April 20

GC- Hardware

Injectors-- gas

• simplest is injecting a gas
  • gas syringe, septum, needle
  • head space samples (vapor over a liquid)
• trap gases on adsorbant
  • heat and desorb into the column
    • likely to be 1-2 minutes (too broad)
• cyofocusing
  • keep column (column head) cold
    • all sample is trapped at head of column; no chromatography
    • then, heat to release
      • can be -78 C accessory; quick reheat
      • can often just be oven at 25^oC; temperature program
• specialized units (natural gas) might use valves

Injector-- liquid

• microsyringe (1, 5, 10, 20 microliter sizes)
• quickly inject into hot area
• injector liner (glass sleeve)
• on column injection (if column itself is hot)
• vaporize in <1 sec
• keep injector hot (varies with sample)
• packed column can handle a milligram of sample
• 10 microliters (10 mg) at 1% is a mg of sample
• more likely 1-5 microliters, 0.01% or a few mg
• of course, minimum is fixed by detector response
• capillary column can handle much less (microgram)
• include a problem with lots of solvent too (can't simply dilute)
• need sample volumes of 0.01-0.1 microliters
• no such syringes
• split injectors
• sample vaporizes
• majority of sample escapes
• perhaps 1-5% enters column
• (may also involve timed valves to control sample going to column)

Detectors

TCD

• Standard, traditional-- thermal conductivity
  • nearly universal, responds to all compounds
  • nearly identical response, per mole, for all species
  • not terribly sensitive
  • rugged, long lived, relatively inexpensive, one gas (He)

FID or Flame Ionization Detector

• C- compounds burn in flame (air, H₂ added)
  • CO₂ forms with small fraction as ions
  • flame is them electrically conductive
  • measure (tiny) electrical currents
• response varies with compound, # carbons
  • non-C species often no response
• can detect much tinier amounts than TCD
  • also a relatively fast response
  • need three gas cylinders
• variations (heated ceramic element) are selective for N, P

ECD Electron Capture

• radioactive Beta source
• sample steals electrons, reduces current detected
• very selective
  • very halogen sensitive
  • often 100-1000 fold less sample than FID
  • great with many pesticides, environmental problems
  • ignores many other common species

GC/MS Definitive Detector

• full mass spectrum for each peak