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Gas Chromatographic Columns

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    Learning Objectives

    All students are expected to:

    • Differentiate different types of columns (i.e., WCOT, PLOT, packed), their dimensions, applicability and diffusion factors affecting the separation
    • Evaluate suitability of common stationary phases for different analytes
    • Distinguish types of interactions between the analytes and stationary phase
    • Justify the selection of a stationary phase based on its interactions, stability, and bleed with the targeted analyte
    • Evaluate and explain the column bleed

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    Quiz GC Columns

    Provide handwritten answers and bring them to the class

    1. Compare capillary and packed columns, their dimensions, the common number of theoretical plates, and factors affecting peak broadening.

     

     

     

    1. Explain the principle and strength of dispersive interactions (London forces), dipole-dipole interactions, hydrogen bonding.

     

     

    1. Differentiate stationary phases and discuss their relationship to the interactions with analytes.

     

     

    1. What is the difference in the interactions with analytes on methyl and phenyl dimethyl polysiloxane stationary phases?

     

     

    1. A homological series of alkanes is analyzed using a linear temperature gradient. What defines the elution order? How does Δtr change for sequentially eluting compounds?

     

     

    1. What are PLOT columns and how are they used?

     

     

    1. What is the column bleed?

     

     

    1. In GC analysis, what is the source of an ion of 207 m/z?

     

     

     

    A.  Stationary Phases and Integrations in GC

    1. Match a suitable stationary phase to the following analytes

    Heptane C7H16

    Heptane.png

    Heptanoic Acid C7H14O2

    HeptanoicAcid.png

    Glycerol C3H8O3

    Glycerol.png

    Nitrobenzene C6H5NO2

    Nitrobenzene.png

    Acetic Acid C2H4O2

    AceticAcid.png

     

    StationaryPhaseA.png    StationaryPhaseB.png    StationaryPhaseC.png    StationaryPhaseD.png

     

     

     

     

    1. List the types of interactions expected between the analytes and stationary phases selected above.

     

     

     

     

     

     

    1. Which of the stationary phases above would be suitable specifically for GC-MS analysis?

     

     

     

    1. Which of these stationary phases is the least thermally stable?

     

     

     

     

    B.  Separation on PLOT columns

    1. What are PLOT columns? Please sketch.

     

     

     

    1. Which of the analytes listed below would be separated on a PLOT column?
      1. Organic solvents
      2. Polycyclic aromatic hydrocarbons
      3. Refinery gases
      4. Air
      5. Fatty acids

     

    1. Which analytes (of those listed above) would not be suitable for a PLOT column and why?

     

     

     

     

    1. Sketch two chromatograms of air on an alumina PLOT column with retention H2< O2 < N2, CO2 using a column with stationary phases of 0.1 µm and 5.0 µm film thickness.

     

     

     

    1. Would you expect to have longer or shorter retention times on a thicker stationary phase and why?

     

    1. How fast would be the elution of the analytes listed above?

     

    1. Based on the points 5 and 6 reconsider your chromatograms.

     

     

    1. Would it better work with higher or lower flowrates (Consider Van Deemter theory)

     

     

    C.  GC Analysis and Column Bleed

    1. Sketch a chromatogram of a solvent (hexane) analyzed using the temperature program from 40 °C to 320 °C with a 25 °C/min gradient.

     

     

     

     

     

     

    1. What is the column bleed? Consider whether it would affect the sketched chromatogram above?

     

     

     

     

    1. Sketch three chromatograms and label all peaks of a series of alkanes (C7-C15) dissolved in n-hexane on a dimethyl polysiloxane stationary phase analyzed using a split injection and
      1. An Isothermal analysis at 200  °C.
      2. A temperature program from 40 °C to 320 °C with a 5 °C/min gradient.
      3. A temperature program from 100 °C to 320 °C with a 5 °C/min gradient.

     

     

     

     

    1. Do you expect any co-elution? Why?

     

     

     

     

    D.  Optimizing Temperature Program

    TemperatureProgram,AnalytesElutionProfile.png

    1. The temperature program and analytes elution profile are shown above. How could you modify the temperature program to improve the analysis?

     

     

     

     

    1. Determine temperatures at which each analyte eluted in the temperature program used above.

     

     

    1. Is your program shorter or longer than the original?

     

     

     

    1. Based on questions 2 and 3, reconsider the proposed program and suggest a new one.

     

     

     

     

     

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