I'm analyzing some compounds by LC/MS. I observed that the derivatives which have a free amino group show systematically a additional mass corresponding to m/z +83. Might it be an adduct formation? Any idea about to what the observed +83 units correspond?

If you're seeing perfect coelution off the column, chances are better they're adducts.

acetic acid + Na+ = 60 + 23 = 83

...if you have acetic acid or acetate in your mobile phase. Or

2CH3CN + H+ = 82 + 1 = 83

Don't know if either of those are possible or not. Anyone else have any ideas?

It's also possible 2CHOH+H20 + H+ = 83

It's also possible 2CH3OH+H20 + H+ = 83
sorry

Thank all for your comments. The mobile phase contain CH3CN, ammonium acetate, 2-propanol and water. Can we possible to exclude some of the hypotheses suggested, knowing that the composition of the mobile phase, the relative intensity (about 20% of the m/z of interest? Can we assume that if the adduct is formed from a compound which is present in sufficient amount in the mobile phase, that the intensity of the adduct would be much higher than that observed?

I don't think you can make that assumption. Your ratio of [other adduct] / [proton adduct] will depend on your lens settings up front (fragmentor, declustering potential, whatever they call it on your instrument). Some compounds will form sodium adducts from only the trace amounts of an additive or contaminant, while others require greater amounts, and still others might not form the adduct at all. If you want to make it go away, and if it's an adduct, try increasing your up-front CID voltage(s).

If you have MS/MS capability, this might give you more information on the mystery mass.

In the presence of Na adduct, it is ussgested to add a bit Na to the mobile phase in order to have high intensity for the adduct and use it for further MS^n. Is this practice relevant with regard to what MG have said about the lens settings.