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Ion Chromatography Slide 2 Ion Exchange Separation is facilitated by formation of ionic bonds between charged samples and charged column packings Slide 3 Ions Ions can be characterised as: organic, inorganic, anion or cation and mono or polyvalent Slide 4 Chemical Considerations Anion or Cation exchanger Slide 5 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. Slide 6 Factors Affecting Ion Exchange Retention Slide 7 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 Slide 8 Exchange Capacity of Anion Exchanges Exchange Capacity: Number of functional groups per unit weight of resin Slide 9 Exchange Capacity of Anion Exchanges pH has no effect on capacity of strong cation exchanges. Weak cation exchanges change dramatically with pH. Slide 10 Control of Ion exchange by Ionic Strength Sample ZoneBGE Zone A - + As the concentration of the eluent ion increases, retention tends to decrease Slide 11 Control of Ion exchange by Eluent Ion cations anions Slide 12 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 Slide 13 Common Acidic Buffers Slide 14 Common Basic Buffers Slide 15 Anion exchange Separation development Sample: Weak or strong? Column: weak or strong? pH ? Slide 16 Anion exchange Separation development Slide 17 Slide 18 Slide 19 Slide 20 Slide 21 Cation exchange method development Column: Strong Cation Exchange Sample: Weak bases pH: acidic (all compounds are ionised) Slide 22 Effect of ionic strength Slide 23 Effect of pH Compare this separation at pH=4.55 and 0.05 in the previous example. Increasing pH reduced retention. Slide 24 Effect of Temperature Increasing temperature increases efficiency, decreases k, and may affect . This is due to improved mass transfer. Slide 25 UV Detection UV detection. Direct detectionUV 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 Slide 26 Concentration Changes Slide 27 UV Detection Slide 28 Direct UV Detection Slide 29 Indirect UV Detection Slide 30 Indirect UV detection Slide 31 Conductivity Detection Ohms law V= IR conductance, G = Non - Suppressed direct - low conducting eluent - high conducting analytes indirect- high conducting eluents - low conducting analytes Slide 32 Ion Conductances Slide 33 Conductivity Detection Slide 34 Direct Conductivity Detection Slide 35 Waters Ion Analysis Method Eluent: Borate/Gluconate Column: IC Pak HR Flow Rate: 1ml/min Injection 50 ul Detection: Direct Conductivity Background: 274 uS Slide 36 Indirect Conductivity Slide 37 Conductivity Detection Suppressed Slide 38 Slide 39 Slide 40 Membrane Suppressor Slide 41 Slide 42 Autosuppression Slide 43 Eluent Generation KOH Slide 44 Eluent Generation MSA Slide 45 Why the difference? Slide 46 Separation of Cations Slide 47 Practical Sessions (1)Analysis of Inorganic Anions by direct conductivity.