Consider the case where we have the salt of a basic drug (citrate, fumarate etc). This salt can be analysed by HPLC under certain chromatographic conditions. Could the free base be analysed under the same conditions? (A minor adjustment in the wave length might be necessary, but apart from that?)
The Mobile phase is Methanol-Phosphate buffer (pH=3.6, at least 2 units below the pKa)-Tetrahydrofurane, 50:48:2.

I am going to make various stabs at this question with the hope one hits the mark.

1. In the solid you may have the salt of the drug, let us say drug-acetate. When you dissolve this salt, in solution you will have a mixture of the free base drug and protonated drug, and no drug-acetate. The acetate will be fully dissociated from the protonated drug in the mobile phase and play no role in the separation or detection. If you were to collect the eluting peak and evaporate the solution you would most likely have a salt of drug-phosphate.

2. During the separation, how much drug is protonated and how much is free base depends on the pK of the base and the proton activity of the mobile phase. (This proton activity may be substantially differect from what ever pH you are assigning to the mobile phase.)

3. You will not separate the drug from its protonated form. These two species will be in rapid equilibrium and the separation you observe will be an average of the forms present in the mobile phase. So, at high pH the spearation will be determined by the properties of the free base drug, while at low pH the separation will be determined by the properties of the protonated drug. If the drug exists half in each form in the mobile pahse the separation will be in the middle of these two extremes.

4.The protonated drug may have a differect spectra from the free base. The spectra you should use for determining a detection wavelength is the spectra of the drug in the mobile phase, which will be an average of the protonated and free base forms as determined by mobile phase proton activity.

If none of this answered your question, try clarifying the question.

Remember that drug-acetates are often esters and not salts ! For instance, hydrocortison-acetate is an ester, and will remain hydrocortison-acetate in solution.
Citrates and fumarates are often salts, and these are immediately dissociated in solution. As far as these salts are concerned, LC conditions are the same for base form or salt form. You will have an equilibrium.
Hope this helps
Jan

I presume we are mostly talking about amines and amine salts, and no one should be confused as to what an amine acetate is.

The conditions, particularly the eluant pH and probably the column, used to spearate an amine will be vastly different from the conditions for separating the respective protonated amine. Because the conditions are different it may well be advantageous, possibly because of other components in the sample, to separate the amine either as the free base or as the protonated amine. Normally one drives the equilibrium fully to one form or the other so that minor variance in eluant pH does not result in retention time shifts.

As stated earlier, if the amine sample was an acetate salt you would get a peak in the chromatogram for the protonated amine and a peak for acetic acid if the spearation was done with an acidic eluant.

Thank you all, ever so much!
:)