liquid-liquid extraction
Liquid-liquid extraction is coaxing a substance to move from one liquid into another that won't mix with it — the way oil and vinegar in a salad dressing stay in separate layers, and a flavour can prefer one layer over the other. Shake water and an oily solvent together in a bottle, let them settle into two layers, and the compound you want gathers in whichever layer it likes best.
Mechanically, the sample (usually in water) is shaken with an immiscible organic solvent in a separating funnel. Each dissolved compound splits between the two layers according to how soluble it is in each — its partition coefficient. The layers then separate by density, and you draw off the one carrying your analyte, leaving much of the unwanted matrix behind in the other.
It is a classic, cheap, and versatile way to pull a target out of a messy sample and away from interferences, and it can concentrate the analyte at the same time. The honest drawbacks: it uses sizeable volumes of organic solvent, can form stubborn emulsions that refuse to separate, and rarely extracts a compound completely in one shake — so chemists often repeat the extraction several times.
To recover pesticide residues from water, the analyst shakes the sample with dichloromethane in a separating funnel; the pesticides cross into the dense organic layer, which is drained off and concentrated.
The target migrates into the layer it prefers, away from the matrix.
How completely a compound transfers depends on its partition coefficient and the pH. Adjusting pH can switch an organic acid or base between a charged form (which stays in water) and a neutral form (which crosses into the solvent) — a simple lever for steering what gets extracted.