KEY2CHEM

Chromatography

Chromatography is a separation technique that relies on differences between interparticle interactions to separate individual components. The more strongly a solute is attracted to the chromatography stationary (or solid) phase (either column or paper material), the longer it will interact with the stationary phase and the longer it will take to elute (pass through). The elution process is aided by using a mobile (often either liquid or gas) phase that has the opposite polarity of the stationary phase. For example, if the stationary phase is a polar material, the mobile phase is a nonpolar substance. Solutes that have very weak interactions with the stationary phase will elute more quickly, since they will be more strongly attracted to the mobile phase.

Example 1.

A solution is to be separated using column chromatography. The solid phase of the column is a very polar material; this is called “normal phase” chromatography. The mobile phase is nonpolar. Which solute is expected to elute most rapidly?

A. $$\require{mhchem}\ce{CH3OH}$$

B. $$\require{mhchem}\ce{CHCl3}$$

C. $$\require{mhchem}\ce{CCl4}$$

Solution

C. $$\require{mhchem}\ce{CCl4}$$

In normal-phase chromatography, the least polar compounds have the lowest attraction to the stationary phase and elute first. The most polar compounds elute last because they are more strongly attracted to the stationary phase.

Example 2.

The solid phase in a chromatographic separation is called the ______ phase.

A. stationary

B. mobile

C. stagnant

Solution

A. stationary

The solid phase that interacts with the eluting solutes is called the stationary phase.

Example 3.

In a chromatography experiment, the stationary phase is nonpolar and the mobile phase is polar; this is called “reverse-phase” chromatography. In this experiment, which will be the last compound to elute?

A. $$\require{mhchem}\ce{C6H14}$$

B. $$\require{mhchem}\ce{CH2Cl2}$$

C. $$\require{mhchem}\ce{CHF3}$$

Solution

A. $$\require{mhchem}\ce{C6H14}$$

The least polar solute will interact most strongly with the nonpolar stationary phase, leading to the longest elution time.