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By bill tindall on Sunday, November 16, 2003 - 08:28 am:
The phenomenon of "phase collapse" has been explained by several authors as the exclusion of polar mobile phase from the less polar column pores. One author, Doshi, even measured the volume of this excluded liquid as exclusion occured.
My question: what fills the pores when this exclusion occurs. Is it just water vapor and hence the pores are under partial vacuum (the vapor pressure of water at column temperature)?
I guess under such conditions we would be doing gas/liquid chromatography. It is interesting to contemplate the fate of a coumpound with high vapor pressure injected under such conditions.
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By HW Mueller on Monday, November 17, 2003 - 12:38 am:
Bill,
couldn´t find the discussion on this (seems I can´t find anything anymore via the keyword search), but it seems that someone suggested it must be a partial vacuum as you surmise. A compound with high partial vapor pressure would then be expected to be retained a bit more than one with low vp? Maybe overlapping (under normal conditions) compounds with different volatility could then be separated after de-wetting? (Probably the whole thing will be coming out near the dead volume and be very pressure, temp. sensitive)
Hans
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By Chris Pohl on Monday, November 17, 2003 - 01:29 pm:
Actually the pores are filled with nitrogen and oxygen.
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By HW Mueller on Monday, November 17, 2003 - 11:21 pm:
Probably it would be better to call this reduced pressure rather than partial vacuum. The latter has the connotation that one compares to atmospheric pressure. Again, one would have to expect anything in there that can have a partial vapor pressure above water.
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By Uwe Neue on Wednesday, November 19, 2003 - 07:28 pm:
I guess its water vapor and air... looks like a good way to degas the water... maybe a new application for C18?
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By Anonymous on Monday, April 5, 2004 - 04:57 pm:
I want to dust off this conversation and pose a dumb question. I always thought phase collapse was simply the phenomenon that occurs when the aqueous content of the mobile phase gets too high and the C-18 (or maybee C-8) ligands begin to dissolve into one another. And this causes the "available stationary phase" to decrease and the retention of all components to decrease. But what's all this stuff about the pores of the column? I don't how that relates to the C-18 ligands collapsing into one another.
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By Uwe Neue on Monday, April 5, 2004 - 06:11 pm:
These may be two different phenomena, and they may have been confused, or it is possible that a true hydrophobic collapse does not exist.
If you bring a nicely hydrophobic C18 packing into water from a water organic mixture, you get initially good retention. When you stop the flow for a moment and then restart it, all retention is gone. We have called this phenomenon a "dewetting" phenomenon. This is the correct expression of what is going on, but others have called it phase collapse (I slipped myself occasionally).
The phenomenon of loosing retention in 100% water and not regaining it until the packing was exposed again to a mobile phase with roughly 50% organic solvent has been reported in the literature, where it was called phase collapse, if my memory serves me correctly. I have seen situations, where the loss of retention is not complete, but only partial. I fundamentally believe that all phase collapse is nothing but a form of dewetting, but I have not looked close enough at the literature.
The underlying phenomenon is the reason why most people do not start a gradient on a hydrophobic packing in 100% water.
In my experiments on retention as a function of the mobile phase composition, I have never seen anything that would need to be interpreted as a loss of retention due to a crumbling up of the stationary phase. But I have seen the dewetting problem. I remember vaguely that there is some literature on this crumbling, but I would need to reread this again to see if the interpretation is correct.
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By Anonymous on Monday, April 5, 2004 - 07:25 pm:
What is the practical significance of the type of phase collapse that you all have been talking about, i.e. the exclusion of polar mobile phase from column pores?
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By DR on Tuesday, April 6, 2004 - 05:36 am:
Just a complete change in the column's k value, little or no retention (and therefore, no resolution) for most or all analytes...
It's unnerving to say the least.
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By Anonymous on Tuesday, April 6, 2004 - 03:37 pm:
Yikes!
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By Anonymous on Wednesday, April 7, 2004 - 05:38 am:
can this phenomenon be reversed or is that once it happens the column is junk and you need to trash it and get a new one?
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By Chris Pohl on Wednesday, April 7, 2004 - 06:09 pm:
The problem is completely reversible by one of several methods. The most common method is to run a mobile phase containing a high percentage of organic solvent. If you pass 100% acetonitrile over the column for 15-30 minutes and then reequilibrate the column with your eluent, the retention will be restored.
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By jimmy on Wednesday, July 7, 2004 - 04:52 pm:
What about all the reversed phase columns on the market that are designed to tolerate 100% aqueous mobile phases: usually due to polar functional groups embedded in the C-18 chains. People are using these columns with 100% aqueous conditions (in fact I've done it myself and they work very well).
How is this possible? Are these columns modified in some way to lessen the phase collapse problem.
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By tom jupille on Wednesday, July 7, 2004 - 05:07 pm:
Yes, they are. That's what the embedded polar functional group does. Think of it as providing "wettability".
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By jimmy on Wednesday, July 7, 2004 - 06:38 pm:
Tom
It makes sense that the polar groups would allow the C18 chains to dissolve into the aqueous mobile phase. But are you saying it also helps with the issue of liquid in the pores being replaced by gas (the issue most of this thread has focused on).
Come to think of it - it does sort of make sense. But some of these "aqueous" column have polar groups on the C18 chains; while some have them also on the surface of the silica. It seems like the second type would be more effective at reducing surface tension.
The phenomenon of "phase collapse" has been explained by several authors as the exclusion of polar mobile phase from the less polar column pores. One author, Doshi, even measured the volume of this excluded liquid as exclusion occured.
My question: what fills the pores when this exclusion occurs. Is it just water vapor and hence the pores are under partial vacuum (the vapor pressure of water at column temperature)?
