Sample preparation is a very time consuming and tedious and easy to introduce analysis process of measurement error. Sample pretreatment method of sample analysis plays a very important.
Interaction mechanism between adsorbent and target compound in solid phase extraction.
Solid phase extraction(SPE) is mainly retained / adsorbed by the force between the functional groups on the solid-phase adsorbent and the functional groups of the target compound. This force can be divided into non-polar force, polar force, ionic force, covalent force and so on.
Nonpolar force: for bonded silica gel and polymer adsorbents, the nonpolar force is generated between the hydrocarbon bond on the functional group of solid-phase extraction material and the hydrocarbon bond of the target compound. It is often used to adsorb and separate the target compound with nonpolar structure from the sample matrix. Such as welchrom ® C18、Welchrom ® C8、Welchrom ® Phynyl et al.
Polar force: polar force occurs between the polar surface of many solid phase extraction materials and the polar functional groups of the target compound in the sample. Common adsorbents with polar force are generally called normal phase chromatographic adsorbents in chromatography. Common polar solid phase extraction materials include silica gel, alumina, Flori silica and bonded silica gel containing cyano, amino and diol groups.
Ionic force: the ionic force occurs between the target compound with opposite charge and the functional group of solid phase extraction adsorbent. Such as strong cation exchanger welchrom ® SCX, strong anion exchanger welchrom ® Sax, weak cation exchanger welchrom ® WCX et al.
Covalent force: the covalent force occurs between the covalent filler and the target compound, and the covalent bond is not easy to be interrupted, but the covalent bond formed by some functional groups is reversible under the condition of changing the solvent environment, such as phenylborate group.
Effect of pH value on Solid Phase Extraction
The pH value can change the degree of ionization or protonation of the target / adsorbent. For strong cation / anion exchange column, because the adsorbent itself is completely ionized, the target must be completely ionized to ensure that it can be completely adsorbed and retained by the adsorbent. The ionization degree of the target is related to the pH value. For example, for weakly alkaline compounds, the pH value must be less than two units of its pKa value to ensure the complete ionization of the target, while for weakly acidic compounds, the pH value must be greater than two units of its pKa value to ensure the complete ionization. For the weak anion / cation exchange column, it is necessary to ensure the complete ionization of the adsorbent to ensure the complete adsorption of the target, and the pH value of the solution must meet certain conditions to ensure the complete ionization.
Operation steps of Solid Phase Extraction
The operation is slightly different according to the different retention mechanism of the filler (the filler retains the target compound or impurities).
* Retention mode of target compound
Solid phase extraction generally has four steps:
① Activation —- remove impurities in the column and create a certain solvent environment.
② Sample loading —- dissolve the sample with a certain solvent, transfer it into the column and keep the components on the column.
③ Leaching – remove interferences to the greatest extent.
④ Elution — elute the tested substance with a small volume of solvent and collect it. (note that the flow rate should not be too fast)
* Impurity retention mode
Solid phase extraction generally has three steps:
① Activation —- remove impurities in the column and create a certain solvent environment.
② Loading – transfer the sample into the column. At this time, most of the target compounds will flow out with the sample base solution, and the impurities will be retained on the column. Therefore, this step should start to collect.
③ Elution — elute the components with a small volume of solvent and collect them, and combine the collected solution. (note that the flow rate should not be too fast)
Application characteristics of solid phase extraction column (SPE)
- The properties of the sample solution system should meet the requirements of solid phase extraction. For example, in the mode of C18 adsorbing the compound to be tested, the proportion of organic solvents in the sample solution system should not be too high; In the mode of C18 adsorbing the sample matrix, the proportion of organic solvents in the sample solution system should not be too low.
- Pay attention to the bearing capacity of fillers to prevent the test results from being inaccurate due to overload of samples.
- The flow rate through the column should be appropriate and should not be too fast or too slow, especially the speed of sample solution on the column and elution should be controlled. Generally, the flow rate through the column is 1 ~ 3ml / min.
- For SPE columns with silica gel matrix, the sample solution shall not be dried up before being put on the column. If it dries up, the small column should be activated again.
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