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

Hello,

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!

0 LC92 9 months ago

Anyone? I see this phenomenon on several steroids in LC/MS ESI-
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0 staceylouie 8 months ago in reply to LC92

If you zoom in, is the next peak for the isotopes at 486 or 487? Wondering if it could be perchlorate (ClO4-, monoisotopic mass 98.9485) or some other Cl-containing adduct. Just making a wild guess though, have never analyzed cholesterols!
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0 LC92 1 month ago in reply to staceylouie

Hello,

Very late answer, so sorry!

Here is the zoom: doesn't look like there is a Cl
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