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An unexpected [M–H]+ ion formation with some isochromene and benzofuran derivatives under electrospray ionization (ES) is observed, and explained by electrochemical oxidation in the electrospray source. This mechanism is supported by comparing the ES spectra recorded using various solvent systems and by comparison with atmospheric pressure chemical ionization (APCI). The oxidation process can be observed even in solvent systems containing water and organic acids optimized to assist ion formation via acid-base reactions. As the compounds studied are not particularly sensitive to oxidation, the analytical results may easily be misinterpreted. In such cases of doubt, comparison of the ES and APCI spectra is strongly recommended.
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Karancsi et al. (1997) studied this question.
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