What is m/z [M+99]- in negative mode?


I analyzed a cholesterol sample on QTOF. The monoisotopic mass is 386.3549 Da, but in negative mode I see m/z 485.3. Any idea why this is [M+99]- and not [M-H]-, or [M+HCOO]-?

In positive mode there is no problem, I see [M-H2O+H]+.

Thank you!

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