Coulomb crystal mass spectrometry in a digital ion trap

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 ${\mathrm{Ca}}^{+}-{\mathrm{CaF}}^{+}$ 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 ${\mathrm{Ca}}^{+}-\mathrm{NH}{}_3{}^{+}-\mathrm{NH}{}_4{}^{+}$ and ${\mathrm{Ca}}^{+}-\mathrm{CaOH}{}^{+}-\mathrm{CaOD}{}^{+}$ crystals—and will facilitate the measurement of ion-molecule reaction rates and branching ratios in complicated reaction systems.

Figure 1

The number of ions in a given ${\mathrm{Ca}}^{+}$ Coulomb crystal can be established to within $\pm 5$ ions by fitting the experimental image to MD simulations. A simulated crystal image with 50 ${\mathrm{Ca}}^{+}$ ions is shown in panel (a), alongside an experimental ${\mathrm{Ca}}^{+}$ crystal image in panel (b). The simulated crystal is overlaid on the experimental image in panel (c), illustrating the excellent agreement between the positions of the ions in the simulated and experimental crystals. This approach is also adopted for multicomponent crystals; in panel (d), a bicomponent ${\mathrm{Ca}}^{+}-{\mathrm{CaF}}^{+}$ experimental crystal (in which only the ${\mathrm{Ca}}^{+}$ ions are fluorescing) is overlaid with a simulated ${\mathrm{Ca}}^{+}-{\mathrm{CaF}}^{+}$ crystal comprising 30 ${\mathrm{Ca}}^{+}$ ions and 35 ${\mathrm{CaF}}^{+}$ ions. Only the ${\mathrm{Ca}}^{+}$ ions are made visible in the simulation, for ease of comparison between the images.

Figure 2

Schematic illustration of the linear Paul trap utilized in this work, with four electrodes in a quadrupole arrangement. Each electrode is composed of three segments, with rf voltages $\left(V_{\text{rf}}\right)$ applied to all segments and additional dc voltages $\left(U_{\text{dc}}\right)$ applied to the endpieces.

Figure 3

(a) The regions of stability for several ionic species in the DIT operating at $\tau$ = 0.25 and ${\omega }_{\text{rf}}=2\pi \times 1.33$ MHz. For clarity, only the stability region of ${\mathrm{Ca}}^{+}$ is shaded black; for all other species, the stability region is enclosed between the relevant colored (shaded) curves. Regions of overlapping stability indicate the operating parameters $(V_{\text{rf}},U_{\text{dc}})$ where species can be cotrapped. (b) A closer view of the experimentally accessible region of the stability diagram.

Figure 4

Experimental digital rf trapping wave form $(-4$ to 0 $\mu \mathrm{s})$ and ejection pulse (0 $\mu \mathrm{s}$ onwards).

Figure 5

(a) The integrated ${\mathrm{Ca}}^{+}$ peak intensity in the TOF spectrum is plotted for each Coulomb crystal against the number of ${\mathrm{Ca}}^{+}$ ions, determined from fitting the experimental images to MD simulations. (b) Schematic illustration of the TOF-MS apparatus, depicting the ion trap electrodes (viewed along the trap axis, with ion ejection perpendicular to this axis), TOF tube, and MCPs. Trajectories of 200 ${\mathrm{Ca}}^{+}$ ions ejected from the trap are shown (in blue online) from the ion trap (left) to the front detector plate (right). The electric potential near the ion trap is positive (shaded red online) and near the front detector plate is negative (shaded green online).

Figure 6

(a) A bicomponent ${\mathrm{Ca}}^{+}-{\mathrm{CaF}}^{+}$ Coulomb crystal and the TOF trace recorded following ejection of the crystal. Peaks arising from species with masses differing by 1 u can be resolved, as demonstrated by the ejection of (b) ${\mathrm{Ca}}^{+}-{\mathrm{CaOH}}^{+}-{\mathrm{CaOD}}^{+}$ and (c) ${\mathrm{Ca}}^{+}-{\mathrm{NH}}_3{}^{+}-{\mathrm{NH}}_4{}^{+}$ multicomponent crystals. The two unlabeled small peaks in panel (a) are due to unidentified minor impurities.

Figure 7

Bicomponent ${\mathrm{Ca}}^{+}-{\mathrm{CaF}}^{+}$ crystal ejection analysis. The integrated TOF peak for (a) ${\mathrm{Ca}}^{+}$ and (b) ${\mathrm{CaF}}^{+}$ ions is plotted against the number of ions of that species in each crystal, ascertained from fitting the crystal images to simulations, for bicomponent crystals of various sizes and compositions.