Solid phase extraction has almost entirely replaced solvent extraction, since it eliminates the use of organic solvents and simplifies the automation of the separation step. In the sorbent extraction technique, analytes are extracted from the liquid phase onto a surface of a solid phase furnished with an appropriate ligand. If the particles, packed in a small column, are hydrophobic (such as surface-bound C-18), any non-charged, organic compound can be extracted from the aqueous phase. This allows the use of all chelating agents that have been applied for trace analysis of metals by solvent extraction, since metal selective ligands (hydroxyquinoline, cupferrone, dithizone, etc.) can be adsorbed on stationary phase, allowing capture of a target metal ion from complex aqueous matrix (Motomizu 2008).
There are many examples of FI and SI– sorbent extraction protocols for trace metal analysis developed for environmental and oceanographic research. Automation of sorbent extraction by Flow Injection or Sequential Injection (Miró 2006) is practical and efficient for sample preconcentration and matrix removal. Indeed FI and SI offer unexplored research opportunities and practical solutions for the development of novel methods for automated assay of a wide range of analytes that are currently processed by labor intensive manual sorbent extraction procedures (Fritz 1999).