Chromatography Forum

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.

pH too low for all but the phosphoric? (Lower amnt of injection doesn´t help?) Higher concentrations of the other acids produces MS problems?
I saw something somewhere of having gotten around the problem with phosphoric, etc. Probably too involved, or specific, too bad I didn´t archive anything on this. (Sure would like to see this "mineral" buffer problem mastered).

Lower concentrations didn't help.

Compounds with higher molecular weight (>160) worked reasonably well, but lower molecular weight ones (MW 120-150) had too much loss in signal due to matrix suppresion and/or chemical noise.

I can use phosphoric acid on my instrument, but hurts detection limits in trace analyses..

Just noticed that yesterday I meant to ask: "pH too high for all but the phosphoric?"
If you can´t go to higher concentrations with the other acids how about a two step chromatography, the first step using your present column with phosphoric...., the second step with a column that holds the analyte until you hit it with a MS compatible solvent. Or maybe HILIC will do? Perhaps even another RP?
Sure would like to see how you solve this.

I think that you might have one of the problems: either you aldehyde/acid is in transitional state at certain pH (pH dependent) or you have chemical reversible interaction (shift base formation??) during the analysis. Based on the peak shape it looks like overloading of some sort.
I found both acids in our lab but no aldehyde. If you send me small sample of aldehyde we will try to solve you problem.
Let me know if you are willing to go this way (my email is below)