For the first complexity, the occasional extra 1 Da in the precursor mass arises when Extract-MSn is used to generate the peaklist and a ’0′ is encountered. In those cases, the mass of the most abundant isotopic peak is chosen, which is the M 1 peak for precursor masses higher than about 1700 Da. Therefore, the extra 1 Da arises not from the instrument itself, but from the way Extract-MSn extracts the information
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