Leaching microwave-assisted

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

In mercury speciation studies, pressurized liquid extraction (PLE), microwave-assisted extraction (MAE), and supercritical fluid extraction (SEE) are employed [33]. In particular, methyl-mercury is extracted by the Westoo method [33,34], which consists in a leaching process with hydrochloric acid, the extraction of the metal chloride into benzene or toluene, the addition of ammonium hydroxide that converts the metal species to hydroxide and the saturation with sodium sulfate. Most of the HPLC methods reported in literature for the determination of organomercury compounds (mainly monomethyhnercury, monoethyhnercury, and monophenylmercury) are based on reversed... 

Seven factors were studied to assess their importance on the acid microwave-assisted leaching of metallic elements... 

E. M. Seco-Gesto, A. Moreda-Pineiro, A. Bermejo-Barrera and P. Bermejo-Barrera, Multielement determination in raft mussels by fast microwave-assisted acid leaching and inductively coupled plasma-optical emission spectrometry, Talanta, 72(3), 2007, 1178-1185. 

Vaisainen, A. and R. Suontomo. 2002. Comparison of ultrasound-assisted extraction, microwave-assisted acid leaching and reflux for the determination of arsenic, cadmium and copper in contaminated soil samples by electrothermal atomic absorption spectrometry. J. Anal. At. Spectrom. 17 739-742. 

J. Szpunar, V. O. Schmitt, J. L. Monod, Rapid speciation of butyltin compounds in sediments and biomaterials by capillary gas chromatography-microwave induced plasma atomic-emission spectrometry after microwave-assisted leaching-digestion, J. Anal. Atom. Spectrom., 11 (1996), 193D199. 

Some contradictory results are obtained in the comparison of results in the determination of arsenic after ultrasonic slurry sampling — the use of sampling is not most appropriate in this case as the sampling step is before the slurry formation — ultrasound-assisted extraction — leaching has been the correct word in this case — and microwave-assisted digestion [28], which will be treated in detail in Chapter 5. 

Ultrasound-assisted leaching versus microwave-assisted leaching... 

Unlike many microwave-assisted leaching methods, USAL requires no moisture adjustment. 

On the other hand, USAL is subject to the following shortcomings relative to microwave-assisted leaching ... 

Ultrasound-assisted methods are usually less robust than microwave-assisted ones because, as noted by Cencic-Kobda and Marcel [51], ageing of the US probe surface can alter the leaching efficiency. 

In many situations, ultrasound-assisted leaching is an expeditious, inexpensive, efficient alternative to conventional extraction techniques and, in some cases, even to supercritical fluid and microwave-assisted extraction. A number of applications to both organic and inorganic analytes in a wide variety of samples exist. Most are conducted by hand. As with automatic extractions, applications involving continuous systems are still very scant and hence one possible target for future research. 

Ultrasound-assisted leaching has also been used to extract natural compounds such as vitamins A, D and E from feeds [57], paclitaxel and related taxoids from leaf tissue of Taxus [58], opiates from hair samples [59] and antioxidants from rosemary [60]. Ultrasounds have so far had much more restricted application in this field than in the previous ones, possibly as a result of the technique being at a disadvantage with respect to alternatives such as microwave-assisted extraction [57] or supercritical CO, extraction [60]. 

Ultrasound-assisted extraction provides several interesting advantages over microwave-assisted leaching, namely ... 

Fig. 5.7. (A) Assembly used in the determination of cadmium and zinc in kidney and liver tissue by simultaneous microwave-assisted acid digestion of 6 samples, FI and AAS CT collector tube, FS flowing sample collector, R recorder. (B) Experimental design for the continuous microwave-assisted leaching, liquid-liquid extraction, sorption/clean-up of phenol compounds in soil samples IV, injection valve, PS membrane phase separator, o.p. and a.p. organic and aqueous phase, W waste. (Reproduced with permission of Elsevier.)...

Fig. 5.8. (A) General scheme of a dynamic focused microwave-assisted extractor. (B) Experimental set-up used to integrate microwave-assisted extraction with the subsequent steps of the analytical process. (1) Leaching step CT controller, MO microwave oven, S sample, R condenser, WR water reservoir, TCPP two-channel piston pump, ER extract reservoir, SV switching valve. (2) Clean-up/preconcentration step M methanol, A air, B buffer, PP peristaltic pump, F filter, EL elution loop, MC mini-column, R retention direction, E elution direction, 1V1-1V3 injection valves, W waste. (3) Individual separation-detection step HPIV high-pressure injection valve, AC analytical column, DAD diode array detector, SR solvent reservoirs.

Cr(VI) Sediments, soils Microwave-assisted analyte leaching UV-Vis 0.012 pg mL-1 (extract) or 0.12 pgg-1 (dry basis) Flow injection system with optional SPE iterative change of flow direction for improving extraction derivatisation with 1,5-diphenylcarbazide [422]... 

Advantages of microwave-assisted leaching vs. conventional Soxhlet can be ascribed to the performance of this heating source based on dielectric loss. The heat appears in the bulk of the irradiated material, thus giving rise to an inverse temperature gradient that is, volume rather than surface heating. Thus, both the extraction time and the volume of solvent required are dramatically reduced. " However, the efficiency of microwaves can be very poor when either the target analytes or the solvents are nonpolar or of low polarity, when these are volatile, and when the solvents used have low dielectric constants. In these cases, Soxhlet extraction is superior to MAE. 

Caballo-Lopez A and Luque de Castro MD (2003) Slurry sampling-microwave assisted leaching prior to hydride generation-pervaporation-atomic fluorescence detection for the determination of extractable arsenic in soil. Analytical Chemistry 75 2011-2017. 

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