Comprehensive Study Guide on Chromatography: Principles, Techniques, and History

Origin and Definition of Chromatography

• Etymology: The term "Chromatography" is derived from the Greek words "chroma," meaning "color," and "graphein," meaning "to write."

• Technical Definitions:

  • It is a non-destructive procedure used for resolving a complex mixture into its individual fractions or compounds.

  • It is also defined as a physical method of separation that distributes components to separate between two phases: one stationary (stationary phase) and the other (the mobile phase) moving in a definite direction.

Historical Development

• Mikhail Tsvet (1906): The technique was first employed in Russia by the Italian-born scientist Mikhail Tsvet during his research on plant pigments.

• Initial Discovery: Although employed in 1906, the method was first described on 30 December 1901.

• Early Methodology: Tsvet used liquid-adsorption column chromatography with the following components:

  • Adsorbent: Calcium carbonate ($CaCO_3$).

  • Eluent: Petrol ether/ethanol mixtures.

  • Targets: Chlorophylls and carotenoids.

• Plant Pigment Characteristics: The different colors of these components gave the technique its name:

  • Carotene: Orange.

  • Xanthophylls: Yellow.

  • Chlorophyll a: Bluish-green.

  • Chlorophyll b: Yellowish-green.

• Evolution (1930s–1950s):

  • New types of chromatography developed in the 1930s and 1940s expanded the utility of the technique for diverse separation processes.

  • Archer John Porter Martin and Richard Laurence Millington Synge: During the 1940s and 1950s, these scientists established the principles and basic techniques of partition chromatography. Their work encouraged the rapid development of paper chromatography, gas chromatography (GC), and high-performance liquid chromatography (HPLC).

• Modern Advancements: Technology continues to improve technical performance, allowing for the separation of increasingly similar molecules.

Primary Components of Chromatography

• Mobile Phase:

  • A solvent that flows in a different direction through the supporting medium to elute the mixture components.

  • It consists of the sample being analyzed and the solvent moving the sample through the column.

  • Possible states include liquid (Liquid Chromatography (LC) and Capillary Electro chromatography (CEC)), gas (GC), or supercritical fluid (SFC).

  • In HPLC, the mobile phase consists of non-polar solvents (e.g., hexane) in normal phase, or polar solvents in reverse phase.

  • The use of binary solvents (two solvents) and ternary solvents (three solvents) as the mobile phase is common.

• Stationary Phase:

  • A layer of solid, gel, or liquid film coating immobilized on the supporting medium.

  • It interacts with the analyte to be separated. An example is the silica layer used in thin-layer chromatography (TLC).

• Supporting Medium: The solid surface on which the stationary phase is bound or coated.

• Eluate (Effluent/Eluant): The mobile phase leaving the column.

• Eluent: The solvent that carries the analyte.

• Eluite: The analyte or the eluted solute.

• Eluotropic Series: A list of solvents ranked according to their eluting power. The order of increasing eluting power is typically: petroleum ether, cyclohexane, benzene, diethyl ether, chloroform, acetone, acetonitrile, alcohol, and water.

Fundamental Principles

• Basic Principle: Components in a mixture have different tendencies to adsorb onto a surface or dissolve in a solvent. Every method requiring separation uses a static part (stationary phase) and a moving part (mobile phase).

• Differential Partitioning: Separation is based on the differential partitioning between the mobile and stationary phases. Differences in a compound's partition coefficient result in differential retention.

• Affinity and Travel:

  • A fraction with a greater affinity for the stationary phase travels slower and covers a shorter distance.

  • A fraction with a higher affinity for the mobile phase travels faster and covers a longer distance.

Core Chromatographic Phenomena

A. Adsorption

• Development: This was the first type developed.

• System: Uses a solid stationary phase and a liquid or gaseous mobile phase.

• Mechanism: Every solute has its own equilibrium between adsorption onto the solid surface and solubility in the solvent. Least soluble or best-adsorbed solutes travel more slowly.

• Result: The mixture separates into bands containing different solutes.

• Examples: Liquid chromatography using a column of silica gel or alumina.

B. Partition

• Concept: Signifies the distribution of solute molecules (analytes) between two liquid phases.

• Mechanism: The stationary phase is a non-volatile liquid held as a thin layer/film on an inert solid surface. The solute distributes between the moving mobile phase and the stationary liquid.

