By Anonymous on Tuesday, June 8, 2004 - 07:29 pm:

Dear experienced LC-MS users,
I get a group peaks with a same interval in ES- mode. Peaks distributed at 249, 385, 521,657, 792,928..., with regularly increasing 136. Anyone could tell me what moeity this is.
Thank you very much.
Michelle

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By Anonymous on Friday, June 11, 2004 - 12:24 pm:

Michelle,

The repetitive ions with a difference of 136 daltons is due to sodium trifluoroacetate (NaTFA). You likely have trifluoroacetic acid in your mobile phase and you are observing NaTFA clusters of the trifluoroacetate ion (m/z 113) in negative ion ESI-MS mode. I would strongly suggest removing TFA from your mobile phase, since is has been well documented that TFA will suppress ion formation of your analytes in negative ion ESI-MS mode. Try instead using either acetic or formic acid if your LC method requires an acid modifier.

Kevin

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By MG on Friday, June 11, 2004 - 03:22 pm:

If TFA were present, a strong signal at 113 would also be observed, if you are scanning that low