Gas Chromatography

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					Gas Chromatography
427 PHC
Gas Chromatograph
• Gas chromatography (GC), is a
  common type of chromatography
  used in analytic chemistry for
  separating and analyzing compounds
  that can be vaporized without
  decomposition.
Principle:
• A method of analysis by which the analyte is
  vaporized and introduced into a stream of
  carrier gas.
• It is conducted through a chromatographic
  column and separated into its constituents.
• These fractions pass through the column at
  characteristic rates, and are detected as they
  emerge in a time sequence.
• The detecting cell responses are recorded on a
  chart, from which the components can be
  identified both qualitatively and quantitatively.
• In GC, the moving phase (mobile phase) is a
  carrier gas, usually an inert gas such as helium
  or nitrogen.

• The stationary phase is a microscopic layer of
  liquid or polymer on an inert solid support,
  inside a piece of glass or metal tubing called a
  column.

• The instrument used to perform GC is called a
  gas chromatograph.
• The gaseous compounds being analyzed interact
  with the walls of the column, which is coated
  with different stationary phases.

• This causes each compound to elute at a
  different time, known as the retention time of
  the compound.

• The comparison of retention times is what gives
  GC its analytical usefulness.
• Gas chromatography is in principle similar to
  column chromatography , but has several notable
  differences:
 ▫ The process of separating the compounds in a mixture
   is carried out between a liquid stationary phase and a
   gas moving phase, whereas in column chromatography
   the stationary phase is a solid and the moving phase is
   a liquid.
 ▫ The column through which the gas phase passes is
   located in an oven where the temperature of the gas
   can be controlled, whereas column chromatography
   has no such temperature control.
 ▫ The concentration of a compound in the gas phase is
   solely a function of the vapor pressure of the gas.
Typical uses of GC include:
• Testing the purity of a particular substance.
• Separating the different components of a
  mixture (the relative amounts of such
  components can also be determined).
• In some situations, GC may help in identifying a
  compound.
• In preparative chromatography, GC can be used
  to prepare pure compounds from a mixture.
GC Instrument
Schematic Diagram of Gas Chromatograph
Carrier gas:
• The carrier gas must be chemically inert.
  Commonly used gases include nitrogen,
  helium, argon, and carbon dioxide.
• The choice of carrier gas is often
  dependant upon the type of detector which
  is used.
• The carrier gas system also associated
  with pressure regulators and flow meters.
• In addition, it contains a molecular sieve
  to remove water and other impurities.
Flow control:
• Flow rates are controlled by a 2 stage pressure
  regulator:
  ▫ At the gas cylinder.
  ▫ Mounted in the chromatograph.

• Inlet pressure 10-50 psi →
F = 25-150 ml/min with packed column
F = 1-25 ml/min with capillary column

The flow rate will be constant if the inlet pressure
 remains constant.
Sample injection port:
• The injector is a piece of hardware attached to
  the column head.
• It provides the means to introduce a sample into
  a continuous flow of carrier gas.
• For optimum column efficiency, the sample
  should not be too large, and should be
  introduced onto the column as vapor.
• Slow injection of large samples causes band
  broadening and loss of resolution.
Common injector types are:
• Microflash vaporizer direct injector:
 ▫ It involves the use of a microsyringe to inject the
   sample through a rubber septum into a flash
   vaporizer port located at the head of the column.
 ▫ The temperature of the sample port is usually
   about 50°C higher than the boiling point of the
   least volatile component of the sample.
 ▫ Used for packed columns, where the sample size
   vary from a few tenth of microliter to 20 ul.
Capillary columns require much smaller
               -3
 samples ( 10 ul), so a sample splitter
 system is used to deliver only a small
 fraction of the injected sample to the
 column head, with the rest going to waste.
• Sample splitter (Split/Splitless) injector:
 ▫ The injector can be used in one of two modes; split
   or splitless.
 ▫ a sample is introduced into a heated small
   chamber via a syringe through a septum (the heat
   facilitates volatilization of the sample).
 ▫ The vaporized sample/carrier gas mixture then
   either sweeps entirely (splitless mode) or as
   portion (split mode) into the column.
 ▫ In split mode, a part of the sample/carrier gas
   mixture in the injection chamber is exhausted
   through the split vent.
 ▫ Split injection is preferred when working with
   samples with high analyte concentrations (>0.1%).
 ▫ Splitless injection is best suited for trace analysis
   with low amount of analyte (<0.01%).
For quantitative work, more reproducible
 sample size are required and this can be
 obtained by a rotary sample valve.
• Rotary sample valve:
  ▫ gaseous samples in collection bottles are
    connected to what is most commonly a six-
    port switching valve.
  ▫ The carrier gas flow is not interrupted while a
    sample can be expanded into a previously
    evacuated sample loop.
  ▫ Upon switching, the contents of the sample
    loop are inserted into the carrier gas stream.
Column:
• There are two general types of column:
 ▫ Packed column
 ▫ Capillary column (open tubular).

