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 this report describes that a regular positive electrospray ionization mass spectrometry ( ms ) analysis of terpendoles often causes unexpected oxygen additions to form [ m + h + o]+ and [ m + h + 2o]+ , which might be a troublesome in the characterization of new natural analogues . 
 the intensities of [ m + h + o]+ and [ m + h + 2o]+ among terpendoles were unpredictable and fluctuated largely . 
 simple electrochemical oxidation in electrospray ionization was insufficient to explain the phenomenon . 
 so we studied factors to form [ m + h + o]+ and [ m + h + 2o]+ using terpendole e and natural terpendoles together with some model indole alkaloids . 
 similar oxygen addition was observed for 1,2,3,4-tetrahydrocyclopent[b]indole , which is corresponding to the substructure of terpendole e. in tandem ms experiments , a major fragment ion at m / z 130 from protonated terpendole e was assigned to the substructure containing indole . 
 when the [ m + h + o]+ was selected as a precursor ion , the ion shifted to m / z 146 . 
 the same 16 da shift of fragments was also observed for 1,2,3,4-tetrahydrocyclopent[b]indole , indicating that the oxygen addition of terpendole e took place at the indole portion . 
 however , the oxygen addition was absent for some terpendoles , even whose structure resembles terpendole e. the breakdown curves characterized the tandem ms features of terpendoles . preferential dissociation into m / z 130 suggested the protonation tendency at the indole site . 
 terpendoles that are preferentially protonated at indole tend to form oxygen addition peaks , suggesting that the protonation feature contributes to the oxygen additions in some degrees . 
  2014 the authors . 
 journal of mass spectrometry published by john wiley & sons , ltd . 
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