Chromatography Forum

Hi everyone,

I have a huge problem with doing the assay of a dextromethorphan HBr DXM HBr syrup from the US Pharmacopeia. The described procedure is a ionpairchromatography.
I followed exactly the described procedure, but with some small changes, like:
the small changes are:
- column : size : l=0,15m, ID=2,1 mm, stationnary phase = octadecylsilica gel.
- flow : 0,5 ml/min: i choosed this flow, bcs with a smaller colomn, it is not possible the use a higher flow, bcs the pressure problem.
- the mobile phase : in stead of docusate sodium I used Na-docasulfonate,the other components where the same

The problem was that I didnt get a peak after injecting my standard DXM HBr after 25 minutes.
What can be wrong with the method? It's a pharmacopeai method, so it should be correct. Thanks

I followed exactly the described procedure, but with some small changes, like:
...
- the mobile phase : in stead of docusate sodium I used Na-docasulfonate, the other components where the same

Docusate sodium is sodium 1,4-bis(2 ethyl hexyl) sulfosuccinate,
and I doubt your ?suphonate is going to behave the same way.

I don't think I'd call that a small change - unless it was a homologue of the original molecule. I'd recommend obtaining the correct ion-pairing agent would be a good starting point.

Bruce Hamilton

My recommendation (for everyone) is to first perform any assay EXACTLY like a compendial or published procedure, THEN investigate whether any/your modifications make things better, worse, or the same. You need to have something to compare to. How much time ($$$) is lost trying to make something work when purchasing the correct chemicals or columns would've made things more efficient and cost-advantageous overall? Of course, maybe I'm fortunate that my employer agrees with me, and we have an adequate supplies budget.

I used sodiumdodecysulphate as ionpair agent. The reason why I have made some changes is that not everything was available in the lab. I am a student and i dont know what I can do to let this succeed. Can someone help me with [b]tips[/b] to set up another method (i have also the oppurtinity to do Capillary electrophoresis) or give me advices to make the current method that I have now more optimal?Thanks a lot.

I often assay a syrup containing ephedrine HCl and dextromethorphan HBr, I recieved a method from the Principal which produced no results. After following the USP method for the assay of Dextromethorphan HBr it works if you do not deviate, sodium dicusate is essential. I tried other ion-pairing agents with poor results. The only condition that i changed was the flow rate - 2ml/min which afforded a RT of +/- 3min for the active.

...
Can someone help me with tips to set up another method (i have also the oppurtinity to do Capillary electrophoresis) or give me advices to make the current method that I have now more optimal?Thanks a lot.

I'm not sure what further information you require. Use the correct ion-pairing agent and follow the method as best you can. Using the correct ion-pairing agent will be a great leap forward.

It should not be too difficult to obtain the reagent, sodium 1,4-bis(2 ethyl hexyl) sulfosuccinate. It's commonly known as Aerosol OT
(AOT) or Dioctyl sodium sulfosuccinate (DSS). I would not expect sodium dodecysulphate (SDS) to be equivalent.

Surely it's easier and more efficient to modify a compendial method than to try and develop a whole new technique?.

Bruce Hamilton