Gas Chromatography

1
Gas Chromatography Instrument components

• Use a gas thats inert (He, N2, Ar) and one that
  minimizes diffusion
• Use a gas thats compatible with the detector He
  for TC detector
• Control temperature of injector and detector to
  prevent condensation for TC detector
  temperature control must be fairly precise

2

• Gas Chromatography
• Sample introduction introduce sample
  instantaneously as a plug of material
• Use a syringe to inject sample into heated inlet
  into the stream of gaseous mobile phase
• Liquid samples must be rapidly vaporized
• Check the peak shape by raising the temperature
  and reducing sample size in an attempt to
  produce Gaussian peaks
• Columns are fabricated from Al, Cu, glass,
  stainless steel, fused silica
• Cu tubing is unsuitable for amines, acetylenes,
  terpenes and steroids because of adsorption or
  chemical reaction
• Packed columns generally use stainless steel
• Length few inches to several 10s of feet -
  depends on pressure
• Radius 0.020 mm to three-fourths of an inch
• For analytical work 1/8 OD, usually 3-12 in
  length
• Capillary columns 20 mm, 25 mm, 53 mm ID with
  0.1-1 mm liquid film, 10-50 m in length
• 3/8 or 1/2 OD packed columns are used for
  preparative work
• Prep columns generally have poor efficiency

3

• Gas Chromatography
• GC detectors
• The two most common detectors are known as
  differentiating detectors
• Response is proportional to concentration or mass
  flow of eluted component
• Thermal conductivity detectors (TC) respond to
  concentration
• RK1C, where C is the concentration of
  component passing through the detector

Need to know how C varies with time Consider the
case where the component zone of analyte enters
the detector as a plug in which the
concentration is independent of time
4

• Gas Chromatography
• GC detectors
• The Flame Ionization Detector responds to mass
  flow

• TC detector design is based on the principle that
  a hot body loses heat at a rate that depends on
  the composition of the surrounding gas
• This rate of heat loss is used to measure gas
  composition

The heated filament can lose heat by several
mechanisms Proper cell design maximizes heat
loss by thermal conduction when gas molecules
strike the filament and rebound with increased
thermal energy
Differences in thermal conductivity of gases are
based on the speeds with which molecules can
diffuse to and from the hot filament Diffusion
speed is related to molar mass less
massive molecules diffuse faster Also,
H2 and He have highest thermal conductivity
5

• Gas Chromatography
• GC detectors
• TC detector design
• The TC filaments are made from materials whose
  resistance change with temperature is large
• W, W with 3 Rhenium, Au sheathed W
• The typical cell
• Use 175 mA current
• Filament is 100 oC above block temperature
• The filament temperature is determined by the
  equilibrium between the electrical power input -
  I2R - and the thermal loss due to conduction
• With a pure gas passing over the filament, the
  heat loss will be constant and the temperature
  - and resistance - constant
• If the gas composition in contact with the
  filament changes, the temperature will change
  and so will its resistance
• Use a Wheatstone bridge to measure the response
  of the cell to changes in gas composition in
  contact with the detector filament

6

• Gas Chromatography
• GC detectors
• TC detector design Gas Chromatography

When all 4 filaments at same temp, S1,S2,R1 and
R2 at same resistance If gas through S1 and S2
changes, an imbalance in the bridge occurs and
shows up on the recorder

• Improve sensitivity by increasing I and R
• Can shorten the life of filament
• Increase Tfil - Tblock
• But Tblock must be high enough to prevent
  condensation
• Use gas with high kcarrier

7

• Gas Chromatography
• GC detectors
• Ionization detectors

R1 is a variable resistance and is determined
by the number of charge carriers in the gap When
pure carrier gas passes through the gap, a
constant current is generated since a constant
number of charge carriers are within the
gap This current can be brought to zero with
the voltage divider
When components from the column enter the gap and
they become ionized, the current increases as R1
decreases this current can be measured from Eo
and R2 An FID passes column eluent through a
hydrogen diffusion flame where they may be
burned Response is to all substances except He,
Ar, Ne, Ze, O2, N2, CS2, COS, H2S, SO2, NO, N2O,
NO2, NH3, CO, CO2, H2O, SiCl4, SiHCl3, SiF4 Lack
of response to air and water makes this a useful
detector for substances dissolved in air or
water Optimum response for H2carrierAir1110
or flow rates of 3030300 ml/min
8

• Gas Chromatography
• GC detectors
• Ionization detectors Electron Capture Detector
  (ECD)
• Makes use of a radioactive source that produces b
  particles that ionize components in the carrier
  gas
• b N2 N2 e- (slow)
• The slow electrons migrate towards the anode
  under fixed potential and the current is
  measured by the electrometer
• If a column eluent contains an electron absorber,
  the current is reduced
• This detector is very sensitive to alkyl halides,
  conjugated carbonyls, nitrites, and nitrates
• Insensitive to hydrocarbons, alcohols, and ketones

9

• Gas Chromatography
• GC detectors
• Performance characteristics of three GC detectors