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 HNO₃) 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 O₂ (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 HNO₃). 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 HNO₃, which is important to minimize the generation of laboratory residues.     

Microwave-assisted digestion procedures for biological samples with diluted nitric acid: Identification of reaction products

Microwave-assisted sample preparation using diluted nitric acid solutions is an alternative procedure for digesting organic samples. The efficiency of this procedure depends on the chemical properties of the samples and in this work it was evaluated by the determination of crude protein amount, fat and original carbon. Soybeans grains, bovine blood, bovine muscle and bovine viscera were digested in a cavity-microwave oven using oxidant mixtures in different acid concentrations. The digestion efficiency was evaluated based on the determination of residual carbon content and element recoveries using inductively coupled plasma optical emission spectrometry (ICP OES). In order to determine the main residual organic compounds, the digests were characterized by nuclear magnetic resonance (¹H NMR). Subsequently, studies concerning separation of nitrobenzoic acid isomers were performed by ion pair reversed phase liquid chromatography using a C18 stationary phase, water:acetonitrile:methanol (75:20:5, v/v/v) + 0.05% (v/v) TFA as mobile phase and ultraviolet detection at 254 nm. Sample preparation based on diluted acids proved to be feasible and a recommendable alternative for organic sample digestion, reducing both the reagent volumes and the variability of the residues as a result of the process of decomposition. It was shown that biological matrices containing amino acids, proteins and lipids in their composition produced nitrobenzoic acid isomers and other organic compounds after cleavage of chemical bonds.     

Comparison of ten digestion procedures for the determination of arsenic in soils by hydride-generation atomic absorption spectrometry

Ten different digestion methods were investigated for the determination of arsenic in soils by hydride-generation atomic absorption spectrometry. These methods included a dry ashing/digestion, several acid-leaching procedures, and digestions in a pressure decomposition vessel or a Kjeldahl apparatus. A certified reference sample was analysed and the results obtained for five other soil samples were compared with the data obtained by spectrophotometry. A nitric/sulphuric acid digestion was the most suitable. A brief interference study is reported.     

Optimization of the Sample Preparation Procedure for the Determination of Trace Elements in Auricularia auricula by Inductively Coupled Plasma-Optical Emission Spectrometry

This study aimed to develop a precise and accurate method of sample preparation of Auricularia auricula for inductively coupled plasma-optical emission spectrometry-based trace element determination and to compare concentrations of seven trace elements (Mg, Fe, Mn, Zn, Ni, Cr, Sr) in six A. auricula samples belonging to three varieties with two cultivation substrates. Five sample preparation procedures, microwave-assisted digestion, nitric acid digestion with hot plate heating, nitric acid and perchloric acid digestion with hot plate heating, nitric acid digestion with ash content, and aqua regia digestion with ash content, were compared. The performance of the procedures was determined based on the precision and accuracy of the results and the limits of detection of the elements. The best results, with limits of detection of 0.60–6.60?ng?·?mL^?1 and recoveries for spiked samples between 93.80 and 105.00%, were found using nitric acid digestion with ash content. Six A. auricula samples were analyzed using the proposed procedure. Among the tested elements, the concentration of Fe was highest in all six A. auricula samples up to a maximum concentration of 284.83?µg?·?g^?1. The concentrations of Mn, Cu, and Ni increased in mixed stands (basswood, birch, and mongolica) compared with pine sawdust cultivation.     

Multielement characterization of soil samples with ICP-MS for environmental studies

The recovery of trace elements of ecotoxic importance has been studied on certified soil and sediment reference samples after pressurized digestions with HNO₃, HNO₃+HF and HNO₃+HCl+HF mixtures, respectively. The acid digests have been analyzed by ICP-MS. The results indicate that digestion with nitric acid alone is satisfactory for the recovery of As, Cd, Co, Cu and Zn. Cr and Pb showed lower recoveries with HNO₃ alone but addition of HF improved their extraction. With appropriate corrections, ICP-MS can be used for the routine analysis of soils and sediments. These digestion procedures, evaluated based on reference samples, have been used for the trace element characterization of soil samples from the German Environmental Specimen Bank.     

