Detailed description of SPE

In conventional SPE, the aqueous samples are passed over a cartridge or a disk, which is packed with a solid sorbent (100 to 1000 mg), to which the analyte adsorbs. Typical sorbents are octadecyl (C₁₈ )-bonded silica or graphitic carbon black for non-polar organic analytes and ion-exchange polymers for polar and ionic organic analytes.

SPE involves the following steps:

1. Sorbent conditioning:
   Activation of the sorbent by passing specific organic and aqueous liquids over the cartridge
2. Sample loading:
   Passing the aqueous sample over the cartridge during which the analyte sorbs to the packing material
3. Removal of interferences:
   Cleaning of the cartridge by passing a small volume (several mL) of distilled, de-ionized water or a 'weak' solvent over it, and drying of the cartridge by sucking air through it, and
4. Elution of retained analyte:
   Drop-wise elution of the analyte from the cartridge using a small volume (1-5 mL) of a 'strong' organic solvent and injection into GC or HPLC.

For high enrichment factors, the analyte needs to be strongly retained in step 1 and readily eluted in step 4. Modern instruments allow to directly couple solid-phase extraction to liquid- or gas chromatographs (instead of step 4). This on-line setup is less susceptible to systematic error as it does not require sample handling between pre-concentration and separation.

In SPE the type and amount of sorbent must be the adjusted to the chosen volume of water sample such that no breakthrough occurs (see Box 10 ) for the analytes. The 'weak' solvent that is used for the removal of interferences (step 3) must be chosen such that no analyte is co-eluted from the sorbent.

In contrast, the 'strong' solvent in step 4 is expected to elute the analytes quantitatively into as little solvent as possible. An optimal design of the whole procedure can be achieved theoretically (i.e. not through trial and error) if all relevant partition coefficients are known (see Box 11 ).

SPE has a number of advantages compared to LLE:

• it can better be automated;
• it allows the extraction of polar analytes by using 'polar' sorbents. This would be impossible to achieve by liquid-liquid extraction because polar solvents would mix with water;
• rather large volumes (up to several liters) of water can be extracted. A major disadvantage of SPE is potential clogging of the cartridge if the water sample contains too much suspended matter.