Basic Principles and Optimization of SPE Cartridge

Solid phase extraction (SPE) is a sample preparation technique to isolate a species in a sample or to clean up a sample before analysis. As the sample passing through the SPE cartridge, the solid adsorbent contained most commonly in a cartridge device can adsorb select species from the solution.

You can choose Reversed Phase C18A SPE Cartridges, SCX SPE Cartridges, SAX SPE Cartridges, Normal Phase PR Grade Florisil SPE Cartridges, and other SPE cartridges using. This cartridge is mainly used in sample pre-treatment of target compounds in various foods, agricultural and livestock products, environmental samples, and biological samples. There are many types of solid-phase extraction cartridges. In specific experimental work, it is necessary to reasonably select suitable packing and reasonable specifications of solid phase extraction cartridges according to the analytical objects, detection methods, and laboratory conditions.

Principle of SPE cartridges

SPE is a physical extraction process including liquid and solid phases; it can also be taken as a simple chromatographic process. The more commonly used method is to pass the liquid sample solution through the adsorbent to retain the tested substance, then use a solvent of appropriate strength to wash away impurities, and then use a small amount of solvent to quickly elute the tested substance, so as to achieve the purpose of rapid separation, purification, and concentration.

Specification of SPE cartridges

The choice of solid phase extraction column is generally considered from two aspects: the choice of adsorbent and column specifications. The most commonly used one is made of polyethylene. The device is equipped with two plugs made of polypropylene or glass fiber, and a certain amount of chromatographic adsorbent is filled between the two plugs.

According to the volume of space above the classic packing, the specifications can be divided into 1 ml, 3 ml, 6 ml, 10 ml, 15 ml, 20 ml, 30 ml, 60 ml, and so on. There are also special specifications produced according to specific application requirements. From the outside, the classic one is similar to the injection syringe and is straight. The other is funnel-shaped at the top, which is convenient for loading more samples at one time during manual operation.

Packing of solid phase extraction cartridges

Solid phase extraction packings are usually chromatographic adsorbents, which can be divided into three categories: the first type is based on silica gel (such as C18, C8, etc.); the second type is based on polymer, such as polystyrene-diethylene Benzene, etc.; the third category is based on inorganic materials, such as Flori diatomaceous earth, alumina, graphitized carbon and so on.

The capacity of solid phase extraction cartridges

The capacity of the SPE column refers to the adsorption capacity of the filler. For the silica gel matrix, its capacity is generally 1~5 mg/100 mg, that is, the column capacity is 1%~5% of the mass of the packing. The capacity of the bonded silica gel ion exchange adsorbent filler is expressed in meq/g, that is, the capacity of each gram of packing is X milliequivalents and 0.5 to 1.5 meq/g is usually used.

When selecting, the column capacity must be considered. Because the sample matrices we face are usually more complicated, such as food, biological samples, and so on. The SPE adsorbent can adsorb the target compound, while also absorbing impurities of the same nature. Therefore, considering the column capacity should be the target compound plus the total amount of impurities that can be adsorbed cannot exceed the column capacity. Otherwise, some target compounds may not be adsorbed during the sample loading process, resulting in a low recovery rate.

Effect of pH values on SPE cartridge

The pH values can change the degree of ionization or protonation of the target or adsorbent. For the strong cation/anion exchange column, because the adsorbent itself is in the fully ionized state, the target must be fully ionized to ensure that it is completely absorbed and retained by the adsorbent. And the degree of ionization of the target is related to the pH value.

The pH value must be less than two units of its pKa value for the weak alkaline compound to ensure the complete ionization of the target, while for the weak acidic compound, the pH value must be greater than its pKa value two units to ensure its complete ionization. For the weak anion/cation exchange column, it is necessary to ensure the complete ionization of the adsorbent to ensure the complete absorption of the target, and the pH value of the second solution must meet certain conditions to ensure its complete ionization.

Factors affecting the extraction efficiency

Factors affecting extraction efficiency refer to choose of an appropriate filler, especially the stationary phase because suitable SPE columns are a prerequisite to ensure ideal results. The strength of the eluting solvent is also a significant factor when the positive phase stationary phase is used, the solvent strength increases with its polarity enhancement; when the reverse phase stationary phase is used, the solvent strength increases with the decrease of polarity.

What’s more, pH values play an important role in extraction efficiency. The pKa of ion-exchange stationary phase, analyte, and interfering substance is varied greatly.

By adjusting the size of the pH, the stationary phase can be charged, and the analyte is charged oppositely so that the interfering substance is not charged; or in turn, the stationary phase is charged, the interfering substance is charged oppositely, and the analyte is not charged. Furthermore, the operation cannot be ignored, it controls the proper flow rate and does not let the column dry when activated.

Operation steps and precautions of SPE cartridge

For the different retention mechanisms of filler, the operation is slightly different, and the specific steps are as follows:

1. Packing retention target compound: solid-phase extraction operation generally has four steps:

• A. Activation: remove impurities in the column and create a certain solvent environment.
• B. Go ahead. Dissolve the sample with a certain solvent, transfer it into the column, and keep the component in the column.
• C. Leaching. Remove the disturbance to the maximum extent, and it is best to drain the column completely after this process.
• D. Elution. It is better to elute the tested substance with a small volume of solvent and collected it.

2. Filler retains impurities: solid phase extraction operation typically involves three steps:

• A. Activation: remove impurities from the column and create a volume environment but do not dry the column throughout.
• B. Sampling: When the sample is transferred in, most of the target compounds flow out with the sample base solution, and the impurities are retained on the column.
• C. Elution: the components were eluted and collected with a small volume of solvent, combining the collection solution. This condition is used to remove pigment from food or agricultural residue analysis.