Ion Chromatography

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Ion Chromatography
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Ion Exchange
Separation is facilitated by formation of ionic
bonds between charged samples and
chargedcolumn packings
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Ions
Ions can be characterised as organic, inorganic,
anion or cation and mono or polyvalent
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Chemical Considerations

• Anion or Cation exchanger

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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.
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Factors Affecting Ion Exchange Retention
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Control of Ion exchange by pH
Changing the pH can eliminate the charge of the
columnif the column is weak, or eliminate the
charge on the ion ifion is weak. Either way, the
retention is reduced.
Strong ion - Weak Exchanger
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Exchange Capacity of Anion Exchanges
Exchange Capacity Number of functional groups
per unit weight of resin
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Exchange Capacity of Anion Exchanges
pH has no effect on capacity of strong cation
exchanges. Weak cation exchanges change
dramatically with pH.
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Control of Ion exchange by Ionic Strength
As the concentration of the eluent ion increases,
retention tends to decrease
-

A
Sample Zone
BGE Zone
BGE Zone
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Control of Ion exchange by Eluent Ion
cations
anions
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The equilibrium constant
At pHpKa 50 is ionised and 50 is neutral At
pHpKa 1 90 is ionised At pHpKa -1 10 is
ionised
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Common Acidic Buffers
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Common Basic Buffers
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Anion exchange Separation development
Sample Weak or strong? Column weak or
strong? pH ?
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Anion exchange Separation development
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Anion exchange Separation development
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Anion exchange Separation development
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Anion exchange Separation development
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Anion exchange Separation development
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Cation exchange method development
Column Strong Cation Exchange Sample Weak
bases pH acidic (all compounds are ionised)
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Effect of ionic strength
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Effect of pH
Compare this separation at pH4.55 and 0.05 in
the previous example. Increasing pH
reducedretention.
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Effect of Temperature
Increasing temperatureincreases efficiency,
decreases k, and may affect ?. This is due to
improvedmass transfer.
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UV Detection

• UV detection.
• Direct detection UV transparent eluent eg,
  bromide, nitrate, nitrite, thiocyanate, _at_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

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Concentration Changes
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UV Detection
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Direct UV Detection
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Indirect UV Detection
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Indirect UV detection
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Conductivity Detection

• Ohms law V IR
• conductance, G
• Non - Suppressed
• direct - low conducting eluent - high
  conducting analytes
• indirect - high conducting eluents - low
  conducting analytes

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Ion Conductances
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Conductivity Detection
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Direct Conductivity Detection
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Waters Ion Analysis Method
Eluent Borate/Gluconate Column IC Pak HR Flow
Rate 1ml/min Injection 50 ul Detection Direct
Conductivity Background 274 uS
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Indirect Conductivity
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Conductivity Detection Suppressed
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Conductivity Detection Suppressed
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Membrane Suppressor
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Autosuppression
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Eluent Generation KOH
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Eluent Generation MSA
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Why the difference?
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Separation of Cations
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Practical Sessions

• Analysis of Inorganic Anions by direct
  conductivity.