Chromatography Forum

To those more learned in these issues,

I have a system of amino acetates and am using a Dionex Surfactant column for separating them. I am running ACN and 100 mM NH4Ac buffer at pH 5 with an ELSD detector. Injection of simple NaCl gives 2 peaks and I do not seem to be able to pick up simple acids (acetic, chloroacetic). So it boils down to a few simple questions:

- why does NaCl give two peaks
- what can I do to start detecting AcOH and chloro-AcOH
- is there a better column out there for separating amino acetates/acids?

Thanks in advance for those who may provide insights.

G.

- why does NaCl give two peaks

Without details as to column dimensions, flow rates, and retention times it's impossible to say for sure but I can speculate that there is just enough ion-pairing going on to separate the tro ions so that what you are seeing is peaks for sodium acetate and ammonium chloride.

- what can I do to start detecting AcOH and chloro-AcOH

You will not be able to reliably quantitate acetic acid in a mobile phase that uses an acetate buffer. Think about what you get when you dissolve acetic acid in water. In any case, I suspect those compounds are too volatile for the ELSD.

- is there a better column out there for separating amino acetates/acids?

"Amino acetates/acids" is too general a description to allow a useful suggestion. What, specifically, are you trying to do?

Tom,

thanks for your reply. By amino acids/acetates think surfactant intermediates:

methylamine + sodium mono chloroacete to get sodium sarcosinate and the dialkyl byproduct -

at low pH the species is cationic - at high pH it is anionic -

I am considering going to pH 7-7.5 with sodium borate buffer to form salt of species andsee if ELSD will catch them - if I want higher pH I will have to switch columns (RP 18 or normal phase silica).

G.

when you inject sodium chloride you see one peak for sodium and one peak for chloride ion which forms non-volatile ammonium chloride salt. You are not going to see acetic acid, otherwise you would never can use ammonium acetate with ELSD or LC/MS. The only acids you can see in ELSD is the one which from non-volatile salt with your ammonium (acetate). In order to analyze acetic acid you need UV, so UV transparent buffer is a must (cutoff of ammonium acetate is 230 nm and you need to look at 200-210 nm). Your problem is detection technique and not column. Your amine requires TFA to monitor as a salt and you chloracetic acid requires ammonia in the mobile phase. I think that it will be impossible to have one method for methylamine, acetic acid and chloracetic acid because of the problem with MONITORING. You can try RI as detection, but I personally avoid RI at any cost .
You choice can be two methods one for acidic compounds with UV detection on something like Primesep D or SB, and mathylamine on something like Primesep A or 100 detected by ELSD.
http://www.sielc.com/upload/file/pdf/SI ... r_2006.pdf
http://www.sielc.com/Compound-Methylamine.html

....and sodium borate is no volatile, you will ruin your ELSD with approaches like this.