Coulomb crystal mass spectrometry in a digital ion trap

Abstract

We present a mass spectrometric technique for identifying the masses and relative abundances of Coulomb-crystallized ions held in a linear Paul trap. A digital radio-frequency wave form is employed to generate the trapping potential, as this can be cleanly switched off, and static dipolar fields are subsequently applied to the trap electrodes for ion ejection. Close to 100% detection efficiency is demonstrated for ${\mathrm{Ca}}^{+}$ and ${\mathrm{CaF}}^{+}$ ions from bicomponent ${\mathrm{Ca}}^{+}\ensuremath{-}{\mathrm{CaF}}^{+}$ Coulomb crystals prepared by the reaction of ${\mathrm{Ca}}^{+}$ with ${\mathrm{CH}}_{3}\mathrm{F}.$ A quantitative linear relationship is observed between ion number and the corresponding integrated time-of-flight (TOF) peak, independent of the ionic species. The technique is applicable to a diverse range of multicomponent Coulomb crystals---demonstrated here for ${\mathrm{Ca}}^{+}\ensuremath{-}\mathrm{NH}{}_{3}{}^{+}\ensuremath{-}\mathrm{NH}{}_{4}{}^{+}$ and ${\mathrm{Ca}}^{+}\ensuremath{-}\mathrm{CaOH}{}^{+}\ensuremath{-}\mathrm{CaOD}{}^{+}$ crystals---and will facilitate the measurement of ion-molecule reaction rates and branching ratios in complicated reaction systems.