Strange method in Pharmacopieal forum...

[Mattias]

I have been asked to have a look at the method for sodium docusate that probably will enter the USP very soon, and I am pussled by the conditions.

The molecules (main peak and degradation products) are very acidic (sulfonic acids). The authors have chosen to add octanesulfonate salt to the mobile phase as ion-pair reagent. Quite strange choice since all molecules are negatively charged? The first degradation product elutes after 0.4 minutes in this method, which is more or less the front.

Can any of you see the motive for adding "the wrong salt"?

[grzesiek]

Have you seen method development report?

I knew people who added ion-pairs ALWAYS, no matter what compounds were in the mixture

[Mattias]

Hi grzesiek,

I have not seen any development data - and I hope you are not right about the ion-pairing part. In this case a the "wrong" ion-pair reagent should lead to that the molecules elute faster than normal. But I cannot see any reason to change the selectivity here... I have tested without ion-pairing (not the same column though) and you really need to work hard to get co-elution.

[grzesiek]

"I have tested without ion-pairing (not the same column though) and you really need to work hard to get co-elution." - as they used to say to me: "but it won't do any wrong, you know"

I'm with you in that I hope this is not the case but I am always little suspectible to people using ion-pair reagents after these events

[mbicking]

Mattias:
If you are looking for scientific reasons for details in USP methods, you will often be disappointed. I have heard stories about how/why some of these methods are developed, but the details are best left confidential.

Many would agree that USP methods are not optimized by current LC standards.