In an electron ionization (EI) mass spectrum, what does the base peak represent?
AThe molecular ion (M⁺)
BThe most abundant ion in the spectrum
CThe ion with exactly half the molecular mass
DThe ion with the highest mass-to-charge ratio
The base peak is simply the most intense peak in the spectrum — it is defined by abundance, not by identity. A common misconception is equating the base peak with the molecular ion. Extensive EI fragmentation often destroys most of the M⁺, so a fragment ion may be far more abundant and become the base peak, while M⁺ appears as a tiny peak or not at all.
Question 2 True / False
In electrospray ionization (ESI), the m/z value of the most intense peak directly gives the molecular mass of the analyte.
TTrue
FFalse
Answer: False
ESI routinely produces multiply charged ions — a molecule of mass M may appear at m/z ≈ (M + z·1)/z for charge states z = 1, 2, 3, … The observed m/z is lower than M for every z > 1. Charge-state deconvolution (identifying the pattern of peaks from multiple charge states) is required to calculate the true molecular mass.
Question 3 Short Answer
Why might the molecular ion (M⁺) peak be absent or very small in an electron ionization (EI) spectrum, even though the sample was successfully introduced into the instrument?
Think about your answer, then reveal below.
Model answer: EI bombards analyte molecules with high-energy electrons (typically 70 eV), far exceeding the ~10 eV needed to ionize most molecules. The excess energy causes rapid fragmentation of the newly formed M⁺ into smaller fragment ions. For labile molecules, nearly all M⁺ fragments before detection, so fragment peaks dominate the spectrum and M⁺ is negligible.
This question targets the misconception that a missing molecular ion means the experiment failed. The M⁺ was formed but then broke apart. Softer ionization methods (ESI, MALDI) were developed precisely to preserve the intact molecular ion for fragile biomolecules.