Mass spectra: distinguish mass-to-charge, isotopes, and fragments

MCAT content library / Chemistry & physics / Mass spectrometry and isotope patterns

Read a mass spectrum as mass-to-charge evidence, then use isotope clusters and fragments without assuming every peak equals a neutral molecular mass.

About 16 minutes, including charge-state and isotope-pattern examples.

The horizontal axis is mass-to-charge

Mass spectrometry detects ions. The horizontal coordinate is m/z: the ion’s mass divided by the magnitude of its charge in elementary-charge units. A singly charged ion with nominal mass 100 appears near m/z 100. If the same species carries a +2 charge, it can appear near m/z 50.

This distinction matters whenever a spectrum contains multiple charge states. A low m/z peak does not automatically mean a low molecular mass.

Concept: charge state can change where the same ion appears

For a simplified ion of mass M and charge z,

m/z ≈ M/z.

If an ion is observed at m/z 60 with z = +2, the corresponding ionic mass is about 120 mass units under the simplified model. Charge information must therefore be interpreted before assigning molecular mass.

Ionization methods differ in how much fragmentation and how many charge states they produce. A passage should identify the relevant ionization behavior if it matters to the inference.

Worked application: read an isotope cluster

An original spectrum shows a singly charged molecular-ion region with peaks at m/z 120 and 122 in an intensity ratio of approximately 3:1. The passage supplies that chlorine’s common isotopes produce an approximate 3:1 M:M+2 pattern when one chlorine atom is present, while one bromine atom produces a pattern closer to 1:1.

The 3:1 pair therefore supports a candidate containing one chlorine atom more strongly than a candidate containing one bromine atom under the supplied model.

The spectrum does not prove the entire structure. It constrains elemental composition. NMR, IR, or fragmentation evidence may still be needed to distinguish constitutional isomers with the same molecular formula.

Pattern: fragments are structural clues, not separate original molecules

During ionization or subsequent gas-phase processes, a molecular ion can fragment. A fragment peak therefore can report a stable charged piece formed from the original molecule. Its m/z can help identify which bond cleavage is compatible with a candidate structure.

For example, if a proposed structure can produce a supplied stable fragment at m/z 43 but another candidate cannot generate that fragment without an implausible rearrangement under the passage’s stated rules, the fragment supports the first candidate.

Do not assume every peak represents a neutral compound that existed in the sample before ionization.

Rule: the most intense peak is not automatically the molecular ion

The tallest peak in a mass spectrum is called the base peak. It is assigned 100% relative intensity, but it may be a fragment rather than the molecular ion. Molecular-ion assignment depends on ionization method, exact mass region, isotope pattern, and other evidence.

Similarly, the highest m/z peak is not automatically the molecular ion if adducts, isotope peaks, or multiple charge states are present. Use the labels and experimental context provided.

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Sources

1. AAMC. Content Category 4E.
2. OpenStax/LibreTexts. Mass Spectrometry and Infrared Spectroscopy.