Mass Spectra of Elements

Atoms of the same element can have different numbers of neutrons, so they have different masses even though they behave the same chemically. A mass spectrometer tells these isotopes apart. It first vaporizes and ionizes the sample, knocking off electrons to make positive ions. These ions are accelerated through a magnetic field, which bends their paths. Lighter ions bend more, heavier ions bend less, so the beam splits apart by mass. A detector counts how many ions land at each position and turns that into a spectrum: a row of peaks, one per isotope, plotted against mass-to-charge ratio, m/z. Since almost every ion formed carries a +1 charge, m/z usually just equals the isotope's mass number.

The height of each peak is not about mass at all — it reports how many ions of that isotope showed up, which means it reports natural abundance. Chlorine has two isotopes, chlorine-35 and chlorine-37, roughly in a 3:1 ratio. The spectrum shows a tall peak at m/z = 35 and a shorter peak at m/z = 37, heights in that same 3:1 ratio. This is why the tallest peak sits at the most common isotope's mass, not at the element's average atomic mass — that average, like chlorine's 35.45, is a weighted blend of both peaks and usually matches no real atom at all.

This topic sets up the next calculation directly: once you can read off each isotope's mass from its peak position and its abundance from its peak height, you can multiply and add to calculate average atomic mass yourself. When two elements are mixed in one sample, their peaks simply appear together on the same spectrum, sorted purely by m/z regardless of which element they came from.