sodium 1-heptanesulfonate usage question

[ccuillie]

Hello,

We just recently started to try a new HPLC method for aspirin, or acetylsalicyclic acid more importantly, and our method calls for 2g of sodium 1-heptanesuflonate. I was just wondering why this is needed for the isolation of aspirin and what the negative effects might become prevalent if we excluded this from our mobile phase, which is were it is called for along with 850ml water and 150ml ACN.

Any feedback would be helpful!

[Bruce Hamilton]

Unless it's changed, that used to be the standard USP method for Aspirin tablets. The sodium heptanesulphonate is an ion-pairing agent, and is essential for the method to work.

From memory, the mobile phase pH was also controlled to around pH=3.4 using acetic acid. Sample were diluted with 1% formic acid in acetonitrile.

One important point about using ion-pairing agents is to ensure that the column is well flushed to remove the agent before storage or use for other analyses.

Please keep having fun,

Bruce Hamilton

[grzesiek]

well if this is USP or validated method then if you change the method (exclude ionpairing) your results are not valid, quite simple

moreover selectivity/retention change will probably occur so the method might become not selective

[HW Mueller]

How does the ion pairing function at pH of 3.4? The pKa of aspirin is ~3.5.

[grzesiek]

How does the ion pairing function at pH of 3.4? The pKa of aspirin is ~3.5.

I see no reason to use chromatography if you expect to see only aspirine, maybe they expect something more in the sample, I usually do

[HW Mueller]

Part of the original question was:
"I was just wondering why this is needed for the isolation of aspirin . . . ".

Why would anybody expect that they might have to retain cations here??

[grzesiek]

Let's just hope this method was not chosen for preparative chromatography and that "isolation" is just a mistake

ionpairing for preparative sounds akward

[Vlad Orlovsky]

There is absolutely no reason to use heptanesulfonate for retention of acids, it does not form Ion-pair because these are two acidic molecules (you need + and - to form ion-pair). I can understand use of HSA if composition has some basic molecules (like a lot of cold and cough medications) - pseudoephedrine, phenylephrine, etc.