Second hydrogen atom abstraction by molecular ions.

Abstract

We report the observation of a new physical phenomenon of the addition of 2 hydrogen atoms to molecular ions thus forming [M+2H]+ ions. We demonstrate such second hydrogen atom abstraction onto the molecular ions of pentaerythritol and trinitrotoluene (TNT). We used both gas chromatography mass spectrometry (GC-MS) with supersonic molecular beam (SMB) with methanol added into its make-up gas and electron ionization (EI) liquid chromatography mass spectrometry (LC-MS) with SMB with methanol as the LC solvent. We found that the formation of methanol clusters resulted upon EI in the formation of dominant protonated pentaerythritol ion at m/z=137 plus about 70% relative abundance of pentaerythritol molecular ion with 2 additional hydrogen atoms at m/z=138 which is well above the 5.7% natural C13 isotope abundance of protonated pentaerythritol. Similarly, we found an abundant protonated TNT ion at m/z=228 and a similar abundance of TNT molecular ion with 2 additional hydrogen atoms at m/z=229. Upon the use of deuterated methanol (CD3 OD) as the solvent, we observed an abundant m/z=231 (M+2D)+ of TNT with 2 deuterium atoms. We found such abundant second hydrogen atom abstraction with butylglycolate and at low abundances in dioctylphthalate, Vitamin K3, phenazine, and RDX. At this time, we are unable to report the magnitude and frequency of occurrence of this phenomenon in standard electrospray LC-MS. This observation could have important implications on the provision of elemental formula from mass spectra that are involved with protonated molecules. Accordingly, while accurate mass measurements can serve for the generation of elemental formula, their further support and improvement via isotope abundance analysis are questionable. Consequently, if a given compound can be analyzed by both GC-MS and LC-MS, its GC-MS analysis can be superior for the provision of accurate elemental formulae if its EI mass spectrum exhibits abundant molecular ions such as with GC-MS with SMB (also known as cold EI).