A mass spectrometer converts particles into positive ions, separates them according to their mass-to-charge ratio, (m/z), and records their relative abundance. In IB Chemistry HL, the resulting mass spectrum is used to identify isotopes and calculate relative atomic mass.
The mechanism
The sample must usually be vaporized so that separate gaseous particles can enter the instrument. The exact design varies, but the process has four main stages:
| Stage | What happens |
|---|---|
| Ionization | High-energy electrons or another ionization source remove electrons from gaseous particles, producing positive ions. For example, . |
| Acceleration | An electric field accelerates the positive ions, producing a focused ion beam. |
| Separation | Ions are separated according to (m/z). In a magnetic-sector instrument, ions with lower (m/z) are deflected more strongly than ions with higher (m/z). |
| Detection | Ions reaching the detector produce an electrical signal. A larger signal indicates a greater relative abundance of that ion. |
The spectrum plots (m/z) on the horizontal axis and relative abundance on the vertical axis. The base peak is assigned a relative abundance of 100, and all other peak intensities are scaled against it. Peak position and peak height therefore provide different information.
For most elemental isotope questions, ions have a charge of , so (m/z) is numerically equal to the isotope's mass number. However, an ion with charge has half the (m/z) value expected from its mass.
Suppose an element has two peaks: with 75% abundance and with 25% abundance. Its relative atomic mass is the weighted mean:
A common misconception is that each peak represents a neutral isotope. The detector actually records ions, and the horizontal axis shows (m/z), not simply mass.
Exam technique
This is AHL content from Structure 1.2, The nuclear atom. In an IB response, name the four stages, define (m/z), read peak heights as relative abundances, and show the complete weighted-mean calculation when asked to determine .