It is (1+) and strongly present with high ACN content at the end of our LC gradients (Setup: Ultimate 3000RSLCnano HPLC coupled to an impactII MS running plant proteomics samples). Didn't find any information about such a mass - it is also present in blanks and we have already exchanged solvents to highest purity (Merck hypergrade ACN) which improves situation but the signal seems to return.
Have you looked in the supp data of this awesome article?
Keller, B. O., et al. (2008). "Interferences and contaminants encountered in modern mass spectrometry." Anal Chim Acta 627(1): 71-81.
Yes, we looked for the list (it's even saved on our MS computers' desktop for reasons) - only 321.13 m/z for PEG would be somewhat in that range, but we have constantly 321.16 m/z and it only occurs with high organic so it must be something in ACN or the path.. which indicates me away from PEG; this also occurs with a "blind injection" (0 uL) and the same LC gradient.
Occuring in the blind injection would imply that it is in your solvents, not injection vials/plates. Does it elute in a single peak or is it always there? If it elutes as a peak, it implies that it is in solvent A and is concentrating on the column to be eluted by solvent B. Do you use plastic pipettes or plastic something to prepare the solvents?
I'm going to have to look in my files, but I can't recall ever seeing this ion in my system which uses stainless, fused silica and glass vials.
It could be something in your plant samples accumulating on your column that is slow to move just with ACN in your mobile phase.
I'd try increasing the solvent strength for buffer B, perhaps 75% ACN/ 25% IPA and then run a few blanks (or alternatively just flush the column for an hour or so while monitoring the signal). If using the new solvent is effective in eluting the mystery compound you can expect the m/z 321.16 signal to increase and then diminish over the first few runs. Then inject another plant sample to see if it returns.
Just to clarify is it a distinct peak or a increasing background ion? Can you post an MS? Is the Instrument new? Have the gas lines been altered recently? I rember I suffers from weird Cu-ACN adducts after my nitrogen generator was installed.
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