Microwave-assisted extractions of active ingredients from plants

Microwave-assisted extraction (MAE) is widely employed in the analysis and the extraction of active compounds from plants. This review summarizes the research done during the last decade on the MAE of active ingredients from plants. Advances and modifications to improve the performance of MAE are presented and discussed in detail. Modified MAE such as vacuum microwave-assisted extraction (VMAE), nitrogen-protected microwave-assisted extraction (NPMAE), ultrasonic microwave-assisted extraction (UMAE), dynamic microwave-assisted extraction (DMAE) and other advancements in MAE are also detailed in this article. In addition, the microwave extraction procedures and the important parameters influencing its performance are also included, together with the advantages and the drawbacks of each MAE techniques.     

An automatic system for multi-element solvent extractions

The automatic system described is suitable for multi-element separations by solvent extraction techniques with organic solvents heavier than water. The analysis is run automatically by a central control unit and includes steps such as pH regulation and reduction or oxidation. As an example, the separation of radioactive Hg²⁺, Cu²⁺, Mo⁶⁺, Cd²⁺, As⁵⁺, Sb⁵⁺, Fe³⁺, and Co²⁺ by means of diethyldithiocarbamate complexes is reported.     

Temperature changes in solvent extractions of cadmium oxinate

The extraction of 10^-5.1M cadmium ion from aqueous solution into chloroform containing oxine has been investigated and the effects caused by changing the temperature were studied. Values of equilibrium constants for all reactions participating in the extraction have been obtained as a function of temperature. Plots of the logarithms of the constants vs. the reciprocal of the absolute temperature gave straight lines. Thermodynamic functions for all reactions involved have been calculated.     

Microwave-assisted enzyme-catalyzed reactions in various solvent systems

The work describes the accelerated enzymatic digestion of several proteins in various solvent systems under microwave irradiation. The tryptic fragments of the proteins were analyzed by matrix-assisted laser desorption/ionization mass spectrometry. Under the influence of rapid microwave heating, these enzymatic reactions can proceed in a solvent such as chloroform, which, under traditional digestion conditions, renders the enzyme inactive. The digestion efficiencies and sequence coverages were increased when the trypsin digestions occurred in acetonitrile-, methanol- and chloroform-containing solutions that were heated under microwave irradiation for 10 min using a commercial microwave applicator. The percentage of the protein digested under microwave irradiation increased with the relative acetonitrile content, but decreased as the methanol content was increased. These observations suggest that acetonitrile does not deactivate the enzyme during the irradiation period; in contrast, methanol does deactivate it. In all cases, the digestion efficiencies under microwave irradiation exceed those under conventional conditions.     

A high-throughput platform for low-volume high-temperature/pressure sealed vessel solvent extractions

A high-throughput platform for performing parallel solvent extractions in sealed HPLC/GC vials inside a microwave reactor is described. The system consist of a strongly microwave-absorbing silicon carbide plate with 20 cylindrical wells of appropriate dimensions to be fitted with standard HPLC/GC autosampler vials serving as extraction vessels. Due to the possibility of heating up to four heating platforms simultaneously (80 vials), efficient parallel analytical-scale solvent extractions can be performed using volumes of 0.5-1.5 mL at a maximum temperature/pressure limit of 200°C/20 bar. Since the extraction and subsequent analysis by either gas chromatography or liquid chromatography coupled with mass detection (GC-MS or LC-MS) is performed directly from the autosampler vial, errors caused by sample transfer can be minimized. The platform was evaluated for the extraction and quantification of caffeine from commercial coffee powders assessing different solvent types, extraction temperatures and times. For example, 141±11 μg caffeine (5 mg coffee powder) were extracted during a single extraction cycle using methanol as extraction solvent, whereas only 90±11 were obtained performing the extraction in methylene chloride, applying the same reaction conditions (90°C, 10 min). In multiple extraction experiments a total of ~150 μg caffeine was extracted from 5 mg commercial coffee powder. In addition to the quantitative caffeine determination, a comparative qualitative analysis of the liquid phase coffee extracts and the headspace volatiles was performed, placing special emphasis on headspace analysis using solid-phase microextraction (SPME) techniques. The miniaturized parallel extraction technique introduced herein allows solvent extractions to be performed at significantly expanded temperature/pressure limits and shortened extraction times, using standard HPLC autosampler vials as reaction vessels. Remarkable differences regarding peak pattern and main peaks were observed when low-temperature extraction (60°C) and high-temperature extraction (160°C) are compared prior to headspace-SPME-GC-MS performed in the same HPLC/GC vials.     

