Analysis of Microemulsion sample

[KarenJ]

Hi,

We are trying to develop an HPLC method for measuring the amount of drug in a microemulsion. When the drug is just prepared in methanol, we get a nice peak at about 6.5 min. on a C18 column with water/methanol mobile phase. If we try to inject the microemulsion with drug in it, we see lots of small peaks between 4 to 7 minutes, but no main drug peak at 6.5 min. The small peaks have a UV spectrum similar to the drug standard. We haven't been able to reproduce those peaks by degrading the standard with peroxide so we're not sure if they are degradation products or matrix.

We tried solid phase extraction with Prevail C18 cartridges. We loaded the microemulsion sample, washed with water, and extracted the sample with methanol. The resulting sample actually has 2 layers. We injected both layers. Since the chromatogram from the lower layer is most similar to that of the untreated sample, we suspect that the drug is going to the lower layer.

Any ideas about why we don't get a drug peak in the presence of the microemulsion or how to clean up the sample to get rid of the microemulsion matirx? Unfortunately, since the emulsion is a proprietory recipe made by our client, we haven't been told the composition of it. However, it is biphasic with the drug dissolved in the inner (organic?) phase we think.

Thanks,

KarenJ

[ym3142]

through google search I found microemulsion is that a liquid or oil contains stablized droplets. I imagine if you inject this microemulsion to LC, the drug inside these numerous droplets fails to be dissolved into the mobile phase completely within a reasonable short time frame or interact with stationery in the mean time, resulting in "sustain release profile" . This theory should explain what you observed. Thus you have to make a homogenous solution before you inject.
hope this helps and good luck

[priz]

Karen,

Maybe you could try a lipid extraction (Folch is the classic- chloroform:Methanol) and shoot both phases and see if your drug partitions. For further clean-up, you could try SPE after the lipid extraction. Be on the look out for what happens at your interface.
(if you have a lot of lipid in your sample, peroxide will encourage lipid oxidation)

good luck,
priz

[DR]

Do you have a good sonicator?

If so, establish a minimum length of time and temperature combination needed to break the emulsion. Once that's done, further clean-up by extraction, SPE etc. is an option.