March 30, 2001

Mass Spectrometry

• Goal is to separate molecules by mass
• generally to identify a species
  • fragmentation pattern provides useful hints
  • parent ion, if present, tells molecular weight
  • at high resolution (+0.01 amu) there's more information
• a major use is as a detector for GC and LC
  • MS pattern can uniquely identify each peak
  • works nicely with automated data systems
• as a rule, MS needs a pure sample
• Also find instruments called RGA
  • residual gas analyzers
  • limited to gases (N₂, O_2, NO, CO, CO₂, H₂O, low hydrocarbons)
  • perhaps to mass 150
• specialized mass spectrometers
  • helium leak detector
  • only looks for Helium
• How to create the ions?
  • First, convert to isolated ions
  • typically sample is a gas/vapor
    • might need vapor pressure of 1-5 torr
  • sample ionized by electron impact
    • at 40-90 eV, can ionize and fragment the species
  • ions are accelerated and focused by electrical fields
  • most work is done with + ions (all we will consider here)
• Vacuum Conditions
  • Ion must travel a path of 2-200 cm
  • any collisions would alter the path in unpredictable manner
  • want low pressure (10^-5 - 10^-7 torr) to reduce collisions
  • Detecting the ions
    • simplest is just a sheet of metal
    • charged negatively
    • collects + ions
    • measure the electrical current
    • really very tiny currents
    • in some simpler devices (RGA, He) that's OK
  • Most use Electron Multipliers
    • act like photomultiplier
    • except first stage accelerates ions to produce electron
    • dynodes multiply electron current
  • Heart of a Mass Spectrometer is separation of ions
  • by their mass
  • (usually really by mass/charge ratio)
• Several Major Schemes
• Magnetic Sectors
  • charged particle moving in magnetic field
  • experiences force perpendicular to motion
  • will follow curve (circular) path
    • radius is fixed by mass
    • affected by (charge, magnetic field strength, ion velocity)
  • magnetic region usually smaller than the circular path
  • one style is 60o (one sixth of a circle)
  • oldest forms (Aston 1910 ?)
  • detect on film or a phosphor screen
  • image shows all masses at different positions
  • (Analogy: spectrograph or diode array spectrometer)
  • more commonly, fixed exit slit
  • reject all ions except one mass
  • scan to look at all the masses
  • (analogy, scanning spectrometer)
• Time of Flight mass Spectrometers
  • create ions in a very short pulse
  • accelerate the ions down a tube (drift tube)
  • determine when ions arrive
    • force on all +1 ions same (e x Voltage)
    • kinetic energy of ions the same
    • velocity proportional to 1/ m^1/2
    • see H+, H2+, He+, N+, O+, N2+, O2+ ...then heavier ions
• Quadrupole Mass Spectrometer
  • ions sent into an analyzer region
  • analyzer has four parallel metal rods
    • typically like four half length wood pencils
    • space down the center a little larger than a similar pencil
  • apply a mixed DC and AC electrical field to the rods
    • ion in the field accelerated up/down/right/left by AC field
  • most ions follow paths of increasing amplitude
    • ions are lost, never make it through the tube
    • one narrow range of masses form stable trajectories
    • ion gets through and gets detected
  • we vary the frequency (RF) or voltages to select the mass
    • can sweep mass range in under a second or over a few seconds
    • useful in GC-MS (peak may last 10 sec, need a fast MS)
• Quadrapole Mass Trap
  • 3D analog of Quarupole Mass Filter
  • ions get trapped inside
    • only one mass stays (others hit walls, lost)
    • can store for minutes if desired
      • like CCD light detector, accumulates signals
    • quick voltage change and ions all hit the wall
    • measure that current
    • can do this rapidly (millisec.) so good GC detector
    • detector is smaller, simpler, less costly than Quad Rod Assembly
    • can accumulate to detect weaker samples
• Fourier Transform Mass Spectrometer
  • older name Ion Cyclotron Trap
  • traps all ions in circular paths
  • quickly change parameters
  • ions travel to detector
  • detect which ions arrive when (similar to TOF)
  • complex signal reversed by Fourier Transform
  • not widely used except in research applications
• Comparisons
  • Magnetic Sector
    • Traditional (1930's to 1970's)
    • Typically an electromagnet
    • Magnet might weigh 1 ton, but 10-20" across
    • need current of 100 amps, cooling water
    • could go 10-4000 amu, + 0.01 amu perhaps
    • scan time varies with resolution
    • some sweep magnetic field, others vary the accelrating voltage
    • typically scan a full spectrum in 1-5 minutes
    • Miniature versions exist
    • lower mass range, lower resolution
    • smaller, lighter
  • Quadrupole (1975 and beyond)
    • smaller and lighter
    • needed to wait on low cost RF electronics
    • cheaper once the electronics evolved
    • initially low mass range, modest resolution
    • gradually replaced most magnetic instruments
  • Quad Ion Trap (1985 and beyond)
    • analyzer more compact, cheaper than quad
    • poorer resolution
    • gradually taking over on GC-MS and LC-MS