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How did J.J. Thomson’s experiments lead to the first mass spectrometer?

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J.J. Thomson’s development of the first mass spectrometer grew out of his research into positively charged rays, building upon earlier work by Wilhelm Wien. Between 1907 and 1913, Thomson refined apparatus to analyze these positive ions by passing them through combined electrostatic and magnetic fields.12

By situating these fields to deflect ions in two perpendicular directions, Thomson caused the particles to strike a photographic plate in a series of parabolic curves. Each parabola corresponded to a specific mass-to-charge ratio, allowing him to separate and identify different ion species. When he applied this method to neon gas, he observed two distinct parabolas, providing the first evidence for the existence of stable isotopes—atoms of the same element with different masses.213

To further estimate the relative abundances of these ions, Thomson replaced the photographic plate with a metal sheet featuring a parabolic slit, allowing him to scan through a mass spectrum by varying the magnetic field. This construction of the parabola spectrograph is credited as the first mass spectrometer, a foundational tool for modern mass spectrometry.2