Microwave-assisted solid-phase synthesis of 2,5-diketopiperazines: solvent and resin dependence

Solid-phase synthesis of diketopiperazines (DKPs) was preformed using various combinations of resins (polystyrene, TentaGel, ArgoGel, and PEGA) and solvents (toluene, tert-butyl alcohol, water, and toluene/2-butanol (1:4, v/v). The DKPs were synthesized from solid-phase bound dipeptides via intramolecular aminolysis. Both thermal and microwave-assisted solid-phase synthesis of DKPs gave high yields of products independently of resin and organic solvent used; however, only the PEGA resin resulted in high yields of DKPs in water independent of heating method. The short reaction times, high yields, and the possibility to run reactions in water when an appropriate resin is used makes the microwave-assisted solid-phase synthesis the method of choice. The method should be suitable for solid-phase synthesis of diketopiperazine-based libraries.     

Microwave-assisted synthesis of quinoxaline derivatives using glycerol as a green solvent

A rapid, efficient, and reliable catalyst-free procedure for the synthesis of quinoxaline derivatives in glycerol under focused microwave irradiation was developed. The reaction proceeded in glycerol without any catalyst making this methodology valuable from both economic and environmental viewpoints.     

The role of the solvent in equilibrium and kinetic aspects of metal chelate extractions

Equilibria and kinetics for the extraction of nickel(II) and copper(II) by 2-hydroxy-5-nonylbenzophenone oxime (LIX 65N) in seven organic solvent systems were studied in order to test the validity of several hypotheses related to the role of the solvent in equilibrium and kinetic aspects of metal chelate extraction. For the nickel—LIX 65N system, the extraction constant is essentially independent of the solvent system, whereas for the copper—LIX 65N system, the extraction constant is not independent of solvent; this indicates that while the stoichiometry of the nickel chelate remains the same in all solvents, that of copper does not. The observed rate constant for the nickel—LIX 65N extraction was found to vary inversely with the LIX 65N distribution constant as predicted from a mechanism involving slow formation of the 1:1 complex. The observed reaction rate constant for the copper—LIX 65N varied inversely with the square of the distribution constant, also in accordance with the previously postulated mechanism of the slow formation of the 2:1 copper complex. This study, therefore, unequivocally eliminates the interfacial mechanism in favor of the homogeneous chemical reaction mechanism for the extraction of metal ions by LIX 65N, as well as by other similar high-molecular-weight extractants.     

Microwave-assisted solvent elution technique for the extraction of organic pollutants in water

Solid phase extraction (SPE) with appropriate solid sorbents has been commonly used in the routine extraction of organic pollutants in water. The elution of analytes from the solid sorbents normally takes place by organic solvents under an applied vacuum. In this study, a microwave-assisted solvent elution technique was developed for the elution of analytes from C₁₈ membrane disks during microwave irradiation from a microwave extraction system (MES). Several parameters, namely, elution solvent, elution temperature, duration of elution and the volume of solvent which may affect the elution efficiency of microwave-assisted solvent elution (MASE) technique towards organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), phthalate esters (PAEs), organophosphorus pesticides (OPs), fungicides, herbicides and insecticides from the membrane disk were investigated. Good recoveries above 75% were obtained for most of the organic pollutants using the optimum SPE-MASE technique. The effect of sodium chloride and humic acid on the recoveries on the target analytes were also investigated.     

Microwave-assisted synthesis of α-aminophosphinic acids from hypophosphorus acid salts under solvent free conditions

A simple, efficient, and general method has been developed for the synthesis of α-aminophosphinic acids from hypophosphorus salts under solvent-free conditions using microwave irradiation. α-Aminophosphinic acids were obtained in high yield under mild conditions by reaction of the amine salts of hypophosphorus acid with aldehydes in the presence of microwave irradiation.     

Microwave-assisted solvent free synthesis of hydroxy derivatives of 4-methyl coumarin using nano-crystalline sulfated-zirconia catalyst

Nano-crystalline sulfated-zirconia solid acid catalyst has been studied for microwave-assisted solvent free synthesis of hydroxy derivatives of 4-methyl coumarin by Pechmann reaction. The catalyst showed good activity for activated m-hydroxy phenol substrates, viz., phloroglucinol and pyrrogallol with ethyl acetoacetate for the synthesis of 5,7-dihydroxy 4-methyl coumarin and 7,8-dihydroxy 4-methyl coumarin, respectively, showing significant yields ranging from 78 to 85% within 5–20 min at 130 °C. However, the less activated phenol and m-methyl phenol was observed to be inactive for the synthesis of 4-methyl coumarin and 4,7-dimethyl coumarin, respectively, under the studied experimental conditions.     

Microwave-assisted chemoselective copper-catalyzed amination of o-chloro and o-bromobenzoic acids using aromatic amines under solvent free conditions

Copper-catalyzed synthesis of N-aryl anthranilic acid derivatives using effective amination of 2-chloro and 2-bromobenzoic acid under microwave irradiation is reported.Some of the advantages of this method are high chemoselectivity,short reaction times,ease of work up procedure and elimination of the need for acid protection.     

