Mass Spectrometry

Mass Spectrometry Overview

• Definition: Mass spectrometry is a technique used to determine the molar mass and chemical formula of compounds.

Process of Mass Spectrometry

• Step 1: Vaporization of the compound

• Step 2: Ionization of the vaporized compound

  • Most common ionization method: Electron Impact (EI)

    • Involves bombardment with high energy electrons (1600 kcal or 70 eV)

    • Causes ejection of an electron from the molecule

Molecular Ion and Fragmentation

• Molecular Ion (M+•): The radical cation formed when no fragmentation occurs initially

• Fragmentation:

  • Often, the molecular ion will fragment, producing smaller ions

  • These fragments can also undergo further fragmentation

• Magnetic Field Deflection:

  • Ions are deflected in a magnetic field; lighter and more charged ions are deflected more

Mass Spectrometer Components

• Ionization Region: Where molecules are ionized

• Acceleration Region: Ions are accelerated towards the magnetic field

• Analyzer Tube: Where ions are separated based on mass-to-charge ratios (m/z)

• Detector Assembly: Detects ions and provides data output

• Computer Data Station: Receives and analyzes spectral data

Analyzing Peaks in Mass Spectrometry

• M+• Peak:

  • Example: In benzene, the M+• peak is considered the base peak

  • This peak does not fragment easily

• Pentane Example:

  • The M+• peak is not the base peak due to easy fragmentation.

Isotopic Patterns in Mass Spectrometry

• (M+1)+• Peak: Represents isotopes differing by neutrons

  • Example: Carbon isotopes

    • 12C: 98.9% abundance

    • 13C: 1.1% abundance

• (M+2)+• Peak:

  • Chlorine isotopes: 35Cl (76%) and 37Cl (24%)

  • Often results in distinctive peak ratios in mass spectra (3:1 for molecules with chlorine)

  • Bromine isotopes: 79Br (51%) and 81Br (49%)

  • Also produces significant (M+•) and (M+2)+• peaks (1:1 ratio)

Fragmentation Analysis

• Peaks corresponding to different molecular fragments can be visualized:

  • The mass-to-charge ratio (m/z) provides information on the fragments.

  • Example: Various fragment peaks from pentane, including losses at m/z 57, 43, and 29.

Summary of Key Concepts

• Mass spectrometry is vital for molecular analysis in organic chemistry. Understanding molecular ions, fragmentation processes, and isotope patterns is crucial for interpreting spectral data.