Gas Chromatography

• mobile phase


  gas
• stationary phase

  nonvolatile liquid/solid
• analyte

  gas/volatile liquid
• gaseous analyte is transported through the column by a gaseous mobile phase called carrier gas
• solvent moving through the column

  mobile phase
• stays in place inside the column

  stationary phase
• fluid entering the column

  eluent
• fluid emerging from the end of the column

  eluate
• process of passing liquid or gas through a chomatography column is called elution
• columns are either packed or open tubular
• solute with greater affinity for the stationary phase remains on the column longer
• retention time is the time elapses between injection of the mixture onto the column and the arrival of that component at the detector
• retention volume is the volume of the mobile phase required to elute a particular solute from the column
• adjusted retention time for a retained suloute is the additional time required to travel the length of the column beyond that required by solvent

  
• relative retention, α, or separation factor, is the ratio of the adjusted retention times

  
• greater the relative retention, greater separation between 2 components
• relative retention is independent of the flow rate
• The unadjusted relative retention is the inverse of the ratio of the speeds at which the two components travel.
• retention factor, k, is the time required to elute that peak minus the time tm required for mobile phase to pass through the column, expressed in multiples of tm.

  
• partition coefficient, K, is Cs/Cm or the concentrations of solute in the stationary phase over the mobile phase
• the farther apart the elution peaks, the better their separation
• the wider the peaks, the poorer the separation
• The longer a solute resides in a column, the broader the band becomes in the chromatogram
• One main cause of band spreading is diffusion, which is the net transport of a solute from a region of high concentration to a region of low concentration caused by the random movement of molecules.
• The diffusion coefficient measures the rate at which molecules move randomly from a region of high concentration to a region of low concentration.
• Plate height is also called the height equivalent to a theoretical plate. It is approximately the length of column required for one equilibration of solute between mobile and stationary phases.
• The smaller the plate height, the narrower the bandwidth.
• The ability of a column to separate components of a mixture is improved by decreasing plate height.
• An efficient column has more theoretical plates than an inefficient column.
• Plate heights are 0.1 to 1 mm in gas chromatography, 10μm in high-performance liquid chromatography
• The ratio of solute concentrations in each phase at equilibrium is called the partition coefficient.
• Many small extractions are more effective than a few large extractions.
• Columns may be packed with stationary phase or may be open tubular, with stationary phase bonded to the inner wall.
• The relative retention of two components is the quotient of their adjusted retention times.
• The retention factor for a single component is the adjusted retention time divided by the elution time for solvent.
• Retention factor gives the ratio of time spent by solute in the stationary phase to time spent in the mobile phase.
• Plate height is related to the breadth of a band emerging from the column.
• The smaller the plate height, the sharper the band.
• Plate height is approximately the length of column required for one equilibration of solute between mobile and stationary phases.
• Resolution of neighboring peaks is the difference in retention time divided by the average width