Amines-Ion Pairing

[cnwangwa]

AMINES!!-
I was recently asked a question about the functional group Amines on ion pairing. The question is would I use an acid or base to determine the what kind of salt and sense the amine is very polar. What would case the amine to have a longer detection time? I hope this makes sense to a genius out here.
Can anyone explain the seperation science of amines on ion pairing in a lay mans terms.
Can anyone explain Ion Chromatography with suppressed conductivity detection?

[gbalock]

Amines are weak bases that will form an association with another weak acid. Typically in IP Chromatography for amines, fatty sulfonic acids are used as the ion pair, for instance heptane or octane sulfonic acid. Also, for ease of preperation these compounds are usually the sodium salt of the acid.

The retnetion mechanism for IPC is that you have a weak base that would not normally be retained on a reverse phase column, associated with a weak acid with a long hydrophobic tail. Beacuse the HP tail does have an attraction for the reverse phase column, the compound is retained. Because thaese are weak acids and bases, the asociation between them is an equilibrium, constnantly breaking and reforming. Also, since the IP has a long non-polar end, the IP can associate itself with the RP staionary phase. This makes your column into an ion exchange column.

The critical factors in controlling IPC are pH, IP concentration and organic modifier concentration.

I hope that this helps.

[cnwangwa]

Amines are weak bases that will form an association with another weak acid. Typically in IP Chromatography for amines, fatty sulfonic acids are used as the ion pair, for instance heptane or octane sulfonic acid. Also, for ease of preperation these compounds are usually the sodium salt of the acid.

The retnetion mechanism for IPC is that you have a weak base that would not normally be retained on a reverse phase column, associated with a weak acid with a long hydrophobic tail. Beacuse the HP tail does have an attraction for the reverse phase column, the compound is retained. Because thaese are weak acids and bases, the asociation between them is an equilibrium, constnantly breaking and reforming. Also, since the IP has a long non-polar end, the IP can associate itself with the RP staionary phase. This makes your column into an ion exchange column.

The critical factors in controlling IPC are pH, IP concentration and organic modifier concentration?

I hope that this helps.

Thanks George!

When would anyone typically use ion pairing chromatography in a laboratory setting? (what would a typical situation be?) What kinds of products and what is an organic modifier concentration?

thank you again.

[tom jupille]

What kinds of products and what is an organic modifier concentration?

You might want to check out a basic tutorial on HPLC (it's free):

http://www.lcresources.com/resources/getstart/

[Markus Laeubli, Metrohm]

Can anyone explain Ion Chromatography with suppressed conductivity detection?

This is the separation of anions on an anion exchanger column using e.g. a carbonate/bicarbonate or KOH eluent. As the background conductivity is quite high direct conductivity would be critical. Therefore a post-column derivatisation called «chemical suppression» is applied.
In the chemical suppressor a cation exchange takes place. The cations of the eluent (and sample) are replaced by H+. For the eluent this ends up in very low conducting dissolved CO2 or just water. On the other hand the sample anions are converted into the respective acids. This encreases the conductivity (at least for the stong ore medium strong acids.

The suppressor reactions:

Eluent: Na2CO3 + H+(supp) —> 2 Na+(supp) + H2O + CO2

Sample: NaCl + H+(supp) —> Na+(supp) + HCl

For more information you may get the Metrohm Monograph «Practical Ion Chromatography - An introduction from your Metrohm agency free of charge.