Building a Solid Foundation for Success: Exploring the Basics of Solid-Phase Extraction

1 Related Terminology:

1. Conditioning: The process of preparing the SPE column to achieve the optimal state for adsorbing the target compounds. (Activation and equilibration)
2. Washing: The purpose is to remove impurities that may interfere with the analysis of the target compounds before elution. (Rinsing)

Requirement: To remove impurities to the maximum extent possible without significant loss of the target compounds.

3. Elution: The process of eluting the target compounds from the SPE column using an appropriate solvent.

Solvent selection: The solvent should be chosen to maximize the elution of the target compounds while minimizing co-elution of impurities.

4. Loading: Also known as sample loading. The process of introducing the sample into the SPE column and passing it through.
5. Interferences: Also known as impurities. Refers to substances that have a negative impact on the extraction or detection of the target compounds.

2 Basic Principles

The basic principle involves the selective adsorption of target compounds and interfering compounds from a liquid sample using a solid adsorbent (packing material), followed by elution with a solvent to remove interferences or concentrate the target compounds.

Essence: The interaction between the functional groups on the solid-phase adsorbent and the functional groups of the target compounds.

Project Case: Liquid Phase Separation
( separation techniques in the liquid phase )

01 Nonpolar Interactions—Reverse phase adsorbents / Nonpolar adsorbents / Hydrophobic adsorbents

The interaction between the hydrocarbon bonds on the functional groups of the solid phase extraction material and the hydrocarbon bonds of the target compound is governed by van der Waals forces and dispersion forces.

Examples: C18 (Si-), C8 (Si-), C4 (Si-), C2 (Si-), PHE (phenyl) (Si-), CH (cyclohexyl) (Si-), graphite carbon black, porous graphitized carbon, PSD, etc. Typically, nonpolar reverse phase SPE columns are suitable for extracting and separating nonpolar and moderately polar target compounds from polar matrices.

02 Polar Interactions—Normal phase adsorbents / Polar adsorbents

The forces between the polar surface of the solid-phase extraction material and the polar functional groups of the target compounds in the sample include hydrogen bonding, induction forces, dipole-dipole interactions, π-π interactions, etc.

Common polar functional groups include hydroxyl groups, amino groups, thiol groups, carbonyl groups, aromatic rings, and heteroatoms such as oxygen, nitrogen, sulfur, phosphorus, etc.

In polar interactions-based solid-phase extraction, the sample matrix is typically nonpolar, while the target compounds often contain larger polar functional groups.

Examples: Si, alumina, Diol (diol-based), NH2 (amino-based), CN (cyanide-based), etc.

Graphite carbon black and porous graphitized carbon can also be used for the extraction of polar compounds.

03 Ionic Interactions—Ion-exchange adsorbents

Ionic interactions occur between target compounds with opposite charges and the functional groups of the solid-phase extraction adsorbent—electrostatic attraction.

Among the three types of interactions, ionic interactions have the strongest strength and the best selectivity.

Conditions for effective extraction:

1. The pH of the environment must result in the target compounds and the adsorbent having opposite charges.
2. The environment should not contain high concentrations of competing compounds with the same charge as the target compounds.

Examples: PSA (Si-), PRS (Si-), SAX (Si-), SCX (Si-), WCX (Si-), MCX (PS-DVB-), MAX (PS-DVB-), PWAX (PS-DVB-), etc.

04 Multiple Interactions – Secondary Interactions
Example: Solid-phase adsorbents with bonded silica gel as the adsorbent material (silanol groups on the bonded silica gel), such as GCB/PSA, GCB/NH2, etc.

05 Molecular recognition mechanism: Immunoaffinity solid-phase extraction (IASPE)
In an immunoaffinity column, a gel containing antibodies bound to the target analyte is present. The sample, which contains the target toxin, undergoes extraction, filtration, and dilution. It is then slowly passed through the immunoaffinity column. Inside the column, the target analyte binds to the antibodies, while other irrelevant substances pass through the column without binding. Subsequently, the immunoaffinity column is washed to remove any unbound irrelevant substances completely. Finally, the target analyte is released by adding methanol and then injected into an analytical instrument for further detection.