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HPLC Analysis of Ionic Compounds. Nicholas H. Snow Seton Hall University. HPLC of Ionic Compounds. Ion Suppression Ion Exchange Chromatography (IEX or IEC) Ion Pair Chromatography (IPC). Weak Acid Equilibrium. LeChatelier Principle. DO NOT chromatograph at pH = pKa !!. Ion Suppression.
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HPLC Analysis of Ionic Compounds Nicholas H. Snow Seton Hall University
HPLC of Ionic Compounds • Ion Suppression • Ion Exchange Chromatography (IEX or IEC) • Ion Pair Chromatography (IPC)
LeChatelier Principle DO NOT chromatograph at pH = pKa !!
Ion Suppression • Weak acid - Use pH 2-3 • acetic or phosphoric acid • Weak base - Use pH 8 • ammonium hydroxide, sodium carbonate • Ionization is suppressed • Neutral molecules chromatographed by reversed phase LC
Ion Exchange Parameters • Increasing pH • Increases ionization of acids - retained longer • Decreases ionization of bases - elute faster • Increasing buffer strength • More competition for exchange sites - decreased retention • Increasing Temperature • Equilibrium favors mobile phase - retention decreases
Ion Exchange Groups • Strong anion exchanger • -NR3+ • Weak anion exchanger • -NR2H+ • Strong cation exchanger • -SO3- • Weak cation exchanger • -CO2-
Ion Chromatography • Originally meant IEX + suppressor device + conductivity detector • Inorganic cations and anions • Organic acids, amines carbohydrates
Limitations of Ion Exchange • Less Efficient Columns • Lot to lot column reproducibility poor • Gradients require long equilibration • Limited choice of packings • Only ionic analytes
Ion Pair Chromatography • Adjust pH to fully ionize analyte • Add ion pair former of opposite charge in mobile phase • acids (anions): tetraalkyl ammonium salt • bases (cations): alkyl sulfonic acids • Use standard reversed phase column