A new procedure for bovine milk digestion in a focused microwave oven: gradual sample addition to pre-heated acid

Milk samples can be efficiently digested using a focused microwave oven, however the conventional procedure of addition of concentrated acids to the liquid sample leads to digestates with elevated acidity and residual carbon concentrations. In this work a focused microwave oven was applied for acid digestion of bovine milk samples using a conventional and an alternative procedure based on gradual sample addition to hot and concentrated acids. A two-level 2³ full factorial design experiment with eight runs was carried out to evaluate the optimum experimental conditions for reducing both the residual carbon and the final acidity of digestates. The three studied parameters were: temperature of the digestion medium for sample addition, addition of sulfuric acid before the sample or during the first step, and number of aliquots of the sample gradually added. The best conditions were attained by adding small aliquots of milk (ten-fold a volume of 0.5 ml added during 5.0 min) to a digestion mixture containing 3.0 ml nitric acid plus 1.0 ml sulfuric acid heated at 105 °C. It was demonstrated that the digestion efficiency of the alternative procedure was better than the conventional procedure, i.e. 98 and 80%, respectively. The alternative procedure was applied for determination of Ba, Ca, Cu, K, Mg, Na, P, and Zn in whole and non-fat bovine milk. The accuracy was proved using two certified reference materials (whole and non-fat milk powder).     

Characterisation of biological RMs for potential use in human brain analysis

Summary In order to investigate the precision and accuracy obtainable with instrumental neutron activation analysis (INAA) and inductively coupled plasma atomic emission spectrometry (ICP-AES), five different biological reference materials were analyzed. Four of them originated from the US National Institute of Standards and Technology (NIST) (Bovine Liver (SRM 1577; 1577/a; 185/A), Orchard Leaves (SRM 1571)) and one of them came from the International Atomic Energy Agency (IAEA) (Animal muscle (H-4)). The decomposition efficiency of microwave acid digestion was examined in comparison to digestions carried out in a Parrbomb. Five to ten subsamples of each material were subjected to the two digestion procedures and the solutions were analyzed by ICP-AES. The dried samples were analyzed by INAA. Up to 16 elements (Al, B, Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, S, Zn) were determined in various materials.     

Effect of acid concentration on closed-vessel microwave-assisted digestion of plant materials

The efficiency of microwave-assisted acid digestion of plants using different concentrations of nitric acid (2.0, 3.0, 5.0, 7.0 and 14 mol l⁻¹) with hydrogen peroxide (30% v/v) was evaluated by measuring the residual carbon content (RCC) using inductively coupled plasma optical emission spectrometry (ICP-OES) with axial viewing. Certified reference materials were used for evaluating the accuracy attained when 2 mol l⁻¹ HNO₃ was employed for digestion. Under all experimental conditions RCC values were always lower than 13% w/v, and even the highest concentration did not cause any interference with element recovery. It seems that the high pressure reached for closed-vessel operation improved the oxidative action of nitric acid due to consequent temperature increase, even when this reagent was not used at high concentrations. According to acid–base titration data, residual acid in the digestates varied from 1.2 to 4.0 mol l⁻¹, depending on the acid concentration initially added. It can be concluded that for plant materials, microwave-assisted acid digestion can be carried out under mild conditions, which implies that digestates do not need extensive dilution before introduction by pneumatic nebulization to ICP-OES. An additional advantage is the lower amount of residue generated when working with less concentrated acid solutions.     

Microwave Single Vessel Acid-Vapor Extraction: Effect of Experimental Parameters on Co and Fe Determination in Biological Samples

Characteristic parameters of a single vessel procedure were investigated for Co and Fe determination in biological samples using nitric acid-vapor extraction under atmospheric pressure. A laboratory-made PTFE support vessel suitable for four cups containing the samples was adapted to fit on the microwave glass vessel. Biological samples (30mg) were weighed directly into these PTFE cups and exposed to nitric acid vapor stemming from 15mL concentrated HNO₃ placed in the bottom of the glass vessel. Sample cups were positioned at 50, 80, 110, and 140mm from the support base. We evaluated the temperature, acidity, and residual carbon content of each sample cup, i.e. parameters that could influence the efficiency of vapor-phase acid digestion. The influence of sample particle size on digestion efficiency was also considered. It was demonstrated that temperature was the main parameter for efficient recovery of both Co and Fe in biological samples, and higher temperatures allowed lower residual carbon contents.     

