When should I choose weak ion exchange SPE phases?

What is a weak ion exchange SPE phase? What is their charge state and how they behave in function of pH conditions? When to choose them? This post will describe Biotage mixed mode weak ion exchange SPE phases - EVOLUTE® EXPRESS WCX and EVOLUTE® EXPRESS WAX - and how to optimize your sample preparation of your analytes of interest.

In the post When should I choose a mixed-mode SPE? we discussed the use of mixed mode polymeric strong ion exchange SPE phases, like EVOLUTE® EXPRESS CX and EVOLUTE® EXPRESS AX. 

EVOLUTE® EXPRESS WCX and EVOLUTE® EXPRESS WAX are similar to the 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.

Figure 1. EVOLUTE® EXPRESS WCX mixed mode weak cation exchange sorbent.

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)
7.  

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.

Figure 2. EVOLUTE EXPRESS WAX mixed mode weak anion exchange sorbent.

Typical protocol for strong acids using EVOLUTE® EXPRESS WAX:

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

Some good applications where these phases are used are:

• Catecholamines and metanephrines in urine
• Catecholamines and metanephrines in plasma
• 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. We have prepared for you plenty of interesting resources on sample preparation extraction techniques and method optimization. For more information, contact your technical sales representative or an applications chemist.