In a mass spectrometry experiment, an organic compound is ionized and fragmented. The peak observed at the highest mass-to-charge ratio (m/z) in the spectrum, excluding isotope peaks, is typically the most informative for determining the compound's molecular weight. What does this highest m/z peak primarily represent?
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The answer
Correct answer: A. The molecular ion, which is the intact molecule that has lost a single electron during ionization.
Tested concept: Mass spectrometry fragmentation
Explanation
The highest m/z peak in a mass spectrum (excluding isotope peaks) corresponds to the molecular ion (M+•). This ion is formed when the intact molecule loses a single electron during the ionization process, retaining its original molecular weight. This peak is crucial for determining the molecular weight of the compound. The base peak is the most intense peak in the spectrum, representing the most abundant and stable fragment, which is not necessarily the molecular ion. Other peaks represent fragments or, in some ionization methods, adducts, but the highest m/z peak (M+•) specifically represents the parent molecule minus an electron.
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