lcms question

[shadow]

Is it possible to see benzene in LCMS (ESI) I have this impurity showing up that has the same mass. Thanks for thehelp

[lmh]

I'd be very, very surprised. What are you doing, and what mass are you seeing?

[Robert Classon]

It is highly unlikely that you would see a benzene ion by ESI. You might see something by APCI or APPI, but not much.

The most likely situation would be that you have some contaminant or a cluster (adduct) ion. Benzene has a mass of 84, so let's consider possible ions near that mass.

At 83 m/z there is a very common acetonitrile adduct [2 AcN +H]+. This is a very strong ion and often dominates the spectrum if the drying gas flow (or temperature) is too low or if the declustering settings for the first skimmer are set incorrectly.
At 85 m/z you can find a fairly common PEG ion (as a sodium adduct). This can be an artifact from autotuning if you used PEG in the tuning solution. It also shows up from time to time in various samples from sample handling. I've seen it in some food samples.
At 86 m/z you can see a common methanol ion as the ammonium adduct.

To see which one you are working with, try switching solvents (AcN to MeOH or vice versa). That will tell you if you are dealing with a solvent adduct. If so, then just adjust the declustering voltage to eliminate. If changing solvent doesn't remove the ion, try adding 10 mM ammonium acetate or 10 mM potassium acetate, make a quick run and see if the ion disappears or changes mass. (don't run with sodium for long). The higher level of the additive will swamp out background levels of sodium or other common ions. That should tell you what kind of adduct you are dealing with.

[shadow]

Thank you very much for the detailed response, I will put in practice what you have described.