Dr Hanif

• Gas chromatography


  Involves both a partition (gas-liquid chromatography – GLC) and adsorption mechanism (gas-solid chromatography – GSC)

  
• Main components of a typical gas chromatography

  • Carrier gas in gas cylinder with flow/pressure control
  • Injection port (injector)
  • Column and column oven
  • Detector
  • Data system

  
• Two basic types of chromatographic column

  • Packed column
  • Capillary column

  
• Packed column


  • Made of glass or stainless steel
  • Normally 1 to 3 m long and 2-4 mm internal diameter
  • Packed with fine particles of an inert solid or solid support that is coated with thin layer of stationary phase (in the range of 1-20 %, at 0.05 to 1.0 m thick)
• Solid supports (packing materials)

  • Consist of small, uniformly size spheres
  • High specific surface area (>1 m2/g)
  • Chemically inert at high temperature
  • Be wetted uniformly by liquid phase
  • Thermally stable
  • Available in uniform sizes
  • Good mechanical strength
• Diatomaceous earth


  A natural spongy siliceous material composed of numerous microscopic unicellular marine organism
• The pressure drop of carrier gas varies inversely as the square of particle diameter
• The pressure differences between top of column and end of column should be less than 50 psi
• The usual support particle diameter size ranges between 170 to 149m (or generally described as 80-100 mesh) or 250 to 170 m (60 to 80 mesh)
• Capillary columns


  • Internal diameter: 0.25-0.75 mm
  • Column length: 10-100 meters
  • Film thickness: 0.2-2.0 m
• Three types of capillary or sometimes called open tubular columns

  • Wall coated open tubular (WCOT)
  • Surface-coated open tubular (SCOT)
  • Porous layer open tubular (PLOT)
• Wall coated open tubular (WCOT)


  • Most commonly used type of capillary column
  • The wall of the long glass or fused silica capillary is coated or chemically bonded with stationary phase as a thin film
  • Typical length: 30-100 m
• Surface-coated open tubular (SCOT)

  • The stationary phase is coated onto a porous layer (~30m thick) that is deposited on the inside wall
  • They have greater surface area per meter and the length is less than 25m
• Porous layer open tubular (PLOT)

  • Gas solid stationary phases are comprised of a thin layer (usually <10 um) of porous small particles adhered to the surface of the tubing
  • Size exclusion and shape selectivity processes occur for this type of capillary column
• The capillary column having inside diameter of 530m is called mega bore columns
• Problems of column that is coated with stationary phase

  • The column deteriorate by oxidation of the stationary phase
  • Has low temperature limits
  • Loss of stationary phase due to 'bleeding'
  • Accumulation of high boiling materials in the column
• Solving the problems


  • Oxidation is minimised by using oxygen trap in the carrier gas line to remove oxygen
  • A stationary phase is chemically bonded to walls of column to stop bleeding and to allow the use of higher temperatures which remove high boiling materials
• Two types of stationary phase (SP)

  • Selective SP
  • Non-selective SP
• Selective SP


  Separation occurs by utilisation of differences in chemical characteristics of components, such as polarity and functional groups
• Non-selective SP


  Separation is achieved on the basis of differences in boiling points of the sample components
• Principle of likes dissolve likes

  Applicable between analytes and SP, similarity in polarities for both analyte and SP also contribute for the interaction between solute and SP
• Polarity of analytes


  • Polar (alcohols, acids, and amines)
  • Medium polar (ethers, ketones, and aldehydes)
  • Non-polar (saturated hydrocarbon)
• The most widely used stationary phase for both packed and capillary columns are polysiloxanes

  
• Polysiloxanes


  • Available in the greatest variety and are the most stable, robust and versatile
  • The most basic polysiloxanes is the 100% methyl substituted
  • When other groups are present, the amount is indicated as the percent of the total number of groups

  
• Examples of Polysiloxanes


  • 5% diphenyl-dimethyl polysiloxane
  • 50% phenyl-methyl polysiloxane
  • 14% cyano-propyl phenyl-dimethyl polysiloxane

  
• Polyethylene Glycols (PEG)


  • Widely used as stationary phases
  • Stationary phases with "wax" or "FFAP" in their name are some types of polyethylene glycol
  • PEG stationary phases are not substituted, thus the polymer is 100% of the stated material
  • They are less stable, less robust and have lower temperature limits than most polysiloxanes
  • PEG stationary phases must be liquids under GC temperature conditions

  
• Gas solid chromatography (GSC) is based upon adsorption of gaseous substances on solid surfaces

  
• Molecular sieves


  • Made of aluminium silicate that consists of interconnected cavities and pores of uniform size
  • The pore size depends on the type of cation present
  • The particle sizes that are available commercially include that of 40-60 mesh to 100-120 mesh
• Polyethylene glycol has unique separation properties that make its liabilities tolerable

  
• PEG stationary phases must be liquids under GC temperature conditions

  
• Some common liquid stationary phases for GLC include SC 3262 GC column

  
• Typical chromatograms from open tubular capillary column
• Gas solid chromatography (GSC)

  Based upon adsorption of gaseous substances on solid surfaces

  
• GSC


  • Useful for separation of analytes that are not normally retained by GLC, such as the components of air, H2S, C2S, NO2, CO, CO2, and rare gases
  • Performed on both packed and capillary columns
• Types of GSC


  • Molecular sieves
  • Porous polymers
• Molecular sieves


  • Made of aluminium silicate that consists of interconnected cavities and pores of uniform size
  • The pore size depends on the type of cation present
  • Particle sizes available commercially include 40-60 mesh to 100-120 mesh
  • Typical molecular sizes are 0.4, 0.5, 1.0 and 1.3 nm
  • Molecules smaller than these size penetrate into the interior of the particles where adsorption takes place
  • Separation by this type of GSC is effective by size as well as polarity and degree of unsaturation of analytes
• Porous Polymers


  • Polystyrene cross-linked with divinylbenzene
  • The pore size of the beads is uniform and can be controlled by varying the degree of cross-linking of the polymer
  • Used for separation of gaseous polar species such as H2S, NOx, H2O, CO2, CH3OH, and vinyl chloride
• Solid Stationary Phases for GSC

  • Molecular sieves (aluminium silicate)
  • Porous Polymers (polystyrene crosslinked with divinylbenzene)

  
• Interfacing GC with spectroscopy

  • GC is often coupled with selective techniques such as spectroscopy and is called the hyphenated methods
  • Provides the chemist with powerful tools to identify individual components in complex mixtures
• Hyphenated methods


  • GC-MS; Mass spectrometry is used as a detector and subsequence analysis of the components separated by GC
  • GC-IR; Infra-red combines capillary column with Fourier transform infrared spectrometers that provides a potent means for separating and identifying the components of difficult mixtures