Chromatography Forum

Dear All,

I am trying to carry out an analysis on SPE SAX 3 ml, 500 mg sorbent bed, for the analysis of acetic and formic acid.

As diluents for the standards I used a 30 mM phosphate buffer at pH 7.0. I made sure the pH of the solution was still 7.0 after I diluted the standards in the buffer.

The SPE extraction procedure was the following:

Condition: 6 ml of methanol followed by 6 ml of 30 mM phosphate buffer pH 7.0

Loading: 6 ml of the standards diluted in phosphate buffer pH 7.0

Elution: 6 ml of H2SO4 20 mM pH 2.0

With these conditions the recovery was poor about 30%. Besides, the standards were present in the sample solution collected after loading and prior to the elution with mobile phase, showing that the acids were not completely retained on the cartridge.

Acetic and formic acid have pKa between 3.7-4.7, thus at pH 7.0 are completely ionized.

I find it difficult to understand why the acids are not being well retained on the cartridge.

I would much appreciate if someone could give me some advice on this issue.

Many Thanks

Emanuele Scollo

You need to provide more information oabout your matrix. For simple sample matrices I'd just dilute with water and try a specialty organic acid column, that's what we do for formic acid. I've never had need to explore with acetic acid.

The ionic strength of the buffer used as diluent and conditioning solution needs to be greatly decreased for that phosphate ions compete for the anion-exchange site to prevent formic and acetic acids from retaining. Can you use D.I. water as the diluent instead?

The reason why I used a phosphate buffer was to keep the acid ionized in solution, as both formic and acetic acid have a pka between 3.8-4.8, thus at pH 7 they would both be ionised. I can try diluting the standards in D.I. At the moment I am just trying standards and no sample