I guess under such conditions we would be doing gas/liquid chromatography. It is interesting to contemplate the fate of a coumpound with high vapor pressure injected under such conditions.
-------------------------------------------------------------------------------------------------------
By HW Mueller on Monday, November 17, 2003 - 12:38 am:
Bill,
couldn´t find the discussion on this (seems I can´t find anything anymore via the keyword search), but it seems that someone suggested it must be a partial vacuum as you surmise. A compound with high partial vapor pressure would then be expected to be retained a bit more than one with low vp? Maybe overlapping (under normal conditions) compounds with different volatility could then be separated after de-wetting? (Probably the whole thing will be coming out near the dead volume and be very pressure, temp. sensitive)
Hans
-------------------------------------------------------------------------------------------------------
By Chris Pohl on Monday, November 17, 2003 - 01:29 pm:
Actually the pores are filled with nitrogen and oxygen.
-------------------------------------------------------------------------------------------------------
By HW Mueller on Monday, November 17, 2003 - 11:21 pm:
Probably it would be better to call this reduced pressure rather than partial vacuum. The latter has the connotation that one compares to atmospheric pressure. Again, one would have to expect anything in there that can have a partial vapor pressure above water.
-------------------------------------------------------------------------------------------------------
By Uwe Neue on Wednesday, November 19, 2003 - 07:28 pm:
I guess its water vapor and air... looks like a good way to degas the water... maybe a new application for C18?
-------------------------------------------------------------------------------------------------------
By Anonymous on Monday, April 5, 2004 - 04:57 pm:
I want to dust off this conversation and pose a dumb question. I always thought phase collapse was simply the phenomenon that occurs when the aqueous content of the mobile phase gets too high and the C-18 (or maybee C-8) ligands begin to dissolve into one another. And this causes the "available stationary phase" to decrease and the retention of all components to decrease. But what's all this stuff about the pores of the column? I don't how that relates to the C-18 ligands collapsing into one another.
-------------------------------------------------------------------------------------------------------
By Uwe Neue on Monday, April 5, 2004 - 06:11 pm:
These may be two different phenomena, and they may have been confused, or it is possible that a true hydrophobic collapse does not exist.
If you bring a nicely hydrophobic C18 packing into water from a water organic mixture, you get initially good retention. When you stop the flow for a moment and then restart it, all retention is gone. We have called this phenomenon a "dewetting" phenomenon. This is the correct expression of what is going on, but others have called it phase collapse (I slipped myself occasionally).
The phenomenon of loosing retention in 100% water and not regaining it until the packing was exposed again to a mobile phase with roughly 50% organic solvent has been reported in the literature, where it was called phase collapse, if my memory serves me correctly. I have seen situations, where the loss of retention is not complete, but only partial. I fundamentally believe that all phase collapse is nothing but a form of dewetting, but I have not looked close enough at the literature.
The underlying phenomenon is the reason why most people do not start a gradient on a hydrophobic packing in 100% water.
In my experiments on retention as a function of the mobile phase composition, I have never seen anything that would need to be interpreted as a loss of retention due to a crumbling up of the stationary phase. But I have seen the dewetting problem. I remember vaguely that there is some literature on this crumbling, but I would need to reread this again to see if the interpretation is correct.
-------------------------------------------------------------------------------------------------------
By Anonymous on Monday, April 5, 2004 - 07:25 pm:
What is the practical significance of the type of phase collapse that you all have been talking about, i.e. the exclusion of polar mobile phase from column pores?
-------------------------------------------------------------------------------------------------------
By DR on Tuesday, April 6, 2004 - 05:36 am:
Just a complete change in the column's k value, little or no retention (and therefore, no resolution) for most or all analytes...
It's unnerving to say the least.
-------------------------------------------------------------------------------------------------------
By Anonymous on Tuesday, April 6, 2004 - 03:37 pm:
Yikes!
-------------------------------------------------------------------------------------------------------
By Anonymous on Wednesday, April 7, 2004 - 05:38 am:
can this phenomenon be reversed or is that once it happens the column is junk and you need to trash it and get a new one?
-------------------------------------------------------------------------------------------------------
By Chris Pohl on Wednesday, April 7, 2004 - 06:09 pm:
The problem is completely reversible by one of several methods. The most common method is to run a mobile phase containing a high percentage of organic solvent. If you pass 100% acetonitrile over the column for 15-30 minutes and then reequilibrate the column with your eluent, the retention will be restored.
-------------------------------------------------------------------------------------------------------
By jimmy on Wednesday, July 7, 2004 - 04:52 pm:
What about all the reversed phase columns on the market that are designed to tolerate 100% aqueous mobile phases: usually due to polar functional groups embedded in the C-18 chains. People are using these columns with 100% aqueous conditions (in fact I've done it myself and they work very well).
How is this possible? Are these columns modified in some way to lessen the phase collapse problem.
-------------------------------------------------------------------------------------------------------
By tom jupille on Wednesday, July 7, 2004 - 05:07 pm:
Yes, they are. That's what the embedded polar functional group does. Think of it as providing "wettability".
-------------------------------------------------------------------------------------------------------
By jimmy on Wednesday, July 7, 2004 - 06:38 pm:
Tom
It makes sense that the polar groups would allow the C18 chains to dissolve into the aqueous mobile phase. But are you saying it also helps with the issue of liquid in the pores being replaced by gas (the issue most of this thread has focused on).
Come to think of it - it does sort of make sense. But some of these "aqueous" column have polar groups on the C18 chains; while some have them also on the surface of the silica. It seems like the second type would be more effective at reducing surface tension.