Comparison of partial digestion procedures for determination of Ca, Cr, Cu, Fe, K, Mg, Mn, Na, P, and Zn in milk by inductively coupled plasma-optical emission spectrometry

A fast procedure was developed for determination of Ca, Cr, Cu, Fe, K, Mg, Mn, Na, P, and Zn in milk samples. This procedure consisted of a partial digestion with hydrochloric acid on a hot plate. The results obtained were compared with 3 other digestion procedures (dry ashing and 2 microwave digestions). All the procedures showed similar precision levels, with coefficients of variation <10% for most analyzed elements. Accuracy was evaluated by using certified reference materials, and the values obtained were within the confidence intervals for these products. The results obtained were not considered statistically different. The partial digestion on a hot plate with HCl can be very practical for laboratories with relatively large numbers of sample analyses.     

Determination of trace elements in Antarctic krill samples by inductively coupled atomic emission and graphite furnace atomic absorption spectrometry

Analytical methods were developed for the determination of trace elements in Antarctic krill samples applying inductively coupled plasma atomic emission spectrometry (ICP-AES) and graphite furnace atomic absorption spectrometry (GF-AAS). Cu, Fe, Mn and Zn were determined by ICP-AES, while Cd and Pb by GF-AAS technique. Two microwave assisted digestion procedures were elaborated for the preparation of 0.5-g krill samples using open and closed vessel systems. The efficiency of the digestion processes was checked by measurements of the total organic carbon content of the solutions obtained. The deviations of the analytical data from the certified values and the relative standard deviations of the concentration measurements were lower for all six elements investigated applying the closed vessel digestion system.     

Accurate determination of selenium in biological materials without perchloric acid for digestion

Selenium (10–80 μg kg^?1) is determined by hydride-generation atomic absorption spectrometry in mixed diet, faecal and urine samples and in standard reference materials (bovine liver, rice flour, wheat flour and horse kidney) after two digestion procedures. No difference was found in digestion efficiency between a nitric/sulphuric acid mixture and a nitric/sulphuric/perchloric acid mixture. The results suggest that the digestion of most biological materials for the determination of selenium does not require the use of perchloric acid.     

Comparison of two digestion procedures for the determination of lead in lichens by electrothermal atomic absorption spectrometry

The efficiency of two procedures for the digestion of lichen was investigated using a heating block and a microwave oven. In the open vessels, concentrated nitric acid was added to the samples, left for 1 h, and the addition of 30% (v / v) hydrogen peroxide completed the digestion. In the closed system, the complete digestion was performed using concentrated nitric acid and hydrogen peroxide, reducing the amount of chemicals, time and contamination risk. Both digestion methods gave comparable results, and recoveries were statistically not different. For a lichen sample spiked with 10 μg Pb, the recovery was 111% and 110% using microwave and heating block digestion, respectively, while it was 100% and 103% for a 100 μg Pb spike. For the determination by electrothermal atomic absorption spectrometry samples were diluted 20 times with water and a volume of 20 μL was injected into the graphite furnace without chemical modifier. Pyrolysis and atomization temperatures of 700 °C and 1500 °C, respectively, were used. The characteristic mass was 8.4 ± 0.6 pg for aqueous calibration solutions and 8.9 ± 0.8 pg for samples. Calibration was against matrix matched standards. The recovery test showed some contamination problem with the lowest concentrations in both procedures. The detection limits were 4.4 μg L^− 1 with microwave oven and 5.4 μg L^− 1 with the heating block in the undiluted blank.     

