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Chemistry LibreTexts

Chromatography - In-class Problem Set #2

Longitudinal Diffusion

After completing this problem, the student will be able to:

  1. Rationalize why a band of a compound in a chromatographic column will spread out in both directions due to normal diffusion processes.
  2. Explain why longitudinal diffusion is more significant in gases than liquids
  3. Predict and rationalize the effect that the mobile phase flow rate has on the contribution of longitudinal diffusion to chromatographic peak broadening.
  4. Write an appropriate term for the van Deemter equation that includes the flow rate dependency of longitudinal diffusion.

Eddy Diffusion

After completing this problem, the student will be able to:

  1. Rationalize why different molecules will have different paths with different path lengths through a packed bed of particles.
  2. Explain why the difference between the shortest and longest path is a function of particle size and is smaller for smaller particles.
  3. Explain the meaning of channeling in a chromatographic column and why channeling contributes significantly to broadening.
  4. Explain why it is more difficult to efficiently pack small particles.
  5. Explain why open tubular capillary columns do not exhibit eddy diffusion.
  6. Rationalize why there is still debate about whether or not eddy diffusion has a flow rate dependency.

Stationary Phase Mass Transport Broadening

After completing this problem, the student will be able to:

  1. Rationalize why molecules in the mobile phase will move ahead of molecules in the stationary phase in a chromatographic column and explain why this contributes to peak broadening in a chromatographic system.
  2. Recognize that molecules must spend a finite amount of time in the stationary phase.
  3. Justify why the contribution of stationary phase mass transport broadening to peak broadening depends on the flow rate and is more significant the higher the flow rate.
  4. Write an appropriate term for the van Deemter equation that includes the flow rate dependency of stationary phase mass transport broadening.
  5. Explain why the dependence on flow rate for stationary phase mass transport broadening represents a limitation to completing the chromatogram in a short period of time.
  6. Rationalize why it is best to have the stationary phase be as thin as possible.
  7. Rationalize why it is best to have a homogeneous coating of the stationary phase.
  8. Explain the process for coating capillary gas chromatographic columns.
  9. Rationalize why smaller particles lead to thinner coating and reduced stationary phase mass transport broadening when using coated phases for gas chromatographic separations that have the same weight percent loading.
  10. Explain why stationary phase mass transport broadening is more significant in liquid chromatography than it is in gas chromatography.
  11. Describe the process used to prepare bonded phase materials for use in liquid chromatography.
  12. Explain the advantage of using bonded over coated stationary phases in liquid chromatography.

Mobile Phase Mass Transport Broadening

After completing this problem, the student will be able to:

  1. Rationalize why it is best for molecules in the mobile phase to be able to encounter the stationary phase as quickly as possible and explain why the finite time it takes for a molecule to encounter the stationary phase contributes to peak broadening in a chromatographic system.
  2. Recognize that molecules must spend a finite amount of time in the mobile phase before encountering the stationary phase.
  3. Justify why the contribution of mobile phase mass transport broadening to peak broadening depends on the flow rate and is more significant the higher the flow rate.
  4. Write an appropriate term for the van Deemter equation that includes the flow rate dependency of mobile phase mass transport broadening.
  5. Explain why the dependence on flow rate for mobile phase mass transport broadening represents a limitation to completing the chromatogram in a short period of time.
  6. Rationalize why a capillary column with a narrower internal diameter will have less mobile phase mass transport broadening.
  7. Justify why capillary liquid chromatography is usually not a practical method to use widely in the laboratory.
  8. Rationalize why packed columns with smaller particles will have less mobile phase mass transport broadening that those with larger particles.
  9. Explain why mobile phase mass transport broadening is much more significant in liquid chromatography than it is in gas chromatography.
  10. Justify why the particles used in liquid chromatographic columns are much smaller than those in gas chromatographic columns.
  11. Explain why, when using bonded liquid chromatographic phases, shorter columns can be used if smaller particles are used.

After completing the development of the van Deemter equation, the student will be able to:

  1. Draw a plot of h versus v for a typical gas and liquid chromatographic packed column.
  2. Rationalize the general form of the plot in (1).
  3. Explain why most analyses are not performed at the flow rate that optimizes the efficiency of the column.
  4. Explain for each of the following (separately for gas and liquid chromatography), referring back to specific terms and processes incorporated into the van Deemter equation, its expected impact of or relationship to column efficiency:
    1. Particle size
    2. Flow rate
    3. Using a gas versus a liquid as the mobile phase
    4. Switching from a packed to a capillary column
    5. Coating a thicker stationary phase
    6. The presence of channels in the packing material
    7. Coating a more homogeneous mobile phase
    8. Carry out a more extensive end-capping of silanol groups on bonded liquid chromatographic phases.
  5. For the same items listed in number (4), explain what other positive or negative effects it has on the chromatographic process. This can include aspects such as analysis time or experimental variables or constraints that it places on the analyst.