solid-phase extraction
Solid-phase extraction pulls a target out of a liquid by passing the sample through a small bed of sticky particles that grab onto the molecules you want, the way a coffee filter holds the grounds while the water runs through. The unwanted liquid drips away; your analyte stays caught on the particles, ready to be rinsed off later in a clean, concentrated form.
In practice, the sample is drawn through a small cartridge or disk packed with a chemically tailored solid. The analyte sticks to that solid while most of the matrix flows past; a wash step rinses away loosely held junk; then a small volume of a stronger solvent elutes the analyte off in pure, concentrated form. Pick a solid that grabs your target tightly but lets the matrix through, and you have cleanup and preconcentration in one.
SPE has largely displaced liquid-liquid extraction for many jobs because it uses far less solvent, almost never forms emulsions, and is easy to automate. The caveats: the cartridge can clog or overload if you push too much through it, and choosing the right solid and solvents for a given analyte takes some method development.
A urine sample is drawn through a small SPE cartridge that traps a drug of interest; the salts and proteins flow to waste, then a splash of methanol washes the clean drug into a tiny vial for analysis.
Particles grab the analyte; the matrix flows to waste.
SPE and the column chromatography used for separation work on the same idea — molecules sticking to and releasing from a solid — but the goals differ. SPE is sample preparation: a quick grab-and-release to clean up and concentrate before the real analysis. Chromatography is the analysis: a long column that separates many compounds from one another.