Application of Leaching Tests for the Assessment of Available Heavy Metals from Domestic and Industrial Sludges

Various sludge samples from different domestic and industrial wastewater treatment plants were analyzed by Flame Atomic Absorption Spectrometry to evaluate their total and available contents of heavy metals (Cu, Cr, Ni, Pb, Zn and Fe). The EDTA and acetic acid single extraction procedures were applied to these samples with the aim to study the leaching behaviour of the metals in the different sludges and also to predict their possible mobility when these wastes are disposed on the environment and landfills. In acidic medium, a higher extraction efficiency was observed for the major part of the elements studied in the industrial sludges, except for Ni and Zn, which were also considerably released from domestic sludges. In contrast, in the presence of the EDTA complexing ligand, a more elevated mobility of metals was found in the domestic sludges and little or no metals were released from the industrial sludges. Moreover, the results of the microwave total digestions were compared, for all metals, with those obtained using a more simplified pseudototal digestion procedure (based on the EPA Method 3051, named microwave assisted acid digestion of sediments, sludges, soils and oils) and a good agreement was found between them, except for Cr and Ni in some particular samples. The between-batch precision (expressed as RSD) of both compared digestion methods was very similar and values lower than 7.7% were obtained in both cases. A certified domestic sludge material (SRM 2781) was employed to validate the two digestion procedures and no significant differences were found between the certified and experimental values for all the elements studied.     

Rapid determination of chemical oxygen demand using a focused microwave heating system featuring temperature control

This paper demonstrates the use of a microwave heating system, employed in the chemical digestion step, for the determination of chemical oxygen demand in wastewater. The results are first compared with those provided by standard methods using reference substances. The problems arising from abrupt heating of the sample and the potential thermal decomposition of potassium dichromate are examined. Two different approaches to sample digestion involving a gradually increasing irradiation time were tested. First, a constant power strategy is applied, and the second proposes a constant temperature approach by using a temperature control system.By optimising the operating conditions, the digestion time was reduced to 8-60 times with respect to the standard method. The reference digestion time is 5 min. In especially difficult digestions, the proposed approach provides a substantially improved degradation with respect to conventional procedures. The procedure was applied to wastewater from various industries and found to ensure thorough digestion of all samples and to provide favourable results in all cases tested.     

Comparison of the efficiencies of on-line and high-pressure closed vessel approaches to microwave heated sample decomposition

Two microwave heated digestion techniques have been tested with a view to the evaluation of the effectiveness of wet decomposition procedures: a high-pressure/temperature focused microwave heated and an on-line microwave heated system. The efficiency of decomposition was studied using a number of biological reference materials as well as urine and sewage plant effluent. Oxidation reagents included mixtures of nitric acid, nitric acid/hydrogen peroxide and peroxodisulphate (Oxisolv). The residual carbon content of the digestate was determined coulometrically. Pressurised decomposition of a TFM-Teflon vessel is the most effective procedure (organic material is totally oxidised with nitric acid in a single-step procedure), whereas urine and sewage plant effluent are incompletely decomposed with on-line microwave heated digestion.     

Digestion of biological materials using the microwave-assisted sample combustion technique

