Solid-Phase Extraction (SPE) is a highly selective mode of sample preparation and an extraction technique that facilitates the separation of the different compounds dissolved in a mixture (liquid). This separation takes place by virtue of the compounds’ physical and chemical properties. This methodology is used to isolate analytes of interest or the undesired impurities from mixtures for analytical purposes.
This extraction process involves a mobile and a stationary phase. The mobile phase of the SPE system is made up of the solvents that are either employed to wash a sample or to elute target analytes. The SPE sorbent serves as a stationary phase. SPE divides a mixture into desirable and unwanted components. It utilizes the affinity of solutes that are dissolved or suspended in a liquid (mobile phase) for a solid through which the sample is passed (stationary phase). As a result, the sample’s undesirable contaminants or the desired analytes of interest are retained on the stationary phase. Depending on whether it contains the desired analytes or undesirable contaminants, the part that goes through the stationary phase is either collected or discarded. If the desired analytes are present in the part that was kept on the stationary phase, they can then be extracted from the stationary phase for collection in a subsequent step in which the stationary phase is washed with the proper eluent.
SPE is a widely used system of sample preparation applications for a wide range of analytes from complex matrix. Although it provides the provision of acquiring a variety of desired analytes, when compared with other techniques of sample preparation like Liquid-Liquid Extraction (LLE), protein precipitation, etc, the SPE demands more effort and time. Nonetheless, its selectivity in extraction makes it eligible to extract the purest samples.
To optimize the process, it is important to know about the fundamentals of the SPE technique so that the correct mode of extraction is chosen. The correct mode also depends on the behavior of analytes and how they interact. While looking for the data which can help get faster recovery of desired analytes, it is necessary to look into the causes for low recovery too, so as to find appropriate remedies.
Principle steps of SPE
- Sample pretreatment: In order to optimize the chemistry of the sample and make it compatible with the SPE procedure, the pretreatment process is undergone. It incorporates the removal of particulates, adjustment of the sample archetype for retention purposes and separating the samples that are bound to each other.
- Conditioning: The SPE sorbent is conditioned in order to remove any kind of impurity that presides since the manufacturing process. This process helps in the activation of the surface of the sorbent with the motive to promote interaction of the analytes.
- Equilibration: This process is performed right before the sample is loaded. In order to increase the compatibility of the sample with the SPE procedure, this process is performed. This process creates an environment of the sorbent which almost is like that of the sample. The most important factor considered during this process is pH.
- Sample loading: In this sample loading process, an environment is created due to which the analyte when added with the loading solvent, is not retained on the sorbent but simply passes through as a clean extract. Linear velocity of the sample needs to be positively judged in this step.
- Column Washing: To remove undesired contents, this column washing step is performed. The washing solvent used to clean the column usually has a higher elusion capacity than the samples due to which they can elute faster, and the samples do not.
- Analyte elution: This is the last step of the process where the analyte is eluted from the column. The elution solvent must be able to disrupt the retentive bonds between the sample and sorbent and concurrently make sure the unwanted contaminants are not eluted.
While there are a lot of things to keep in mind while performing SPE procedures, the main thing that must be addressed as a priority is the sorbent utilized. If not, recovery of the samples will be hampered. Analyte functional groups and their polarity should be used to judge the use of sorbents for SPE. Also, the sample matrix needs to be taken into consideration while deciding which sorbent to use. Most manufacturers of sorbent would have charts that could help in the decision-making process.
References:
- “Solid Phase Extraction (SPE).” Solid Phase Extraction (SPE), www.sigmaaldrich.com/IN/en/applications/analytical-chemistry/sample-preparation/solid-phase-extraction.
- “Solid-phase Extraction – Wikipedia.” Solid-phase Extraction – Wikipedia, en.wikipedia.org/wiki/Solid-phase extraction.
