April 6, 2001

Mass Spectrometry

Ion Sources

Several Ways to Catalog Sources

• how is the sample introduced
  • as a gas / vapor
  • as a solid and somehow desorbed
• then how is the sample ionized
  • simple electron impact
  • indirect ionization (collide with another atom)
  • field ionization

What would you like a source to do?

• Ionize efficiently
• avoid fragmentation? (find parent ions)
• create characteristic fragmentation? (unique pattern)
  • produce a reproducible result
  • reliable fragmentation pattern
• libraries
• usable on other instruments

• produce reproducible ionization efficiency
  • for quantitatively relating ion current to sample concentration

When sample is gas

• Can include solids and liquids with VP of 5 torr at 300^oC
• Introduce sample
  • Vapor enters source through a small pinhole (leak)
  • Overall pressure is perhaps 10-3 to 10-4 torr in the source
  • Need to continuously pump the source

Typically electron impact

• electrons from either hot filament or heated oxide coated metal
• electrons accelerated (50-90 volts)
  • collisions with molecules produce ions
• Ion could be simply M+ (ignoring M-)
  • Excess energy in molecule
  • It can fragment into smaller units
  • It can rearrange (isomerize)
    • If this dissociates, get ion fragments not in the original species
  • can even get M+ + M collisions.... (at higher pressure)
• Drawbacks
  • often too much fragmentation, esp. with large molecules
  • often ionization efficiency is very low
• Advantages:
  • cheapest and simplest source
  • basis of most MS libraries
  • fragmentation pattern fixed by voltage,
    • much less dependent on details of the source (portable data)

Alternative-- add another species

Chemical Ionization Sources

• species selected as one that produces ions efficiently
  • CH₄ produces CH₄+, CH₃+, CH₂+ in good yield
• rely on [ion+] + M ---> M+ [neutralized ion]
  • can often work at lower energies
  • much less fragmentation
  • much higher efficiencies

Field Ionization

• much less commonly used
• high voltage gradients can rip off electrons (ionize)
  • fine needles at 4000 V
  • field is often 4000V / 5 A -- a very strong field
  • text shows fuzzy needle (very high voltage gradients)
    • gas molecule gets ionized
    • adsorbed molecules can be ionized and desorbed
• Field Ion Microscopy is oldest technique with atomic resolution (1960's)

GC-MS

• probably the most important use of MS these days
  • (at least by number of instruments, frequency of use)
• sample arrives as a gas
  • but diluted in flow of He
  • pressure typically too high for source operation
  • but conc. is too low just to restrict the flow
• molecular beam method
  • let gas (He + sample) flow through jet into vacuum
  • light molecules have higher random velocity
  • all molecules have same lateral velocity
  • forms a defined beam (not diffusion)
    • beam is a cone of fat molecules, surrounded by He
    • skim out the center of the beam, discard the rest

Solid samples that don't vaporize easily

• desorb techniques
• MALDI
  • mix sample in solid matrix
  • vaporize matrix (IR laser pulses usually)
    • molecule now finds itself free in space (a gas like species)
    • often ionized in the process
    • no need for the molecule to be volatile
    • relatively little impact or fragmentation
  • with large molecules, Tome of Flight MS works best