Adsorption Chromatography

Introduction

Adsorption chromatography is the most accepted modern High-Performance Liquid Chromatography (HPLC) method. The method involves the division of components in a mixture introduced into a chromatography system based on the relative differences in adsorption of components to the stationary phase present in the chromatography column.

The molecules or components of the mixture pass through at different speeds because of the differences in the affinity towards the stationary phase. The stationary phase is a solid element on which the sample compound is to be adsorbed. The mobile phase is either a liquid (Solid-Liquid Chromatography) or a gas (Gas-Solid Chromatography) because the adsorption aspect is an essential part of solids and hence it is followed by only the solid stationary phase chromatography.

It is based on the interaction between the solute molecules and active sites in the stationary phase. This interaction depends upon the polarity of solutes (“polar like polar”). Tswett established the system around 1900 and noticed its use by separating plant pigments. Gas chromatography, thin-layer chromatography, and open column chromatography come under this chromatography type.

Principle of Adsorption Chromatography

It includes the systematic partition of a chemical mixture (gas or liquid) by giving it over an adsorbent bed that adsorbs various compounds at various rates. Solute in a liquid (or gas) phase is associated with adsorption sites on the solid surface (finely alienated particles for the maximum surface area).

A mixture of liquids is separated by being passed over a solid substrate in the same manner as gases are separated by being passed over a liquid substrate. The liquid solvent is the mobile phase and the solid is the stationary phase. As with gas chromatography, the variation in the flow rate of the components of the mixture depends on how much time is spent by each component in the stationary phase and how much is spent in the mobile phase.

A proper inert solution is used as the mobile phase in the same direction as an inert carrier gas is used in gas chromatography. The ratio of time spent by the sample in the solvent and on the surface of the stationary phase depends on the solubility of the sample in the solvent and the adsorption affects the stationary surface on the sample molecules.

Each of these parameters in the Van Deemter equation is usually appropriate in liquid-solid chromatography, i.e., geometry, diffusion, mass transfer, and flow rate.

Theory

The adhesion of atoms, ions, or molecules from dissolved solid, liquid, or gas to a surface is termed adsorption. A layer of the adsorbate is created on the surface of the adsorbent, by this method. The term “Adsorbent” is defined as the stationary phase in adsorption chromatography while “adsorbate” is the mobile phase.

The course of action of adsorption can be studied in graphs known as adsorption isotherm. Adsorption isotherm is the graph formed between the quantity of adsorbate adsorbed on the surface of the adsorbent and pressure at a consistent temperature. Different adsorption isotherms are Freundlich and Langmuir.

Based on the kind of forces present between the adsorbent and the adsorbate molecule, two types of adsorption are there:

• Physical adsorption:
If the forces of attraction take place between adsorbate and adsorbent are Vander Waal’s forces then the adsorption is known as physical adsorption. By reducing the pressure or by heat, this type of adsorption can be reversed.

• Chemical adsorption:
If the forces of attraction being held between, adsorbate and adsorbent are of the same energy as chemical bonds then the adsorption is called chemical adsorption. This type of adsorption is also referred to as chemisorption or Langmuir adsorption. They are unchangeable.

Types of Forces in Adsorption Chromatography

• Dipole-dipole attraction:
It is the variety of forces taking place between polar adsorbent and polar solutes.

• Hydrogen bonding:
They are weaker than covalent bonds. They are formed between OH group hydrogen and electronegative atoms such as nitrogen, oxygen in solutes. Eg. OH-Si.

• Polarizability forces:
It takes place between polar adsorbents and solutes that can polarize. Weak covalent bonds:
It continues during complex formation.

• Vander Waals forces:
Non-polar forces of magnetism occur between the atoms of nuclei and electrons of other atoms.

• Elution:
Solutes lead to liquefy and transfer with the mobile phase.

• Displacement:
Where the solvent molecules collide with the solutes for the adsorption sites on the stationary phase, this competition makes the solutes travel at various speeds.

Methodology

Suppose a tubular column with the top open to the atmosphere and a restricted outlet. This column is placed with a finely distributed solid such as silica, alumina, calcium carbonate, or charcoal, which will physically adsorb but will not chemically react with the solute components.

The equilibrium is attained between the suspended solutes in the liquid phase and the adsorbed solutes in the solid phase. When a given volume of a proper solvent containing two components X and Y, is included at the top of the column, both of the components will be adsorbed near the top of the packing. An eluent is added, the minor amount of strongly adsorbed Y will be selectively removed and will migrate before X until the whole partition is attained.

Limitations

• The sample molecules are adsorbed only at active sites.

• The active sites are influenced by the dimensions of the crystal lattice which makes the surface.

• The correct choice of solvent and the solid substrate is involved in liquid-solid chromatography.

• The solid substrate loses its position with time to handle samples of suitable size.

Applications

• It is used to establish the identity or non-identity of two substances; the two substances under test are mixed and the solution is poured over a vertical adsorbent column.

• It is used for the partition of high molecular weight compounds, this cannot be analyzed by gas chromatography owing to their high boiling points.

• It is used as a presumptive test for the examination of samples collected for workplace drug testing.

• It is used for the examination of explosives.

• It is used for the examination of colored materials.

• It is used for the detection of the sample in the case of performance-enhancing drugs used by athletes.

• It is used for the detection of the samples which are collected in cases of alleged drug-facilitated sexual- assault.