Gas Chromatography

• Gas chromatography


  First instrumental chromatographic method developed commercially
• Gas chromatography


  • It is relatively easy to produce a stable flow and pressure for the mobile phase - carrier gas
  • All that is really needed is a tank of compressed gas, pressure regulator and a valve

  
• Gas chromatography


  1. Components of a vaporized sample are separated as a consequence of being partitioned between a mobile gaseous phase and a liquid or a solid stationary phase held in a column

  2. Sample is vaporized and injected onto the head of a chromatographic column

  
• Elution


  Brought about by the flow of an inert gaseous mobile phase
• Mobile phase


  Does not interact with molecules of the analyte
• Types of gas chromatography


  • Gas solid chromatograpy
  • Gas liquid chromatography

  
• Gas liquid chromatography (GLC or GC)


  • Stationary phase: liquid adsorbed or chemically bonded to an inert solid support
  • Retention is due to partitioning

  
• Gas-solid chromatography


  • Stationary phase: solid
  • Retention is due to physical adsorption
  • Not widely used due to tailing and semipermanent retention of polar/active compounds
  • Useful for the separation of species not retained by gas-liquid columns such as the components of air, hydrogen sulfide, carbon disulfide, nitrogen oxides, carbon monoxide, carbon dioxide, and the rare gases

  
• Instruments for GC


  • Carrier gas system
  • Sample injection system
  • Columns
  • Detection systems
• Carrier gas


  • The mobile phase which transports the analyte through the column, chemically inert gas e.g. He, N2, Ar, H2
  • Equipped with pressure regulator which regulates the flow rate
• Injection port


  • Purpose is to flash evaporate the sample and introduce it into the column
  • Injection is through a septum
  • Septum must be stable at the injection temperature and replaced regularly to maintain seal
• Liner


  • Provides a known area for the flash vaporization
  • Typically made of glass although metal liners may be used
  • It can and should be replaced at regular intervals - all non-volatile materials and degradation products end up here
• Syringes


  • Used to introduce a known volume of a liquid or gas samples
  • Adapters can be used to help control the volume injected
  • When filling, ensure that there is no air in the syringe
  • Sample must be injected rapidly so that the sample is introduced as a small "plug"

  
• Sample size


  • Liquids 0.10 – 10 µl is typical
  • Gases 0.5 – 5 mL is typical
  • Injection precision with a syringe is +/- 1%

  
• Columns


  • Heart of the separation process
  • Can be classified by tubing diameter and packing type
• Column configurations

  • Packed columns
  • Open tubular/capillary column
• Types of GC columns

  • Wall coated open tubular (WCOT)
  • Support coated open tubular (SCOT) or porous layer open tubular (PLOT)
  • Fused silica open tubular (FSOT)
  • Packed columns
• Wall-coated open tubular (WCOT) columns


  Capillary tubes coated with a thin film of stationary phase
• Support-coated open tubular (SCOT) columns


  • Also called porous layer open tubular (PLOT)
  • Inner surface of a capillary is lined with a thin film of a support material such as diatomaceous earth
  • This column holds as much as stationary phase as does the WCOT
• Fused silica open tubular (FSOT) columns


  • Capillaries are drawn from specially purified silica that contains minimal amount of oxides
  • Have much thinner walls than their glass counterparts
  • With protective polyimide coating on the outside layer as protection
  • Flexible

  
• Packed columns


  Tubes are densely packed with a uniform, finely divided packing material, or solid support, that is coated with a thin layer of the stationary liquid phase
• Typical GC columns


  • FSOT
  • WCOT
  • SCOT
  • Packed
• Column configurations and column ovens


  • The column sits in an oven
  • The optimum column temperature depends on the boiling point of the sample and the degree of separation required
  • If the temperature is held constant during the entire analysis, it is isothermal elution
  • For samples with broad boiling range, temperature programming is often employed
• With homologues, the retention time increases exponentially with the number of carbon


