1.2 Mass Spectroscopy of Elements

How a mass spectrometer works

A mass spectrometer vaporizes a sample, then ionizes it into positive ions. These ions are accelerated by an electric field and passed through a magnetic field, which deflects them based on their mass-to-charge ratio (m/z). Lighter ions deflect more; heavier ions deflect less. A detector records where each ion lands, producing a spectrum.

Reading the spectrum

Each peak on the spectrum represents an isotope, with its position showing mass and its height/intensity showing relative abundance. Since isotopes of an element have the same number of protons but different numbers of neutrons, they have different masses but land at different peaks with different heights.

Calculating average atomic mass

The average atomic mass reported on the periodic table is a weighted average of all naturally occurring isotopes. Multiply each isotope's mass by its fractional abundance (percent ÷ 100), then sum the results: Average mass = Σ(isotope mass × fractional abundance).

Key terms

Mass spectrometer

An instrument that ionizes, accelerates, and deflects particles using a magnetic field to determine mass-to-charge ratios.

Mass-to-charge ratio (m/z)

The value that determines how much an ion deflects in a magnetic field; used to identify isotope mass.

Peak position vs. peak height

Position shows the isotope's mass; height shows its relative abundance in the sample.

Average atomic mass formula

Σ(isotope mass × fractional abundance) — a weighted average of all naturally occurring isotopes.

Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons, giving different masses.

Try an AP-style question

What is the average atomic mass?

1. About 36.5 amu
2. 35.0 amu
3. 36.0 amu
4. About 35.5 amu