Joaquim A. Nóbrega

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

Abstract 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.

Focused-microwave-assisted strategies for sample preparation

Abstract In this work a general discussion is presented about extraction and digestion procedures, assisted by focused-microwave radiation. Applications involving inorganic, organic, and organometallic analytes in different types of samples are presented, taking into account recent literature data. The main advantages of using focused-microwave radiation are highlighted, such as safety, versatility, control of microwave energy released to the sample, and programmed addition of solutions. All these features can be applied properly in sample preparation for speciation analysis. New routes of development are discussed considering partial digestion by acid-vapor and gradual addition of a liquid sample to hot concentrated acids. Some preliminary results using these strategies are presented to demonstrate their potentiality.

Flow analysis strategies to greener analytical chemistry. An overview

An overview of the strategies adopted in flow analysis towards cleaner analytical methods is presented. The discussion deals with reagentless procedures, replacement of hazardous chemicals, strategies for waste minimization as well as on-line waste treatment or recycling. The potential of flow approaches such as sequential injection, multicommutation and monosegmented flow is emphasized. Automation, employment of solid-phase reagents and miniaturization are highlighted as alternatives for waste minimization.

Direct Determination of Major and Trace Elements in Milk by Inductively Coupled Plasma Atomic Emission and Mass Spectrometry

The direct determination of major elements (Ca, K, Mg, Na and P) and Zn by inductively coupled plasma atomic emission spectrometry (ICP-AES) and trace elements (Al, Ba, Cu, I, Mn, Mo, Pb, Rb, Se, Sr and Zn) by inductively coupled plasma mass spectrometry (ICP-MS) in powdered, skimmed and whole milk was developed. Samples were diluted with a 5 or 10% v/v water-soluble, mixed tertiary amine reagent at pH 8. This reagent mixture dissociated casein micelles and stabilized liquid phase cations. Analyte emission or mass intensities losses were not observed. Reference solutions were prepared in 10% v/v mixed amine solution, and no internal reference element was needed for ICP-AES. The quantitative ICP-MS procedure applied the standard additions method with an yttrium internal reference. Except for Se in whole milk powder, all determined values in NIST Standard Reference Material (SRM) Non-Fat Milk (SRM 1549) and Whole Milk (SRM 8435) Powders agreed with certified values at the 95% confidence level. This direct technique is as fast as slurry approaches without particle size effects and sensitivity losses.

A critical evaluation of digestion procedures for coffee samples using diluted nitric acid in closed vessels for inductively coupled plasma optical emission spectrometry

The efficiency of diluted nitric acid solutions for digesting regular coffee samples was evaluated employing two closed vessel procedures: one was based on microwave-assisted heating and the other was based on conductive heating using pressurized Parr bomb. The efficiency of digestion was evaluated by determining residual carbon content (RCC) and residual acidity. The digestion was effective using both procedures, i.e. there were no solid residues after the decomposition reactions when using up to 3.5 mol L(-1) nitric acid solutions. It was demonstrated that the digestion procedures are critically dependent on reactions occurring in liquid and gas phase and that the formation of NO and its conversion to NO2 by O2 exerts a major effect in the oxidation of organic matter. These processes are more effective in closed vessels heated by microwave radiation due to the greater volume of these flasks and the temperature gradient that exists during the first step of the digestion process. The proposed model for the digestion processes in diluted nitric acid solution is corroborated by data about consumption of acid during the digestion and by measuring the pressure during the whole process.

Evaluation of inductively coupled plasma optical emission spectrometers with axially and radially viewed configurations

b ´ Abstract The analytical performance of two inductively coupled plasma optical emission spectrometers with axially and radially viewed configurations, equipped with charge coupled device solid-state detectors was evaluated using Ar, Ba, Mg, and Ni as test elements. Both instruments have similar Echelle optical arrangements and radio-frequency generators, differing only in the viewing mode and in the diameter of the central injector tube, i.e. 2.3 for the axially and 1.4 mm for the radially viewed ICP. The figures of merit evaluated were: warm-up time, short- and long-term stabilities, UV and VIS spectral resolution and limit of detection (LOD) for Ni in 0.14 moll HNO and 1000 y1 3

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 ((1)H 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.

