Chromatography Forum

Hey guys,

I wanted to get some people's thoughts on a specific part of a method for separating methamphetamine-d and methamphetamine-l using LC-MS.

Most papers I've read regarding it use triflouroacetic acid or a mixture of acetic acid and ammonium hydroxide in their mobile phase. From what I gather, TFA is used because of its ion pairing capabilities, but I may be wrong...

My actual question: Would it be possible to separate these enantiomers (obviously using a chiral column) using formic acid in the mobile phase instead of these other acids?

My colleague believes that a)formic acid would not work to separate and b)TFA would 'gunk' up our mass spec and affect our other methods that use formic acid, even if it would be run through a separate LC channel at a different time.

Thanks to anyone that can shed any light on this. Happy to give more info.

Side note: if anyone has an old chiral column that they aren't using, I would love to take it off your hands to attempt this before purchasing one. Could trade for one of our old columns (phenyl-hexyl or C18)

Afaik chiral phase is required to separate optical isomers. There are several stationary phases vendors f.e. chiralpack. Also there are some methods with chiral mobile phase and transition metals ions.

Hi Morey,

you're right, TFA is not advisable with LC-MS (or you'll never use negative mode again...)

For basic compounds like methamphetamines you need alkaline pH if you don't have an ion-pairing reagent like TFA. You could look up the application from Phenomenex: http://az621941.vo.msecnd.net/documents ... 9a8ad2.pdf

Thank you for the replies!

The document you have linked from phenomenex helps a lot.
Also good to hear confirmation about TFA.

EDIT: Currently, we only use 0.1% formic acid in water/meoh for mobile phases with no buffers. What is the opinion of using ammonium bicarbonate based mobile phase on the same system? Could this affect other methods? would there have to be an extensive flush/rinse between methods? Sorry if this is a silly question.

moreysalzman wrote:
Currently, we only use 0.1% formic acid in water/meoh for mobile phases with no buffers. What is the opinion of using ammonium bicarbonate based mobile phase on the same system? Could this affect other methods? would there have to be an extensive flush/rinse between methods? Sorry if this is a silly question.

It's not a silly question. I wouldn't worry too much if you stick to concentrations <10mM and use a different column for the high pH method (able to work at your pH!). We perform a similar switch often, without extensive flushing, on our new LCMS and don't have one method affecting the other.

If in doubt, you could check your instrument manufacturers guidelines for pH compatibility.

Awesome! Thank you all for your inputs. It was all very helpful. Now, next step is to talk the Boss into buying the column for testing