Chromatography Techniques Reference (TLC, Paper, Column, GC) Cheat Sheet

Key Facts

• The retention factor is calculated by $R_f = \frac{\text{distance from baseline to spot center}}{\text{distance from baseline to solvent front}}$.
• A valid $R_f$ value is between $0$ and $1$ because a solute spot cannot travel farther than the solvent front in normal TLC or paper chromatography.
• In normal-phase TLC, a more polar compound usually has a smaller $R_f$ because it interacts more strongly with the polar stationary phase.
• In column chromatography, compounds that interact weakly with the stationary phase move faster and elute earlier.
• In gas chromatography, retention time $t_R$ is the time from injection to the peak maximum for a compound.
• The partition coefficient is $K = \frac{C_s}{C_m}$, where $C_s$ is concentration in the stationary phase and $C_m$ is concentration in the mobile phase.
• Chromatographic resolution can be estimated by $R_s = \frac{2(t_{R2} - t_{R1})}{w_1 + w_2}$, where $w_1$ and $w_2$ are peak widths.
• A larger number of theoretical plates means a more efficient column, often estimated by $N = 16\left(\frac{t_R}{w}\right)^2$.

Vocabulary

Stationary phase

The phase that stays fixed in place and interacts with sample components during chromatography.

Mobile phase

The solvent, liquid, or gas that moves through the system and carries sample components.

Retention factor

The ratio $R_f$ that compares how far a solute spot travels to how far the solvent front travels.

Retention time

The time $t_R$ required for a compound to pass through a chromatographic system and reach the detector.

Elution

The process of washing a compound through and out of a chromatography column.

Chromatogram

A visual record of separated components, often shown as spots on a plate or peaks on a detector graph.