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 microwave- assisted 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.