Clean Up Cation Exchange

MECHANISM OF ION EXCHANGE BONDING

Compounds are retained on the sorbent through ionic bonds.  Therefore, it is essential that the sorbent and the analyte to be extracted are charged.  Generally, the number of molecules with charged cationic groups increases at pH values below the molecules pKa value.  The number of molecules with charged anionic groups decreases at pH values below the molecule’s pKa value.  To ensure 99% or more ionization, the pH should be at least two pH units below the pKa of the cation and two pH units above the pKa of the anion.  Elution occurs by using a solvent to raise the pH above the pKa of the cationic group or to lower the pH below the pKa of the anion to disrupt retention.  At this point, the sorbent or compound is neutralized.

These sorbents are composed of a silica backbone bonded with carbon chains terminated by a negatively or positively charged functional group.  Ion exchange interactions occur between a sorbent that carries a charge and a compound of opposite charge.

This electrostatic interaction is reversible by neutralizing the sorbent and/or analyte.  Ion exchange bonds can also be disrupted by the introduction of a counter ion to compete with the analyte for binding sites on the sorbent.

CATION EXCHANGE SORBENTS & STRUCTURES

Sorbent                                                               Structure                                                              pKa            

Carboxylic Acid                                                -Si-CH2COOH

Propylsulfonic Acid                                          -Si-(CH2)3SO3H                                                 <1

Benzenesulfonic Acid                                     -Si-(CH2)2  Benzene ring SO3H                          Always charged

Benzenesulfonic Acid High Load                 -Si-(CH2)2 Benzene ring SO3H                           Always charged

Triacetic Acid                                                    -Si-(CH2)3NH-(CH2)2N(CH2COOH)2
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CH2COOH