pH, ion mode and acid/basic analytes

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

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I´ve learned that I should use high pH for acids to produce ions in negative mode and vice versa (correct?), but then I read that an adduct formed from formic acid is
[M-H+HCO2H]-. And I, myself, have experience of acid analytes that form [M-H]- in ES- with formic acid. Would the response be better in ES+ or with basic pH?

Can anyone explain a little bit more about this (pH, ion mode and acid or basic analyte)?

If the compound only has an acidic functional group (no basic group), the response will almost certainly be better in ESI negative, even in acidic pH. You can try raising the pH to see if response improves, but you must also consider the chromatography, which will usually be better with an acidic mobile phase (when chromatographing acids).
2 posts Page 1 of 1

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