What Are Those Forces During SPE?

Solid Phase Extraction (SPE) is a widely used technique for the selective extraction, purification, and concentration of specific compounds from complex sample matrices. Several forces and interactions play a crucial role in the SPE process. These include:

Adsorption: Adsorption is the fundamental force in SPE. It involves the attraction of target analytes to the solid-phase sorbent. Adsorption can occur through various mechanisms, including van der Waals forces, dipole-dipole interactions, hydrogen bonding, and ion-exchange interactions, depending on the nature of the sorbent and the analytes.

Hydrophobic Interactions: In reversed-phase SPE, hydrophobic interactions are prominent. Nonpolar analytes tend to be attracted to hydrophobic sorbents. This is achieved by using nonpolar solvents (elution) or polar solvents (washing) to selectively desorb or retain analytes based on their hydrophobicity.

Ion-Exchange Interactions: In ion-exchange SPE, charged analytes are attracted to oppositely charged functional groups on the sorbent. Cation-exchange sorbents attract positively charged ions (cations), while anion-exchange sorbents attract negatively charged ions (anions).

Ion Interaction refers to an attraction force (known as Coulomb force) between target compounds with a positive/negative charge and absorbents with the opposite charge.

Size Exclusion: Size exclusion, also known as gel filtration, is used when the sorbent has pores or channels that exclude molecules above a certain size. Smaller analytes enter these pores and are retained, while larger molecules are excluded and eluted.

Affinity Interactions: Affinity SPE uses specific binding interactions, such as antibody-antigen, receptor-ligand, or enzyme-substrate interactions, to selectively capture target analytes. This is particularly useful for highly specific extraction of certain biomolecules.

Van Der Waals Forces: Van der Waals forces are weak attractive forces that arise from temporary fluctuations in electron density around atoms and molecules. They play a role in the adsorption of nonpolar analytes to sorbent surfaces.

Hydrogen Bonding: Hydrogen bonding occurs when hydrogen atoms are shared between the sorbent’s functional groups (e.g., hydroxyl groups) and polar analytes. This interaction is essential in polar sorbents or for polar analytes.

Dipole-Dipole Interactions: Dipole-dipole interactions occur when polar analytes are attracted to the dipoles of polar functional groups on the sorbent. This interaction can be significant for analytes with substantial dipole moments.

Polar interaction is the interaction between the polar functional group on the target compound and the polar functional group on the adsorbent. Polar interactions include Hydrogen Bonding, Dipole/Dipole, Induced Dipole/Dipole, π-π (Pi-Pi), and many other interaction forces. This force can be better reflected in a weakly polar or non-polar solvent environment.

Non-polar interaction is the interaction between a non-polar functional group on a target compound and a non-polar sorbent, which is better reflected in a polar solvent environment, especially in a water environment. Also known as hydrophobic interactions, such as the interaction between phthalate compounds and C18 in aqueous environments.

π-π Stacking: π-π stacking interactions involve the stacking of aromatic rings in analytes with those in the sorbent. This is common in reversed-phase SPE with aromatic analytes.

Electrostatic Interactions: Electrostatic interactions occur when charged analytes are attracted to oppositely charged functional groups on the sorbent. This is especially important in ion-exchange SPE.

Hydrophilic Interactions: Hydrophilic interactions can be used to selectively retain polar analytes on polar sorbents. This is achieved by using aqueous eluents and selecting analytes that interact favorably with the sorbent’s polar functional groups.

The choice of sorbent and the optimization of SPE conditions (such as sample loading, washing, and elution solvents) are critical for achieving selective and efficient analyte extraction based on these forces and interactions. Understanding the specific interactions involved in a given SPE method is essential for successful sample preparation and purification.

Secondary Interaction, for reversed-phase silica bonds and adsorbents, the residual silanol groups on the surface of the particles will interact polarly with polar compounds, and some of the silanol groups will undergo ionic interaction with basic compounds after dissociation.

These forces are secondary to non-polar interactions and therefore are referred to as secondary interactions. Secondary interactions are undesirable for reversed-phase silica sorbents and can usually be eliminated by the End capping technique.