The 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 solution.
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 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 Extraction Efficiency
Factors affecting extraction efficiency refer to choose of 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 are 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 proper flow rate, do not let the column dry when activated.
Operation Steps and Precautions of SPE Cartridge
For the different retention mechanism 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 into the column, and keep the component on 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.
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.
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