Lucimar L. Fialho

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.

Determination of P, S and Si in biodiesel, diesel and lubricating oil using ICP-MS/MS

The feasibility of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) to overcome polyatomic interferents in severely affected isotope determination is demonstrated here. This instrument arrangement presents an octopole reaction/collision system (ORS) in between two quadrupole mass analyzers. The first quadrupole operates as a mass filter which allows just the target mass-to-charge ratio (m/z) to enter the ORS and rejects all other species with m/z different from the target analytes m/z. Once in the ORS, analytes react with the reaction gas introduced into the ORS, and the second quadrupole selects the product of the reactions m/z (mass shift operation mode). We describe here a procedure for accurate determination of P, S and Si in biodiesel, diesel and lubricating oil by ICP-MS/MS. Fuel standard reference materials (SRMs) were microwave-assisted acid-digested using nitric acid and hydrogen peroxide. The oxygen gas flow rate in the ORS was optimized considering the limit of detection (LOD) values and accuracies reached for P, S and Si determinations in the lubricating oil SRM and the best oxygen gas flow rate was 0.75 mL min−1. Adopting optimized conditions, limits of detection were 1.2, 0.49, 0.31, 0.33 and 0.78 μg L−1 for 28Si16O+, 29Si16O+, 31P16O+, 32S16O+ and 34S16O+, respectively. Adequate precision, accuracy and sensitivity were obtained when using the mass shift mode and recoveries for biodiesel, diesel and lubricating oil digests ranged from 95.0 to 113%. No significant differences were observed between the certified values and the ones obtained using ICP-MS/MS at a 95% confidence level.

A novel strategy to determine As, Cr, Hg and V in drinking water by ICP-MS/MS

This work demonstrates the potential of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) in trace element analysis. Aiming Hg determination in water, sample preservation requires chloride addition, which presents formation of polyatomic interfering ions as a drawback for the determination of other analytes (e.g. As and V). Thus, the effect of chloride on As, Cr, V and Hg determination and the feasibility of using ICP-MS/MS to circumvent the interference were evaluated. A standard reference material (NIST SRM 1643e) and recoveries from tap water spiked with As, Cr, Hg and V were used for accuracy assessment and for the evaluation of matrix effects. The reaction between Hg and oxygen gas is not favorable in the octopole reaction system (ORS3). Therefore, Hg was accurately determined as a single atom ion (202Hg+) under the same analytical conditions adopted for As, Cr and V determination by monitoring 75As16O+, 52Cr16O+ and 51V16O+ while the ORS3 was pressurized with oxygen gas. The oxygen reaction mode significantly improved the accuracy for As, Cr and V, especially at the lowest spiked level, which is strongly affected by matrix effects. The signal gain in m/z 75 for a solution containing 1 μg L−1 As in 0.48 mol L −1 chloride medium was 1974% when employing single quadrupole mode. On the other hand, the signal increased only 7% when 0.75 mL min−1 O2 was used in mass shift mode proving the efficiency of the MS/MS mode. Thus, ICP-MS/MS and ORS3 pressurized with O2 gas effectively reduced the interference, and enhanced the accuracy and sensitivity for some specific isotopes in a complex medium. The limits of detection for V (51V16O+), Cr (52Cr16O+), As (75As16O+) and Hg (202Hg+) were estimated to be 2, 3, 1.6 and 38 ng L−1 at the 99.7% confidence level, respectively.

Combining contamination indexes, sediment quality guidelines and multivariate data analysis for metal pollution assessment in marine sediments of Cienfuegos Bay, Cuba

The purpose of the present work was to combine several tools for assessing metal pollution in marine sediments from Cienfuegos Bay. Fourteen surface sediments collected in 2013 were evaluated. Concentrations of As, Cu, Ni, Zn and V decreased respect to those previous reported. The metal contamination was spatially distributed in the north and south parts of the bay. According to the contamination factor (CF) enrichment factor (EF) and index of geoaccumulation (Igeo), Cd and Cu were classified in that order as the most contaminated elements in most sediment. Comparison of the total metal concentrations with the threshold (TELs) and probable (PELs) effect levels in sediment quality guidelines suggested a more worrisome situation for Cu, of which concentrations were occasional associated with adverse biological effects in thirteen sediments, followed by Ni in nine sediments; while adverse effects were rarely associated with Cd. Probably, Cu could be considered as the most dangerous in the whole bay because it was classified in the high contamination levels by all indexes and, simultaneously, associated to occasional adverse effects in most samples. Despite the bioavailability was partially evaluated with the HCl method, the low extraction of Ni (<3% in all samples) and Cu (<55%, except sample 3) and the relative high extraction of Cd (50% or more, except sample 14) could be considered as an attenuating (Ni and Cu) or increasing (Cd) factor in the risk assessment of those element.

