Solid Phase Extraction

Courtesy of Alltech Inc.

Figure 20. Separation of Some Chlorinated Pesticides Removed from Drinking Water by Solid State Extraction.

Devices for multiple extractions are available, the extraction tubes are mounted in racks of 12, 24 or even 96 and vacuum or pressure manifolds are also obtainable. The conditioning, extraction and regeneration can be automated and the racks used in conjunction with an automatic sampler to process batches of assays at one time.

Hard or brittle solid samples must be reduced to a particle size of less than 20 mm using the simple pestle and mortar or the so-called 'diamond' mill. This will usually provide sufficient surface area for efficient extraction. If the analyte is present in very small quantities, and the matrix is hard and not porous the particle size may need to be further reduced. Softer materials can be ground in a laboratory ball mill and very soft materials of biological origin, (e.g. animal and vegetable tissue, processed foods etc.) can be minced, crushed or shredded in a suitable blender. Substances such as plastics and rubber can be ground after freezing in liquid nitrogen or sometimes ground with solid carbon dioxide.

If the analyte is preferentially distributed in the extracting solvent, the sample can be shaken or stirred with sequential amounts of solvent. Extraction can be aided by heating or by a microwave generator. Solution of the analyte can be accelerated by sonication. Sonication will also disrupt cell walls, and release cell contents for extraction.

The continuous solvent extraction of solids can be achieved with a Soxlet extractor and filter thimble. However, the technique has been improved by immersing the thimble in the boiling solvent producing a much faster extraction with less solvent. Traditional Soxhlet extraction is used for very insoluble materials where pure solvent is necessary to isolate all the material.