Mass spectrometry is an analytical technique used to identify substances by measuring the mass-to-charge ratios of their ions. It matters because tiny samples can reveal molecular masses, isotope patterns, and structural clues. Chemists use it in medicine, environmental testing, forensics, materials science, and research.
A mass spectrum acts like a fingerprint that helps distinguish one compound from another.
In a mass spectrometer, molecules are first vaporized and ionized so they can be accelerated and steered by electric or magnetic fields. Ions separate because their paths depend on m/z, where m is mass and z is charge. The detector counts ions at each m/z value and produces peaks on a spectrum.
The molecular ion peak gives the molar mass of the original molecule, while fragment peaks give evidence about how the molecule broke apart.
Key Facts
- Mass spectrometry measures mass-to-charge ratio, written as m/z.
- For a singly charged ion, z = 1, so m/z is numerically equal to the ion mass in atomic mass units.
- The molecular ion peak, M+, corresponds to the intact molecule after losing an electron.
- Fragmentation forms smaller ions that help identify structural parts of the molecule.
- Relative abundance = peak height compared with the base peak, often scaled so the base peak is 100%.
- In a magnetic sector instrument, lighter ions or ions with higher charge are deflected more than heavier ions with the same charge.
Vocabulary
- Mass-to-charge ratio
- The quantity m/z that compares an ion's mass to its charge and determines how it moves through the analyzer.
- Molecular ion
- The ion formed when a molecule loses an electron but remains otherwise intact, often shown as M+.
- Base peak
- The tallest peak in a mass spectrum, assigned a relative abundance of 100%.
- Fragment ion
- A smaller charged piece produced when a molecular ion breaks apart inside the mass spectrometer.
- Detector
- The part of the instrument that records arriving ions and converts their impacts into an electrical signal.
Common Mistakes to Avoid
- Confusing mass with m/z is wrong because the instrument measures mass divided by charge, not mass alone. For ions with charges greater than 1, the m/z value is smaller than the actual mass.
- Assuming the tallest peak is always the molecular ion is wrong because the base peak is simply the most abundant ion. The molecular ion may be small, missing, or not the tallest peak.
- Ignoring isotope peaks is wrong because elements such as chlorine, bromine, carbon, and sulfur create predictable peak patterns. These patterns can provide strong evidence for a molecular formula.
- Reading only one peak to identify a compound is wrong because many compounds can share a similar molecular mass. A reliable identification uses the molecular ion, fragment pattern, isotope pattern, and sometimes a library match.
Practice Questions
- 1 A singly charged ion has an m/z value of 72. What is the mass of the ion in atomic mass units?
- 2 A mass spectrum has a base peak height of 80 units and another peak height of 20 units. What is the relative abundance of the second peak?
- 3 A compound shows molecular ion peaks at m/z 78 and 80 with nearly equal heights. Explain what this pattern suggests about the element present and why.