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There is a broad literature describing the use of microwave-assisted acid digestion for preparing agriculture samples. The use of closed vessels operated at high pressure allows exploring the oxidizing properties of nitric acid at high temperatures and avoids the use of perchloric acid. This is a major achievement coming from microwave technology and by itself justifies its large use for digestion of plant tissues. Additionally, it was demonstrated in several analytical procedures the feasibility of working with diluted acid nitric solutions and the effect of hydrogen peroxide in the recovery of nitric acid during the initial steps of digestion. Then, we may summarize that most digestions of plant tissues can be performed by using an oxidizing mixture composed by a nitric acid solution containing from 2 to 7 mol/L plus a small volume of concentrated hydrogen peroxide. Maximum temperatures around 220°C should lead to efficient oxidation processes resulting in digests containing residual carbon contents compatible with further measurements using spectro-analytical techniques. For plant tissues containing high amounts of silicon, such as grass and sugarcane leaves, the use of HF may be needed depending on the analytes that will be determined. In this case, residual HF must be removed or reacted with boric acid for avoiding chemical attack to glass and quartz parts of the equipment, such as ICP-OES and ICP-MS.

Selected applications deal with microwave-assisted digestions of fruit juices, coffee, milk, beer and wine samples aiming the determination of several elements using different analytical techniques. Standard digestions conditions are proposed and all procedures used mixtures containing nitric acid and hydrogen peroxide. Most procedures have used concentrated reagents, but it seems feasible to use nitric acid diluted solutions. Sample volumes should be chosen considering the volume of the closed digestion vessels to avoid over pressurization. The choice of the vessel is also related to the requested sample throughput. These applications represent typical examples where microwave radiation can efficiently be used for oxidation of organic compounds and reducing sample volumes in a clean closed environment compatible with trace analysis requirements using modern spectro-analytical techniques.

Generally ceramic materials have a refractory behavior with a high thermal and chemical resistance. These characteristics are useful for a plethora of technological applications, but make this class of materials hardly digested using conventional acid mixtures. Just as a parallel, we may say that nowadays most samples containing high amounts of organic samples can be digested using a mixture of nitric acid and hydrogen peroxide at high temperatures in a closed vessel able to support high pressure. On the other hand, the digestion of ceramic samples is critically dependent on the choice of the acid mixture. Despite its hazardous, frequently HF must be used for digesting materials with high amounts of silica or resistant metals, such as molybdenum, tungsten, and vanadium. All selected papers here listed used Milestone technology for digesting samples of alumina, aluminum nitride, boron nitride, silicon nitride, cements, niobium oxides, cobalt-substituted lithium nickelate, and pottery. Digestion mixtures were composed by nitric, hydrofluoric, boric, phosphoric acids and hydrogen peroxide. Time of heating programs varied up to 140 min and in general temperatures were not stated. It is important to point out that when using modern microwave technology for digesting pottery samples, nitric acid without any other acid lead to an efficient digestion. In this case, Rončević et al. emphasized that “the utilization of upgraded microwave digestion that allows pressure higher for an order of magnitude and higher pressure in reaction vessels has resulted in more effective decomposition of archaeological pottery shards” (Anal. Methods,4:2506-2514,2012). The reading of respective papers for specific and detailed information is strongly recommended.

Literature data about application of microwave-assisted digestion for clinical samples is abundant. Once again, the use of closed vessels operated at high pressure allows exploring the oxidizing properties of nitric acid at high temperatures and avoids the use of perchloric acid. Most selected procedures recommended the use of a digestion mixture composed by nitric acid and hydrogen peroxide. One work shown below has exploited the use of alkaline conditions, particularly tetra-methyl ammonium hydroxide, which is a convenient reagent for samples of animal tissues containing high amounts of fats. Two critical points to be considered for clinical samples are contamination and sample throughput. Contamination is a critical issue because the widespread use of inductively coupled plasma mass spectrometers with extremely high sensitivity. Most contamination and losses are solved by using closed vessels built using specially designed materials. Sample throughput may be addressed by proper choice of rotors with higher number of vessels and careful control of both pressure and temperature during digestion.

