Whose research first explained isotopes as atoms of the same element with different masses?

Francis Aston built and refined the mass spectrograph in the early 20th century. Using it, he demonstrated that atoms of the same element can have different masses, which are now called isotopes. This was a major breakthrough because it explained why atomic weights are often not whole numbers. Aston’s work helped connect chemistry with nuclear physics and gave scientists a more precise way to measure elements. He won the 1922 Nobel Prize in Chemistry for this research.