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Chapter 26. Other Methods. Ion-Exchange Chromatography. The mechanism of separation will be the exchange of ions from the column to the solution. Water softening – exchange Na ions for Ca and Mg. Water deionization – exchange H ions for cations and OH ions for anions. Leaving water.

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chapter 26

Chapter 26

Other Methods

ion exchange chromatography
Ion-Exchange Chromatography
  • The mechanism of separation will be the exchange of ions from the column to the solution.
  • Water softening – exchange Na ions for Ca and Mg.
  • Water deionization – exchange H ions for cations and OH ions for anions. Leaving water.
  • Can be larger scale. The support is modified to allow for the ion exchange equilibrium.
  • Can be natural materials or synthetic
gels vs resins
Gels vs Resins
  • Resins are firm and can stand greater pressure.
  • Gels are softer – have lower charge densities and are made from polymeric sugars.
  • Polyacrylamide can also be used a the backbone.
ion exchange selectivity
Ion Exchange Selectivity
  • Equilibrium system
    • R-Na+ + Li+ = R-Li+ + Na
    • K = [R-Li+][Na+]/[R-Na+][Li+]
    • K is called the selectivity coefficient
which ions have greater affinity
Which ions have greater affinity
  • Higher charge, higher polarizability and decreased hydrated radius.
  • Pu4+>>La3+>Ce3+>Pr3+>Eu3+>Y3+>Sc3+>Al3+ >> Ba2+> Pb2+ > Sr2+ > Ca2+ > Ni2+ > Cd2+ > Cu2+ > Co2+ >Zn2+ > Mg2+ > UO2+ >> Ti+> Ag+> Rb+> K+ >NH4+> Na+> H+> Li+
  • Reconditioning by having higher concentration of the less tightly held ion.
donnan equilibrium
Donnan Equilibrium
  • Concentration of ions outside the resin will be higher than the inside concentration.
  • Cations will be excluded from the inside of an anion exchanger. (Has same charge as resin site)
  • Ion Exclusion Chromatography
  • Non charged species can migrate in but not ions.
ion exchange
Ion Exchange
  • Types
    • Resins
    • Gels
    • Inorganic exchangers (Zeolites)
  • Use a gradient to remove stronger bound ions.
applications
Applications
  • Preconcentration
    • Pass much water over a resin and then elute with a high concentration of acid.
    • Cation exchange to trap cations
    • Chelex -100 to trap transition metals.
  • Water deionization.
    • Cation exchange from cation removal.
    • Anion exchange for anion removal.
  • Water softening
ion chromatography
Ion Chromatography
  • HPLC ion exchange.
    • Detection is an issue. Ions do not absorb uv/vis light.
    • Conduction is used to detect ions but the mobile phase will have high electrolyte like KOH
    • We use ion suppression
unsuppressed ion chromatography
Unsuppressed Ion Chromatography
  • The ions have higher conductivity than the eluent. Carboxylic acids used as eluent.
  • Indirect Detection. Mobile phase has a light absorbing ion. Phthalate ion.
ion pair chromatography
Ion Pair Chromatography
  • Separate ions on a reverse phase column. (Ammonium ions)
  • Add a surfactant to the mobile phase.
    • Such as sodium octane sulfonate.
molecular exclusion chromatography
Molecular Exclusion Chromatography
  • Separation Based on Size Only
    • Gel Filtration
    • Gel Permeation
  • Large molecules can not get into the internal diameter so the elute more quickly.
slide25
Vt = Vo + Vi + Vg + Vec
  • Vt is the total volume of the system. If we ignore volume outside the column then we have
  • Vt’ = Vo + Vi + Vg
    • Vo is the elution volume for large molecules
    • Vo + Vi is the elution volume for small molecules
elution
Elution
  • Ve = Vo + KVi
  • Kave assumes that Vg is very small and I suggest you not use it.
  • K will fall between 0 and 1 unless there is another mechanism in the column.
stationary phase
Stationary Phase
  • A solid support with internal volume of fixed size. There are many options available. Both low pressure and high pressure (HPLC)
determination of molecular weight
Determination of Molecular Weight
  • Plot Log (MW) vs elution volume
affinity chromatography
Affinity Chromatography
  • Stationary phase is made so that it has a very specific interaction that can cause binding to a specific substrate.
  • Elution is carried out by disrupting this interaction. (Change pH is an example)
capillary electrophoresis
Capillary Electrophoresis
  • Motive force is no longer pressure but electrical migration.
    • Cations migrate to the cathode
    • Anions migrate to the anode
  • High electric field place across a capillary column.
slide38
CZE
  • Very high resolution due to the lack of no packing or stationary phase, no A term or c term in the van Deempter equation.
  • H = A + B/ux + Cux
  • Just longitudinal diffusion plays a role.
mobility
Mobility
  • Ion of charge q will accelerate in the potential field until the frictional force counter balances it and it travels at constant speed.
  • uep = q/f*E = mepE
  • mep is electrophoretic mobility
  • Relates speed and charge
  • Directly related to charge, indirectly related to size
stokes equation
Stokes Equation
  • F = 6phr
  • h is the measure of solution viscosity
electroosmotic flow eof
Electroosmotic Flow (EOF)
  • ueo = meoE
    • Units of the electroosmotic mobility is m2/[V.s]
joule heating
Joule Heating
  • Capillary tubes must be narrow enough to get rid of the excess heat. 50 mm tubes are ok but 1 mm would be a real problem. Some are cooled.
  • Heat is related to I2R
apparent mobility
Apparent Mobility
  • Two mechanisms for movement. Electrophoresis and Electroosmosis.
  • Can be going the same direction or the opposite.
  • mapp = mep + meo
apparent mobility1
Apparent Mobility
  • Speed divided by electric field.

Ld isthe length to the detector and Lt is the total length.

separation is based on size and charge
Separation is based on size and charge
  • Bovine carbonic anhydrase – acetylated at the lysine residues R-NH2
plates and resolution
Plates and Resolution
  • N = Ld/s2
  • Or
  • N = mappV/2D* Ld/Lt
resolution
Resolution
  • Same as for GC or HPLC
injection
Injection
  • Two Modes
    • Hydrodynamic Injection
    • Electrokinetic Injection
detection
Detection
  • UV is most common.
elution order
Elution order
  • In CZE
    • Cations – highest mobility first
    • Neutrals – unresolved
    • Anions – highest mobility last
mekc micellar electrokinetic chromatography
MEKC – Micellar Electrokinetic Chromatography
  • Add a surfactant to the mobile phase.
  • Micelles form above the CMC
  • Neutral species will partition into the micelles and flow at that rate
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