Salt Analysis on RP HPLC

[Chandresh K. Soni]

Is there any Method for Analysis or Separation of salts on HPLC with Routine Columns Like C8,C18 , Amino , Phenyl, Cyano etc.

[DR]

1) LC works as a function of different partition coefficients of the analytes you're trying to separate. Ionic salts will rarely have different partition coefficients when it comes to typical LC column packings and mobile phases.

2) Most LC uses UV detection. With salts, you would generally have no UV chromophore (associated with aromatic bonds, double bonds...). There are other modes of detection available (RI could be used if you got around point 1), but they're either less sensitive or do not fit "traditional" HPLC setups.

So, in short, a qualified "No" is in order.

Qualifier: it has been assumed that you're not analyzing salts of compounds containing double bonds and enough of a nonpolar moiety to be retained on a traditional LC column (examples of which can be found in the CalBioChem catalog).

[Chandresh K. Soni]

Thanks for Reply
Can I Use Amino Column for the Salt Analysis Having Double Bond or Containing Sulphur or NH2 Group on UV At 200nm.
And What is the role of Amino column ( its NH2 group works as ion pairing reagent.)

[Mattias]

I think you need to specify what salt that you want to analyse.

Many drugs are present as salts, and have still good retention on RP columns.

[shaun78]

Yes, yes. Indeed the salt needs to be specified.

Though, salts can be done on traditional RP columns, but they require a great deal of work and usually an RI detector.

Anyway, the sample prep and mobile phase need to be at "ultra low pH", meaning you probably want something around 1 and definately below 2 for most ionic salts (I will use the well studied sodium tartrate as a reference). Using low pH in the sample prep will cause the salt to shift to its acid form and low pH in the mobile phase keeps it there. Additionally, the low pH of the mobile phase allows for slight ion exchange on your RP column.

If you have no other choice but a UV detector, then you can always select and drop the pH of the mobile phase using something like HCl and still keep your detection at 200 nm.

When selecting a column, make sure it can handle the pH and near 100% aqueous conditions. This essentially leaves you with a polymeric column of some sort. I always had good luck with XTerra's, Synergi's, and Gemini's when doing this sort of analysis .... when for what ever reason (probably due to other components) I was forced to make RP work.

[tom jupille]

I think that we may have some terminology issues here.

When you say "salt", are you interested in the anion, the cation, or both?

And which (if any) of the ions is organic?