There are few applications of microwave-assisted digestion for preparing cosmetics samples. This sector is increasingly controlled because of public health concerns and this aspect certainly will lead to an expansion of these procedures. An overview of the literature shows that digestion is generally based on a reagent mixture composed by nitric acid plus hydrofluoric acid. The former acts as an oxidant since cosmetics samples typically contain high amounts of organic constituents; the latter is important for digesting silica and silicate compounds frequently present in cosmetics matrices. The use of hydrofluoric acid requires its removal by evaporation or its complexation with boric acid before measurements using inductively coupled plasmas for avoiding chemical attack to the sample introduction components and to the quartz torch.

Determination of metals and organic compounds in environmental samples is required for many purposes and legislation clearly puts a demand on the development of modern analytical procedures able to prepare large amounts of samples in as short as possible time. Sample throughput and green chemistry procedures are critical needs and microwaveassisted procedures are helping to deal with this demand. Digestion and extraction procedures are frequently applied for preparing solutions before determinations using either spectro-analytical or chromatographic techniques. Taking into account the typical huge number of samples in environmental studies with temporal and spatial sampling the capability of treating large batches of samples simultaneously is required and using microwave technology this aspect is associated with fast heating and suitable control of losses and contamination. Additionally, the use of lower amounts of reagents and energy is compatible with modern trends in green chemistry.

Microwave-assisted digestion of food samples may be considered similarly to plant tissues, animal tissues or beverages depending on the main constituents of the matrices. Consequently, most procedures will rely on the combination of nitric acid and hydrogen peroxide. Selected procedures demonstrated the application of tetra-methyl ammonium hydroxide, extraction with an acetone-water mixture, use of hydrofluoric acid for a silicon-rich sample, and diluted solutions of nitric acid. As mentioned for other sample matrices containing high amounts of organic compounds, it is advisable to add the reagent to the sample and to left the vessel open in order to avoid the fast formation of gases that may cause sudden increase of pressure during the first step of microwave-assisted heating. Conditions established using closed vessels are fully compatible with trace analysis determination further carried out using spectro-analytical techniques. Contamination and losses are completely controlled by using closed vessels.

The use of microwave-assisted digestion for geochemistry is not so spread as it is for plant and animal tissues. It is well known that for organic matrices the critical point is to establish strong oxidizing conditions essentially based on reactions promoted by a mixture of nitric acid plus hydrogen peroxide at high temperatures. On the other hand, the digestion of geochemical samples is critically dependent on the choice of an acid mixture able to break chemical bonds among refractories compounds, such as certain oxides, carbides, and silicon compounds. Even microwave-assisted heating at high temperatures in closed vessels does not promote proper digestion if the acid mixture is not strong enough for the type of matrix. Frequently the use of hydrofluoric acid is needed and this causes difficulties with further step of analyte determination. Some selected examples are shown below, but we cannot point out major developments in the last years. We may foresee that the evolution of the sample preparation for geochemistry analysis will rely on future developments using muffle furnaces and crucibles heated by microwave radiation. The ratio of sample and flux masses may be eventually decreased, but major gains probably will come from a significant reduction in energy consumption.

Microwave-assisted digestion of metals and alloys relies on a careful choice of the reagents mixture. Frequently, hydrochloric acid should be combined with nitric acid and the use of aqua regia or inverted aqua regia is also recommended depending on the chemical constituents of the alloy and the chemical bonds in the material. Addition of small amounts of hydrofluoric acid may also be needed. Usually, there is not an intense formation of gases, but the formation of hydrogen gas must be taken into account to avoid safety issues in a closed pressurized vessel. The main advantage of using microwave radiation is the possibility of implementing fast heating programs to reach high temperatures, however the combination of high temperature and proper choice of reagents is essential for efficient conversion of alloys to representative solutions. Long times of heating at high temperatures will not be effective if the reagent mixture was not properly selected. There is not an expressive recent literature about this application, but some representative papers are presented below.

