Cezar A. Bizzi

Microwave-assisted digestion in closed vessels: effect of pressurization with oxygen on digestion process with diluted nitric acid

The efficiency of diluted nitric acid solutions under oxygen pressure for decomposition of bovine liver was evaluated using microwave-assisted wet digestion. Calcium, Cu, Fe, Mg, Mn and Zn were determined by inductively coupled plasma optical emission spectrometry (ICP OES). Efficiency was evaluated by determining the residual carbon content (RCC) by ICP OES and residual acidity in digests. Samples (up to 500 mg) were digested using nitric acid solutions (0.1, 0.5, 1, 2, 3, 7, and 14 mol L−1 HNO3) and the effect of oxygen pressure was evaluated using pressures of 0.5, 1, 1.5 and 2 MPa. It was demonstrated that 2 and 0.5 mol L−1 nitric acid solutions may be used for efficient digestion of 500 and 100 mg of bovine liver, respectively, with oxygen pressures ranging from 0.5 to 2 MPa. Using these conditions, less than 0.86 and 0.21 mL of concentrated nitric acid were necessary to digest 500 and 100 mg of sample, respectively. Similar digestion efficiencies for both conditions were obtained under pressures of O2 ranging from 0.5 to 2 MPa. The residual acidities in final digests were lower than 24% when compared to the initial amount of acid used for digestion. The accuracy of the proposed procedure was evaluated using certified reference materials of bovine liver and bovine muscle. Using a solution of 2 mol L−1 with oxygen pressure of 0.5 MPa for 500 mg of sample, the agreement with certified values ranged from 96 to 105% (n = 5). Using the proposed procedure with diluted nitric acid it was possible to obtain RCC values lower than 15% that is important for minimizing the generation of laboratory residues and improving limits of detection.

Evaluation of oxygen pressurized microwave-assisted digestion of botanical materials using diluted nitric acid.

The feasibility of diluted nitric acid solutions for microwave-assisted decomposition of botanical samples in closed vessels was evaluated. Oxygen pressurized atmosphere was used to improve the digestion efficiency and Al, Ca, K, Fe, Mg and Na were determined in digests by inductively coupled plasma optical emission spectrometry (ICP OES). Efficiency of digestion was evaluated taking into account the residual carbon content (RCC) and residual acidity in digests. Samples were digested using nitric acid solutions (2, 3, 7, and 14 mol L(-1) HNO(3)) and the effect of gas phase composition inside the reaction vessels by purging the vessel with Ar (inert atmosphere, 1 bar), air (20% of oxygen, 1 bar) and pure O(2) (100% of oxygen, 1 bar) was evaluated. The influence of oxygen pressure was studied using pressures of 5, 10, 15 and 20 bar. It was demonstrated that a diluted nitric acid solution as low as 3 mol L(-1) was suitable for an efficient digestion of sample masses up to 500 mg of botanical samples using 5 bar of oxygen pressure. The residual acidities in final digests were lower than 45% in relation to the initial amount of acid used for digestion (equivalent to 1.3 mol L(-1) HNO(3)). The accuracy of the proposed procedure was evaluated using certified reference materials of olive leaves, apple leaves, peach leaves and pine needles. Using the optimized conditions for sample digestion, the results obtained were in agreement with certified values. The limit of quantification was improved up to a factor of 14.5 times for the analytes evaluated. In addition, the proposed procedure was in agreement with the recommendations of the green chemistry once it was possible to obtain relatively high digestion efficiency (RCC<5%) using only diluted HNO(3), which is important to minimize the generation of laboratory residues.

Evaluation of a digestion procedure based on the use of diluted nitric acid solutions and H2O2 for the multielement determination of whole milk powder and bovine liver by ICP-based techniques

Many efforts have been made in order to reduce the amount of reagents and waste produced in analytical laboratories. However, suitable digestion efficiency must be considered, and depending on the sample preparation procedure, incomplete digestion can result in severe matrix effects during analysis by spectrometric techniques such as ICP OES and ICP-MS. In the present work a procedure based on the use of H2O2 was developed in order to minimize the consumption of HNO3 without decreasing the efficiency of digestion. Although H2O2 has been used combined with HNO3 for sample digestion, its role is still not completely elucidated even as its action combined with O2 in pressurized systems. The performance obtained using H2O2 was similar to that observed when adding O2 to the reaction vessel, driving the better understanding of the role of H2O2 in closed digestion procedures. Digestion using H2O2 allowed the use of HNO3 solutions as diluted as 1 mol L−1 to digest sample masses of biological materials as high as 500 mg. The proposed procedure allowed a reduction of up to 14 and 9.3-fold in the HNO3 amount normally used in whole milk powder and bovine liver digestions, respectively, without decreasing the digestion efficiency. Calcium, Cu, Fe, K, Mg, Mn, Mo, Na, and Zn were determined by ICP OES, while Cd, Co, and Pb were determined by ICP-MS. Using diluted HNO3 solution low blank values were obtained resulting in relatively lower limits of detection and relative standard deviations. The accuracy was evaluated by using certified reference materials of milk powder and bovine liver (agreement was better than 95% to certified values for all evaluated analytes).

