Advanced Topic 911 of 1102 Analytical, Lab & Spectroscopy
Molecular Ion Peak
The peak in a mass spectrum corresponding to the intact ionized molecule.
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In-depth explanation of Molecular Ion Peak
The molecular ion peak corresponds to the intact molecule that has lost one electron, M+, and it normally appears at the highest m/z value in the spectrum, apart from isotope peaks. Its position gives the molar mass, the most direct experimental determination of molecular weight. By the nitrogen rule, a compound with an odd nominal molar mass contains an odd number of nitrogen atoms, so the parity of the molecular ion helps narrow the possible structures. Chlorine and bromine produce characteristic M+2 peaks, with a 3:1 ratio for chlorine-35 and chlorine-37 and a 1:1 ratio for bromine-79 and bromine-81. Weak or absent molecular ions occur in compounds that fragment very readily.
Going deeper: Molecular Ion Peak
Key ideas: Check for isotope clues: M and M+2 in 3 to 1 means one Cl, 1 to 1 means one Br, while M+2 of 4.4 percent suggests one sulfur. The nitrogen rule says an odd molecular mass means an odd number of N atoms. Common losses from M+: 15 (CH3), 17 (OH), 18 (H2O), 28 (CO or C2H4), 29 (CHO or C2H5), 31 (OCH3). Use high resolution to obtain exact formulas.
Common mistakes and connections: Students assume the highest peak must be the molecular ion, but impurities or isotope peaks may sit above it, and some molecules (tertiary alcohols, highly branched alkanes) show almost no M+. The molecular ion has an odd electron count, unlike most fragments. Using an average molar mass instead of monoisotopic mass shifts the result. Links to isotopes and fragmentation.
Visualization of Molecular Ion Peak
Bromoethane shows molecular ion peaks at m/z 108 and 110 in a near 1:1 ratio, the signature of one bromine atom (79Br and 81Br). The interactive lesson shows this as a live animation or chart.
Worked example and use case
Bromoethane shows two peaks of nearly equal height at m/z 108 and 110, indicating one bromine atom.
Frequently asked questions about Molecular Ion Peak
What is Molecular Ion Peak?
The peak in a mass spectrum corresponding to the intact ionized molecule.
Is Molecular Ion Peak a beginner, intermediate or advanced topic?
Molecular Ion Peak is a advanced topic in Analytical, Lab & Spectroscopy. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Molecular Ion Peak?
Study these earlier topics in Analytical, Lab & Spectroscopy first: HPLC, Retention Time, Mass Spectrometry Basics.
What comes after Molecular Ion Peak?
The next topics in Analytical, Lab & Spectroscopy are Fragmentation Patterns and UV-Visible Spectroscopy.
Can I practise Molecular Ion Peak with quizzes?
Yes. The interactive lesson includes a self-check question on Molecular Ion Peak, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Where Molecular Ion Peak fits in Analytical, Lab & Spectroscopy
Molecular Ion Peak is topic 35 of 99 in Analytical, Lab & Spectroscopy.