Laser-Cooling-Assisted Mass Spectrometry

Mass spectrometry is used in a wide range of scientific disciplines including proteomics, pharmaceutics, forensics, and fundamental physics and chemistry. Given this ubiquity, there is a worldwide effort to improve the efficiency and resolution of mass spectrometers. However, the performance of all techniques is ultimately limited by the initial phase-space distribution of the molecules being analyzed. Here, we dramatically reduce the width of this initial phase-space distribution by sympathetically cooling the input molecules with laser-cooled, cotrapped atomic ions, improving both the mass resolution and detection efficiency of a time-of-flight mass spectrometer by over an order of magnitude. Detailed molecular-dynamics simulations verify the technique and aid with evaluating its effectiveness. This technique appears to be applicable to other types of mass spectrometers.

Figure 1

Schematic of the LAMS apparatus. The LQT has a field radius of $R_0=6.85\mathrm{mm}$ and a length of 91 mm. Its axis is aligned perpendicularly to the TOF drift tube to enable the radial extraction. The Yb and ${\mathrm{BaCl}}_2$ ablation targets are mounted below the LQT. The laser-cooling beams are roughly aligned with the axis of the LQT. The TOF drift tube contains two Einzel lenses and has a total length of 275 mm. Ions are detected by using a channel electron multiplier (CEM), which is shielded by a grounded stainless-steel mesh, and the complete assembly is held under vacuum at a pressure of approximately $10^{-9}\mathrm{mbar}$. The fluorescence of the laser-cooled species is imaged through an objective lens (not shown), facing the TOFMS, onto an electron-multiplying CCD camera and allows estimation of the number of atoms and molecules in the assay.

Figure 2

$\mathrm{LAMS}[{}^{174}{\mathrm{Yb}}^{+}]$ versus traditional TOFMS for trapped Yb ablation products. Samples consisting of approximately 1000 ions are loaded into the LQT, and ${}^{174}{\mathrm{Yb}}^{+}$ is laser cooled. The LAMS curve represents the average of 20 spectra. The labels denote the mass of the ions of the corresponding peak and are determined by a fit of a Gaussian curve to the LAMS spectrum. For comparison, a traditional TOFMS curve, which is an average of 20 spectra taken without laser cooling, and a simulation of a LAMS spectrum (see the text) are shown.

Figure 3

$\mathrm{LAMS}[{}^{138}{\mathrm{Ba}}^{+}]$ versus traditional TOFMS for trapped $\mathrm{Ba}{\mathrm{Cl}}_2$ ablation products. Compared to Fig. 2, slightly larger assays are used for the LAMS (of the order of few thousand ions) and ${}^{138}{\mathrm{Ba}}^{+}$ is laser cooled. The LAMS curve represents the average of 20 spectra. The labels denote the mass of the ions of the corresponding peak and are determined by a fit of a Gaussian curve to the LAMS spectrum. For comparison, a traditional TOFMS curve, which is an average of 20 spectra taken without laser cooling, and a simulation of a LAMS spectrum (see the text) are shown.

Figure 4

Mass resolution of an ideal Wiley-McLaren TOF [8] as a function of the width of the velocity ($\mathrm{\Delta }v$) and spatial distribution ($\mathrm{\Delta }x$) of the input assay. The simulation assumes an input assay consisting of 1000 ${}^{174}{\mathrm{Yb}}^{+}$ ions and LQT parameters as used in Fig. 2. The ion-ion Coulomb repulsion prohibits further squeezing of the assay’s spatial distribution at increasingly lower temperatures (Coulomb-forbidden region). Points correspond to distributions belonging to different cooling methods.