Gas Liquid Chromatography Mcat

BC = first-semester biochemistry
OC = two-semester sequence of organic chemistry

Chemical and Physical Foundations of Biological Systems Section: Content Category 5C

Analysis of complex mixtures of substances ― especially biologically relevant materials ― typically requires separation of the components. Many methods have been developed to accomplish this task, and the method used is dependent on the types of substances which comprise the mixture. All of these methods rely on the magnification of potential differences in the strength of intermolecular interactions.

The content in this category covers separation and purification methods including: extraction, liquid and gas chromatography, and electrophoresis. The topics and subtopics are below:

Topic Level Key:

The abbreviations found in parentheses indicate the course(s) in which undergraduate students at many colleges and universities learn about the topics and associated subtopics. The course abbreviations are:

BC = first-semester biochemistry
OC = two-semester sequence of organic chemistry

Please note topics that appear on multiple content lists will be treated differently. Questions will focus on the topics as they are described in the narrative for the content category.

Separations and Purifications (OC, BC)

• Extraction: distribution of solute between two immiscible solvents
• Distillation
• Chromatography: Basic principles involved in separation process
  • Column chromatography
    • Gas-liquid chromatography
    • High pressure liquid chromatography
    • Electrophoresis
    • Quantitative analysis
    • Chromatography
      • Size-exclusion
      • Ion-exchange
      • Affinity

      Additional Review: Khan Academy MCAT ® Collection Tutorials

      To support your studies, see the following video tutorials below from the Khan Academy MCAT Collection. The videos and associated questions were created by the Khan Academy in collaboration with the AAMC and the Robert Wood Johnson Foundation.

      Separations and Purifications

      Extraction (Distribution of Solute Between Two Immiscible Solvents

      Organic phase	Aqueous phase
      Solvent	Nonpolar solvent	Water
      Solute	Nonpolar solutes dissolve here	Ionic and polar solutes dissolve here
      Density	The organic phase does not always float on top. Chloroform, for example, sinks below the aqueous phase.	Water is usually denser than other solvents, but some organic solvents are even denser.

      Distillation

      • Separates liquids based on boiling point. The stuff with the lower boiling point is boiled off and collected; the higher boiling point stuff remains behind.
      • Simple distillation = done with a normal column = can separate two liquids if the difference in boiling point is large.
      • Fractional distillation = done with a fractionating column = can separate two liquids with smaller differences in boiling point.
      • Vacuum distillation = done under lower pressure (vacuum) = lowers the boiling point for all liquid components so you don’t have to crank up the temperature so high (chemical might decompose).

      Chromatography (Basic Principles Involved in Separation Process)

      • Gas-liquid chromatography
        • Good if analyte can be promoted to gas phase.
        • Gas-liquid chromatography (GLC) is the same thing as gas chromatography (GC).
        • The gas part is the mobile phase, the liquid part is the stationary phase coated to the inside walls of the column.
        • Substrate equilibrates between mobile (gas) and stationary (liquid coat) phase.
        • Those with greater affinity for the stationary phase comes out of the column slower. Polar substrate has more affinity for polar stationary phase, and hydrophobic substrate has more affinity for hydrophobic stationary phase.
        • Classically used to separate pigments in dyes.
        • Solvent = mobile phase. Paper = stationary phase.
        • Pigments in dyes stick to paper, solvent tries to wash them along, those with greater affinity to paper stays behind, those with greater affinity to solvent gets washed along.
        • R_f value = distance traveled by pigment / distance of solvent front.
        • R_f = 0 means that pigment has not moved.
        • R_f = 1 means that pigment moved as far as the solvent front.
        • Thin-layer chromatography = advanced paper chromatography.
        • Instead of paper, you have a plate coated with a specific stationary phase of your choosing.
        • R_f value used in the same way as paper chromatography.

        Recrystalization (Solvent Choice from Solubility Data)

        • Recrystalization = barely dissolving your compound, then let it recrystalize out of solution = compound ends up being more pure.
        • Barely dissolving = use just enough to fully dissolve your compound under warm temperature = saturated solution.
        • Recrystalize = solution cools, solubility decreases, compound comes out of solution.
        • Solvent choice = choose a solvent in which your compound is soluble in at warm temperature, but not at cool temperature. Also, choose a solvent in which impurities are highly soluble.
        • Impurities should remain dissolved in the solvent even when your compound recrystalizes out.

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