PA TOPIC 5 PART 2

• Higher solubility in SP


  Component is more likely to be absorbed in the SP, longer retention time
• Higher boiling point


  Component requires a higher temperature to be vaporized, longer retention time
• Other factors that can affect the separation

  • Flow rate of the carrier gas: Higher flow rate, shorter analysis time
  • Type of carrier gas: Different carrier gas, different optimal flow rate
  • Temperature of the column: Higher column temperature, shorter analysis time
• 4 main components in GC

  • Injector
  • Carrier gas (MP)
  • Column (SP)
  • Detector
• Injector / Injector port

  An inlet system that allows for the introduction of the sample into the GC
• Injection

  • Can be done manually or be automated
• Types of injector

  • Packed column injector (Packed columns)
  • Split / Splitless injector (Capillary columns)
• Packed column Injector

  • Temperature is kept elevated by the heated metal block
  • Sample is vaporized upon injection
  • All sample enters the column
• Split/Splitless Injector

  • Split vent can be controlled
  • Only a fraction of the sample will enter the column
  • Commonly used split ratio is 1/100 (1% of the sample enters the column)
• Carrier gas (MP)

  • Must be chemically INERT: He, Ar, N2, H2
  • Must be PURE (99.999%) and dry
  • Most common MP: He, N2, Ar (expensive), H2 (dangerous)
• Two types of columns

  • Packed columns
  • Capillary columns
• Packed column

  • Can accommodate a larger sample size & more convenient for use
  • Column material: Stainless steel / glass
  • Packing materials composition: silica & other impurities
  • Limited applications mainly in separation of permanent (fixed) gases eg CO2
• Capillary column

  • Has higher separation efficiency & requires less sample volume
  • Mostly used now as it offers better peak resolution than packed column
  • Has lesser sample capacity than packed column
• Factors to consider when choosing SP

  • Thermal stability
  • Chemical inertness
  • Solvent characteristics
  • Different partition ratios for different solutes - separation of components
  • Good solubility of sample components
  • Film thickness: Varies from 0.1 – 5 μm, Thick films used for analyte with high volatility, Thick films has higher capacity, useful for larger analyte volume
• Detector

  • A device that senses the presence of a sample component and converts that information to an electrical signal
  • Magnitude of signal is proportional to the concentration or amount of component in the sample
• Common detectors in GC

  • Thermal Conductivity Detector (TCD)
  • Flame Ionization Detector (FID) - most common
  • Mass Spectrometer (MS)
• TCD

  • As a gas passes over a heated filament wire, the temperature and resistance of the wire will vary according to the thermal conductivity of the gas
• FID

  • Effluent from the column passes through a hydrogen-air flame, Organic molecules break down and produce ions, Positive ions produced are attracted to negatively charged electrodes and the resulting ion current is measured as an electrical signal, Number of ions collected indicates the amount of analyte present in the sample
• MS

  • Separation is based on mass-to-charge (m/z) ratio, m/z is equivalent to the molecular weight of the fragment if the ion is singly charged
• Advantages


  • TCD: low cost; can be used for many substances; has large linear dynamic range; non-destructive to sample
  • FID: most widely used; good sensitivity (ppb); has wide linear dynamic range; not sensitive to water (good for analysis of aqueous sample)
  • MS: can be used for any type of substances; sensitive (ppb); most definitive identification of compounds
• Limitations

  • TCD: poor sensitivity; may not be suitable for use in capillary column
  • FID: destroys sample; may not be suitable for inorganic solutes; not sensitive in monitoring of air contaminants eg SO2, halogens
  • MS: expensive