Laser-cooled fluorescence mass spectrometry using laser-cooled barium ions in a tandem linear ion trap

We report on a mass spectrometry that uses laser-cooled barium ions to optically detect sample ions in a linear radio-frequency-quadrupole ion trap. To obtain a mass spectrum, we observe the intensity modulation of laser-induced fluorescence emitted by the barium ions, as we electrically excite the secular motions of sample ions that are sympathetically cooled by the barium ions. In a fluorescence mass spectrum of sample xenon ions, we identified the major isotopes of xenon. To estimate the absolute amount of the sample ions that contribute to the mass spectrum, we detect the ions using an electron multiplier that is absolutely calibrated by crystallized laser-cooled barium ions. We enhance transfer efficiency to the electron multiplier using a linear radio-frequency-quadrupole ion guide that is placed in tandem with the ion trap. Typically 30 ions of xenon isotope 131Xe+ were detected with a signal-to-noise ratio of ∼30 in the fluorescence mass spectrum.