Bromine and chlorine determination in cigarette tobacco using microwave-induced combustion and inductively coupled plasma optical emission spectrometry

(NH4)2CO3 as absorbing solution and 5 min of reflux. Temperature during combustion was higher than 1400 °C and the residual carbon content in digest obtained after MIC was lower than 1%. Up to eight samples could be processed simultaneously and a single absorbing solution was suitable for both Br and Cl. Limit of quantification by MIC and further ICP OES determination was 12 and 6 µg g -1 for Br and Cl, respectively.

Halogen determination in food and biological materials using plasma-based techniques: challenges and trends of sample preparation

Halogens are a group of reactive elements, which are capable of making stable bonds with metals and nonmetals. Due to their properties, they play an important biochemical role in living organisms; some, such as F, Cl and I, are essential for humans, whereas others, such as Br, are non-essential and can cause key toxicological effects. This is probably the main reason why this group of elements has been extensively studied in food and biological materials, especially in recent years. The determination of halogens at trace levels requires suitable instrumentation, and plasma-based techniques, such as inductively coupled plasma optical emission spectrometry (ICP-OES) and especially inductively coupled plasma mass spectrometry (ICP-MS), are particularly well suited for this purpose. Although they are very versatile for performing multielemental determination, halogens present high ionization potentials and low wavelength emission lines, and they are prone to interferences due to matrix effects. Such drawbacks are difficult to overcome without changing instrumental features, such as mass or optical resolution. On the other hand, some interferences due to the presence of dissolved carbon in solution, enhancement or suppression of ionization, and related matrix effects can be minimized by applying an appropriate sample preparation step. In this sense, the present review covers the main sample preparation methods applied for halogen determination in food and biological materials by using ICP-OES and ICP-MS. Methods based on dilution, solubilization, extraction or those involving matrix decomposition (e.g., acid digestion and combustion) are discussed based on key applications selected in the last 20 years.

Effect of simultaneous cooling on microwave-assisted wet digestion of biological samples with diluted nitric acid and O2 pressure

The present work evaluates the influence of vessel cooling simultaneously to microwave-assisted digestion performed in a closed system with diluted HNO3 under O2 pressure. The effect of outside air flow-rates (60-190 m(3) h(-1)) used for cooling of digestion vessels was evaluated. An improvement in digestion efficiency caused by the reduction of HNO3 partial pressure was observed when using higher air flow-rate (190 m(3) h(-1)), decreasing the residual carbon content for whole milk powder from 21.7 to 9.3% (lowest and highest air flow-rate, respectively). The use of high air flow-rate outside the digestion vessel resulted in a higher temperature gradient between liquid and gas phases inside the digestion vessel and improved the efficiency of sample digestion. Since a more pronounced temperature gradient was obtained, it contributed for increasing the condensation rate and thus allowed a reduction in the HNO3 partial pressure of the digestion vessel, which improved the regeneration of HNO3. An air flow-rate of 190 m(3) h(-1) was selected for digestion of animal fat, bovine liver, ground soybean, non fat milk powder, oregano leaves, potato starch and whole milk powder samples, and a standard reference material of apple leaves (NIST 1515), bovine liver (NIST 1577) and whole milk powder (NIST 8435) for further metals determination by inductively coupled plasma atomic emission spectroscopy (ICP-OES). Results were in agreement with certified values and no interferences caused by matrix effects during the determination step were observed.

Microwave-assisted ultraviolet digestion of petroleum coke for the simultaneous determination of nickel, vanadium and sulfur by ICP-OES.

