Letícia M. Costa

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.

Use of factorial design for optimization of microwave-assisted digestion of lubricating oil

This work proposes the use of factorial design for optimization of microwave-assisted digestion of lubricating oil. The accuracy of digestion procedures is affected by critical experimental parameters, such as sample amount, concentrated acid volumes, microwave radiation applied power, and digestion time. The effects of these key variables on the microwave-assisted digestion efficiency were investigated. The residual carbon content and the acidity were determined in all digestates after microwave-assisted digestion as response of the factorial design. Calcium, Cu, Mg, P, S, and Zn were determined in oil digestates obtained by using two systems: a cavity- and a focused-oven. The accuracy was checked using one standard reference material, NIST SRM 1848 - Lubricating Oil Additive Package. All determined and certified values are in agreement at a 95% confidence level. The digestion time, microwave applied power, sample mass, and the interaction between these variables were significant according to P-values when the analysis of variance (ANOVA) was used.

Arsenic and Mercury mobility in brazilian sediments from the São Francisco River Basin

Os sedimentos possuem propriedades de acumulacao de poluentes e a determinacao de elementos toxicos nessas amostras permite avaliar o seu ciclo geoquimico. Arsenio e mercurio foram determinados em 28 amostras de sedimento da Bacia do Rio Sao Francisco, Minas Gerais, Brasil, por geracao de hidretos acoplada a espectrometria de absorcao atomica (HG AAS). Foram empregados procedimentos de digestao pseudo-total empregando HCl:HNO3 (3:1 v/v) e extracao parcial com HCl 1 mol L-1. Realizou-se ainda a determinacao de Hg nas amostras solidas, utilizando-se um analisador direto de mercurio. Observou-se que 46% das amostras apresentaram concentracoes de As acima do limite de nivel 1 estabelecido pela Resolucao 344/2004 do CONAMA e que 14% excederam o limite de nivel 1 para Hg. As amostras com altas concentracoes de Hg foram tambem analisadas por termodessorcao acoplada a espectrometria de absorcao atomica (TDAAS) para especiacao e os termogramas apontaram somente a presenca da especie Hg(II)

Planejamento fatorial aplicado à digestão de amostras de feijão assistida por radiação microondas

The use of factorial design was evaluated for optimization of focused-microwave-assisted digestion of bean samples. Calcium, Fe, Mg, Mn and Zn percentual recoveries were determined in digestates after focused-microwave-assisted digestion according to factorial design procedures. A cavity microwave digestion was carried out to certify the elemental compositions obtained. The accuracy was checked using a standard reference material, the NIST SRM 8433 - Corn Bran. Results are in agreement with certified values at the 95% confidence limit when the Student t-test was used. Volumes of nitric and sulfuric acid, temperature, and the interplay between HNO3 and H2SO4 initial volumes were significant variables according to P-values in the analysis of variance (ANOVA).

Efficient removal of Cd2+ from aqueous solutions using by-product of biodiesel production.

In this study, chemically modified radish cake, a by-product of biodiesel production, was used to remove Cd(2+) from aqueous solutions. The chemical modification was carried out by treating the radish cake with citric acid (CRC), NaOH (NRC) or the combination of citric acid and NaOH (CNRC). The sorbents were characterized by elemental analysis, surface area analysis, infrared spectroscopy (FTIR), potentiometric titration (PT), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and thermogravimetric analysis (TGA). The effect of the chemical treatment and contact time on cadmium sorption was evaluated. The equilibrium data were analyzed using Freundlich and Langmuir models. The maximum sorption capacity obtained by the Langmuir isotherm was 58.5mg/g and 64.10mg/g for the CRC, and CNRC sorbents, respectively. Complete desorption of Cd(2+) was achieved using 0.1 mol/L HNO(3). The results obtained demonstrate that chemically modified radish cake has potential as a sorbent for Cd(2+) removal from aqueous solutions.

DIRECT DETERMINATION OF Dy, Sm, Eu, Tm, AND Yb IN GEOLOGICAL SAMPLES BY SLURRY ELECTROTHERMAL VAPORIZATION INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

Slurry sample introduction with electrothermal vaporization (ETV) has been applied to inductively coupled plasma mass spectrometry (ICP-MS) for the direct determination of Dy, Sm, Eu, Tm, and Yb in geological samples. The slurries were prepared by mixing 50 mg of the powdered sample, sediment or soil (≤50 µm) with 0.8 mL of 65% m/v nitric acid, 0.4 mL of 40% m/v hydrofluoric acid and 0.4 mL of 30% v/v hydrogen peroxide in PTFE flasks. The mixture was submitted to an ultrasonic bath for 36 min and then diluted to 14 mL with water. The best compromise conditions for the furnace temperature program, the nebulizer gas flow rate, the RF power, and the amount of NaCl used as carrier were determined. By introducing 20 µL of 3.57 mg mL−1 sediment slurry, using 50 µg of NaCl as a physical carrier, and pyrolysis and vaporization temperatures of 1000°C and 2800°C, respectively, the obtained values for four geological reference materials were well within the certified range, using external calibration with aqueous solutions. Under the optimum operating conditions, the detection limits for 164Dy, 152Sm, 153Eu, 169Tm e, 174Yb were of 200, 30, 20, 10, and 10 pg g−1, respectively. The precision was better than 10%.