Compensation of inorganic acid interferences in ICP-OES and ICP-MS using a Flow Blurring® multinebulizer

A new and easy method has been proposed for the compensation of inorganic acid matrix effects in ICP-OES and ICP-MS. The method consists of an on-line standard addition calibration using a Flow Blurring® multinebulizer (FBMN-based system). Experimental conditions of the FBMN-based system are optimized for both ICP-OES and ICP-MS. Under optimized conditions recovery values obtained in the analysis of synthetic acid samples were close to 100% for HNO3 and HCl (with acid concentrations of up to 15% (w/w)) and H2SO4 (up to 10% (w/w) for both plasma-based spectrochemical techniques. The applicability of the proposed method has been evaluated analyzing two whole milk powders, a certified reference material and a commercial product, showing excellent recovery values. Compared with other calibration strategies and experimental setups used, the on-line standard addition calibration using the FBMN-based system is faster, easier to handle and significantly reduces reagent and sample consumption.

Avaliação da combinação da nebulização discreta e processos de microextração aplicados à determinação de molibdênio por espectrometria de absorção atômica com chama (FAAS)

Simple and sensitive procedures for the extraction/preconcentration of molybdenum based on vortex-assisted solidified floating organic drop microextraction (VA-SFODME) and cloud point combined with flame absorption atomic spectrometry (FAAS) and discrete nebulization were developed. The influence of the discrete nebulization on the sensitivity of the molybdenum preconcentration processes was studied. An injection volume of 200 µL resulted in a lower relative standard deviation with both preconcentration procedures. Enrichment factors of 31 and 67 and limits of detection of 25 and 5 µg L-1 were obtained for cloud point and VA-SFODME, respectively. The developed procedures were applied to the determination of Mo in mineral water and multivitamin samples.

Investigation of analyte losses using microwave-assisted sample digestion and closed vessels with venting

Microwave-assisted sample digestion using closed vessels is becoming the standard for trace analysis because contamination and losses can be better controlled. Gases are generated during digestion and there is an increment of the internal vessel pressure. Consequently, vessels venting may occur depending on the design of the vessel and the maximum pressure it can stand for. In the present work it was observed that it is possible to allow venting during the digestion without losing volatile analytes, such as As, when properly controlling heating and chemical conditions. Recoveries for As in certified reference materials of animal tissues ranged from 94 to 112% despite mass losses as high as 62%mm(-1) observed in the digests. However, for Hg(II) in medium containing chlorides recoveries were poor. The efficiency of digestion was measured by determination of organic carbon contents in digests and they ranged from 0.10 to 0.19% for plant and animal tissues. The temperature gradient along the vessel height is important for avoiding losses of volatile elements.

Qualitative and Quantitative Chemical Investigation of Orthopedic Alloys by Combining Wet Digestion, Spectroanalytical Methods and Direct Solid Analysis

In this study, two laser-based techniques, laser-induced breakdown spectroscopy (LIBS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were used for analytical signal evaluation of Ti, Al, and V and investigation of possible harmful elements eventually present as minor elements in Ti alloys. Due to the lack of certified reference materials, samples were also analyzed by wavelength dispersive X-ray fluorescence (WDXRF) and inductively coupled plasma optical emission spectrometry (ICP OES) after microwave-assisted digestion. To maximize the efficiency of LIBS and LA-ICP-MS, operational conditions were adjusted aiming to find optimal analytical performance. LIBS showed several Ti emission lines and few signals for Al and V. LA-ICP-MS was able to detect all three major constituents. For quantitative analysis, the correlation of intensity signals from LIBS analysis with reference values obtained by ICP OES was not successful, showing that there are still difficulties for quantification using solid samples. Measurements using ICP OES showed that additionally to major constituents, only Fe was present in concentrations around 0.2%. Analysis by WDXRF confirmed the presence of Fe. Results using both methods, i.e., ICP OES and WDXRF, were in good agreement.