hptlc

[pansuriyagopal]

i am trying to develope a hptlc method for combine dosage form. drug A has pka 9.67 and drug B has pka 4.5.
i had tried lots of mobile phase but drug A remain at the place of application or run very slightly while drug B reach at the solvent front. if i reduce the polarity to decrease Rf of B than Drug A remain at the spot of application.
both drugs are soluble in methanol as well as water.
Which mobile phase should i try?
is there any criteria other than polarity index for the seperation?

I wish to publish my research work of HPTLC method.
As i had not published any paper, can anyone suggest name of reputed journals with good impact factor?

[tom jupille]

First of all, are you using normal-phase or reversed-phase plates (both are available)?

Second, are the drugs acidic (they give up a proton when they ionize) or basic (they accept a proton when they ionize)?

Third, what was the actual mobile phase you used?

[pansuriyagopal]

First of all, are you using normal-phase or reversed-phase plates (both are available)?

Second, are the drugs acidic (they give up a proton when they ionize) or basic (they accept a proton when they ionize)?

Third, what was the actual mobile phase you used?

Thank you for your reply.
1. I am using normal phase precoated HPTLC plates mfg by merck. coated with silica gel-g F254

2. Drug A is weak base (Pka 9.64) and drug B is also weak base (pka 4.5). both of them accept a proton when they ionize.

3. I had tried mobile phase
methanol,Hexane, toluene,ethyl acetate, butanol and chloroform in a combination of 2 or 3 solvents with or without small addition of acetic acid or ammonia.

[tom jupille]

Ok, it may be that the polarities are sufficiently different that getting reasonable retention of both drugs will be impossible. In column chromatography (HPLC) one can get around that problem by using a gradient (one of the reasons HPLC is much more widely used than is TLC).

You can approximate that by using a mixture of a very polar solvent (ispropanol? ethyl acetate?) with a very non-polar solvent (hexane), probably with a bit of acetic acid included to protonate both drugs (if possible; remember that pKa values s are defined for *aqueous* systems) in order to take advantage of "demixing" to generate a gradient. The idea is that the more polar solvent will be preferentially retarded by the silica; in extreme cases, you can actually see a secondary solvent front. The problem is that it takes a lot of fiddling and tweaking to get exactly the right solvents and ratios.

Despite its apparent simplicity, TLC is actually a more complex technique to manipulate than is column chromatography.

Since your compounds have good water solubility, you might want to consider getting reversed-phase plates; that will let you take advantage of pH to manipulate selectivity.

[krickos]

Hi

A hint that may help you. When you add ammonia conc. you actually add water as well that may not be desireble. Typically you would put 100ml of your disered mobile phase composition into a separation funnel, then add like 15ml ammonia, shake, let layers separate and discard water phase. Also it is not uncommon to add 2 small beakers in development chamber with like 30ml of ammonia conc.(let saturate for like 30mins).

[pansuriyagopal]

Hi

A hint that may help you. When you add ammonia conc. you actually add water as well that may not be desireble. Typically you would put 100ml of your disered mobile phase composition into a separation funnel, then add like 15ml ammonia, shake, let layers separate and discard water phase. Also it is not uncommon to add 2 small beakers in development chamber with like 30ml of ammonia conc.(let saturate for like 30mins).

Thanks for your suggestion.