It is well known that for organic matrices the critical point is to establish strong oxidizing conditions essentially based on reactions promoted by a mixture of nitric acid plus hydrogen peroxide at high temperatures. On the other hand, the digestion of mining samples is critically dependent on the choice of an acid mixture able to break chemical bonds.

The critical point about petroleum and its derivatives is the extremely high contents of organic compounds with different reactivity towards oxidizing reagents. Sample mass is a critical issue and sample preparation procedure must be designed to avoid sudden increase of pressure during the initial steps of digestion. Digestion can be efficiently performed using oxidizing mixtures composed by nitric acid plus hydrogen peroxide and it is recommendable to add nitric acid to the sample aliquot and let the mixture without any external heating for at least 1 h. This strategy will promote the initial decomposition of easily oxidized compounds and it will avoid fast increase of pressure when starting the heating program. An important point about microwave-assisted digestion in closed vessels is that the use of more critical reagents, such as sulfuric and perchloric acids, can be avoided.

After long decades relying on old-fashion comparative visual measurements based on formation of metal ions precipitate with sulfide, all strategies for determination of metals in pharmaceuticals are changing based on new USP 232 (Elemental Impurities – Limits) and USP 233 (Elemental Impurities – Procedures) protocols. Nowadays, there are efforts towards harmonization of legislation among USP, European Community, Japan and other pharmacopeias and it seems clear that metals determination will strongly rely on microwave-assisted closed vessel digestions and determinations using ICP-based techniques, i.e. ICP OES and ICP-MS. This is clearly affecting the pattern of papers published in the area and they are moving from specific applications considering type of samples and analytes to general procedures with analytical capability for determining a large number of analytes in several types of samples. Efforts in this direction are underway and there is plenty of room for the development of microwave-assisted digestion procedures. It is clear that the development of these procedures will be critically dependent on the choice of the acid digestion mixture and the use of closed vessels to support high pressures and, consequently, allowing operation at high temperatures.

There are scarce papers in the literature about the application of microwave radiation for digestion of pigment samples. Both examples listed below required the use of hydrofluoric acid for complete digestion of titanium dioxide samples. One paper demonstrated that microwave-assisted heating led to proper digestion using a reagent mixture composed by aqua regia plus hydrofluoric acid in a 25-min heating time, including 5 min for cooling down the vessels. Despite needing hydrofluoric acid, it is advantageous not using sulfuric and phosphoric acids. Particle size effects and oxides crystalline phases should also be considered.

Microwave-assisted digestion of polymers is critically dependent on the choice of acid mixture. The type of polymer and the concentrations of additives are critical aspects for selecting the reagent mixture. A mixture composed by oxidizing agents, such as nitric acid and hydrogen peroxide, plus hydrofluoric acid is frequently needed. Usually conventional wet ashing in open vessels is based on the use of sulfuric acid and hydrogen peroxide. In this case, the use of sulfuric acid is justified by its high boiling point at normal pressure. However, when working with open vessels losses of volatile elements may be observed and procedures are too long. Microwave-assisted digestion using closed vessels provides comparatively faster procedures and the use of sulfuric acid may be avoided due to the high boiling point of nitric acid at high pressure. It is interesting to avoid the use of sulfuric acid because it may cause precipitation of some sulfate salts and it increases the viscosity of the digests which may cause transport interference effects when introducing solutions by pneumatic nebulization in spectroanalytical instruments. Selected applications also show the use of microwave-assisted heating for extraction of antioxidants and digestion of polymers with tetra-methyl ammonium hydroxide for determination of bromine as a fast first screening of brominated flame retardants.