Current Microwave Chemistry (v.4, #1)

Meet Our Associate Editor by Jiaguo Yu (1-1).

Editorial (Thematic Issue: Microwave-assisted Sample Treatments) by Arabinda K. Das, Miguel de la Guardia (4-4).

Microwave-enhanced Speciation Analysis of Environmental Samples by Arabinda K. Das, Ruma Chakraborty (5-15).
Background: For acquiring information on accessibility, mobility, chemical behavior and fate of elements, it is necessary to determine their specific chemical species. Relevance of microwave energy for chemical speciation analysis has been extensively investigated over a wide range of applications since more than last three decades. The aim of writing this review would be to discuss salient features of important procedures applying microwave-assisted extraction followed by multi-elemental speciation through sequential extraction and speciation of several selected elements with complicated speciation chemistry.

Methods: The use of microwave assisting dissolution, extraction and derivatization leads to a breakthrough in sample pretreatment for speciation analysis. After extraction, analytes were determined by atomic spectrometry such as AAS and ICP MS. A higher leaching rate could be achieved by applying focused-microwave followed by simultaneous multi-elemental detection through ICP TOFMS then that had been applied in continuous leaching followed by high resolution ICP HRMS.

Results: A drastic reduction in time viz., from 68 h in the classical method to < 3.5 h in the microwave enhanced multi-elemental speciation methodology was reported and the scheme reduces the risk of contamination and involves a clean process technology. Detection limit of the order of picogram was reported for inorganic arsenic species in coal using microwave treatment followed by HPLC-HG AFS. Open focused microwave extraction using low power and less time (40-60W, 2-4 min) leads to not only reproducible and quantitative recovery but also saves the organo- mercury species intact in nanogram level. On-line non-chromatographic method for monitoring inorganic selenium species employing microwave has been found to be extremely useful for real-time speciation with picogram detection limit. Microwave-assisted extraction followed by HPLC-ES MS allows an environmentally clean method of tin speciation with femtogram detection limit.

Conclusion: The use of microwave energy to prepare sample for speciation is now a necessity because tedious sample preparation procedures can not only be drastically reduced in time and simplified, but also it brings about the possibility of on-line automation. The pretreatment protocols proposed for metal species determination allow to extend the potential of these rapid sample preparation procedures to environmental matrices. Several methods have immense possibility of applying microwave-enhanced speciation in future because they are green in nature.

Use of a Constrained Mixture Design to Optimize a Procedure for Closed Vessel Microwave-Assisted Digestion of Vegetal Leaves by Jacira T. Castro, Marcos A. Bezerra, Mois&#233;s A.L. Neto, Maria G.A. Korn, Cleber G. Novaes, Ricardo E. Santelli (16-24).
Background: Reliable analytical methods for metals determination in tea is a demand of the nutritional and toxicological sciences. In this work, a constrained mixture design has been applied to develop an efficient closed-vessel microwave-assisted digestion procedure for Ca, Cu, Fe, K, Mg, Mn, Na, Sr and Zn determination in vegetal leaves used for tea preparation.

Methods: The optimization step was carried out using a constrained mixture design to find digested samples of good characteristics such as clarity, low residual carbon and low final acidity. The studied metals and residual carbon were determined by simultaneous inductively coupled plasma optical emission spectrometry (ICP OES).

Results: The optimum proportions found by applying the mixture design were 31.3, 56.2 and 12.5% (v/v) for H2O, HNO3 and H2O2, respectively. The accuracy of the method was confirmed by analysis of two certified reference materials: apple leaves (NIST 1515) and spinach leaves (NIST 1570a). The proposed method was applied for the determination of studied metals in vegetal leaves used for the preparation of infusions.

Conclusion: The developed method has presented adequate analytical characteristics to determining of studied elements from tea samples with quickly, accuracy, precision and reliability.

Microwave Assisted Synthesis of Chalcogenides by Sabir Ahammed, Debasish Kundu, Nirmalya Mukherjee, Brindaban C. Ranu (25-35).
Background: The microwave reactor has been an important tool in organic synthesis. During last two decades microwave radiation has been successfully used in a variety of reactions including carbon- carbon and carbon-heteroatom bond formation. The application of microwave energy for carbonchalcogen bond formation leading to the synthesis of chalcogenides have been reported by various groups. Considering the importance of organo-chalocogenides in pharmaceutical industries this review is initiated to assemble these results in one article.

Methods: Microwave radiation has been applied for carbon-chalcogen bond formation producing a variety of diversely substituted chalocogenides.

Results: A variety of organo-sulfur, organo-selenium and organo-tellurium compounds have been synthesized starting from readily available and inexpensive starting materials quite efficiently. It was also demonstrated that microwave heating is superior to conventional heating in many reactions giving higher yield, clean product and shorter reaction time.

