Ultrasound Assisted Extraction

Extraction techniques are widely accepted as a prerequisite for analytical determination of both organic and inorganic analytes in a large variety of samples.

As a part of an analytical process, sample preparation is considered to be an essential step so that the entire process can be simplified. In this case, the ability of many analytical systems to handle liquid samples has brought about the development of separation methods which fulfil a main objective, i.e. to obtain quantitative analyte leaching from the solid matrix using a suitable solvent, with little or no matrix release, so that matrix effects can be kept to a minimum during the measurement steps. For speci-ation applications, a last condition of a solidliquid extraction method must be the maintenance of the species integrity during treatment.

Table 1 shows the most relevant methods for treatment of solid samples based on analyte extraction. An important requirement of most techniques shown is that solvents at high temperature (i.e. at boiling point) or pressure must be used. In contrast, operation with ultrasonic processors can be performed at ambient temperature and normal pressure, and mild chemical conditions can be used in most cases.

Sonication is usually recommended for pretreat-ment of solid environmental samples for the extraction of nonvolatile and semivolatile organic com pounds from solid, such as soils, sludges and wastes. When comparing the different methods available for analyte extraction from solid samples, sonication is considered as an effective method since unsophisticated instrumentation is required and solid-liquid separations can usually be performed in a short time using diluted reagents and low temperatures. To date, most of applications of ultrasonic extraction have been carried out for organic compounds, but the usefulness of ultrasound for element extraction is still to be explored. Some examples of solid-liquid extraction of some elements with the use of ultrasound are shown in Table 2. It should be pointed out that for many applications reported in this table, operation conditions were intended to obtain a homogeneous slurry so that a representative aliquot could be sampled; specific optimization of the variables influencing ultrasound-assisted extraction processes was not performed. Significant variables influencing the solid-liquid extraction process with a probe-type sonicator are sonication time, vibrational amplitude of the probe, acid concentration, particle size and solid concentration in the liquid. In general, the presence of an acidic liquid is an important prerequisite for quantitative extraction to be achieved; nitric acid at low concentration (e.g. 3-5% v/v) is usually chosen for extraction of elements from solid samples.

Quantitative extraction can be achieved for some analytes such as As, Cu, Pb, Cd, etc., from plant and animal tissues. Nevertheless, incomplete extraction has been observed from samples containing a typical inorganic matrix (e.g. sediment). It is believed that this finding is related to the ability of ultrasound to penetrate the solid material. A further variable that influences the solid-liquid extraction is the analyte-matrix interaction. Thus, strongly bound analytes should be more difficult to extract, thereby requiring more stringent extraction conditions. A relationship between extractability and binding characteristics of elements in the sample is yet to be established.

The extraction efficiency obtained with ultrasound could be increased by addition of glass beads which promote particle disruption by focusing the energy released by cavitation, and by physical crushing. Particle disruption could also be enhanced by increasing hydrostatic pressure and viscosity. The use of a bubbling gas during sonication gives rise to an enhanced formation of H2O2 and hydroxyl radicals (OH • ) thus aiding analyte extraction from oxidizable materials. In general, the use of probe-type sonicators at the appropriate vibrational amplitude and sonica-tion time is required so that extraction efficiency can be improved for strongly-bound elements.

Table 1 Extraction methods from solid samples.

Sample pretreatment method

Principles ofthe technique

Accelerated solvent

Automated Soxhlet

Forced-flow leaching

Gas phase

Homogenization Pervaporation

Solid-liquid extraction Sonication Soxhlet extraction Supercritical fluid

Thermal

Sample is placed in a sealed container and heated to a temperature higher than its boiling point, causing pressure in the vessel to rise.

A combination of hot solvent leaching and Soxhlet extraction; sample in thimble is first immersed in boiling solvent and then the thimble is raised for Soxhlet extraction with solvent refluxing.

Sample is placed in a flow-through tube, and solvent is pumped or pushed through high-pressure nitrogen gas, while the tube is heated near the boiling point of solvent.

After equilibrium, analytes partition themselves between a gas phase and the solid phase at a constant ratio; with static headspace extraction, volatiles are sampled above the solid; with dynamic headspace extraction, volatiles are sampled by continuously purging the headspace above a sample with inert gas, trapping them on a solid medium, and then thermally desorbing them into a gas chromatograph.

Sample is placed in a blender, solvent is added, and sample is homogenized to a finely divided state; solvent is removed for further work-up.

Volatile substances present in a heated donor phase placed inside a pervaporation module evaporate through a porous membrane and the vapour condenses on the surface of a cool acceptor stream on the other side of the membrane.

Sample is shaken together with the appropriate solvent in a container and the liquid separated by filtration

Finely divided sample in a container is immersed in ultrasonic bath with solvent and subjected to ultrasonic irradiation; an ultrasonic probe or cell disrupter can also be used.

Sample is placed in a disposable, porous container (thimble); constantly refluxing solvent flows through the thimble and leaches out analytes that are collected continuously.

Sample is placed in flow-through container and a supercritical fluid (e.g. CO2) is passed through sample; after depressurization, extracted analyte is collected in solvent or trapped on adsorbent and desorbed by rinsing with solvent.

A form of dynamic headspace analysis, but the sample is heated (controlled) to much higher temperatures (as high as 350°C).

Contents based on Majors RE (1996) LC-GC International, 638 and Luque de Castro MD and Papaefstathiou I (1998) Trends in Analytical Chemistry 17: 41.

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