Analytical Separations

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Analytical Separations

• Gas Chromatography

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Instrument
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Types of Gas chromatography

• Gas solid chromatography (GSC)
• Solid stationary has limited application owing
  to semipermanent retention if active or polar
  molecules and severe tailing of elution peaks.
• Gas Liquid chromatography (GLC)
• A liquid phase immobilizes on surface of an
  inert solid.

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Liquid Phase for GLC

• Low volatility-the boiling point of the liquid
  should be 100oC higher than the maximum operating
  temperature.
• Thermal stability
• Chemical inertness
• Solvent characteristics-k and a values for the
  solutes to be resolved fall within a suitable
  range.

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Principle of Gas chromatography

• Retention volume
• The effect of pressure and temperature
• Retention volumes rather than the retention times

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Retention volume
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Carrier Gas

• The carrier gas must be chemically inert.
• Commonly used gases include N2, He, Ar.
• The choice of carrier gas is often dependant upon
  the type of detector which is used.
• The carrier gas system also contains a molecular
  sieve to remove water and other impurities.

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Columns- Packed columns

• There are two general types of column, packed and
  capillary (open tubular).
• Packed columns- contain a finely divided, inert,
  solid support material (commonly based on
  diatomaceous earth) coated with liquid stationary
  phase.
• Most packed columns are 1.5 - 10m in length and
  have an internal diameter of 2 - 4mm.

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Columns- Capillary columns

• Capillary columns have an internal diameter of a
  few tenths of a millimeter (0.25mm or 0.42mm).
• Two types-wall-coated open tubular (WCOT) or
  support-coated open tubular (SCOT).
• WCOT -a capillary tube whose walls are coated
  with liquid stationary phase.
• SCOT - the inner wall of the capillary is lined
  with a thin layer of support material such as
  diatomaceous earth, onto which the stationary
  phase has been adsorbed.
• SCOT columns are generally less efficient than
  WCOT columns.

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WCOT column

• These columns are flexible and can be wound into
  coils.
• They have the advantages of physical strength,
  flexibility and low reactivity.

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Instrumental ComponentsSample injection

• Two modes-split or splitless.
• The injector contains a heated chamber containing
  a glass liner into which the sample is injected
  through the septum.

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Split Mode

• The carrier gas enters the chamber and can leave
  by three routes
• The sample vaporizes to form a mixture of carrier
  gas, vaporized solvent and vaporized solutes.
• A proportion of this mixture passes onto the
  column, but most exits through the split outlet.
• The septum purge outlet prevents septum bleed
  components from entering the column.

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Injection Method

• For optimum column efficiency, the sample should
  not be too large
• Sampleshould be as a "plug" of vapour - slow
  injection of large samples causes band broadening
  and loss of resolution.
• Microsyringe is used to inject sample
• The temperature of the sample port is usually
  about 50C higher than the boiling point .
• For packed columns, sample size ranges from
  tenths of a microliter up to 20 microliters.
• Capillary columns need much less sample,
  typically around 10-3 mL.

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Column temperature

• The optimum column temperature is dependant upon
  the boiling point of the sample.
• Minimal temperatures give good resolution, but
  increase elution times.
• If a sample has a wide boiling range, then
  temperature programming can be useful.

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Flame Ion Detector

• The effluent from the column is mixed with
  hydrogen and air, and ignited.
• A large electrical potential is applied at the
  burner tip, and a collector electrode is located
  above the flame.
• The current resulting from the pyrolysis of any
  organic compounds is measured.

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Advantages of FID

• Changes in mobile phase flow rate do not affect
  the detector's response.
• The FID is a useful general detector for the
  analysis of organic compounds
• It has high sensitivity, a large linear response
  range, and low noise.
• Unfortunately, it destroys the sample.

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Thermal Conductivity Detector (TCD)

• One pair is placed in the column effluent to
  detect the separated components as they leave the
  column
• One pair is placed in a separate reference
  column.

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TCD

• The temperature of the sensing element depends on
  the thermal conductivity of the gas flowing
  around it.
• The TCD is not as sensitive as other detectors.
• Advantages
• It is non-specific and non-destructive.
• Simplicity

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Electron Capture Detector (ECD)

• Uses a radioactive e- to ionize some of the
  carrier gas and produce a current between
  electrodes.
• Molecules capture some of the electrons and
  reduce the current measured between the
  electrodes.

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Advantages of ECD

• Specific for the organic molecules contain
  electronegative functional groups, such as
  halogens, phosphorous, and nitro groups.
• The ECD is as sensitive as the FID but finds its
  greatest application in analysis of halogenated
  compounds.