I am in the process of sending some samples for LC-ESI-MS analysis and was curious about removing any salts that might be in my extracts. The samples I am sending out are biosurfactants/bioemulsifiers from bacteria grown in M9 supplemented with glucose. I extract my compounds using organic solvents (the supernatant is filtered prior to extraction) and let the solvents evaporate in a fume hood. Sometimes I get crystallized extracts, sometimes a powder, and usually an oily residue. I am worried that any salts that carry over from the medium will interfere with the analysis. I am only interested in obtaining the molecular ion peak and some fragmentation patterns. Any suggestions will be appreciated, thanks.
Hi, firstly you should specify "organic solvent". If you use relatively polar organic solvent as methanol or ethanol, there may be salts present in your samples. Such samples may be desalted solid-phase extraction using C18 or similar polymeric sorbent (HLB from waters, Strata-X from Phenomenex etc.).
On the other hand, if organic solvent is rather non-polar such as chloroform, hexane etc., no further desalting is needed due to limited solubility of polar (ionic) molecules.
Thank you for the suggestions. I currently use non-polar solvents (CHCl3 and/or Hexane) but I must note that these are surfactants (biosurfactants) so when I do the extraction with the non-polar solvents, I get an emulsion. These emulsion could be either water-in-oil (w/o) or oil-in-water (o/w), so some of the aqueous phase (M9) gets carried over with the extract. I noticed that my extracts are usually soluble only in water which could make analysis more difficult. I did read a dissertation (I only read certain parts) regarding rhamnolipids and they would solubilze their extracts in water adjust the pH to 8 with NaOH. Then they would lyophilize the sample to obtain a powder but they mention that this could deprotonate the compound if its originally charged. I plan on just sending an extracted compound as is and maybe one that has been modified. What are some ways to desalt extracts that are soluble in water?
You may want to acidify (or basify) your crude material before the extraction. It will ensure that the compounds exist in their organic form rather than as a salt, thus improving the extraction efficiency. You may also observe better phase separation during the extraction using this method.
Second, when you inject the sample on to the LC column, set an isocratic wash with the polar solvent before starting the gradient. This way any salts will elute out in the earlier fractions while retaining the non-polars on the column. The C-18 itself acts as a desalting column.
Thank you for the suggestion, I will definitely give it a try. I am not running the LC but will be sending it out for LC-ESI-MS. I want to note that some biosurfactants/bioemulsifiers are actually lipopeptides and the method to extract those involves precipitation by acidification (pH < 2), so its definitely worth a shot.
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