Microwave-assisted regioselective addition of P(O)-H bonds to alkenes without added solvent or catalyst

The addition of P(O)-H bonds to alkenes has been accomplished using microwave irradiation in the absence of added solvent and catalyst. In addition to single addition reactions, tandem hydrophosphinylation reactions with alkynes afforded unsymmetrical species such as phosphine oxide-phosphinates. [reaction: see text]     

Comparison of hot Soxhlet and accelerated solvent extractions with microwave and supercritical fluid extractions for the determination of polycyclic aromatic hydrocarbons and nitrated derivatives strongly adsorbed on soot collected inside a diesel particulate filter

Several methods of extraction were optimized to extract polycyclic aromatic hydrocarbons (PAHs), their nitrated derivatives and heavy n-alkanes from a highly adsorptive particulate matter resulting from the combustion of diesel fuel in a diesel engine. This particular carbonaceous particulate matter, collected at high temperatures in cordierite diesel particulate filters (DPF), which are optimized for removing diesel particles from diesel engine exhaust emissions, appeared extremely refractory to extractions using the classical extracting conditions for these pollutants. In particular, the method of accelerated solvent extraction (ASE) is described in detail here. Optimization was performed through experimental design to understand the impact of each factor studied and the factors’ possible interactions on the recovery yields. The conventional extraction technique, i.e., Soxhlet extraction, was also carried out, but the lack of quantitative extractions led us to use a more effective approach: hot Soxhlet. It appeared that the extraction of the heaviest PAHs and nitroPAHs by either the optimized ASE or hot Soxhlet processes was far from complete. To enhance recovery yields, we tested original solvent mixtures of aromatic and heteroaromatic solvents. Thereafter, these two extraction techniques were compared to microwave-assisted extraction (MAE) and supercritical fluid extraction (SFE). In every case, the only solvent mixture that permitted quantitative extraction of the heaviest PAHs from the diesel soot was composed of pyridine and diethylamine, which has a strong electron-donor character. Conversely, the extraction of the nitrated PAHs was significantly improved by the use of an electron-acceptor solvent or by introducing a small amount of acetic acid into the pyridine. It was demonstrated that, for many desirable features, no single extraction technique stound out as the best: ASE, MAE or SFE could all challenge hot Soxhlet for favourable extractions. Consequently, the four optimized extraction techniques were performed to extract the naturally polluted diesel soot collected inside the DPF. Comparisons with the NIST standard reference material SRM 1650b showed that the soot collected from the DPF contained 50% fewer n-alkanes, and also markedly lower levels of PAHs (44 less concentrated) than SRM 1650b, and that the ratio of nitroPAHs to PAHs was increased. These results were attributed to the high temperatures reached inside the particulate filter during sampling runs and to the contribution of the catalytic DPF to aromatic and aliphatic hydrocarbons abatement.     

Membrane approach to solid-phase extractions

A novel material for separations has been developed in which efficient chromatographic particles are enmeshed in a network of PTFE fibrils to form a strong, porous sheet or “membrane”. The membrane can have a variety of sorptive or reactive properties, which are determined by the characteristics of the particulate chosen. The membranes, in the form of 47- or 25-mm disks, are used with standard filtration equipment. The use of hydrophobic octyl- and octadecyl-bonded silica particulates for reversed-phase extractions of environmental pollutants from an aqueous matrix, replacing conventional liquid-liquid extraction (LLE) is described. The water sample is filtered through the disk and adsorbed organic species are eluted from the disk with a small volume of an appropriate solvent for subsequent chromatographic separation. The technique is rapid, less labor intensive then LLE and eliminates the need for large volumes of hazardous and expensive solvents. Specific applications investigated include the analysis of ground, surface and tap waters for pesticides, polychlorinated biphenyls and phthalates at the μg l^?1 level.     

Effects of supercritical fluid extractions on metal ion partitioning as indicated by sequential extractions

We have studied the partitioning of stable elements and uranium to two sediments using sequential extractions (SE). These matrices were then subjected to supercritical fluid extraction (SFE) to attempt to remove the uranium. SEs were repeated on the material remaining after SFE to determine any changes in partitioning. We have confirmed that under suitable chemical conditions, ligand-assisted SFE can be used to extract many metal ions from such matrices. Under the conditions we used, the "easily leachable" metals were extracted, but the refractory metals were not. In addition, SFE appears to redistibute the remaining metal cations among the remaining geochemical after SFE treatment. Our results are presented and discussed in the context of developing decontamination strategies for contaminated soils and sediments.     

