Background Ions

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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I am getting background ions of 110.1, 154.9, and 436.3 throughout each sample acquisition, but I am mainly concerned with m/z 436.3. The mobile phases I use are 0.1% formic acid in H2O and 100% ACN. I thought the contamination was inside the mass spec because whatever I infused would give me m/z 436.3 so I cleaned the Ion Transfer tube, changed needle insert and also cleaned the skimmer cone in case any build-up was present. Afterwards, I infused my mobile phases starting with ACN and it came back clean with minimal background ions. I infused the 0.1% formic acid in H2O mobile phase and it showed m/z 436.3 like before. Then I infused straight H2O (Millipore reagent water) and it was still there so I moved to deionized water and had no luck. I even saw trace amounts of it when I infused MeOH.

Is it possible that BOTH water sources are contaminated and what exactly is m/z 436.3? I can't find it anywhere online. Is there anything else I should try or has anyone else had this kind of problem before? Also, any ideas about the other two ions present in my spectrum?
Do you have a spare ESI probe to check? Or perhaps an APCI probe to exclude the probe?

Be careful making conclusions that it is not present in plain ACN. It's also possible that it just doesn't ionize in plain ACN.

I would only worry about it if it actually impacts your analysis. I personally wasted time trying to get control over background in full scan GC and LC-MS/MS. Looking back it wasn't worth it. If you look at full scan background in LCMS there is always stuff going on.

Perhaps you could post a screenshot of the full scan MS spectrum while infusing mobile phase so we get a better view of what's going on.
Could it possibly be a sodium adduct of PFOA (Perfluorooctanoate) which is mass 413? Since you most likely have teflon lines in the system somewhere, it could be a possibility, that along with the fact that sodium is usually present even in the best water at trace levels.
The past is there to guide us into the future, not to dwell in.
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