In this study a procedure for sample digestion based on sample combustion assisted by microwave radiation is proposed. Combustion is started by microwave radiation in the presence of oxygen under pressure using ammonium nitrate as aid for ignition. The system was adapted in a microwave oven with quartz closed vessels. A quartz piece is used simultaneously as a sample holder and as protection to the cap of quartz vessel from the flame generated in the combustion process. Sample was pressed into a pellet and placed on a disc paper in the holder and 50 μl of 50% m/v ammonium nitrate solution was added. The influence of the absorption solution (diluted and concentrated nitric acid or water) on the recoveries for Cu and Zn was evaluated. About 3 s of microwave irradiation was necessary to start the combustion. The combustion process was evaluated in relation to the influence of sample mass on the ignition time, combustion time and maximum operation pressure. Bovine liver, milk powder and oyster tissue certified reference materials were used to evaluate the accuracy of the procedure for determination of copper and zinc. Good agreement for zinc (96% to 103%) was obtained from bovine liver certified reference material when microwave combustion and microwave combustion followed by reflux were used to sample decomposition, even if water was used for absorption of analyte. For copper, the combustion followed by reflux of 5 min allows an agreement from 96% to 100%. Similar results were obtained for oyster tissue samples. However, for milk powder good agreement close to 100% was obtained only if 4 mol l^− 1 HNO₃ was used with a reflux step. Results from the proposed procedure were also compared to those from conventionally used procedures for biological samples decomposition, such as wet digestion in open vessels and microwave-assisted digestion in closed vessels. The advantages of this procedure include the complete sample decomposition in less time than other procedures and the acid consumption was always lower than 2%. Another advantage is the low residual carbon content, less of 1.4% without reflux and less than 0.3% with the reflux step and the possibility of use of diluted acid as absorbing solution. Moreover, the new holder allows an effective protection of the vessel cap to burnt high masses.     

Minimization of volatile nitrogen oxides interference in the determination of arsenic by hydride generation atomic absorption spectrometry

In this study emphasis was given to minimize the interference of volatile nitrogen oxides from digestion procedures with nitric acid on the determination of arsenic by hydride generation atomic absorption spectrometry (HG AAS). Sulfamic acid (SA) is proposed to minimize this interference by employing three procedures for the digestion of hair in closed systems: conventional and microwave (MW) heating in polytetrafluorethylene (PTFE) vessels and by MW heating in glass vials. Hair samples were digested with H₂SO₄+HNO₃ or HNO₃+H₂O₂ mixtures. Concentrated hydrochloric acid was added for the digestion for the procedure in glass vials. The accuracy of the procedures with PTFE vessels was verified by the spike recoveries of organic (p-aminobenzenearsonic acid and dimethyl arsinic acid, from 92 to 101%) and inorganic (sodium arsenate, from 98 to 102%) arsenic compounds. For the procedure in glass vials the recovery was from 86 to 97% for organic As and from 97 to 102% for inorganic As. The results obtained for a certified hair reference material using the three digestion procedures were well within the 95% confidence interval of the certificate when SA was added to the solutions. However, when SA was not added, recoveries were low and non-reproducible signals and high background levels were observed. Urea, benzoic acid and hydroxylamine hydrochloride were also studied (maximum As recovery of 90% using hydroxylamine hydrochloride) but the best results were obtained with use of SA.     

酸对微波消解-氢化物发生原子荧光光谱法测定食品中锗的影响

以微波消解为前处理手段,采用氢化物发生原子荧光谱法测定食品中的锗,研究了作为载液的磷酸体积分数和消解过程中残留硝酸对测定结果的影响. 磷酸体积分数从5%增加至30%时,标准曲线斜率缓慢上升. 认为10%~20%体积分数磷酸适宜作为原子荧光光谱法测锗的载液. 硝酸在样品中体积分数超过1%后,锗的荧光信号严重减弱. 优化条件后对市售大蒜样品进行加标试验. 为进一步确定对有机锗的消解效果,对某批次锗-132胶囊进行测定. 大蒜加标试验回收率为109.7%,胶囊测定值为标签标识值的81.4%. 两次试验结果均较为满意,说明方法具有可靠性.     

Improved measurement of iodine in food samples using inductively coupled plasma isotope dilution mass spectrometry

A new ICP-MS method for the determination of iodine in food samples is presented. The method makes use of a new miniature cyclonic spray chamber and a concentric glass nebulizer that is designed for low sample uptakes and is operated in a self-aspirating mode. As a consequence the wash-out was accelerated over conventional systems. This configuration allows the direct determination of iodine in mineralized solutions following digestion with nitric acid only. No strong oxidizing reagents such as perchloric acid or lengthy sample preparation were necessary to alter the chemical form of potentially volatile species. The isotope dilution technique using the long-lived isotope 129I was applied to obtain freedom from matrix effects. The present study reports on results for total iodine in selected nutritional and biological reference materials and makes a comparison with instrumental neutron activation analysis.