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Chemistry LibreTexts

Chromatography set #2 – Peak Broadening

Development of Longitudinal Diffusion Broadening in Chromatography

Consider a “band” of a compound in a chromatographic column.  The band has the following concentration profile.

a) What would happen to this profile if the flow of the column were stopped and the column was allowed to sit?

b) Would this phenomena be more significant (i.e., happen faster) in a gas or a liquid?

c) Does this phenomena contribute more to band broadening at higher or lower flow rates?

Development of Eddy Diffusion Broadening in Chromatography

Consider molecules flowing through a packed bed of particles.

a) Would different molecules have different path lengths as they passed through the bed?

b) Is the difference in length between the shortest and longest path dependent at all on the diameter of the particles?  If so, which particles (smaller or larger) would lead to a greater difference?

c) Some packed columns exhibit channeling.  What do you think is meant by this term?

d) Would channeling be more likely to occur with smaller or larger particles?  In other words, which is more difficult to pack efficiently, larger or smaller particles?  A column is packed efficiently when the particles are in a uniform bed with the minimum amount of voids.

e) Do open tubular capillary columns exhibit eddy diffusion?

f) Does this phenomena exhibit any dependence on the flow rate?

Development of Stationary Phase Mass Transport Broadening in Chromatography

Consider a compound that has distributed between the mobile and stationary phase within a plate in a chromatographic column.  The following might represent the concentration distribution profiles in the two phases (note that the compound as depicted has a slight preference for the mobile phase).

a) What would happen to these concentration profiles a brief instant of time later?

b) Will what happens in (a) contribute to band broadening?  Explain.

c) Does the contribution of this phenomenon to band broadening exhibit a dependence on flow rate?  If so, are there any troublesome aspects to this dependence?

d) What happens to this effect as the stationary phase coating is made thicker?

e) Capillary GC columns have very thin coatings.  Describe one advantage of these columns.

f) Compare the effect of this phenomenon on a uniform versus non-uniform stationary phase coating.

g) Is this effect of more concern in gas or liquid chromatography?

Development of Mobile Phase Mass Transport Broadening in Chromatography

Consider a capillary column as shown below.

The dot represents a molecule that has just left the stationary phase is about to diffuse across the mobile phase and re-encounter the stationary phase on the other side of the column.

a) Draw a line representing the path of the molecule.

b) What would this path look like if the flow rate were doubled?

c) Is it important for the molecule to encounter the stationary phase?  Think about a situation in which the flow was so fast that the molecule never re-encountered the stationary phase.

d) If using capillary columns, what does this suggest about the desirable diameter for such a column?

e) Is this phenomenon worse in gas or liquid chromatography?

f) How does the contribution to band broadening depend on flow rate?

g) Would this effect be observed in a packed column?  If so, how?

h) How could you reduce this effect in a packed column?

i) Say something about the length of column needed if you undertake what you have suggested in

(h), assuming that you have maintained a constant thickness of stationary phase.