prefi

• Gas Chromatography


  A chromatographic technique used for the separation of volatile compounds

  
• Gas Chromatography


  • Useful for gaseous analyte
  • Gaseous mobile phase called carrier gas (usually He, Ne, H2)

  
• Types of Gas Chromatography

  • Gas – Solid Chromatography (GSC): analyte is adsorbed directly on solid particles of stationary phase
  • Gas – Liquid Chromatography (GLC): stationary phase consists of non-volatile liquid immobilized inside of a column by a fine solid support

  
• Basic Operation in the Gas Chromatograph

  1. Volatile liquid or gaseous sample is injected through a septum into a heated port which rapidly evaporates

  2. Vapor is carried through the column by the carrier gas. Separation is based on the affinity of the solute particles to the carrier gas and the stationary phase

  3. Separated analytes flow through a detector, which is maintained at a temperature higher than the column. This will ensure that your analyte is still in a gaseous phase

  4. The detected response is displayed in the computer as a chromatogram

  
• Advantages of an open tubular or capillary column over packed column

  • Higher resolution
  • Shorter analysis time
  • Greater sensitivity
  • Lower sample capacity
• Most GC applications used packed columns but now industry are switching to capillary columns due to efficiency and faster analysis time
• Length of these columns can range from 2 m to 60 m or even more
• Columns are housed in thermostated ovens
• How to protect the Columns

  1. Installation of guard columns and retention gaps

  2. Use of high-quality gases
• Guard Columns


  Installed before the chromatography column to remove non-volatile substances to avoid contamination
• Retention Gaps


  Improves peak shape by separating volatile solvent from less volatile solutes prior to chromatography
• Carrier gases


  Should be purified to remove O2, H2O, and traces of organic compounds that will degrade the stationary phase
• Stationary Phase


  The function is to separate the different components in a solution causing each to exit the column at a different time
• Choosing Stationary Phase


  • Related to chemical nature of solute and stationary phase
  • Must have different k (retention factor) for each solute in mixture
  • The retention factor must not too large or small
  • Apply " like dissolves like" rule
• Polar solutes need


  • Polar stationary phases –CN, CO, and –OH
• Non-polar solutes need


  • Hydrocarbon like stationary phase or dialkyl silanes
• Intermediate solutes need


  • Something in between polar and non-polar stationary phases
• 6 common stationary phases are applicable 90% of application
• Most stationary phases are based on polydimethylsiloxane or polyethylene glycol
• Polarity of the stationary phase can be changed by derivatization with different functional group
• Temperature and Pressure Programming

  1. Varying kinds of solutes react differently with stationary phase

  2. Programming temperature may improve the situation

  3. When oven condition is set at a particular temperature, say 150∘C, volatile compounds will elute first but the less volatile ones remain in the column

  4. When temperature is increased from 50 ∘C to 250 ∘C at constant flow rate, elution of all compounds may occur uniformly

  5. Retention time of the analytes will also decrease

  6. The highest temperature should not be set at a point where stationary phase and some analytes would start to decompose

  7. Pressure programming is also a plausible option when temperature programming is not possible, as when decomposition of analytes take place at high temperature
• Carrier Gas/ Mobile Phase

  • Stored in pressurized tanks
  • Does nothing in GC but to transport the compounds. NOT involved in separation mechanism
• Carrier Gases


  • Helium, He
  • Hydrogen, H2
  • Nitrogen, N2
• Helium


  Most commonly used carrier gas as it is compatible with most detectors
• Hydrogen


  • Very fast separation, fastest optimal flow rate with low penalty on resolution
  • Its drawbacks include: possible reaction with unsaturated compounds on metal surfaces, incompatibility with mass spectrometric detector, and formation of explosive mixture with air
• Nitrogen


  • Low detection limit, when this gas is the carrier
  • Optimal flow rate is also low
• Sample Injection


  1. Sample injections are done using calibrated microsyringes for manual injection

  2. Injection volume is 0.1 – 10 𝜇L generally

  3. Slow injection or oversized sample injection will cause band broadening and poor resolution

  4. Sample ports must be heated to 50∘C or to a temperature more than the least volatile sample component

  5. Autosamplers/headspace autosamplers are also available for analyzing large sample quantities
• Types of Sample Injection

  • Split injection
  • Splitless injection
  • On-column injection
  • Programmed temperature vaporization (PTV) injection
• Detection devices for a GC must respond rapidly and reproducibility to the low concentrations of the solutes emitted from the column
• Concentration dependent detectors


  • Thermal Conductivity Detector (TCD)
  • Electron Capture Detector (ECD)
  • Photo-ionization Detector (PID)
• Mass flow dependent detectors

  • Flame Ionization Detector (FID)
  • Nitrogen Phosphorus Detector (NPD)
  • Flame Photometric Detector (FPD)
• The most widely used detectors are TCD, FID, and ECD
• Sampling Techniques in Gas Chromatography

  • Headspace Technique
  • Thermal Desorption Sampling Technique
  • Derivatization Technique

  
• Headspace Sampling Technique


  • Isolates volatile analytes from the mixture prior to injection into the gas chromatograph
  • Ideal way of introducing a sample in a GC
  • Avoids the introduction of non volatile or high boiling contaminants from the sample matrix
  • Can be used for the trace or ultra-trace determination of volatile organics with little or no additional sample preparation
• Headspace Sampling Technique


  1. The sample is placed in a closed vial with an overlaying air space and allowed to equilibrate between the sample and the overlaying air

  2. The vapor phase portion will be extracted using a syringe and injected into the gas chromatograph
• Thermal Desorption Sampling Technique

  1. The sample is place in a glass-lined , stainless steel tube

  2. The sample will be heated after purging with carrier gas that remove any O2

  3. Volatile analytes are swept from the tube by an inert gas and carried to the GC

  4. Volatile analytes will be concentrated at the top of the column by cooling the column inlet (cryogenic focusing)

  5. Once volatization is complete, the column inlet is heated rapidly, releasing the analytes to travel through the column
• Derivatization Technique


  • Process of chemically modifying a compound to produce new compound which has properties that are suitable for analysis using a GC and HPLC
  • The chemical structure of the compound remains the same and just modifies the specific functional groups to make them detectable and analyzable
• Reasons for Derivatization in GC

  • Necessary in the analysis by GC of compounds that exhibit low volatility, poor thermal stability, contain "active" groups that can lead to loss of sample to intermolecular hydrogen bonding or adsorption on the inlet or column, and/ or demonstrate poor sensitivity at the detector
• Methods of Derivatization in GC

  • Silylation
  • Acylation
  • Alkylation
• Derivatization Technique

  • Increases volatility
  • Enhances sensitivity
  • Increases detectability
  • Increases stability (thermostability)
  • Reduce adsorption of polar samples on active surfaces of column walls and solid support