19.2.2 Mass Spectrometry

• Mass spectrometry determines the mass-to-charge ratio of ions
• How does mass spectrometry identify the molecular mass of a compound?

  Molecular ion peak
• Fragment ions in mass spectrometry help determine structural fragments of a compound.
• Match the feature of mass spectra with its description:

  Molecular ion peak ↔️ Represents the intact molecule

  Fragment ions ↔️ Indicate structural subunits

  Mass-to-charge ratio ↔️ Shows mass divided by charge

  Isotope peaks ↔️ Confirm elemental composition
• What is the $m / z$ value of the molecular ion peak in the mass spectrum of methane ($\text{CH}_{4}$)?


  16
• The molecular ion peak in a mass spectrum is usually the highest m / z value in the spectrum.
• Fragment ions appear at $m / z$ values lower than the molecular ion peak.
• What is the primary function of the ionization source in a mass spectrometer?

  Converts sample to ions
• Match the mass spectrometer component with its function:

  Ionization source ↔️ Converts sample to ions

  Mass analyzer ↔️ Separates ions by $m / z$

  Detector ↔️ Records ion intensity

  Vacuum system ↔️ Maintains low pressure
• Electron Ionization (EI) and Electrospray Ionization (ESI) are two common ionization methods in mass spectrometry.
• What type of particles are used in Electron Ionization (EI) to bombard the sample molecules?

  High-energy electrons
• Electrospray Ionization (ESI) is ideal for large, polar compounds.
• Match the ionization method with its characteristic:

  Electron Ionization (EI) ↔️ Extensive fragmentation

  Electrospray Ionization (ESI) ↔️ Minimal fragmentation

  EI ↔️ Positive ions only

  ESI ↔️ Positive or negative ions
• What type of fragmentation involves cleavage adjacent to a functional group?

  Alpha cleavage
• Homolytic cleavage results in two radicals.
• What occurs during fragmentation in mass spectrometry?

  Molecules break down into smaller ions
• Alpha cleavage in mass spectrometry involves cleavage adjacent to a functional group
• Homolytic cleavage in mass spectrometry results in two radicals.
• What is the Retro-Diels-Alder reaction in fragmentation?

  Reversion of a Diels-Alder product
• The McLafferty rearrangement involves the transfer of a hydrogen atom followed by cleavage
• Match the functional group with its fragment and mass loss in mass spectrometry:

  Alcohols (R-OH) ↔️ Loss of H2O, 18

  Ketones (R-CO-R') ↔️ Loss of CO, 28

  Carboxylic Acids (R-COOH) ↔️ Loss of OH, 17
• What does mass spectrometry measure?

  Mass-to-charge ratio
• The molecular ion peak in a mass spectrum represents the intact molecule.
• Fragment ions in a mass spectrum are found at lower m / z
• What information do fragment ions provide in a mass spectrum?

  Structural subunits
• What are the five main components of a mass spectrometer?

  Ionization source, mass analyzer, detector, vacuum system, data system
• Match the component of a mass spectrometer with its function:

  Ionization source ↔️ Converts sample to ions

  Mass analyzer ↔️ Separates ions by m/z

  Vacuum system ↔️ Maintains low pressure

  Detector ↔️ Records ion intensity
• Electron Ionization (EI) is a hard ionization method that leads to extensive fragmentation.
• What is the ionization process in Electron Ionization (EI)?

  $\text{M} +$$e^ - \rightarrow \text{M}^{ + .} +$$2e^ -$
• Electrospray Ionization (ESI) is a soft ionization method that preserves the molecular ion
• Steps to interpret mass spectra:

  1️⃣ Identify the molecular ion peak

  2️⃣ Analyze fragment ions

  3️⃣ Use common fragment losses