A method for the simultaneous determination of Ni, V and S in petroleum coke by inductively coupled plasma optical emission spectrometry (ICP-OES) after microwave-assisted ultraviolet digestion (MW-UV) in closed vessels was proposed. Digestion was performed using electrodeless discharge lamps positioned inside quartz vessels and turned on by microwave radiation. The following parameters were evaluated: HNO3 concentration (15 mL of 1, 4, 7, 10 or 14.4 mol L(-1)), volume of H2O2 (30%, 1 or 3 mL), sample mass (100, 250 or 500 mg) and heating time (40 or 60 min) with or without the use of UV lamps. Digestion efficiency was evaluated by the determination of the residual carbon content (RCC) in digests. Using UV lamps lower RCC was obtained and the combination of 4 mol L(-1) HNO3 with 3 mL of H2O2 and 60 min of heating allowed a suitable digestion of up to 500 mg of petroleum coke (RCC< 21%). The agreement with the reference values for Ni, V and S (obtained by digestion of petroleum coke by microwave-induced combustion) and with a certified reference material of petroleum coke was between 96 and 101%. The proposed method was considered as advantageous when compared to American Society for Testing and Materials method because it allowed the simultaneous determination of Ni, V and S with lower limit of detection (0.22, 0.12 and 8.7 µg g(-1) for Ni, V and S, respectively) avoiding the use of concentrated nitric acid and providing digests suitable for routine analysis by ICP-OES.

Microwave-assisted wet digestion with H2O2 at high temperature and pressure using single reaction chamber for elemental determination in milk powder by ICP-OES and ICP-MS.

In this work a green digestion method which only used H2O2 as an oxidant and high temperature and pressure in the single reaction chamber system (SRC-UltraWave™) was applied for subsequent elemental determination by inductively coupled plasma-based techniques. Milk powder was chosen to demonstrate the feasibility and advantages of the proposed method. Samples masses up to 500mg were efficiently digested, and the determination of Ca, Fe, K, Mg and Na was performed by inductively coupled plasma optical emission spectrometry (ICP-OES), while trace elements (B, Ba, Cd, Cu, Mn, Mo, Pb, Sr and Zn) were determined by inductively coupled plasma mass spectrometry (ICP-MS). Residual carbon (RC) lower than 918mgL(-1) of C was obtained for digests which contributed to minimizing interferences in determination by ICP-OES and ICP-MS. Accuracy was evaluated using certified reference materials NIST 1549 (non-fat milk powder certified reference material) and NIST 8435 (whole milk powder reference material). The results obtained by the proposed method were in agreement with the certified reference values (t-test, 95% confidence level). In addition, no significant difference was observed between results obtained by the proposed method and conventional wet digestion using concentrated HNO3. As digestion was performed without using any kind of acid, the characteristics of final digests were in agreement with green chemistry principles when compared to digests obtained using conventional wet digestion method with concentrated HNO3. Additionally, H2O2 digests were more suitable for subsequent analysis by ICP-based techniques due to of water being the main product of organic matrix oxidation. The proposed method was suitable for quality control of major components and trace elements present in milk powder in consonance with green sample preparation.

Microwave-assisted digestion methods: towards greener approaches for plasma-based analytical techniques

In recent years, the evolution of instrumental techniques such as inductively coupled plasma (ICP)-based techniques has allowed multielemental analytical capability at trace levels and for a variety of matrices. Such information can be strongly affected by matrix-related effects causing spectral and non-spectral interferences and several alternatives have been proposed to overcome these drawbacks. Sample preparation methods with high digestion efficiency have been the most convenient for subsequent ICP analysis. Conventionally, high digestion efficiency can be obtained following protocols based on the addition of a relatively high amount of concentrated reagents combined with operational conditions using high temperature and pressure. Modern instrumentation and a better understanding of chemical reactions related to sample digestion have enabled the achievement of high digestion efficiency even using dilute reagents. In this sense, this review presents the main advances for the digestion of organic samples, based on the reduction of reagents without impairing digestion efficiency for further multielement determination by ICP-based techniques. Recent approaches are highlighted, such as the use of dilute acid solutions with the aid of O2, H2O2 and UV-assisted digestion, high pressure flow digestion, and combustion methods, all assisted by microwave radiation. The main features and applications reported in the past ten years are also covered. Finally, a critical view of the suitability of metrics for the evaluation of greener digestion methods, mainly for matrices considered hard to be digested using conventional protocols, is presented.

Bromine and Iodine Contents in Raw and Cooked Shrimp and Its Parts

The concentration of bromine and iodine was determined in shrimp and its parts (tissue and shells), and changes in the analyte concentration were evaluated after the cooking procedure. Bromine and iodine concentrations were determined by a method recently developed by our research group based on microwave-induced combustion for sample preparation and inductively coupled plasma mass spectrometry for analyte determination. The accuracy was evaluated using a reference material (NIST 8414) that was digested using the proposed method. No statistical difference was observed between certified and determined values (Students t test, 95% confidence level). Suitable limits of detection (Br, 0.02 μg g(-1) and I, 0.01 μg g(-1)) were obtained for both analytes. Higher concentrations of both analytes were observed in shrimp shells in comparison to shrimp tissue for raw and cooked samples. Moreover, losses of Br and I (between 24 and 43%) were observed after cooking.