Conclusion: This review, in general highlights the microwave-assisted carbon-chalcogen bond formation leading to the synthesis of a variety of chalcogenides. It covers the recent developments on various types of reactions including C-H activation under microwave irradiation producing a variety of sulfides, selenides and tellurides. The prominent among them are Promazine, VX-745, a potent inhibitor with p38 activity, 8-arylsulfanyl adenine, a core structure present in several inhibitors of heat shock protein 90 (Hsp90) which is an attractive target in cancer and neurodegenerative diseases, and unsymmetrical diheteroaryl selenides and tellurides which are of much potential for pharmaceutical activity. We hope, this will be of much interest to a wide spectrum of chemists in academia and industry.

Comparison of Sample Treatment Methods for Bismuth and Tellurium Determination in Yogurt by Manuela Ruiz-de-Cenzano, Arancha Rochina-Marco, Patricia Cava-Montesinos, M. Luisa Cervera, Miguel de la Guardia (36-40).
Background: Dairy products are rich in nutrients that are essential for a high bone mineral density. Furthermore, several studies relate additional health benefits to the regular consumption of fermented milk products such as yogurt. However, Te and Bi have been determined in milk samples but there is not available information about Te and Bi determination in yogurt samples.

Methods: Three sample treatment approaches including: i) microwave-assisted digestion, ii) ultrasoundassisted extraction at room temperature, and iii) microwave assisted extraction have been investigated for the determination of bismuth and tellurium in yogurt by hydride generation atomic fluorescence spectrometry.

Results: Highest levels of Bi and Te found in a variety of commercially available samples were 10.0 ± 0.8 ng mL-1 and 0.91 ± 0.05 ng mL-1, respectively. These results are on the same order or at lower concentration than in milk indicating that these two elements are not modified in a significant way throughout the technological process involved in yogurt manufacture.

Conclusion: All three methods gave coincident results in samples elaborated with skimmed or partially skimmed milk, nevertheless sonication gave lower values for samples with major fat content. The use of microwave-assisted extraction is recommended because it offers a less time consuming methodology.

High Temperature Microwave Oven Treatments by Angel Morales-Rubio, Miguel de la Guardia (41-48).
Background: A literature survey on the use of High Temperature Microwave Oven Treatments focused on the sample pretreatment of different matrix types, from the digestion of samples with high organic content such as foods, to those with high mineral content such as rocks, and also considering microwave assisted synthesis of products including nanomaterials has been made. From the first application published in 1992, not too many papers have been found in the literature. An additional topic, like microwave pyrolysis as a remediation technology, has been considered in this review.

Methods: Examples of different applications are presented, i) Microwave high temperature treatments for the determination of analytical parameters, ii) Microwave high temperature for ceramic synthesis, iii) Microwave high temperature for alloying and magnetic ores separation, iv) Microwave high temperature in nanoparticle synthesis, v) Microwave high temperature waste treatment.

Results: It seems clear from this literature update that microwave-assisted synthering has been the most useful application found in the last years for high temperature microwave treatments and it is a promising field which will continue to provide exciting possibilities in the development of new materials and new properties of old products synthesized by traditional processes. On the other hand, the reduced consume of energy and the increased efficiency of microwave-assisted treatments in front of those based on classical convective heating systems is a guarantee of its general use in waste treatment and waste valorization.

Conclusion: In spite that the pioneering studies were focused on dry-ashing microwave assisted treatments, this way has been scarcely exploited due to the advances in the development of high sensitive techniques for mineral profile determination of samples. However, we are confident that the tremendous advantages offered by microwave-assisted dry ashing will provide new developments on this field in a near future, in which, probably one of the aspects which remains practically unexploited is the search for alternative energy catcher materials suitable to replace the traditional use of silicon carbide.

On-Line Microwave Sample Pretreatment for Organic and Inorganic Analyte Determination by Instrumental Techniques: A Review by Aline S. Freire, Bernardo F. Braz, Delmo S. Vaitsman, Ricardo E. Santelli (49-63).
Background: Microwave energy has been used in chemistry for years. Regarding analytical chemistry, microwave energy has been employed for extraction, digestion, reaction, fractionation and speciation of selected analytes from several matrix samples. Within this goal, automation has of concern in these applications.

Methods: This review presents some applications of microwave use in flow systems for the determination of selected inorganic and organic analytes from several types of samples. A literature survey was done with the aim to identify relevant applications of the use of microwave energy in flow systems.

Results: Application of on-line microwave sample-preparation techniques, based on coupling a microwave unit with extraction, reaction, digestion, speciation, or fractionation, were selected from the current literature describing instrumental techniques and detection. The versatility of closed and open microwave systems as well on-line configurations for static and dynamic are presented.

Conclusion: The aim of this paper is to review the most important literature describing the application of microwave radiation in analytical chemistry using on-line coupling with modern instrumental measurement techniques. This review is not intended to be exhaustive, but describes relevant applications of on-line microwave sample pretreatment.

