17.1 Analysis of Isotopes using Mass Spectrometry

This section explains how a mass spectrum can be used to determine the number of isotopes for a given element and their relative abundances.

Interpreting a Mass Spectrum

A mass spectrometer separates ions based on their mass-to-charge ratio ($m/z$). The resulting graph, a mass spectrum, provides two crucial pieces of information about the isotopic composition of an element.

1. Number of Isotopes

The number of distinct peaks that appear on the mass spectrum directly corresponds to the number of isotopes present in the sample. Each peak represents a different isotope with a unique mass.

Example:

• Chlorine (Cl): The mass spectrum of chlorine shows two peaks, indicating that there are two naturally occurring isotopes of chlorine (${}^{35}\mathrm{Cl}$ and ${}^{37}\mathrm{Cl}$).
• Hydrogen (H): The mass spectrum of hydrogen shows three peaks, corresponding to its three isotopes: protium (${}^1\mathrm{H}$), deuterium (${}^2\mathrm{H}$), and tritium (${}^3\mathrm{H}$).

2. Relative Abundance of Isotopes

The height (or intensity) of each peak is proportional to the relative abundance of that specific isotope. A taller peak signifies a more abundant isotope, while a shorter peak indicates a less abundant one. This information is often expressed as a percentage.

Example:

• For chlorine, the peak for ${}^{35}\mathrm{Cl}$ is significantly taller than the peak for ${}^{37}\mathrm{Cl}$, indicating that ${}^{35}\mathrm{Cl}$ is the more abundant isotope (approximately 75.77%) compared to ${}^{37}\mathrm{Cl}$ (approximately 24.23%).

The x-axis of the spectrum represents the mass-to-charge ratio ($m/z$), and the y-axis represents the relative intensity or abundance.

Basis of Separation in Mass Spectrometer

The fundamental basis of separation in a mass spectrometer is the mass-to-charge ratio ($m/z$) of the ions. Ions are separated as they pass through magnetic and electric fields based on this ratio.

The process involves several stages:

1. Vaporization: The sample is converted into gas.
2. Ionization: High-energy electrons knock off electrons from the atoms to form positive ions.
3. Acceleration: Ions are accelerated through an electric field.
4. Deflection: Ions are deflected by a magnetic field based on their $m/z$.
5. Detection: The detector records the ions hitting it.

For more advanced analytical techniques, you can also explore IR Spectroscopy→.