GPC peak broadening problem [July 1, 2004]
Posted: Tue Aug 24, 2004 7:02 pm
By Anonymous on Thursday, July 1, 2004 - 09:49 am:
I am trying to determine the MW distribution of a neutral
polymer containing piridyl groups by GPC on a Waters
Breeze with a set of 3 Styragel columns, an UV-vis
detector and at 30 ºC.
I believe that the polymer is being adsorbed by the
stationary phase, because when I use pure CHCl3 as
eluent only very broad and low molecular weigth peaks
are observed, but when 0.5% triethylamine is added a
peak with MW around 20000 Da is observed, but is is
still broad and unsymmetrical, decreasing smoothly on
the low molecular side.
I will be most grateful if someone could suggest me any
additive or change in the conditions to improve the
chromatogram
The
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By Chris Pohl on Saturday, July 3, 2004 - 12:25 pm:
It's quite difficult to produce polymeric particles without any anionic impurities on the exterior surface. A likely explanation is an electrostatic interaction. Since the anionic surface impurities are typically weakly acidic, one option would be to lower the pH through the addition of a bit of acetic acid. Alternatively, you might want to try a bit more lipophilic cationic additive such as dimethyldodecyl amine.
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By Juan Gil on Monday, July 5, 2004 - 03:43 am:
Thanks a lot for the suggestions.
Why dimethyldodecyl amine is a "cationic" additive?
And also why sometimes salts like LiCl are used as
additives in GPC?
-------------------------------------------------------------------------------------------------------
By Chris Pohl on Saturday, July 10, 2004 - 02:26 pm:
Jaun,
Dimethyldodecyl amine will be predominantly in the form of a cation at pH 7 or less, hence the term "cationic".
Certainly, another option is to add electrolytes to mobile phase in order to minimize electrostatic interaction between your analyte and the surface of the support particles. I'm not sure why lithium chloride would be chosen for this application, however. Lithium exhibits the weakest elution power of any common cation so it should be the least effective at minimizing the problem. Cesium chloride should be much more effective (although also more expensive).
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By Juan Gil on Friday, July 16, 2004 - 02:16 am:
Thanks for the information.
I will try adding dimethyldodecylamine. What
concentration is usually used?
Another question about electrolyte additives: Salts like
Lithium or Cesium chloride are not soluble in my
mobile phase (CHCl3). Will ammonium salts like
tetrabutylammonium chloride work in the same
way?
-------------------------------------------------------------------------------------------------------
By Chris Pohl on Sunday, July 25, 2004 - 04:23 pm:
Juan,
Use as little as possible. Start with adding 1mM (but you may need to be patient when using low concentrations as the effect may be minimal at first). Also, it's possible that dodecyldimethyl amine won't work in pure CHCl3 as it may not be sufficiently ionized in this solvent. You may need to use dodecyltrimethylammonium chloride (at a similar concentration) for your system.
Regarding your other question, you can substitute smaller quats for cesium cations. In my experience, the effect of tetramethylammonium ion is roughly equal to the effect of cesium ion.
I am trying to determine the MW distribution of a neutral
polymer containing piridyl groups by GPC on a Waters
Breeze with a set of 3 Styragel columns, an UV-vis
detector and at 30 ºC.
I believe that the polymer is being adsorbed by the
stationary phase, because when I use pure CHCl3 as
eluent only very broad and low molecular weigth peaks
are observed, but when 0.5% triethylamine is added a
peak with MW around 20000 Da is observed, but is is
still broad and unsymmetrical, decreasing smoothly on
the low molecular side.
I will be most grateful if someone could suggest me any
additive or change in the conditions to improve the
chromatogram
The
-------------------------------------------------------------------------------------------------------
By Chris Pohl on Saturday, July 3, 2004 - 12:25 pm:
It's quite difficult to produce polymeric particles without any anionic impurities on the exterior surface. A likely explanation is an electrostatic interaction. Since the anionic surface impurities are typically weakly acidic, one option would be to lower the pH through the addition of a bit of acetic acid. Alternatively, you might want to try a bit more lipophilic cationic additive such as dimethyldodecyl amine.
-------------------------------------------------------------------------------------------------------
By Juan Gil on Monday, July 5, 2004 - 03:43 am:
Thanks a lot for the suggestions.
Why dimethyldodecyl amine is a "cationic" additive?
And also why sometimes salts like LiCl are used as
additives in GPC?
-------------------------------------------------------------------------------------------------------
By Chris Pohl on Saturday, July 10, 2004 - 02:26 pm:
Jaun,
Dimethyldodecyl amine will be predominantly in the form of a cation at pH 7 or less, hence the term "cationic".
Certainly, another option is to add electrolytes to mobile phase in order to minimize electrostatic interaction between your analyte and the surface of the support particles. I'm not sure why lithium chloride would be chosen for this application, however. Lithium exhibits the weakest elution power of any common cation so it should be the least effective at minimizing the problem. Cesium chloride should be much more effective (although also more expensive).
-------------------------------------------------------------------------------------------------------
By Juan Gil on Friday, July 16, 2004 - 02:16 am:
Thanks for the information.
I will try adding dimethyldodecylamine. What
concentration is usually used?
Another question about electrolyte additives: Salts like
Lithium or Cesium chloride are not soluble in my
mobile phase (CHCl3). Will ammonium salts like
tetrabutylammonium chloride work in the same
way?
-------------------------------------------------------------------------------------------------------
By Chris Pohl on Sunday, July 25, 2004 - 04:23 pm:
Juan,
Use as little as possible. Start with adding 1mM (but you may need to be patient when using low concentrations as the effect may be minimal at first). Also, it's possible that dodecyldimethyl amine won't work in pure CHCl3 as it may not be sufficiently ionized in this solvent. You may need to use dodecyltrimethylammonium chloride (at a similar concentration) for your system.
Regarding your other question, you can substitute smaller quats for cesium cations. In my experience, the effect of tetramethylammonium ion is roughly equal to the effect of cesium ion.