Solid phase extraction (SPE) is a sample pretreatment technology, which is developed from the combination of liquid-solid extraction and column liquid chromatography. It is mainly used for the separation, purification and concentration of samples. Compared with the traditional liquid extraction method, it can improve the recovery of analytes, more effectively separate analytes from interfering components, reduce the sample pretreatment process, and is simple, time-saving and labor-saving. Liquid extraction is mainly to do some weak polar or non-polar targets, which is not applicable to those targets with too strong polarity or ionic targets, and there is no impurity removal process. The treated samples will contain non-polar impurities, which will not only pollute the chromatographic column and instrument, but also cause a certain matrix effect. In theory, non-polar, polar and ionic targets can be applied to solid phase extraction, which is also the reason why solid phase extraction technology is becoming more and more popular in recent years.
SPE solid phase extraction can be divided into two categories according to the retained substances: one is the retained target compound; The other is retained impurities (similar to the pretreatment operation of QuEChERS). SPE solid phase extraction can be divided into three types according to the type of filler: nonpolar retention type; Polarity retention type; Ion exchange type. The following figure shows the SPE processing process.

Step 1: activation

SPE is generally activated by a certain volume of organic reagent and a certain volume of water-soluble reagent through the column step by step. Activation has two purposes:

  • The organic reagent and water-soluble reagent pass through the column to remove some impurities in the small column, so as to avoid system pollution and influence on the sample loading effect.
  • Make the functional groups on the surface of the filler stretch more orderly, increase the contact area for the later sample solution and play a better role in retention.

    Before activation, the structural arrangement of polymers is irregular, and many structures are tightly held together like “wool balls”. This will cause many functional groups to be wrapped in the “thread group”, which can not make good contact with the target and retain it. Therefore, after activation, the arrangement of these structures will become very orderly, increase the contact area with the target, and retain the target well.

Step 2: Sample loading

After the sample is activated, we can carry out the pretreatment. After loading the sample, the target will be firmly retained on the SPE filler. Of course, some impurities with similar properties to the target will be retained on the spe. The remaining impurities will flow out directly with the loading solution.

Step 3: Rinsing

As the name suggests, the purpose of elution is to elute the impurities, avoid the pollution of chromatographic columns and instruments, and reduce the interference of impurities to the detection of target objects. This reduction of interference is the so-called reduction of matrix effect.

Step 4: Elution

Elution is to use reagents to destroy the force between the target and the filler, so as to separate and collect the target from the filler.

Step 5: Nitrogen blowing and re-dissolution

This process is an optional step, and there are usually two situations that require nitrogen blowing and re dissolution. First, there is a large difference between the eluent and the initial flow. If the sample is injected directly, there may be serious solvent effect and affect the peak type. Therefore, it is necessary to conduct sample injection analysis after nitrogen blowing and volatilization and re dissolving with the initial mobile phase. Second, when the sensitivity of the instrument cannot meet the analysis requirements of the actual sample, it can be re dissolved with a small volume of re solution after nitrogen volatilization, and the sample can be injected for analysis after increasing the concentration of the target substance.

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