Chromatography Forum

Posted: Fri Oct 06, 2006 12:26 pm Post subject: Problem with Aldehyde containing species in LC-MS method

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I am trying to develop a method to chromatography 4-carboxybenzoic acid in terephthalic acid. I need the method to be MS friendly.

The chromatogram at:

http://littledomain.com/aldehyde_problem.pdf

shows the bad peak shape I am observing. A gradient separation was used on a Zorbax C18 stablebond column using 0.01 % formic acid/methanol and water. The peak shape looks fine if you increase the concentration of formic acid to 0.1-0.5%.

Unfortunately, the higher concentration of formic acid suppresses the response for lower MW species such as benzoic acid.

The same bad peak shape is noted with 0.5% acetic, 0.01% trifluoroacetic acid, but is not note with 0.14% phosphoric.

Thought it might be due to methanol forming an acetal during separation with the aldehyde group, but same problem is noted with acetonitrile and tetrahydrofuran.

Anyone know why such poor peak shape?
Could it be avoided by switching to a different type C18 reverse phase column?
Derivatization of the aldehyde?

Thanks for the help

What's your sample diluent? At first glance it looks like the sample diluent may be stronger than the starting mobile phase.

If you want to try a different column I would suggest ion-exclusion chromatography on a USP L17 packing with a mobile phase of 0.1% acetic acid with up to 10% MeOH. I have used this with electrospray lc-ms in negative ion mode and gotten good sensitivity on some organic acids.

Usually have to use stronger solvent to dilute organic acids such as those in the chromatogram.

The stronger solvent didn't seem to hurt the peak shape of 2 out of 3 of the components.

Will see if I can dilute sample enough to use solvent system more like the one used at the starting of the gradient.

Thanks for the suggestion.