An evaluation of microwave-assisted fusion and microwave-assisted acid digestion methods for determining elemental impurities in carbon nanostructures using inductively coupled plasma optical emission spectrometry

Elemental Analysis of Human Blood Serum by Microwave Plasma-Investigation of the Matrix Effects Caused by Sodium Using Model Solutions

Human blood is a complex sample matrix when elemental analysis is considered. In this study, the effects of Na, a natural component of serum samples, was investigated in the quantitative determination of Ca, K, Mg, Cu, Zn, and Fe by microwave plasma atomic emission spectrometry. The robustness of the microwave plasma was tested by evaluating MgII 280.271 nm/MgI 285.213 nm by varying two adjustable operating parameters, the read time, and the nebulizer pressure. The read time has no influence on the robustness while the MgII/MgI ratio decreased when the nebulizer pressure was increased during the analysis. The threshold concentrations of the interfering Na were determined at the analytical lines used for the measurement of other elements. The matrix effect of the commercially available microwave plasma was studied by a series of model experiments with human blood. The increasing concentration of Na in the matrix within the normal ranges reported for blood serum increased the intensities of the measured atomic lines. According to a factorial design-where two applied factors were the concentration of Na matrix and the measured elements as well as their levels were considered as factorial points-it was found that the Na concentration in a serum sample after acid digestion and 10 times dilution affected the intensity values of the measured elements. For Ca, Cu, and Fe, a statistically significant effect was observed, while for Zn, Mg, and K, an interaction effect was also found. However, after calculating the percentage errors caused by the shift, the relative difference was observed to be quite small (< 10%).

Chemical and Structural Analysis of Carbon Materials Subjected to Alkaline Oxidation

Redox species such as transition metals may, unknowingly, integrate carbon materials that are produced (or supplied) for the assembling of electrodes in batteries, supercapacitors, and fuel cells. The extent to which these species alter the electrochemical profile of carbons and affect the performance and/or degradation of energy storage systems is still not fully appreciated. Alkaline oxidation (or fusion) is a promising approach to disintegrate nanocarbons for the subsequent study of their chemical composition by routine analytical tools. In this work, three commercial carbon powders, relevant for electrochemical applications and bearing varied textural orientation (point, radial, and planar), were selected to evaluate the versatility of fusion as a pretreatment process for elemental analysis. Additionally, the interaction of the flux, a lithium borate salt, with the carbons was elucidated by examining their post-fusion residues. The degree of structural degradation varied and, generally, the doping with Li and/or B (whether substitutional or interstitial) was low to nonexistent. With future developments, fusion could become a relevant pretreatment method to analyze the composition of carbon materials, even when complex mixtures (e.g., cycled battery electrodes) and larger batch scales are considered.

Determination of inorganic contaminants in carbon nanotubes by plasma-based techniques: Overcoming the limitations of sample preparation

In this work, sample preparation of carbon nanotubes (CNTs) for further determination of inorganic contaminants was investigated using a microwave-assisted wet digestion single reaction chamber system (MAWD-SRC). Analytes (Al, As, