“SPE is a method of sample preparation that concentrates and purifies analytes from solution by sorption onto a sorbent contained in either a disposable solid phase cartridge or a 96-well plate, followed by elution of the analyte with a solvent and reconstitution into a mobile phase that is appropriate for the instrumental method used for analysis.”  (E.M.Thurman and M.S.Mills, Solid-Phase Extraction – Principles and Practice, John Wiley, New York, 1998)

SPE Cartridges Classification 
According to principle of reaction, SPE is divided into the following three categories.
1. Normal-phase extraction- Adsorbents used in normal phase extraction are polar. 
2. Reversed-phase extraction- The sorbent and the target analytes by reversed-phase extraction is usually the non-polar or weak polar, relying mainly on non polar and non polar interactions, is a fan of Edward force or dispersion force.
3.Ion exchange extraction- is relying on the interaction between target analytes and sorbent.

Operation Steps
According to the different sorbent retention mechanism (sorbent retain target analytes or retain impurities), the operation is slightly different. Sorbent retain target analytes.
SPE operations are generally four step :
 Conditioning and equilibration: Solvent is passed through the SPE material to wet the bonded functional groups => ensures consistent interaction. Sorbent/ phase is treated with a solution that is similar (in polarity, pH, etc.) to the sample matrix => maximizes retention.
 Sample Load: Introduction of the sample = analytes of interest are bound/ extracted onto the phase/sorbent.
 Washing: Selectively remove unwanted interferences co-extracted with the analyte without prematurely eluting analytes of interest.
 Elution: Removing analytes of interest with a solvent that overcomes the primary and secondary retention interactions between sorbent and analytes of interest.

Specification Maximum Sample Load Volume Minimum Elution volume
500-100mg 2.5mg-5mg 1mL 100-200mL
100-200mg 5mg-10mg 3mL 200mL-500mL
200mg-1g 10mg-50mg 6mL 500mL-6mL
500mg-2g 25mg-100mg 12mL 3mL-10mL
2g-5g 100mg-250mg 25mL 10mL-50mL
5g-10g 250mg-500mg 60mL 20mL-50mL