Ion Chromatography Ion Exchange Separation is facilitated by formation of ionic bonds between charged samples and charged column packings Ions Ions can be characterised as: organic, inorganic, anion or cation and mono or polyvalent Chemical Considerations Anion or Cation exchanger Strong vs Weak Exchange Materials Strong exchangers stay ionised as pH varies between 2 and 12. Weak exchangers can lose ionisation as a function of pH. Factors Affecting Ion Exchange Retention Control of Ion exchange by pH Changing the pH can eliminate the charge of the column if the column is weak, or eliminate the charge on the ion if ion is weak. Either way, the retention is reduced. Strong ion - Weak Exchanger Exchange Capacity of Anion Exchanges Exchange Capacity: Number of functional groups per unit weight of resin Exchange Capacity of Anion Exchanges pH has no effect on capacity of strong cation exchanges. Weak cation exchanges change dramatically with pH. Control of Ion exchange by Ionic Strength As the concentration of the eluent ion increases, retention tends to decrease - + A BGE Zone Sample Zone BGE Zone Control of Ion exchange by Eluent Ion cations anions The equilibrium constant At pH=pKa 50% is ionised and 50% is neutral At pH=pKa +1 90% is ionised At pH=pKa -1 10% is ionised Common Acidic Buffers Common Basic Buffers Anion exchange Separation development Sample: Weak or strong? Column: weak or strong? pH ? Anion exchange Separation development Anion exchange Separation development Anion exchange Separation development Anion exchange Separation development Anion exchange Separation development Cation exchange method development Column: Strong Cation Exchange Sample: Weak bases pH: acidic (all compounds are ionised) Effect of ionic strength Effect of pH Compare this separation at pH=4.55 and 0.05 in the previous example. Increasing pH reduced retention. Effect of Temperature Increasing temperature increases efficiency, decreases k’, and may affect . This is due to improved mass transfer. UV Detection UV detection. Direct detection UV transparent eluent eg, bromide, nitrate, nitrite, thiocyanate, @214nm Indirect Detection UV absorbing electrolyte. Anions are detected via “vacancies” in background absorbance universal detection usually used when other modes of detection are unavailable. Very specific Concentration Changes UV Detection Direct UV Detection Indirect UV Detection Indirect UV detection Conductivity Detection Ohms law V= IR 1 conductance, G = R Non - Suppressed direct - low conducting eluent - high conducting analytes indirect - high conducting eluents - low conducting analytes Ion Conductances Conductivity Detection Direct Conductivity Detection Waters Ion Analysis Method Eluent: Borate/Gluconate Column: IC Pak HR Flow Rate: 1ml/min Injection 50 ul Detection: Direct Conductivity Background: 274 uS Indirect Conductivity Conductivity Detection Suppressed Conductivity Detection Suppressed Membrane Suppressor Autosuppression Eluent Generation KOH Eluent Generation MSA Why the difference? Separation of Cations Practical Sessions (1) Analysis of Inorganic Anions by direct conductivity.