Ion pairing on Carbon Based Stationary Phase

[M. Farooq]

Hello list,

Is there anyone in the list who has experience with carbon based stationary phases. Anyways, certain suggestions would be welcome.

My stationary phase is a commercially available Zirchrom-Carb, which is pyrolytic carbon coated via vapor deposition on ZrO2. Apparently free ZrO2 is not exposed because phosphate elutes very near the injection peak. I am trying to use 1.5 mM tetrabutylammonium hydroxide-1 mM phthalate system at pH -5.5 (the second pKa of phthalic acid) adjusted via titration rather than external acid base. Note I am trying to separation inorganic anions via indirect spectrophotometry. However three ions sulfate ion are causing the most problems:

1. Chloride & HPO4(2-) elute near or in the water dip
2. When SO4 (2-) is present, it disturbs the baseline very near the injection where other simple anions have their peaks.

a. With purely aqueous system, baseline does not remain stable after injection of aqueous anions.
b. With 5% ACN in the eluent, sulfate elutes with the negative system peak.
c. With 10% ACN in the eluent, sulfate causes such a huge disturbance near the negative injection peak of water that sulfate can not be seen, even if elutes. Interestinly system peaks vanish with this eluent but the retention of others anions is decreased very badly that chloride now comes out with the water dip.

Suggestions to increase retention would be welcome and taming sulfate behaviour would be welcome. Note changing pH independently via NaOH or inorganic acids is being avoided because one of the articles said that number of system peaks increase.

Thanks.

M. Farooq

[danko]

I think you should try IC (ion chromatography) or HILIC (hydrophilic interaction chromatography) for these analytes.

Best Regards

[M. Farooq]

Thanks. Consider my original query as an academic "idea" for the time being.

M. Farooq

I think you should try IC (ion chromatography) or HILIC (hydrophilic interaction chromatography) for these analytes.

Best Regards

[tom jupille]

You need more retention. I can think of three ways to get it (each of which have their drawbacks!)

1. Increase the TBA concentration to increase the effective ion exchange capacity of your column. That will raise the pH and effectively convert more of the phthalate to the divalent form which is a stronger eluant; I don't know which effect will predominate

2. Decrease the phthalate concentration. If the charge stayed constant, that should make the eluant weaker, but it will also effectively increase the pH, so the above comment also applies.

3. Decrease the pH by adding a monovalent strong acid that you're not interested in (methane sulfonic?). That would shift the phthalate toward the monovalent (weaker eluant) form. But the additional acid may hurt your sensitivity.

[M. Farooq]

Thanks,

I was thinking of reducing phthalate concentration and reducing the pH via external means. Methane sufonate is a good suggestion.

Besides these, system peaks after injecting salt solutions is another big problem. Greater the charge on the ion, more upset is the baseline right after injection.

Researchers on carbon phases have even tried TBA-Na2CO3 on graphite and successfully separated common anions but this one can not be adapted to spectrophotometric detection.

Pyrolytic carbon is behaving very oddly!

You need more retention. I can think of three ways to get it (each of which have their drawbacks!)

1. Increase the TBA concentration to increase the effective ion exchange capacity of your column. That will raise the pH and effectively convert more of the phthalate to the divalent form which is a stronger eluant; I don't know which effect will predominate

2. Decrease the phthalate concentration. If the charge stayed constant, that should make the eluant weaker, but it will also effectively increase the pH, so the above comment also applies.

3. Decrease the pH by adding a monovalent strong acid that you're not interested in (methane sulfonic?). That would shift the phthalate toward the monovalent (weaker eluant) form. But the additional acid may hurt your sensitivity.

[Kostas Petritis]

On a 1999 I think Journal of Chromatography A paper, Chaimbault and Elfakir published a paper on the separation and analysis by ELSD of inorganic ions by using porous graphitic carbon (Hypercarb)... maybe you should start from there...