The key difference between ion pair and ion exchange chromatography is that, in ion pair chromatography, ions in the sample can be “paired” and separated as the ion pair whereas, in ion exchange chromatography, ions in the sample can be separated as cations and anions separately.
Chromatography is an important technique that leads to the separation of different components in a mixture. Ion pair and ion exchange chromatography are analytical techniques we can use to separate ions and polar molecules in a mixture, based on the electrical charge that they carry with them.
CONTENTS
1. Overview and Key Difference
2. What is Ion Pair Chromatography
3. What is Ion Exchange Chromatography
4. Side by Side Comparison – Ion Pair vs Ion Exchange Chromatography in Tabular Form
5. Summary
What is Ion Pair Chromatography?
Ion pair chromatography is an analytical technique where ions in the sample are paired and separated as the ion pairs. Ion pairing refers to the neutralization; when cations pair with anions, their electrical charges neutralize. Here, this separation technique is done in a reverse-phase column. In this process, we need to use ion-pairing agents in order to form ion pairs and separate the ions in the sample. Usually, the ion-pairing agents are compounds containing hydrocarbon chains. These ion-pairing agents should have the opposite electrical charge to that of the ions in the sample; otherwise, ions will not pair (ions with the same charge does not pair, because they repel each other). In addition, these ion-pairing agents can increase hydrophobicity and retention as well.
Furthermore, using ion-pair agents as the mobile phase allows us to separate ionic and highly polar substances easily. For example, if we add a reagent which has a hydrophobic functional group, the stationary phase can retain this hydrophobic functional group; thus, paired ions are also retained on the stationary phase along with the added hydrophobic functional group. 1-pentylsodiumsulfonate and 1-hexylsodiumsulfonate are important as the anionic counter ions for the cations present in the sample and 1-pentanesulfonate is important as a cationic counter ion for anions.
Advantages of Ion Pair Chromatography
There are several advantages of ion pair chromatography when compared to ion-exchange chromatography;
- Easy to prepare required buffer solutions
- Can choose a wide range of carbon chain lengths in ion-pairing agents
- Results are highly reproducible
- Can obtain an improved peak shape
- Reduced separation time
What is Ion Exchange Chromatography?
Ion exchange chromatography is a form of liquid chromatography in which we can analyze ionic substances. We often use it to analyze inorganic anions and cations (i.e. chloride and nitrate anions and potassium, sodium cations). Though it is less common, we can analyze organic ions as well. Moreover, we can use this technique for the purification of proteins because proteins are charged molecules at certain pH values. Here, we use a solid stationary phase to which the charged particles can attach. For example, we can use the resin polystyrene-divinylbenzene copolymers as solid support.
To explain this further, the stationary phase has fixed ions such as sulfate anions or quaternary amine cations. Each of these should associate with a counter ion (an ion with opposite charge) if we are to maintain the neutrality of this system. If the counter ion is a cation, then we name the system as a cation exchange resin. But, if the counter ion is an anion, the system is an anion exchange resin. Moreover, there are five major steps in ion-exchange chromatography:
- Initial stage
- Adsorption of the target
- Starting of elution
- End of elution
- Regeneration
What is the Difference Between Ion Pair and Ion Exchange Chromatography?
Ion pair and ion-exchange chromatographies are analytical techniques we can use to separate the ions and polar molecules in a mixture. The key difference between ion pair and ion exchange chromatography is that, in ion-pair chromatography, we can make ions in the sample “paired” and separate it as the ion pair, whereas in ion-exchange chromatography, we can separate the ions in the sample as cations and anions separately.
Below infographic shows more comparisons related to the difference between ion pair and ion exchange chromatography.
Summary – Ion Pair vs Ion Exchange Chromatography
Ion pair and ion exchange chromatography are analytical techniques we can use to separate the ions and polar molecules in a mixture. The key difference between ion pair and ion exchange chromatography is that in ion-pair chromatography, ions in the sample can be “paired” and separated as the ion pair whereas, in ion-exchange chromatography, ions in the sample can be separated as cations and anions separately.
Reference:
1. “Ion-Pair Chromatography.” Practical High-Performance Liquid Chromatography, Sept. 2010, pp. 217–223., doi:10.1002/9780470688427.ch13.
Image Courtesy:
1. “Our Cecil Instruments Ion Chromatography system” By Khanom – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Metrohm 850” By Datamax – Own work (Public Domain) via Commons Wikimedia