  As tR increases, width increases and the height decreases, making detection impossible after a few peaks have eluted
• Since solubility of a gas in a liquid decreases as temperature goes up

  The retention of a material can be reduce by increasing column temperature
• Liquid stationary phases


  • Low volatility (BP should be 100 oC higher than the maximum operating temperature for the column)
  • Thermal stability
  • Chemical inertness
  • Solvent characteristics such that k and α values for the solute to be resolved fall within a suitable range
• Types of liquid stationary phases


  • Polar stationary phases – contain functional groups such as –CN, –CO, and –OH
  • Nonpolar stationary phases – hydrocarbon type and dialkyl siloxane
  • Highly polar stationary phase – polyesters
• Polydimethyl siloxane


  R groups can all be methyl group or a percentage of the methyl group can be replaced by any of the phenyl, trifluoropropyl, or cyanopropyl groups
• Polyethylene glycol


  • Separation of polar species
  • Bonded and cross-linked stationary phase – to provide a longer lasting stationary phase
• Desirable properties of detection systems


  • Adequate sensitivity (10-8 to 10-15 g solutes/s)
  • Good stability and reproducibility
  • A linear response to solute that extends to over several orders of magnitude
  • Nondestructive of samples
  • A temperature range from room temperature to at least 400oC
  • High reliability and ease of use
  • Similarity in response toward all solutes or alternatively a highly predictable and selective response toward one or more classes of solutes
  • A short response time that is independent of flow rate
• Types of detection systems

  • Flame ionization detectors (FID)
  • Thermal conductivity detectors (TCD)
  • Electron capture detectors (ECD)
  • Mass spectrometer
• Flame ionization detectors (FID)


  • Effluent from the column is directed into a small air/hydrogen flame
  • Detection involves monitoring the current produced by the ions and electrons when organic compounds are pyrolyzed due to the air/hydrogen flame
  • The current produced is measured using a picoampere (10-12 A)
  • Applicable samples: hydrocarbons
• Flame ionization detectors (FID)


  • Changes in flow rate of the mobile phase have little effect on detector response
  • Insensitive toward noncombustible gases such as H2O, CO2, SO2, and NOx (enables analysis of samples contaminated with water and the oxides of sulfur and nitrogen)
  • Highly sensitive (10-13 g/s) with large linear response and low noise, easy to use
  • Destroys the sample during the combustion step
• Thermal conductivity detectors (TCD)


  • Consists of an electrically heated source whose temperature at constant electric power depends on the thermal conductivity of the surrounding gas
  • Detection involves measuring the change in temperature of the detector due to the decrease in the thermal conductivity of the carrier gas in the presence of organic compounds or other analytes
• Thermal conductivity detectors (TCD)


  • Applicable samples: organic and inorganic species (universal detector)
  • Simple, large linear dynamic range, general response to both organic and inorganic species, nondestructive which permits collection of solutes
  • Low sensitivity
• Electron capture detector (ECD)


  • Consists of a radioactive β-emitter, usually nickel-63
  • Detection involves measuring the decrease in current when molecules containing electronegative functional groups such as halogens, peroxides, quinones, and nitro groups capture electrons
• Electron capture detector (ECD)


  • Applicable samples: halogenated compounds (found wide used for pesticide residue analysis and PCBs)
  • Highly sensitive, nondestructive
  • Low linear response, radioactive detector, requires license
• Mass spectrometry


  • Measures the mass to charge ratio (m/z) of ions that have been produced from the sample
  • Ionization can be caused by electron impact (EI) ionization source where molecules are bombarded with a high energy stream of electrons or chemical impact (CI) where molecules are ionized in the presence of gaseous ions
• Mass spectrometry


  • Enables the determination of the molecular weight of the samples
  • Applicable samples: tunable for any species
• Thermionic detector


  • Similar in construction with the FID
  • Widely used for organophosphorus pesticides and pharmaceutical compounds