Max. ion pair conc.

[Rob Burgess]

Is there a max. working limit to the amount of ion pair conc. in the mobile phase that you can readily work with.
Our present method uses a 20mM SDS + 0.05% TFA as MP A with a 5 cm column. However, we'd like to increase our LOD for the basic solute... presumably by simply injecting a larger volume. By gaining more retention on our column by adding more ion pair reagent will the system be able to tolerate a larger injection volume or should we simply increase the length of our column to increase column injection volume. (Please note that our sample diluent is fixed and cannot be changed thus eliminating on-column conc. techniques).
Finally for best results with ion pair reagents, is it best to pre-mix your mobile phase with the organic portion or is it just as well to let the pump mix online? I have tried using a C16 sulphonate but it didn't dissolve in the aqeous portion of the mobile phase. Perhaps I should have premixed with the organic component?

[DR]

The amount of analyte your column can hold is a function of the cross sectional area of the column. Once you exceed the column's capacity, you will start to see fronting of the main peak and loss of resolution. No amount of ion pairing reagent can 'fix' this as it is (mostly) a function of the amount of stationary phase (and the partition coefficient between it & analyte) you have. The ion pairing agent, at ~20mM, is enough to keep any silanols busy and shouldn't have much effect on retention times.

That said, see this... http://www.sepsci.com/chromforum/viewto ... highlight=

[Uwe Neue]

The surface saturates with the ion pair reagent at a rather high concentration on the stationary phase. Saturation is achieved already at a very low concentration of the ion-pair reagent in the mobile phase. Additional reagent in the mobile phase does not increase the concentration of the pairing ion on the stationary phase, it just speeds up the column equilibration process. At higher concentrations, you get into a world where micelles form in the mobile phase, and then you are in a different world of chromatography.

[Einar Ponten]

Whenever you like to get a mess use "20mM SDS + 0.05% TFA".

Why don't you convert to HILIC for your "basic compound"? That will make everything more easy for you!