Experimental and numerical investigations of under-expanded gas flows for optimal operation of a novel multipole differential ion mobility filter in the first vacuum-stage of a mass spectrometer

Summarization: The integration process of a higher-order multipole differential mobility spectrometer in the first vacuum stage of a mass spectrometer is described and separation of ions under the influence of a pair of antiphase asymmetric rectangular RF waveforms is demonstrated experimentally. The superposition of quadrupole RF and DC components to the main dipole field is investigated and directional focusing of ions controlled via adjustments in the electrical potentials applied to the pole-electrodes is accomplished. The requirement for laminar flow to minimize perturbations on the oscillatory motion of the ions inside the separation field of the differential mobility spectrometer is obtained by incorporating a carefully dimensioned duct to confine the free jet expansion and suppress turbulence typically observed in the far-field region of the expanding flow. The transitions of the flow and relaminarization effects established within the duct are visualized by particle tracking velocimetry. Monodisperse and polydisperse nanoparticles produced by spark discharge are employed as tracers to visualize the gas dynamic effects at low pressure. Particle tracking velocimetry is extended to free jets and experimental data are contrasted to numerical solutions of the flow to understand time relaxation effects of particles and evaluate the applicability of numerical methods at low pressure.Presented on: International Journal of Mass Spectrometr