In mass spectrometry, what does a molecular ion M+ indicate?

• A. The molecular weight of the compound
• B. The base peak
• C. A fragment containing little information
• D. The isotope pattern

Answer: (A)

The molecular ion peak represents the intact molecule that has lost one electron during ionization, forming a radical cation. Its mass-to-charge ratio corresponds to the molecular weight of the neutral compound, so this peak directly gives you the molecular weight. The base peak is just the most intense fragment, not the whole molecule, and fragmentation products carry less direct information about the original molecule. Isotope patterns come from heavier isotopes and appear as additional peaks near the molecular ion, not the definition of the molecular ion itself. Thus, the molecular ion M+ is a signal of the molecule’s molecular weight.