Improved technique for toxicological extractions

Summary Two designs of extractors were developed and tested for removal of small amounts of acidic and basic poisons from biological materials. The first of these operated with heavier-than-water solvents placed between two aqueous phases. By adjusting the chemical composition of the two aqueous phases, a double extraction was performed, first to the organic solvent and, secondly, from it into the second aqueous phase. The second extractor was similar except that it operated with lighter-than-water solvents.Rapid extraction of basic poisons such as alkaloids was achieved with formation of pure preparations. Extraction of a single barbiturate gave an unsatisfactory yield but produced a pure product. The extractors reduced greatly the amount of labor and time involved in separating small amounts of poisons from materials of toxicological interest.

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Zusammenfassung Zwei Ausführungsformen von Extraktoren für die Isolierung saurer und basischer Gifte aus biologischem Material wurden geprüft. Die erste Ausführungsform ist für Lösungsmittel bestimmt, die schwerer sind als Wasser. Dieses Lösungsmittel trennt zwei wäßrige Phasen, derenph so eingestellt wird, daß eine Doppelübertragung der Gifte — erst in das organische Lösungsmittel und dann in die zweite wäßrige Phase — möglich wird. Die zweite Ausführungsform funktioniert nach demselben Prinzip, aber mit Lösungsmitteln, die leichter sind als Wasser.Die Schnellextraktion alkalischer Gifte (Alkaloide) lieferte die Substanzen in reiner Form. Ein Versuch mit einem Barbiturat gab ebenfalls ein reines Produkt, aber in schlechter Ausbeute. Die beschriebenen Extraktoren ersparen viel Arbeit und Zeit in der Abtrennung kleiner Mengen Giftstoffe für toxikologische Zwecke.

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Résumé Deux modèles d'appareils ont été étudiés pour l'extraction de petites quantités de poisons acides et basiques des milieux biologiques. Le premier d'entre eux comporte des solvants plus denses que l'eau placés entre deux phases aqueuses. En ajustant la composition chimique des deux phases aqueuses, il a été possible d'effectuer une double extraction, tout d'abord celle du solvant organique et ensuite, à partir de ce solvant dans la seconde phase aqueuse. Le second extracteur est semblable, mais il comporte des solvants moins denses que l'eau.Il a été possible d'effectuer des extractions rapides de poisons basiques, tels que les alcaloïdes avec obtention de préparations pures. L'extraction d'un barbiturate isolé se fait avec un rendement qui n'est pas satisfaisant mais fournit un produit pur. Les extracteurs ont permis de réduire considérablement le travail et le temps consacrés à la séparation de petites quantités de poisons de substances soumises à un examen toxicologique.

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Dedicated to Prof.H. Lieb on the occasion of his 70th birthday. Aided by grants from the National Institutes of Health, U. S. Public. Health Service, and the Research Committee, University of California.     

Liquid-liquid extractions with microporous hollow fibers

Liquid—liquid extractions in microporous hollow fiber modules are unusually fast because of the large surface area per volume in these modules. Extractions of p-nitrophenol into amyl acetate show no membrane resistance, and hence are especially rapid. Extractions of acetic acid into methyl amyl ketone are controlled by the membrane resistance. The results show that extractions in hollow fibers will be fastest when the fibers are wet by the fluid in which the solute is more soluble. They also show when fiber modules are a sound inexpensive alternative to centrifugal extractors.     

A comparison of carbon and nitrogen stable isotope ratios of fish tissues following lipid extractions with non-polar and traditional chloroform/methanol solvent systems

Stable isotope ratios act as chemical tracers of animal diet, and are used to study food web dynamics. Because carbon stable isotope values are influenced by tissue lipid content, a number of extraction methods have been used to remove lipid bias, but, in some species and tissues, extractions also alter nitrogen isotope values. We have analyzed delta(13)C and delta(15)N in Atlantic bluefin tuna liver and white muscle, and whole Atlantic herring, fish tissues covering a wide range of lipid content (bulk C:N 3.1-12.5). In order to compare delta(13)C and delta(15)N values from traditional chloroform/methanol extractions with non-polar solvent alternatives, we analyzed samples following (1) no treatment, (2) lipid removal using chloroform/methanol (2:1), and (3) Soxhlet extractions using chloroform, diethyl ether or hexane. Chloroform/methanol and chloroform extractions produced the lowest C:N values and highest delta(13)C values. In bluefin tuna, chloroform and hexane extractions significantly altered liver delta(15)N, and all methods significantly altered delta(15)N values in white muscle. Whole Atlantic herring delta(15)N was not altered by any extraction method, while the 2:1 chloroform/methanol extraction most completely removed fish tissue lipid components. Our results indicate that delta(15)N effects are not limited to common chloroform/methanol extractions and suggest that chloroform/methanol is the most effective extraction for delta(13)C correction. Given evidence for delta(15)N alteration among all tested methods, mathematical correction approaches should be further explored as an alternative to lipid correction.