Chromatography Forum

I'm trying to modify a published SPME procedure since I'm using a different derivatization reagent. The logP of the derivative in the publication is estimated to be 4.07. The logP of my derivative is much lower, 2.05. In the published procedure a mixture of pentane-ether (1:3) is used to extract the compound from the aqueous matrix before exposition of extract to the fiber.

The question is which solvent or solvent mixture might be suitable to pull the compound with such lower logP from the aqueous phase?

I am unfamiliar with this approach. Can you provide more details on your methodology? Analyte(s), matrix, derivatization chemistry, etc.

It seems as though you are extracting the analyte from the matrix with a solvent and then performing the SPME/derivatization. Is that correct? If you're going to the effort of solvent-extracting your analyte out of the matrix, why bother with SPME at all?

The only on-fiber derivatization work I've done was to determine formaldehyde in a gas sample. In that method, you load up some water with 2,3,4,5,6-pentafluorobenzene hydroxylamine, use heated headspace sampling of that solution to saturate the fiber with the derivatizing agent, then expose the saturated SPME fiber to the gaseous sample to trap the formaldehyde. GC analysis after that. That analysis is very time dependent. It's easy to consume a majority of the derivatizing agent on the fiber very quickly.

We are comparing two different approaches to sample prep, liquid-liquid extraction vs. SPME, but with a derivatization reagent different from the one in the publication.

Initially a derivatization is done, then an extraction solvent is added, shaken, centrifuged and then organic layer was evaporated to dryness in a vial. Water and salt are added and SPME is performed. It has nothing to do with on-fiber derivatization.

The sample is urine, and some volatiles which are present in certain diseases are measured.

If you've already had to go to all of that effort to get your derivative out of the matrix with the solvent, does SPME really help you in the end? It seems to me that when you add water/salt, you're just diluting your sample again.

Why not just evaporate the extraction solvent to 50-100 µL or so and inject it directly?

SPME really shines when you can eliminate the solvent extraction part.

I agree that the sample preparation procedure sounds needlessly long at first sight.

About the original question, if I understand correctly you are looking for a solvent (mixture) to extract your derivatized compound from the aqueous phase, which is more polar than the one in the original procedure. I'd try dichloromethane or ethylacetate. If that fails, try acetonitrile and use salt to separate the phases.

Pynball wrote:
We are comparing two different approaches to sample prep, liquid-liquid extraction vs. SPME, but with a derivatization reagent different from the one in the publication.
transfers Amiens
Initially a derivatization is done, then an extraction solvent is added, shaken, centrifuged and then organic layer was evaporated to dryness in a vial. Water and salt are added and SPME is performed. It has nothing to do with on-fiber derivatization.

The sample is urine, and some volatiles which are present in certain diseases are measured.

I agree with you