The Determination of Acid-Volatile Sulphide in Sediments Using Microwave- Assisted Chemical Vapour Generation: An Alternative to the US EPA Method by Ricardo E. Santelli, Aline S. Freire, Bernardo F. Braz, Francis A. Meireles, Maria de F.B. de Carvalho, John E.L. Maddock (64-72).
Background: Acid-volatile sulphides (AVS) and simultaneously extracted metals (SEM) have been used as sediment quality criteria in both contaminated or non-contaminated sediments. The AVS/SEM model is based on the criterion that metal toxicity is absent when the sum of the acid-labile divalent metal concentrations ([ΣSEM]) is less than the concentration of acid-volatile sulphides ([AVS]) expressed as a molar ratio.

Method: The US EPA-recommended method for AVS extraction and determination uses 6 mol L-1 HCl solution for 1 hour and NaOH solution to absorb the H2S generated in the previous step. Sulphide ions can be determined by different procedures, such as gravimetry, spectrophotometry and ion-selective electrode.

Results: The aim of this work was the use of microwave radiation as a fast alternative for the generation of H2S. For this procedure, a glass apparatus was developed, adapting a digestion tube that is generally used in a focused microwave oven. The chemical extraction conditions were evaluated. A flow injection analysis (FIA) system was applied to the determination of sulphide.

Conclusion: The developed procedure results in a total time for the AVS determination method that is one-tenth of that of the recommended method (12 minutes compared to approximately 2 hours). The results obtained show that AVS was accurately quantified in a range of 1.5 to 290 mol S g-1 sediment (RSD < 13 %). Surficial and core sediment samples from several impacted areas were analysed by both the standard (US EPA) procedure and the developed method, and no significant differences were observed.

Background: The use of microwave irradiation is an important development for the synthesis of benzimidazoles in the 21st century, and in recent years, microwave-assisted heterocyclic synthesis has gained significant attention in modern drug discovery studies. Benzimidazoles are an important group in the field of medicinal chemistry because of a wide range of biological activities. This review article provides a collection of studies on the synthesis of benzimidazole derivatives by microwave techniques over a twenty-one years period for an alternative to conventional heating.

Methods: Microwave irradiation has been applied for the synthesis of benzimidazoles. The most used method for the preparation of benzimidazoles is condensation of o-diaminobenzene with carboxylic acids or derivatives.

Results: Benzimidazoles have a seminal role in drug discovery and these derivatives show an excellent scaffold for the development of novel drugs, polymers, ligands and dys. Microwave heating is a useful technique for the synthesis of benzimidazole derivatives. When compared to the conventional heating methods, many advantages, such as reduction in reaction times, greater yield and purity provision, and less energy requirement, and creation of environmentally friendly conditions have been noted.

Conclusion: This review aimed to demonstrate the applications of microwave for the synthesis of benzimidazole and their derivatives from 1994-2016. Microwave-assisted heterocyclic synthesis is a method that can be used to quickly explore and increase diversity in modern chemistry. In the light of the these studies, it could be argued that nearly all of the previously conventionally heated reactions could be performed using the microwave heating. We hope that this review will be helpful to synthesize new potential bioactive benzimidazole derivatives by using this effective method. Time is a very important dimension for researches people. If you save time, you can research much more. In this respect, the microwave technique helps researchers to make effective studies.

Catalyst-free Straightforward Synthesis of Propargylic Ethers from their Carbonates and Alcohols under Microwaves Irradiation by Shiva Kalhor-Monfared, Raja Ben Othman, Claire Beauvineau, Christian Girard (102-106).
Background: Propargylic derivatives are important reagents and building blocks in organic synthesis. They can be obtained starting with propargylic alcohols or their protected forms. The actual methods rely on the use of acidic or metal-catalyzed approachs. These efficient methods however need to use solvents and metals, that are sometimes toxic or expensive. Ways to prepare those derivatives without metals or solvents can thus be an interesting alternative.

Methods: During our work on propargyl carbonates, we found out that reflux in an alcoholic solvent was producing in part the corresponding ether. In this paper we present the study of reaction conditions under microwaves in order to accelerate them. The reaction conditions were optimized using a model substrate and alcohol in order to find the best ratio, temperature and power mode to obtain propargylic ethers form their methyl carbonate. The conditions were then applied to different propargylic derivatives to evaluate the scope of this reaction.

Results: The reaction was applied to different propargyl methyl carbonates and alcohols to evaluate the scope of the reaction. Propargyl ethers can be synthesized from carbonates bearing an aromatic ring and alcohols in good yields, by irradiating them together by microwaves. The limitations include hindered alcohols and propargyl carbonates without aromatic rings as substituents on the carbon atom bearing the carbonate.

Conclusion: During this study, we were able to find out conditions to prepare propargylic ethers from propargyl carbonate and the alcohol, as a solvent and reagent. The ethers can be easily and readily prepared in 0.5 h by microwave irradiation. The reaction can be applied to derivatives bearing groups that can stabilize the carbocationic intermediate proposed in the mechanism. Even if some limitations were encountered, the reaction gives the opportunity to prepare propargylic ethers from their carbonates without the use of acidic- or metal-catalyzed conditions, or solvent other than the reagents, in a simple and straightforward approach.