• Result: The more soluble component in the mobile phase reaches the end of the column first. Separation is based mainly on differences in solubility.

• Quantification: The distribution is quantified using the solubility coefficient/partition coefficient ($K_D$):     $K_D = \frac{\text{Concentration of solute in the stationary phase}}{\text{Concentration of solute in the mobile phase}}$

• Examples: Paper chromatography, some TLC, and gas chromatography.

C. Ion Exchange

• Mechanism: The stationary phase is a coated solid referred to as a resin (e.g., agarose, cellulose). Resin has ions covalently bonded to it (cations or anions) and ions of the opposite charge electrostatically bound to the surface.

• Retention: Analyte molecules are retained based on coulombic (ionic) interactions. The surface displays ionic functional groups ($R-X$) that interact with analyte ions of opposite charge.

• Subcategories:

  1. Cation-exchange: Retains positively charged cations because the stationary phase has negatively charged functional groups.

  2. Anion-exchange: Retains negatively charged anions because the stationary phase has positively charged functional groups.

• Application: Domestic water softeners operate on this principle.

D. Molecular Exclusion (Size Exclusion)

• Mechanism: There is no equilibrium state established between the solute and the stationary phase. The mixture passes through a porous gel.

• Process: The pore size allows large solute particles to pass through uninhibited. Small particles permeate the gel pores and are slowed down.

• Result: The smaller the particle, the longer it takes to pass through the column. Separation is strictly according to particle size.

Classification of Chromatographic Methods

1. By Physical State of the Mobile Phase

• Gas Chromatography (GC): Uses a gas mobile phase.

• Liquid Chromatography (LC): Uses a liquid mobile phase.

2. By Stationary Phase Type

• GC Sub-divisions:

  • Gas-solid chromatography: Solid, underivatized support.

  • Gas-liquid chromatography: Liquid-coated support.

  • Bonded-phase gas chromatography: Chemically-derivatized support.

• LC Sub-divisions:

  • Adsorption chromatography: Solid, underivatized support.

  • Partition chromatography: Liquid-coated or derivatized support.

  • Ion-exchange chromatography: Support containing fixed charges.

  • Size exclusion chromatography: Porous support.

  • Affinity chromatography: Support with immobilized ligand.

3. By Packing/Geometry

• Column Chromatography: The stationary phase is contained within a tube called a column.

• Planar Chromatography: The stationary phase is configured as a thin two-dimensional sheet or flat surface.

  • Paper Chromatography: A sheet or strip of paper serves as the stationary phase.

  • Thin-layer Chromatography: A thin film of solid particles (e.g., bound with calcium sulfate) is coated on glass, plastic, or metal.

4. By Force of Separation

Chromatographic Theory and Terminology

• Chromatogram: The visual output of the chromatograph equipment. Peaks correspond to different components.

  • X-axis: Retention time ($t_R$).

  • Y-axis: Signal response (proportional to concentration).

• Chromatograph: The actual sophisticated equipment enabling separation.

• Retention Time ($t_R$): The characteristic time an analyte takes to pass from the column inlet to the detector.

  • Void time ($t_M$): Time for the mobile phase to pass through.

  • Baseline width ($W_b$): Peak width at the baseline in time units.

  • Half-height width ($W_h$): Peak width at half the maximum height.

Retardation Factor ($R_f$)

• Definition: The ratio of the distance traveled by the center of a spot to the distance traveled by the solvent front.

• Mathematical Formulas:     $R_f = \frac{\text{Distance travelled by a Solute}}{\text{Distance travelled by a Solvent}}$     $R_f = \frac{\text{Distance traveled by component from application point}}{\text{Distance traveled by solvent from application point}}$

• Interpretation of Values:

  • If $R_f = 0$, the solute remains in the stationary phase and is immobile.

  • If $R_f = 1$, the solute has no affinity for the stationary phase and travels with the solvent front.

• Consistency: $R_f$ is a function of the partition coefficient and is constant for a given substance provided system conditions are constant.

• Factors Affecting $R_f$ Value:

  1. Temperature.

  2. Purity of the solvents used.

  3. Quality of the paper or adsorbents used.

  4. Impurities present in the adsorbents.

  5. Chamber saturation techniques.

  6. Methods of drying and development.

  7. The absolute distance traveled by the solute and solvent.

  8. Chemical reactions between the substances being partitioned.

  9. pH of the solution.