When should I choose weak ion exchange?

In a recent post, we discussed the use of mixed mode polymeric strong ion exchange SPE phases, like EVOLUTE EXPRESS CX and EVOLUTE EXPRESS AX.  This post will describe EVOLUTE EXPRESS WCX and EVOLUTE EXPRESS WAX mixed mode weak ion exchange SPE phases and when to use them.

They are similar to the EVOLUTE EXPRESS CX and AX strong ion exchange phases in that they are mixed mode, offering retention by reversed phase and ion exchange mechanisms.  The difference is in the functional groups on the SPE phase and their charge state.  The strong ion exchange phases are called “strong” because the sulfonate cation or quaternary ammonium anion exchange group remain charged over the entire pH range.  They are always positively or negatively charged, regardless of pH conditions.  Weak ion exchange phases have weak acids or bases as their ion exchange group and are charged only under certain pH conditions.  What does this mean for sample prep?  If your analyte of interest is a strong acid or strong base where the charge of the molecule can’t be turned on or off for retention and elution by adjusting the pH, you can turn the charge of the SPE sorbent on or off by changing the pH to control retention and elution.

Figure 1 shows the EVOLUTE EXPRESS WCX mixed mode weak cation exchange sorbent.  It has a carboxylic acid group that is negatively charged at pH >7, and neutral at pH <3.  Positively charged compounds, like strong bases, are retained at a pH >7, and eluted at a pH <3.

Here is a typical protocol for strong bases using EVOLUTE EXPRESS WCX:

  1. Pre-treat sample with water or buffer.
  2. Condition with methanol & equilibrate with water
  3. Load pre-treated sample
  4. Wash with water and/or 5 % NH4OH in water
  5. Wash with an organic solvent like methanol
  6. Elute with 2 % formic acid in methanol (strong bases eluted)

Figure 2 shows the EVOLUTE EXPRESS WAX mixed mode weak anion exchange sorbent.  It has a secondary or tertiary amine group (depending on the pH and charge) that is positively charged at pH <8, and neutral at pH >10.  Negatively charged compounds (like strong acids) are retained at a pH <8 and eluted at a pH >10.

Here is a typical protocol for strong acids using EVOLUTE EXPRESS WAX:

  1. Pre-treat sample with acid
  2. Condition with methanol & equilibrate with water
  3. Load pre-treated sample
  4. Wash with water and/or 2 % formic acid in water
  5. Wash with an organic solvent like methanol
  6. Elute with 5 % NH4OH in methanol (strong acids eluted)

Some good applications of these phases are catecholamines and metanephrines in urine and plasma, and polyfluorinated compounds in water.  Remember, these phases are polymeric, so they are stable across the entire pH range, and can also retain compounds by reversed phase.  Why not give them a try?  It you don’t know where to start, we can help.

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