Solid-phase extraction, abbreviated as SPE, is a sample pre-treatment device developed from chromatographic columns for extraction, separation, and concentration. It is developed by combining liquid-solid extraction cartridges with liquid chromatography technology and is mainly used for sample separation, purification, and concentration. Compared with traditional liquid-liquid extraction methods, SPE can improve the recovery rate of analytes, more effectively separate analytes from interfering components, reduce sample preparation processes, and is simple, time-saving, and labor-saving to operate. It is widely used in fields such as medicine, food, environment, customs inspection, and the chemical industry.
With widespread use, many experiments may fail due to choosing the wrong size of solid-phase extraction cartridge, which causes the purification volume to exceed the capacity of the cartridge. Many people are distressed because they do not know how to choose the correct size of solid-phase extraction cartridge. The following will explain how to choose the appropriate size.

SPE columns are mostly injection syringe-like devices made of polypropylene (PP) material (see Figure 1).

Column tube
It measured in milliliters (mL), is the carrier for the entire solid phase extraction cartridge. It is selected based on the packing material and sample requirements. Common specifications include 3mL, 6mL, and 12mL. Under the same packing material specifications, some experiments can use similar cartridge tubes as substitutes. For example, a 200mg/3mL and a 200mg/6mL C18 solid phase extraction cartridge can be used interchangeably. Apart from the difference in maximum sample volume, the experimental results will not be affected.
The empty tube is equipped with two Frits made of polyethylene (PE) or glass fiber material, one on top and one on bottom, which mainly serve to fix the adsorbent and filter. The frit size is generally selected according to the inner diameter of the empty tube.
  The space between the two frits is filled with a certain amount of chromatographic adsorbent, which can be selected according to the concentration and amount of the purified sample. The core of the solid-phase extraction cartridge is the packing material, which is used in two modes: target retention (see Figure 1) and impurity retention (see Figure 2). The purpose of the retention mode is to enrich and concentrate the target analytes, while the impurity retention mode is mainly used for purification. The core of the purification process in both modes relies on the operation of the chromatographic adsorbent in the middle of the frit.

Common adsorbents include silica gel-based adsorbents, polymer-based adsorbents, and ion exchange adsorbents. They are selected according to different experimental needs.

Figure 2 Target retention mode
Figure 3 Retention of interferents

Silica-based matrix 
Translation: Silica gel-bonded adsorbents are suitable for a pH range of 2-8, showing rigidity and neither contracting nor expanding during solvent transformation. They can quickly reach equilibrium in new solvents and are a relatively ideal choice.

Polymer matrix
pH ranges from 0 to 14, making it widely applicable and compatible with most organic solvents. Its surface is non-polar and lacks active hydroxyl groups, eliminating the negative impact of secondary adsorption on the recovery of alkaline compounds. It has the advantages of high adsorption capacity, high recovery rate, and good reproducibility for most organic compounds. It overcomes the drawbacks of traditional silica-based packing materials and has been increasingly used in SPE.

 Ion exchange adsorbent
According to the nature of the charged groups covalently bound to the matrix, ion exchange adsorbents can be divided into cation exchange adsorbents and anion exchange adsorbents. Cation exchange adsorbents can dissociate anionic groups and retain basic compounds by adsorbing cations (alkaline substance ions) in the solution. Strong cation exchange columns can retain weakly basic compounds. Anion exchange adsorbents can dissociate cationic groups and retain acidic compounds by adsorbing anions (acidic substance ions) in the solution. Strong anion exchange columns can retain weakly acidic compounds.
Solid-phase extraction capacity refers to the amount of extracted adsorbent that can adsorb and retain the compound. It is usually expressed in mass units, i.e. the maximum mass of the retained compound per 100 milligrams of adsorbent (mg/100mg). The formula for calculation is:C (column capacity) = mass of retained compound (mg) / mass of extraction filler (100mg)For a given solid-phase extraction column, the total amount of compound that can be retained is:C (total amount) = C (column capacity) × total mass of extraction column.
For solid phase extraction adsorbents based on silica gel, their capacity is generally between 1-5 mg/100 mg, that is, 1%-5% of the adsorbent mass. The capacity of polymer-based fillers is generally 3-5 times that of silica gel-based fillers, and their maximum capacity does not exceed 15% of the adsorbent mass. The capacity of bonded silica gel ion exchange adsorbents is expressed in mep/g adsorbent and is generally between 0.5-1.5 mep/g.
So, when selecting a solid-phase extraction column, do we only need to determine the size based on the amount of target compound? Of course not, the maximum capacity of the solid-phase extraction column is not all for adsorbing the target compound. In many analyses, the content of the target compound is often very low and generally does not exceed the capacity of the solid-phase extraction column. However, in the experimental sample processing, there are many other compounds in addition to the target compound, which are commonly referred to as impurities. Therefore, when considering the extraction column capacity, the total amount of the target compound and possible retained impurities must be taken into account. That is to say, the total amount of the target compound plus the possible retained impurities must be less than the column capacity, otherwise, the column will be overloaded, which will result in some of the target compound being lost with the sample matrix during the sample passing through the column. Therefore, for the target compound, the actual capacity of the solid-phase extraction column is:C (actual capacity) = (retained compound (mg) – retained impurities (mg)) / adsorbent weight

In summary, before selecting a solid phase extraction column, first confirm the required adsorbent for the experiment, then determine the column loading based on the quantity and concentration of the sample to be purified, and finally determine the adsorbent carrier specifications.

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