Direct determination of Cu and Zn in fruit juices and bovine milk by thermospray flame furnace atomic absorption spectrometry.

In the present work, thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) was employed for Cu and Zn determination in bovine milk and fruit juice samples without any pretreatment. TS-FF-AAS system was optimized and a sample volume of 300mul was injected into the carrier stream (0.014moll(-1) HNO(3) at a flow rate of 0.4mlmin(-1)), and it was introduced into a hot Ni tube. The detection limits obtained for Cu and Zn in aqueous solution were 2.2 and 0.91mugl(-1), respectively, and 3.2mugl(-1) for Cu in a medium containing water-soluble tertiary amines. The relative standard deviations varied from 2.7 to 4.2% (n=12). Sample preparation was carried out by simple dilution in water or in water-soluble tertiary amines medium. Accuracy was checked by performing addition-recovery experiments as well as by using reference materials (whole milk powder, non-fat milk powder, and infant formula). Recoveries varied from 97.7 to 105.3% for Cu and Zn. All results obtained for reference materials were in agreement with certified values at a 95% confidence level.

Microwave-assisted digestion of organic samples: how simple can it become?

Advancements in sample preparation for performing trace analysis of inorganic analytes are coming from the dissemination of microwave-assisted procedures, but there is still room for improvements by looking for simple and easily applied procedures. Recently it was proposed a new approach called single reaction chamber with capability for digestions at high pressures and temperatures using simple vials and racks. This was a limitation of former cavity microwave ovens with closed vessels. It was demonstrated here that the use of single reaction chamber approach allows the implementation of efficient digestions using diluted solutions of nitric acid and also allows addressing a critical need of sample preparation for inorganic analysis by running mixed batches of samples. The feasibility of this procedure was demonstrated for organic samples and accuracy was proved by using certified reference materials of apple leaves, bovine liver and whole milk powder. Digestions performed of whole milk powder and bovine liver using 2.0 mol L(-1) nitric acid solution plus concentrated hydrogen peroxide at 240 °C led to residual carbon contents of 0.825 and 1.50% and residual acidities of 1.04 and 0.618 mol L(-1), respectively. These parameters are fully compatible with further measurements using ICP OES or ICP-MS. Al, Cu, Fe, Mn, Mo, Rb, Se, Sr, and Zn were accurately determined by ICP OES or ICP-MS depending on their concentrations in digests.

Simultaneous determination of cadmium and lead in wine by electrothermal atomic absorption spectrometry

A method has been developed for the direct simultaneous determination of Cd and Pb in white and red wine by electrothermal atomic absorption spectrometry (ET-AAS) using a transversely heated graphite tube atomizer (THGA) with longitudinal Zeeman-effect background correction. The thermal behavior of both analytes during pyrolysis and atomization stages were investigated in 0.028 mol l−1 HNO3 and in 1+1 v/v diluted wine using mixtures of Pd(NO3)2+Mg(NO3)2 and NH4H2PO4+Mg(NO3)2 as chemical modifiers. With 5 μg Pd+3 μg Mg as the modifiers and a two-step pyrolysis (10 s at 400°C and 10 s at 600°C), the formation of carbonaceous residues inside the atomizer was avoided. For 20 μl of sample (wine+0.056 mol l−1 HNO3, 1+1, v/v) dispensed into the graphite tube, analytical curves in the 0.10–1.0 μg l−1 Cd and 5.0–50 μg l−1 Pb ranges were established. The characteristic mass was approximately 0.6 pg for Cd and 33 pg for Pb, and the lifetime of the tube was approximately 400 firings. The limits of detection (LOD) based on integrated absorbance (0.03 μg l−1 for Cd, 0.8 μg l−1 for Pb) exceeded the requirements of Brazilian Food Regulations (decree #55871 from Health Department), which establish the maximum permissible level for Cd at 200 μg l−1 and for Pb at 500 μg l−1. The relative standard deviations (n=12) were typically <8% for Cd and <6% for Pb. The recoveries of Cd and Pb added to wine samples varied from 88 to 107% and 93 to 103%, respectively. The accuracy of the direct determination of Cd and Pb was checked for 10 table wines by comparing the results with those obtained for digested wine using single-element ET-AAS, which were in agreement at the 95% confidence level.