• All the GC studies in the early 1950s were
  carried out on packed column.

• In the late 1950s capillary column were
  constructed that much superior in speed and
  column efficiency (≈ 300000 plates).
• Capillary columns did not gain widespread until
  the late 1970s due to several reasons:
  ▫ Small sample capacity.
  ▫ Difficulties in coating the column.
  ▫ Tendencies of columns to clog.
  ▫ Short lifetimes of poorly prepared columns.
  ▫ Fragileness of columns.
  ▫ Mechanical problems in sample introduction
    and connection to the detector.
Capillary column:
• Capillary columns have an internal diameter of a few
  tenths of a millimeter.
• They were constructed of stain-less steel, aluminum,
  copper, plastic, or glass.
• They can be one of two types; wall-coated open tubular
  (WCOT) or support-coated open tubular (SCOT).
• Wall-coated columns consist of a capillary tube whose
  walls are coated with liquid stationary phase.
• In support-coated columns, the inner wall of the
  capillary is lined with a thin layer of support material,
  onto which the stationary phase has been adsorbed.
• SCOT columns are generally less efficient than WCOT
  columns, but both types are more efficient than packed
  columns.
• In 1979, a new type of WCOT column appeared,
  the Fused Silica Open Tubular (FSOT) column.
• It was drawn from specially purified silica that
  contains metal oxides.
• These have much thinner walls than the glass
  capillary columns, and are given strength by an
  outside protective polyimide coating.
• These columns are flexible and can be bent into
  coils.
• They have the advantages of physical strength,
  flexibility and low reactivity.
Packed column:
• Packed columns contain a
  finely divided, inert, solid
  support material coated
  with a thin layer of liquid
  stationary phase.
• Most packed columns are
  1.5 - 10m in length and
  have an internal diameter
  of 2 - 4mm.
• They are made from glass,
  metals, or Teflon.
Solid support materials:
• Hold the liquid stationary phase.
• Consists of small, uniform, spherical particles
  with good mechanical strength.
• It should be inert at elevated temperature.
• The most widely used is prepared from the
  naturally occurring diatomaceous earth
  (skeleton of thousands of single-celled plants-
  diatoms- lives in lakes and seas).
• The efficiency of a GC column increases with
  decreasing particle size of the solid support.
Properties and characteristics of GC columns
                    FSOT       WCOT        SCOT      Packed

  Length, m        10-100      10-100      10-100      1-6
   Inside          0.1-0.53   0.25-0.75     0.5        2-4
diameter, mm
  Efficiency,     2000-4000   1000-4000   600-1200   500-1000
   plates/m
Sample size, ng     10-75      10-1000    10-1000     10-106
Relative back       Low         Low         Low        high
  pressure
Relative speed      Fast        Fast        Fast       slow
  Chemical          Best         →           →       Poorest
  inertness
   Flexible          Yes         No         No         No
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posted:11/1/2012
language:English
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