CHEM220-Section08

Mass Spectrometry Overview

  • An analytical technique for identifying unknown compounds, quantifying known compounds, and determining molecular structure and chemical properties.

  • Important in life sciences, including protein analysis and drug discovery.

Mass Spectrometer Components

  • Definition: Instrument measuring individual molecules' masses as ions.

  • Unit of Measure: Dalton (Da) is used instead of kilograms or grams for molecular masses.

  • Operation Steps:

    • Ionization: Conversion of gaseous molecules into ions.

    • Separation: Ions are separated based on their mass-to-charge ratio (m/z).

    • Detection: Measurement of ions and generating a mass spectrum.

Mass Spectrum

  • Molecular Ion Peak: Ion with the same elemental composition as the original molecule (denoted as M+ or M+•).

  • Base Peak: The peak with the highest abundance, assigned a value of 100.

Operating Conditions

  • Vacuum Requirement: Mass spectrometers must operate in high vacuum (10^-5 to 10^-6 torr) to prevent collisions with neutral air molecules.

Ionization Methods

Electron Ionization (EI)

  • Description: Most common method; uses a beam of high-energy electrons to ionize gaseous molecules.

  • Results in fragmentation, leading to various ions including the original molecular ion.

Chemical Ionization (CI)

  • Description: Soft ionization technique; uses a reagent gas that reacts with an electron-ionized gas to produce fewer fragments.

  • Results in ions like protonated molecules, not necessarily the molecular ion.

Electrospray Ionization (ESI)

  • Description: Charges droplets of the sample solution as it exits a capillary.

  • Used primarily for polar compounds and large biological molecules; little fragmentation occurs.

Matrix-Assisted Laser Desorption Ionization (MALDI)

  • Description: Uses a laser to ionize large biological molecules dispersed in a solid matrix; effective for proteins and DNA fragments.

Alternative Ionization Techniques

  • Fast-atom bombardment (FAB)

  • Secondary ion mass spectrometry (SIMS)

  • Thermal ionization (TIMS)

  • Plasma and glow discharge methods.

Mass Analyzers

  • Purpose: Separate ions based on mass-to-charge ratio.

Types of Mass Analyzers

  1. Magnetic Sector: Uses magnetic fields to bend ion paths and separate based on m/z.

  2. Quadrupole: Uses a combination of DC and alternating voltages to filter ions by mass.

  3. Time-of-Flight (TOF): Measures ions' drift times based on their velocities (lighter ions move faster).

  4. Fourier Transform Ion Cyclotron Resonance (FT-ICR): Separates ions by their motion in electric and magnetic fields, using resonance for detection.

Ion Detectors

Common Detectors

  • Electron Multiplier: Amplifies ion signals using secondary electron emissions.

  • Faraday Cup: Measure ion-induced currents but has lower sensitivity.

  • Photomultiplier: Detects photons from secondary emissions, operates in a vacuum for contamination prevention.

  • Micro-Channel Plate: Amplifies signals through a cascade of electrons in tiny channels.

Tandem Mass Spectrometry

  • Description: Two mass spectrometers connected in series with a collision cell for ion fragmentation.

  • Used for structural analysis and sequencing based on fragment patterns.