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Supercritical Fluid Chromatography. Theory Instrumentation Properties of supercritical fluid Critical temperature Above temperature liquid cannot exist Vapor pressure at critical temperature is critical pressure T and P above critical T and P Critical point Supercritical fluid.

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supercritical fluid chromatography
Supercritical Fluid Chromatography
  • Theory
  • Instrumentation
  • Properties of supercritical fluid
    • Critical temperature
      • Above temperature liquid cannot exist
      • Vapor pressure at critical temperature is critical pressure
    • T and P above critical T and P
      • Critical point
      • Supercritical fluid
supercritical fluid
Supercritical fluid
  • Above the critical temperature
    • no phase transition regardless of the applied pressure
  • supercritical fluid is has physical and thermal properties that are between those of the pure liquid and gas
    • fluid density is a strong function of the temperature and pressure
    • diffusivity much higher a liquid
      • readily penetrates porous and fibrous solids
    • Low viscosity
    • Recovery of analytes
      • Return T and P
supercritical fluid chromatography1
Supercritical fluid chromatography
  • Combination of gas and liquid
  • Permits separation of compounds that are not applicable to other methods
    • Nonvolatile
    • Lack functional groups for detection in liquid chromatography
supercritical fluid extraction
Supercritical Fluid Extraction
  • near the critical point properties change rapidly with only slight variations of pressure.
    • inexpensive,
    • extract the analytes faster
    • environmentally friendly
  • sample is placed in thimble
  • supercritical fluid is pumped through the thimble
    • extraction of the soluble compounds is allowed to take place as the supercritical fluid passes into a collection trap through a restricting nozzle
    • fluid is vented in the collection trap
      • solvent to escapes or is recompressed
  • material left behind in the collection trap is the product of the extraction
    • batch process
capillary electrophoresis
Capillary Electrophoresis
  • Separations based on different rate of ion migration
    • Capillary electrochromatography separates both ions and neutral species
    • Electroosmotic flow of buffer acts as pump
  • Principles
  • Applications
planar electrophoresis
Planar electrophoresis
  • porous layer
  • 2-10 cm long
    • paper
    • cellulose acetate
    • polymer gel
      • soaked in electrolyte buffer
  • slow
  • difficult to automate
capillary electrophoresis1
Capillary Electrophoresis
  • narrow (25-75 mm diameter) silica capillary tube
    • 40-100 cm long
  • filled with electrolyte buffer
  • fast
  • complex but easy to automate
  • quantitative
  • small quantities
    • nL
separation
Separation
  • Movement of ions function of different parameters
    • molecular weight
    • charge
      • small/highly-charged species migrate rapidly
    • pH
      • Deprotonation HAH+ + A-
    • ionic strength
    • low m
      • few counter-ions
      • low charge shielding
    • high m,
      • many counter-ions
      • high charge shielding
migration rate
Migration rate
  • v= migration velocity
  • me=electrophoretic mobility (cm2/Vs)
  • E=field strength (V/cm)
  • For capillary
    • V=voltage
    • L=length
  • Electrophoretic mobility depends on net charge and frictional forces
    • Size/molecular weight of analyte
    • Only ions separated
  • Plate height (H) and count (N)
    • Function of diffusion and V
plates
Plates
  • Planar electrophoresis
    • large cross-sectional area
    • short length
    • low electrical resistance, high currents
    • Sample heating Vmax=500 V
    • N=100-1000 low resolution
  • Capillary electrophoresis
    • small cross-sectional area
    • long length
  • high resistance
  • low currents
    • Vmax=20-100 kV
  • N=100,000-10,000,000 high resolution
    • As comparison, HPLC N=1,000-20,000
zone broadening
Zone Broadening
  • Single phase (mobile phase) - no partitioning
  • three zone broadening phenomena
    • longitudinal diffusion
    • transport to/from stationary phase
    • multipath
  • planar
    • no stationary phase
  • capillary
    • no stationary phase or multipath
transport
Transport
  • ions migrating in electric field
    • cations to cathode (-ve)
    • anions to anode (+ve)
  • Electroosmosis movement in one direction
    • anode (+ve) to cathode (-ve)
  • Components
    • Analyte dissolved in background electrolyte and pH buffer
    • Silica capillary wall coated with silanol (Si-OH) and Si-O-
    • Wall attracts cations - double-layer forms
    • Cations move towards cathode and sweep fluid in one direction
  • Electroosmotic flow proportional to V
    • usually greater than electrophoretic flow
bulk flow properties
Bulk flow properties

hydrodynamic

ion

buffer

techniques
Techniques
  • Electropherogram
    • migration time analogous to retention time in chromatography
  • Isoelectric focusing
    • Gradient
      • No net migration
    • pH gradient with weak acid
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