REDUCED ISOTOPIC SCRAMBLING IN NEUTRALIZATION REIONIZATION MASS-SPECTRA

Facile ion isomerization compromises isotopic position characterization by mass spectrometry, particularly for hydrocarbons. For example, hydrogen atom loss does not provide such characterization for [C6D5CH3]+. ions from electron impact ionization of toluene-ar-d5, as this loss is preceded by 89% H/D scrambling. Similarly, the corresponding low-energy molecules formed from these ions by neutralization with Hg yield, on reionization, mass spectra that exhibit 78% scrambling. This is true for reionization to either positive or negative ions, collection of neutralized products scattered at larger angles and after collisional activation of the fast toluene neutral species. However, when these ions are neutralized with Cs vapor to form excited (approximately 5 eV) toluene molecules, half of the resulting losses occur before H/D scrambling. The resulting neutralization-reionization mass spectra are thus more useful for isomeric isotopic characterization. The method is not effective for ions such as [styrene]+. with an isomerization pathway(s) requiring far less energy than that for direct dissociation.