ION FOCUSING AT ATMOSPHERIC PRESSURE USING A VOLTAGE GRADIENT (iEdison No. 0685901-24-0097)
This invention was developed to enhance atmospheric pressure ion detection through ion focusing and reducing ion loss. This invention is demonstrated with the atmospheric flow tube (AFT) coupled to either a mass spectrometer (MS) or a Faraday plate detector, and involves ion manipulation (e.g., extraction, focusing, and confinement) at atmospheric pressure. The invention includes an atmospheric flow tube at a uniform electric field through which ions are passed using a controlled flow of air. The end of the tube is located near a conducting plate placed perpendicular to the flow tube. Placing an electric field differential between the flow tube and the plate creates electric field lines that extend into the tube and focus the ions towards a spot on the conducting plate. The focal point of the spot is determined by the axial center of the flow tube. A voltage difference between the AFT and the plate causes ion focusing. A larger voltage difference results in greater focusing. This focusing was demonstrated using both a mass spectrometer and a Faraday plate detector. For the mass spectrometer, a hole was placed in the conducting plate that was aligned with the inlet to the mass spectrometer. The AFT was mounted on an adjustable height table with the center of the AFT aligning with the inlet. As the voltage between the AFT and the conducting plate increased, the signal measured by MS also increased. In a different experiment, while the ion signal was monitored by the MS, the table was raised and lowered sweeping the focused ion beam across the inlet, which allowed the ion beam profile to be measured. The width of the beam decreased as the voltage difference increased. A narrower beam width demonstrates increased ion focusing. In a different experiment, a Faraday plate was mounted at the end of the AFT. Using a Faraday plate with a diameter approximately equal to that of the AFT, similar ion currents were measured at increasing voltages between the tube and the plate. Smaller faraday plates showed a much smaller ion current with no focusing, however as the voltage increased between the AFT and the Faraday plate, the signal rapidly increased, approaching the same signal observed with the larger plate. This suggests that at higher voltages, the ion beam can be focused nearly completely onto the smaller plate. Data are provided in the attached PowerPoint file.