Has anybody got a good method for separating Phenylpropanolamin+brompheniramine and norpseudoephedrine. My current method is:
Mob Phase: 2.83g of Dioctyl sodium sulfosuccinate +550ml MEOH + 340ml H2O + 110ml THF + 4ml HAC.
Column: Luna C-18, Wavelength = 260nm.
This method works well but the brompheniramine peak elutes at about 26min. Has anybody and easier method or suggestions on how I could reduce the run time without reducing the resolution between phenylprop + norpsehdoephedrine.( resolution between these peaks are about 1.8 with the above mob phase.)

Any suggestions would be greatly appreciated.

How about just increasing the flow rate?

With my current flow rate of 1.3ml/min the back pressure of my system is ±2500psi. Therefore increasing the flow rate will increase the presssure to over 3000psi. I would not like to work at this high pressure because of the extra "stress" on my system.

Thanks for the input.

Is your pump able to run a gradient? That would probably help you.

I'm using Waters Alliance 2690 system but found with this mob phase that there is a slight shift in ret time. I've tried to equilibrate over night but still found that the post stds differ by about 0,5min.i.e Pre-stds -brompheniramine = 26min
Post stds - brompheniramine = 26.5min. I know mob phases with THF tend to require a long time to equilibrate and as a result I would rather prefer to use an alternative method.

Many thanks

A Waters Alliance 2690 system can quite easily work at system pressures above 3000 psi without extra 'stress' on the system. You can safely increase the flow rate as long as the pressure stays below 4000 psi. The other option is to go to shorter, smaller diameter columns (you did not mention column dimensions) at increased flow rates. This stratagy can sometimes work. I find it odd that you have this retention time shift while running a gradient. The instrument should be able to do much better than what you described (even with THF), I would guess that some small procedural changes in your gradient method would solve that quick. One of the things that cause that kind of RT shift are incorrect equilibration time. Use the following formula to calculate the minimum equilibration time: 3 times system volume + 5 times column volume. The other thing to do with the 2690 is to make sure that all 4 degasser chambers are full of solvent (even the ones you are not using), this help alot.

A.A.

Are you using helium sparging to degas? If so, you may be removing the THF from your mobile phase there, thus causing retention time shift.

I'm using the in-line degasser of the 2690. After having this problem I decided to set the degasser on continous. This still did not reduce the %RSD of ret time between my pre and post stds.
At present the %RSD between pre and post stds is about 1.5%. All my lines of the 2690 are in solvent. i.e Line A - Mob Phase; B - MEOH/H2O 50:50; C - 100% H2O; D - 100% MEOH.
I'm running isocratically but still have the ret time shift. All solvent lines have freshly degassed solvents. I've equilibrated my column(4.6mm X 15cm) @ 0.2ml/min for 15hrs but the ret shift still occurs.

Thanks

Does the temperature of the room change throughout the day? Or are you using a column heater to compensate for this?

The temperature of our lab is controlled throughout the day i.e 20 - 25șC. I'll try using a column heater set at about 30șC to reduce my ret time shift.

Thanks

Have had luck with similar cough/cold remedies using the following mobile phase:
A = Water with 0.1% H3PO4 and 0.1% Octane-1-sulfonate
B = Acetonitrile, C = Methanol
50:10:40 A:B:C; fr=1.5 on a 150x4.6mm Luna C8(2) column

*Pseudo and Phenylprop elute 3-4min, brom about 8min
*Can increase retention of these bases by increasing concentration of ion-pairing agent