Organic Acids with SPE-SAX

Hello,

We are working on a method to determine organic acids in food samples with HPLC-DAD. We are using a strong anion exchange SPE cartridge (Strata-X-A 33u Polymeric Strong Anion) from Phenomenex for the purification of organic acids (Formic acid, malic acid, lactic acid, acetic acid, propionic acid and citric acid).

The SPE-cartridge has a capacity of 0.3 meq/g. Because we’re using cartridges of 200 mg, our capacity is 0.06 meq. I’ve calculated that the six organic acids in a phosphate-buffer have a capacity of 0.056 meq in 0.5 ml.
The procedure was:

1) Conditioning: 6 ml MeOH, followed by 6 ml H2O
2) Sample loading: 0.5 ml of organic acid standards (500 ppm) in a phosphate-buffer (0.1 M)
3) Drying for 5-10 seconds
4) Eluating with 4 ml of 0.5M H2SO4

We became high recoveries (80-90 %) with this procedure, but when we use a washing step (4 ml MeOH or 4 ml Phosphate-buffer), the recovery decreases to 20-40 %.

Can anyone help and explain why this is happening? Or what solution we can use to keep the high recoveries? We will need a washing step when we use this purification on food samples, because they contain a lot of interferences.

Thanks for your input!






I didn't find the right solution from the internet.
References:
www.chromforum.org/viewtopic.php?t=25844#p122872

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