Cadmium and lead cloud point preconcentration and determination in tobacco samples by thermospray flame furnace atomic absorption spectrometry

A thermospray flame furnace atomic absorption spectrometer (TS-FF-AAS) was employed to determine Cd and Pb in different Brazilian tobacco samples. Sample pretreatment consisted of acid decomposition and cloud point (CP) preconcentration optimized by Doehlert design. To promote phase separation in the cloud point procedure, polyoxyethylene(8) isooctylphenyl ether (Triton® X-114) and 2-(bromo-2-pyridylazo)-5-diethyl-amino-phenol (Br-PADAP) were used as a nonionic surfactant and a complexing agent, respectively. The variables of the cloud point preconcentration procedure optimized by Doehlert design were: complexing agent volume, Triton X-114 volume, and pH. A response surface maximum point was obtained for Cd and Pb. For TS-FF-AAS, a sample volume of 50 µL was introduced into the hot Ni tube using air carrier at a flow rate of 0.4 mL min-1. The limits of detection and relative standard deviations obtained for Cd and Pb when CP and TS-FF-AAS were associated were 4.0 and 13 µg kg-1, and 3.1 and 0.7%, respectively. Accuracy was checked by addition-recovery experiments, as well as by using two certified reference materials (tea and peach leaves). Cadmium and lead recoveries ranged from 93 to 103 and 93 to 107%, respectively. All results obtained for reference materials were in agreement with certified values at 95 or 99% confidence levels by t-Student test.

Methyl oleate as matrix simulacrum for the simultaneous determination of metals in biodiesel samples by flame atomic emission spectroscopy.

A measurement procedure for direct and simultaneous quantification of Na, K and Ca in biodiesel by flame atomic emission spectroscopy (FAES) was developed. A lab-made device was constructed by coupling a nebulizer/combustion system from a commercial photometer to a continuous emission detector in a spectral range of 255 to 862 nm. Instrumental optimizations were carried out evaluating the most important variables, such as gas flow rates and sample introduction temperature, indicating that a temperature of 50°C enhances the analytical signals and assures good precision. The direct analysis method was properly validated and presented limits of quantification of 0.09, 0.07 and 0.43 μg kg(-1) for Na, K and Ca, respectively. Accuracy of the proposed procedure was checked by comparing the results with those obtained by the standard procedure described in ABNT NBR 15556 and the standard addition method.

DIGESTÃO DE ÓLEO LUBRIFICANTE ENCAPSULADO EM FORNO DE MICROONDAS COM RADIAÇÃO FOCALIZADA POR ADIÇÃO DE AMOSTRA AO REAGENTE PRÉ-AQUECIDO

The applicability of the recently proposed procedure based on gradual sample addition to microwave-assisted pre-heated concentrated acid is limited by the sample viscosity. In this work, lubricating oil samples with high viscosity were encapsulated and manually added to the microwave-assisted pre-heated concentrated digestion mixture. The procedure was applied for determination of Al, Ca, Cr, Cu, Fe, Mg, Ni, P, Pb, Si, Sn, Sr, V, W, and Zn in lubricating oil by inductively coupled plasma optical emission spectrometry (ICP OES). Determined and certified values for Ca, Mg, P, and Zn in lubricating oil were in agreement at a 95% confidence level.

Evaluation of signal-to-background and Mg II/Mg I ratios as response for the optimization of rare earth elements determination by inductively coupled plasma optical emission spectrometry

Rare earth elements (REEs) is a peculiar group of chemical elements. Their determination in geological samples is important and complex due to their low concentrations and problems with spectral and non-spectral interferences. Herein, the operating conditions of an inductively coupled plasma optical emission spectrometer were optimized for the determination of REEs using experimental designs. The signal-to-background ratio (SBR) of the REEs emission signals and the Mg II/Mg I line intensity ratio were evaluated as response functions. According to the results obtained, three different sets of operating conditions were selected and classified as robust, semi‑robust and best SBR conditions. Limits of detection (LOD), SBR and relative standard deviation of the blank solution (RSDblank) were determined. The semi-robust condition (RF power of 1250 W and nebulizer gas flow rate of 0.90 L min-1) exhibited lower LOD and RSDblank than others tested operating conditions.