What’s Solid Phase Extraction?

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Solid Phase Extraction (SPE) is the name of a process that separates solid and liquid substances in a chemical solution. Extraction is often done by passing the sample through a column filled with an absorbent, which allows the liquid to pass through the apparatus while the solid remains collect above. The purpose of solid phase extraction is to concentrate and purify samples for further study. These analytes could be contained in the solid or liquid matter. If the material to be analyzed is the solid, it will then be purified by a solvent rinse.

A general solid phase extraction uses a disposable cartridge that looks like a medical syringe. This cartridge sits atop a vacuum manifold to maximize the rate at which solvent flows through the column. The cartridge contains C18 silica, which acts as an absorbent and is held in place with frits. A collection tube is placed under the SPE column to receive the liquid removed from the mixture.

The chemist typically uses a four-step process when performing a SPE. The first step is to equilibrate the cartridge by wetting the surface with a non-polar or only slightly polar solvent. This accomplishes penetration of the covalently bonded phase so that separation can begin.

Second, water or a buffer similar in nature to the sample being used is applied to the column. This liquid washes the entire column. It also saturates the surface of the cartridge with silica, which may begin to dissolve.
Only in the third step is the actual sample inserted into the cartridge. The sample passes through the stationary phase of the extraction process and the analytes begin to interact and remain on the absorbent material of the column. Liquids, including salts, solvent and various impurities, proceed through the column to be collected in the vessel below. A vacuum apparatus could be employed to facilitate the process.

The final step is to elute or wash the column to remove further impurities from the cartridge, which contains the sample. The elution used is a buffer that has the appropriate potency hydrogen (pH) level or a non-polar solvent. The analyte is then ready for further chemical analysis.

Solid phase extraction is often considered preferable to liquid-liquid extraction because it takes less time and can be less expensive. SPE is similar to chromatography in that a liquid mixture is dispensed through a packed column which removes analytes and impurities. The procedure can be performed on a large scale by using manifolds to hold multiple columns so that several samples can be processed simultaneously.

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