The retention mechanism of solid-phase extraction can be divided into two categories. Most compounds apply this mechanism that fillers retain target compounds, filler retains its target component and a small amount of impurity, a small amount of impurity adsorbed on the column is removed by elution step, and finally, a suitable solvent is selected to elute the target component. According to the retention mechanism of adsorbent, it can be further divided into:
(1) Inverse phase: it comprises C18, C8, CN, Phenyl, C4, and C1. Analytics is from non-polar to moderate polarity. Matrix is water solubility. Methods are as followed: a. activation: usually activated with water-soluble organic solvents such as methanol, followed by water balance; b. elution: elution of impurities by buffer solution containing 0-50% polar solvents; c. elution: polar or nonpolar solvent elution of target.
(2) Positive phase: it comprises Silica, Florisil, Diol, and NH2. The analyte is from moderate to strong polarity. The matrix is from nonpolar to medium polarity. The specific methods are as followed: a. activate nonpolar organic solvent; b. elute nonpolar organic solvent.
(3) Cation exchange: the specific methods include activation, sample loading, and elution. Firstly, activation: when used in a sample in a nonpolar organic solvent, the sample solvent may be used to activate; when used in a sample in a polar solvent, the water-soluble organic solvent may be used after the column, then balanced with water, and finally with a buffer solution of appropriate pH value. Secondly, sample loading: the pH value of the sample solution is less than its pKa two units to ensure its charge.
(4) Anion exchange: the methods are as followed: firstly, activation: sample solvents are available for use in samples in nonpolar organic solvents; water-soluble organic solvents are available for use in samples in polar solvents, then balanced with water and, finally, with buffer solutions of appropriate pH values. Secondly, sample loading: the value pH the sample solution is greater than its pKa two units to ensure that it is charged. Third, elution: the value pH the elution solution should be less than its two units pKa.
This mechanism is used to remove impurities such as pigments in food. Fillers retain impurities without or only a very small number of target components, so after the sample began to collect the target components, then the target in the solvent is further elution, and finally combined upper sample and eluent effluents.