Solid phase extraction (SPE) is a sample preparation technique. It relies on a solid adsorbent (or sorbent) in a cartridge or disk. As a liquid solution with a matrix (such as plant material) passes over the cartridge, the sorbent retains the target analyte. SPE can thus isolate certain compounds for extraction or cleanup.
Following retention, the target compound is eluted (or removed) from the sorbent with a solvent. Alternatively, SPE may adsorb the non-desired matrix compounds and leave the target compound behind.
The combination of solute and sorbent depend on hydrophobic, polar, and ionogenic properties.  Sigma Aldrich notes several SPE methods:
- Normal Phase: polar analyte, mid- to non-polar sample matrix (e.g., hexane), and polar stationary phase (e.g., silica gel). A solvent more polar than the matrix is used for elution.
- Reversed Phase: mid- to non-polar analyte, polar sample matrix (e.g., acetic acid), and nonpolar stationary phase (e.g., end-capped silica). A nonpolar solvent is used for elution.
- Ion Exchange: based on electrostatic attraction between a charged compound and a charged sorbent surface. Different types of bonded silica may be used based on whether the target is positively or negatively charged.
Silica is a common choice for sorbent since it can withstand heat and does not swell.  Manufacturers modify the surface with functional groups to change the chemical properties of silica depending on the application/method. 
Thermo Fisher Scientific outlines the steps involved in SPE:
- Pre-treat the sample: ensure appropriate dilution/ionic strength and pH; filter out particulates
- Condition the column: use a solvent similar to the analyte and prevent drying of sorbent
- Re-equilibrate the column: use the pre-treatment solvent to buffer the column
- Apply the sample: slower flow rates (e.g., 1 ml/min) may help ensure analyte retention
- Wash the stationary phase: use an intermediate solvent to remove compounds clinging to the sorbent other than the analyte of interest
- Elute the analyte of interest: apply strong solvent to disrupt the analyte-sorbent attraction
SPE can simplify complex matrices, which is critical when analyzing samples using mass spectrometry. It can also isolate and purify target compounds, even those present at low levels. Applications are manifold. As an example, in the cannabis industry, SPE is used to test pesticide levels and refine crude extracts. Automation may empower its scalability for extractors.
Image: Solid-phase extraction cartridges; Jeff Dahl, CC BY-SA 3.0
- Żwir-Ferenc A, Biziuk M. Solid phase extraction technique – trends, opportunities and applications. Polish J of Environ Stud. 2006;15(5):677-690. [Impact Factor: 1.383; Times Cited: 224]