Extraction from Solids

It is estimated that 40% of all analytical samples are solids. This significant portion of the analytical sample load represents the most difficult extraction challenge, since solute interactions with the sample matrix must be overcome and the solute must then diffuse through the solid sample. As a result, the development of extraction methods for solids has focused on improving the diffusion issues.

Leaching Leaching simply involves soaking the sample in the extraction solvent for a prescribed period, and is a batch process. Because of the adsorp-

tive properties of the sample and the slow diffusion through a solid, leaching is not a very efficient extraction method. Improvements to simple leaching can be made by placing the extraction vessel on a heat source (such as a heating plate or steam bath). Agitation, as in shake-filter methods, and a decrease in the sample particle size can also improve recoveries. An adaptation of leaching is forced-flow leaching. In this case the sample is placed in a tube and solvent flow is forced (under pressure) through the tube. In many instances the solvent is heated to near its boiling point and forced-flow leaching can be a continuous process.

Soxhlet extraction This common procedure, which uses the device shown in Figure 2A, was developed nearly 100 years ago and is still in routine use. The sample is placed into a porous container (called a thimble) and the volatile extraction solvent is continuously refluxed and condensed through the sample. Although the method can be slow (12-24 h Soxhlet methods are not uncommon), the apparatus can be left unattended with multiple extractions being performed by a bank of Soxhlet extractors. As with any technique using applied heat, loss of volatile compounds and thermal degradation are concerns. Because of its routine use in established analytical procedures, Soxhlet extraction is undoubtedly the extraction method to which other methods for extracting solids are compared.

New developments in Soxhlet extraction include a high pressure system, developed by J & W Scientific, which allows liquid carbon dioxide to be used as the extraction solvent, and an automated version. The automated Soxhlet extractor allows the thimble to be immersed in the boiling extraction solvent for a prescribed period, before the extraction is completed in the more traditional Soxhlet approach. This two-step process can be 4-10 times faster than conventional Soxhlet extractions and use about half of the solvent volume.

Sonication Sonication or ultrasound extractions can be considered a development of leaching, in which ultrasonic energy is applied to disrupt solute-sample interactions and facilitate solute diffusion. The use of ultrasonic probes can be quite efficient.

Accelerated solvent extraction (ASE) (also called pressurized fluid extraction) This technique, developed by the Dionex Corporation, is commonly discussed using the trademarked name, accelerated solvent extraction. However, the more generic term pressurized fluid extraction is becoming more widely used. In this technique the sample is placed into a sealed container and solvent is pumped through this extraction vessel. Because a modest pressure is applied, temperatures much greater than the atmospheric boiling point can be used with liquid extraction solvents. The technique is automated. This application of temperature greatly enhances solute solubility, diffusion, and viscosity, resulting in extractions that are qualitatively and quantitatively equivalent to Soxhlet in minutes instead of hours, and with significantly less solvent.

Microwave-assisted extractions In some respects microwave extractions can be thought of as a form of leaching with the addition of microwave irradiation. The microwave irradiation, when absorbed by materials with a permanent dipole, leads to heating. In a closed system, the approach is like ASE in the respect that temperatures greater than the atmospheric boiling point of the solvent can be achieved. This form of the technique is generally used with polar solvents (which absorb microwave energy). Open-cell approaches are generally used with non-absorbing solvents and samples with a high water content (or that otherwise possess a high dielectric constant). In this case localized heating in the sample allows extraction efficiencies to be improved.

Supercritical fluid extraction (SFE) SFE employs solvents, generally carbon dioxide (neat or with added co-solvents), at temperatures and pressures near or above the critical point. These high-temperature, high-pressure solvents have gas-like diffusion, liquid-like solvation properties, and do not possess surface tension. Hence, SFE can be quite rapid. With the use of carbon dioxide, the deleterious effects (e.g. cost, health and environment concerns, etc.) of organic solvents can be minimized. Another advantage of SFE is that solvating properties can be modified as a function of temperature and pressure, adding a selectivity advantage to the technique. In SFE the sample is placed in an extraction vessel and the supercritical fluid passes through the vessel in a series of static and dynamic steps. Upon depressurization of the extracting fluid the extracted solute remains in a solute collection region.

Gas-phase methods When volatile compounds are being extracted they can often be forced from the solid into the gas phase and subsequently trapped. In static methods the volatile compounds above the sample (often after heating) are simply trapped. Dynamic methods are exemplified by the purge-and-trap technique. In purge-and-trap, a continuous purge of the sample with an inert gas takes place and the volatile solutes are trapped onto a solid support. Thermal desorption is similar to the purge-and-trap technique, except the sample is heated ballistically to higher, controlled temperatures to force the solutes into the gas phase. Each of these gas-phase methods have been modified for use with the SPME approach to solute trapping.

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