Three Types of SPE Cartridges: Silica Gel, High Polymer, Inorganic Material

Solid phase extraction(SPE) is the separation principle of liquid chromatography using selective adsorption and selective elution. The more commonly used method is to pass the liquid sample solution through the adsorbent to retain the test substance, and then use a solvent of appropriate strength to wash away impurities, and then use a small amount of good solvent to quickly elute the test substance, so as to achieve the purpose of rapid separation, purification, and concentration.

Also, it is able to selectively absorb interfering impurities and make the measured material flow out. Or absorb impurities and the tested substance at the same time, and then use an appropriate solvent to selectively elute the tested substance.

On the market, there are various types of SPE cartridges. In the specific experimental work, it is necessary to select appropriate packing and reasonable specifications of SPE columns according to the analysis object, detection method, and laboratory conditions. Consider the extraction capacity of the SPE column for the analytical object, the volume of the sample solution, the final volume of the solution after elution, and the total amount of analytes and interferences in the sample solution.

In addition to the required specifications, the key to selecting an SPE cartridge is its packing material that determines the separation performance. When choosing an extraction column, you must choose a suitable packing according to the type of sample to be tested and its physical and chemical properties. The packings are usually chromatographic adsorbents, which can be roughly divided into three categories, which are based on silica gel, polymers, and inorganic materials.

Silica gel
The first type is based on silica gel, such as C18 solid phase extraction cartridge.

Being polar and weakly acidic, the silica gel is can be used in two separation modes: normal phase or reverse phase. The performance is weaker than silica gel, and the non-polarity is weaker than C18 or C8 in reverse extraction. 

With good extraction effect on steroids, usually it is used to extract non-polar or weak polar compounds or to remove polar impurities. It is mainly used in blood samples, urine samples, drugs and their metabolites, polypeptide desalination, trace organic compound enrichment in environmental samples, organic acids in beverages.

Polymers
Secondly, it is based on polymers, taking polystyrene-divinylbenzene for example. The extraction cartridge packed with high-purity, high-crosslinking styrene-divinylbenzene polymer as a stationary phase has a high sample load, can withstand extreme pH conditions and different solvents and has excellent resistance to polar compounds retention capacity.

It can be used as a general-purpose adsorbent for acidic, neutral, and basic compounds usually used to retain hydrophobic compounds containing hydrophilic groups under reversed conditions, such as phenols, nitroaromatics, nitroamines, nitrates, etc.

Inorganic materials
Thirdly, it is based on inorganic materials, for example, graphitized carbon, alumina, and Flory diatomaceous earth, and so on. Florisil is a kind of polar silica gel adsorbent composited with magnesium oxide. The matrix based extraction cartridge is suitable for absorbing polar compounds such as POLYchlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and organochlorine residues from non-polar matrices.

Graphitized carbon black (CARB) extraction cartridge, with graphitized carbon black as the filler, the extraction process is very rapid, and the adsorption capacity for compounds is more than double that of silica gel. It has a strong affinity for planar molecules and is very suitable for the extraction and purification of many organic materials, especially for the separation, thanks to the regular six-member ring structure on the surface of graphitized carbon black.

Or remove various substrates such as pigments, sterols, phenols, and other substances in fruits and vegetables; fillers based on alumina include acid, alkali, and neutral types, which are suitable for separation and extraction of acidic, alkaline, and neutral solvents.