid-phase microextraction


    support-coated open tubular column

    theoretical plate

    void time

    This page titled 12.10: Chapter Summary and Key Terms is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by David Harvey.

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