april 18

return to ch 26

Assume solute is distributed between

• mobile phase (vapor, solvent)
• stationary phase (liquid form, solid)
  • K = C_S/C_M
  • equil cnst, partition coef, distribution coef
  • (won't actually evaluate directly)
  • (don't measure conc inside the column)
• When a peak emerges we measure
• retention time, t_R
  • perhaps retention volume, V_R
• t_M (to usually) is a non-retained peak
  • v= vel of solute = L/ t_R
    • different for each peak, of course
    • varies with temp, solvent polarity
  • u= vel of mobile phase = L/t_M
• related,
  • v =u x (fraction of time in mobile phase)
  • v= u x (fraction of solute in mobile phase)
    • v = u x [ C_MV_M / (C_MV_M+C_SV_S)]
    • v = u [1 / (1 + K (V_S/V_M)]
      • varies but V_S/V_M typically 3 (75% packing, 25% void area)
  • v = u [1/ (1+k')]
  • k' = (t_R-t_M)/t_M
  • this we can evaluate from chromatogram
• when comparing two solutes
• a = K_B/K_A =( t_RB-t_M) / (t_RA-t_M)
• prefer large a value
• related-- Resolution
  • R = 2 DZ /(W_A+W_B)
    • separation / average width
    • really want R>1
• N- plate count, number of theoretical plates (100, 1000, 10,000)
  • N= 16 (t_R / W)²
  • can compute N from the chromatogram
    • for similar solutes, N should be nearly independent of species and t_R
• H or HETP height of a theoretical plate
  • HPLC, N=3000, L = 100 mm
    • H= 100/3000 = 0.03 mm
  • GC, packed column, N=2000, L= 2 meters
    • H=2/1000 = 0.001m = 1 mm
  • GC, capillary, N=20,000, L=40 meter,
    • H=40/20,000 = .002 m = 2 mm
• H and N vary with operating conditions
  • optimal flow rate can be found
  • in GC, N and H might vary by 50% over a 2 fold change in flow rate
  • if separation is adequate, may operate at faster than optimum flow rates