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- Posts: 9
- Joined: Mon May 21, 2012 9:36 am
Hi folks,
I have a question relating to the reduced velocity (v) of the mobile phase, as defined by the equation:
v = u * dp/Dm,
where u is the average interstitial velocity of the eluent, dp is the particle diameter of the packed column and Dm is the diffusion coefficient for the solute.
When increasing temperature, we increase the diffusion coefficient of the solute within the mobile phase as per the Wilke-Chang equation. The corresponding effect is to reduce the magnitude of the v term as per the above equation. My question, therefore, relates to how increasing the flow rate can compensate for this effect and so return us to an optimum value for h (the reduced plate height), offering maximum chromatographic efficiency? I’m interested to understand, at a mechanistic level, what exactly is causing the value of v to decline when temperature is increased?
From what I can deduce, there are two possible options:
1) A reduced velocity of solute diffusion in to the particle pores – though with an enhanced diffusion coefficient, I’ve no idea how that could make sense as I’d imagine molecules to move more freely in to the mobile phase within the pores.
2) A reduction in the B term of the Knox equation resulting from use of a greater flow rate.
Option 2 makes the most sense to me personally, however, due to its absence in the above equation, I wonder whether I have perhaps misunderstood.
Option 1, meanwhile, confuses me somewhat. If we were to have a reduced velocity of solute diffusion in to the pores of a packed column’s particles, would we not expect an increase in flow rate to enhance the spreading of a solute band? My thoughts on this come from the fact that we are considering an equilibrium processes whereby, as mobile phase passes the stationary phase, some of the solute molecules are retained within/on the stationary phase (resistance to mass transfer of the stationary phase). These retained molecules lag then behind what will eventually become the apex (point of highest concentration) of a chromatographic peak within the detector. By increasing flow, I imagine they would be left even further behind – so creating even greater spread and hence, reduced efficiency.
As a final part to the post, could someone perhaps confirm or deny whether or not by increasing flow rate in response to elevated temperature, we might improve efficiency, but in so doing, reduce our resolution? I’d imagine it’d be a trade off between two events: 1) narrower peaks that provide an improved resolution if the retention values remained unchanged; 2) the higher flow rates would lead to a reduction in retention factor (k), in turns means reduced separation of analytes.
All the best,
U0mrj1K
I have a question relating to the reduced velocity (v) of the mobile phase, as defined by the equation:
v = u * dp/Dm,
where u is the average interstitial velocity of the eluent, dp is the particle diameter of the packed column and Dm is the diffusion coefficient for the solute.
When increasing temperature, we increase the diffusion coefficient of the solute within the mobile phase as per the Wilke-Chang equation. The corresponding effect is to reduce the magnitude of the v term as per the above equation. My question, therefore, relates to how increasing the flow rate can compensate for this effect and so return us to an optimum value for h (the reduced plate height), offering maximum chromatographic efficiency? I’m interested to understand, at a mechanistic level, what exactly is causing the value of v to decline when temperature is increased?
From what I can deduce, there are two possible options:
1) A reduced velocity of solute diffusion in to the particle pores – though with an enhanced diffusion coefficient, I’ve no idea how that could make sense as I’d imagine molecules to move more freely in to the mobile phase within the pores.
2) A reduction in the B term of the Knox equation resulting from use of a greater flow rate.
Option 2 makes the most sense to me personally, however, due to its absence in the above equation, I wonder whether I have perhaps misunderstood.
Option 1, meanwhile, confuses me somewhat. If we were to have a reduced velocity of solute diffusion in to the pores of a packed column’s particles, would we not expect an increase in flow rate to enhance the spreading of a solute band? My thoughts on this come from the fact that we are considering an equilibrium processes whereby, as mobile phase passes the stationary phase, some of the solute molecules are retained within/on the stationary phase (resistance to mass transfer of the stationary phase). These retained molecules lag then behind what will eventually become the apex (point of highest concentration) of a chromatographic peak within the detector. By increasing flow, I imagine they would be left even further behind – so creating even greater spread and hence, reduced efficiency.
As a final part to the post, could someone perhaps confirm or deny whether or not by increasing flow rate in response to elevated temperature, we might improve efficiency, but in so doing, reduce our resolution? I’d imagine it’d be a trade off between two events: 1) narrower peaks that provide an improved resolution if the retention values remained unchanged; 2) the higher flow rates would lead to a reduction in retention factor (k), in turns means reduced separation of analytes.
All the best,
U0mrj1K
