Gas Chromatography

Gas Chromatography

1. Gas-solid Chromatography

   • -solid stationary phase -retention of analytes is a result of physical adsorption -limited application due to semi-permanent retention of active or polar molecules and severe tailing of elution peaks -applied to the separation of some low molecular weight gaseous species

2. Gas-liquid Chromatography

   • -Stationary phase is a liquid immobilized on the surface of an inert solid -Retention of analytes is a result of the partition of the analyte between mobile and stationary phases -most common -aka partition chromatography

3. Parts of the Instrument

   • Carrier Gas Supply
     • Chemically inert (sometimes certain gases are required for certain detectors
     • Contains molecular sieve
     • Flow rates are controlled by pressure and regulators attached to the tank and chromatograh
   • Sample Injection System
     • Use microsyringe to inject liquid or gaseous samples through a silicone septum
       • sample size=few tenth of microliter to 20 microliters
       • Column splitter for capillary colums
     • injected at head of column where it is vaporized
     • Alternative sample injector=rotary valve
       • more reproducible sample sizes injected
   • Column and Oven
     • Packed columns
       • Cu, Al, stainless steel, glass, or teflon tubes
       • packed with stationary phase
         • solid support
           • -ideal support: small, uniform, spherical particles with good mechanical strength and a surface area of at least 1 m^2/g -inert at elevated temps -evenly wetted by stationary phase
         • liquid stationary phase
           • Squalene-hydrocarbons
           • polyglycol-alcohols and aromatics
           • polyesters-fatty acid methyl esters
           • silicones substituted with various groups
           • Characteristics:
             • Thermally stable
             • Non-volatile
             • unreactive to samples
             • good solven for sample
             • held tightly to support
         • Porous organic polymers
           • use as both support and liquid stationary phase
           • adjusted chemically to produce substances with widely varying absorption properties
           • Advantage: used at very high temps
           • Disadvantage: absorb solute so well that they must be run at high temps
     • Capillary Columns
       • advantages over packed columns: higher resolution, shorter analysis time, greater sensitivity
       • Disadvantages over packed columns: Less sample capacity
       • Types
         • Wall Coated
           • liquid coated stationary phase is coated on the inside wall of the column
         • Support coated
           • liquid stationary phase is coated on a solid support
   • Detector
     • Ideal:  adequate sensitivity,  good stability and reproducibility,  linear response over several order of concentration magnitude,  temp range from RT to 400C,  short response time that is indepdent of flow rate,  high reliability and ease of use,  similar response to all analytes or a highly predictable and selective response to a few analytes,  nondestructive
     • Flame Ionization
     • Thermal Conductivity
     • Thermionic
     • Electron Capture
     • Atomic Emission
     • Mass Spec
     • Infrared Spec