Diatomaceous earth, solid-phase extraction

A pivotal step in the analytical process is sample preparation. Frequently liquid-liquid extractions (LLEs) are used. Solvents, pH, and multiple back extractions are all manipulated to increase selectivity and decrease unwanted contaminants before injection on the GC system. Solid phase extraction (SPE) is more convenient than it used to be because of an increase in commercially available SPE columns. SPE columns are packed with an inert material that binds the drug of interest, allowing impurities to pass through. As with LEE, solvent choices and pH affect retention and recovery. There are three commercially available types of SPE columns, diatomaceous earth (which uses the same principles as LLE), polystyrene-divinylbenzene copolymer, and mixed mode bonded silica (Franke and de Zeeuw, 1998). 

Solid-phase extraction (SPE) is nearly always required prior to the supercritical fluid extraction of PAHs from liquid samples alternatively, samples can be spiked to a solid material such as diatomaceous earth. A comparison of SPE, SFE, SPE-SFE and liquid-liquid extraction (LEE) for the removal of PAHs from drinking water revealed the SPE-SFE combination to provide the best results and LLE the poorest [188]. 

PAs can be extracted with organic solvents from aqueous systems and solid samples made basic with ammonia, fri this case dichloromethane or chloroform have often been used as the organic phase. PAs can conveniently be transferred from an aqueous extract into an organic solvent by means of solid phase extraction (SPE) using columns packed with inert diatomaceous earth such as Extrelut [8]. The aqueous extract is supported on the column matrix and the PAs eluted by passing organic solvents through. The solid matrix prevents the formation of emulsions. This method is much less satisfactory for the more polar PANOs. 

The product from Step 1 (13.4 mmol) was dissolved in 100 ml apiece THE and a saturated solution of NH4CI, placed in a water bath, and activated zinc dust (230 mmol) added. The mixture stirred 20 minutes and was then filtered through a pad of diatomaceous earth. The organic layer was separated and the aqueous layer extracted with 20 ml THF. The organic phase was washed with brine, dried, and concentrated. The residue was triturated with 20 ml water and the product isolated in 71% yield as a tan solid. NMR data supplied. 

In solid-supported LLE (SS-LLE) or liquid-liquid cartridge extraction, the aqueous sample is applied on to a dry bed of inert diatomaceous earth particles in a flow-through tube or in 96-well plate format. After a short equihbration time (3-5 nun), organic solvent is added. The organic eluate is collected, evaporated to dryness, and reconstituted in mobile phase. Compared to conventional LLE procedures, SS-LLE avoids the need for vortex-mixing, phase separation by centrifugation, and phase transfer by aqueous layer freezing. 

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