Amino acetates and NaCl

[ghum@sialco.com]

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.

[tom jupille]

- 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?

[ghum@sialco.com]

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.

[Vlad Orlovsky]

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.