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HPLC ion exchange chromatogram nasty

Posted: Mon Dec 04, 2017 3:00 pm
by LabPro
I am new with ion exchange HPLC columns. I am following an EPA method for DiThioCarbamates (Metham sodium) which uses a Supelcosil LC-SAX1 column (25cm x 4.6mm x 5um) and has a 45% MeOH to 55% water pH buffered to 6.9. There also is a component in the buffered water of HexaDecylTriMethyl Ammonium Bromide. Embarrassed to say that I don't know what this does.

This first injection looked like it had a problem - but I thought could be worked with and might work. The chromatogram looked like 2 peaks where the first was twice the size of the second which melded into a shoulder where they weren't resolved really at all.Image
The subsequent injections - even after running a MeOH and a the 6.9 buffer water eluent rinse injections looked even worse.Image

Can anyone explain what I am seeing? BTW, the Metham Sodium target analyte is stable at a pH of 6.9 but unstable below that breaking down to Methyl IsoThio Cyanate (MITC). Since the Metham is stable above 6.9 pH I should get one solitary peak not two, and certainly not some ski slope tail after the first peak comes out.

Thanks in advance!

Re: HPLC ion exchange chromatogram nasty

Posted: Sun Dec 24, 2017 11:13 pm
by mattmullaney
Hi LabPro,

More detail regarding the method would be helpful, such as retention time(s), flow rate, and maybe an attached chromatogram.

I couldn't find the method online to read for myself. The easy part is the HexaDecylTriMethyl Ammonium Bromide (a.k.a. cetrimonium bromide), this compound is a cationic surfactant which ought to form a nice ion-pair with the methyl dithiocarbamate ion (anion). This ion-pair should increase the retention of your analyte--if you were running a reverse phase separation. I'd think that the SAX phase wouldn't require the use of the cetrimonium bromide at all...method details would be helpful here, too.

Guess this will do for a start...