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It's possible to identify a solution with several compounds* without the coupled liquid chromatography (LC) before to isolate the compounds?

On the university where I study, the LC is broken, so all we can do is inject the solution directly on the mass spectrometer. I wonder if I will be able to identify the metabolites? I don't need quantitative analysis, I just want to know what's in there.

(*) To be precise: A bacterial culture, that has gone under organic solid phase extraction, freeze dried and resuspended in methanol.

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The practical answer really depends on two things: do you know exactly what you're looking for and can your instrument do tandem MS?

If you know exactly which metabolites you're looking for and they're present in great enough abundance, it may be possible to resolve them from the the background, depending on the resolving power of the instrument and what else is in there (if you have an ICR instrument, it may not be a problem at all, but if it's just a single quadrupole, you may be out of luck).

If you know exactly which metabolites you're looking for (or know that they share some common structural feature that can be knocked off by CID or the like) and your instrument can do tandem MS, you're in a much better position. Tandem MS is often used to avoid needing to do a slow LC step as it's possible to identify two compounds that aren't resolved on a precursor ion scan by looking at their fragment ions: A messy primary ion scan can be dealt with if you know that your analyte has a [M-18] transition or whatever, by CID. Also, a given class of compounds often shares certain CID reactions, which can be picked up with a neutral loss scan.

If you have tandem, but aren't sure exactly what you want to find, it's a bit more work, but it just means you need to look at product ion scans of peaks that are ambiguous and try to work out what they are.

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It all depends on the selectivity of your SPE for the class of compounds you're looking for. If you have some confidence that the SPE has let only a few components of the bacterial lysate pass into the eluate, then you might see few enough m/z signals on the MS that qualitative analysis would be possible. If your SPE is relatively unselective, though, you will probably have a dense array of peaks that won't tell you much.

It (probably?) can't hurt to try running the sample(s), though. If you see relatively few peaks, then you could rerun the samples, spiked with known amounts of known compounds, to see whether:

  • A new peak shows up (the spiked chemical is absent in the sample);
  • An existing peak gets bigger (modest indication that the chemical is present); or,
  • No change is observed (the spiked chemical is absent in the sample or the spike was too small)

If you're looking for more than a few chemicals, this promises to be a pretty drawn-out process, with lots of samples pushed through the MS. Might still be faster than waiting for the LC, though!

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