Solid phase extraction (SPE) cartridges: a practical introduction

What's inside an SPE cartridge

A packed bed of sorbent material—silica-based, polymeric, or specialty chemistries—held between frits inside a barrel, typically used with vacuum or positive pressure to draw sample through. The sorbent's surface chemistry determines what it retains and releases.

Reversed-phase sorbents

The most common starting point for extracting nonpolar-to-moderately-polar organic analytes from aqueous samples—the sorbent surface is hydrophobic, so it retains organic analytes while polar matrix components pass through. C18-bonded silica is the classic example.

Normal-phase and ion-exchange sorbents

Normal-phase sorbents (polar stationary phase) suit nonpolar sample matrices where you're extracting polar analytes—less common than reversed-phase but useful for specific applications. Ion-exchange sorbents (cation or anion) retain analytes based on charge rather than polarity, useful when your analyte is ionizable and you want charge-based selectivity instead of, or in addition to, polarity-based retention.

Mixed-mode sorbents

Combine two retention mechanisms (commonly reversed-phase plus ion-exchange) in one sorbent bed, giving more selective, often cleaner extractions for analytes that benefit from dual-mechanism retention—frequently used for basic or acidic drug compounds in complex biological matrices.

Where to start

Match sorbent chemistry to your analyte's polarity and charge behavior first, then confirm cartridge size (sorbent mass and bed volume) against your sample volume and expected analyte concentration. For a regulated or validated method, follow the method's specified cartridge—this is a starting framework for method development, not a substitute for a validated protocol.