Based on numerical simulation of the air jet in the first stage of the differential pumping system of the interface between the ionization source at atmospheric pressure to a mass spectrometer, it is shown that at a background gas pressure larger than 600 Pa, an efficient retarding of the supersonic jet can be achieved at the passage of the gas through a tube with an optimal diameter depending both on the gas pressure and the nozzle diameter of the first differential pumping system stage. For nozzle diameters between 0.5 and 1 mm, at a distance of 60 mm downstream from the nozzle, the gas flow is slowed to speeds of 20–50 m/s, which is 5–6 times slower than the speed of a freely propagating jet. Ion transmission through an exit aperture with a diameter of 1 mm at this speed achieves 60–80% and has a maximum around 1300 Pa. The proposed method of gas jet retardation provides for high ion transmission through the first pumping stage with an increased diameter of the nozzle separating the atmospheric pressure region from this pumping stage. This allows increasing the sensitivity of a mass spectrometer using ionization at atmospheric pressure.
Pomozov et al. (Mon,) studied this question.