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 Sample ZoneBGE Zone A - + As the concentration of the eluent ion increases, retention tends to decrease

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

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 detectionUV transparent eluent eg, bromide, nitrate, nitrite, 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 conductance, G = 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

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.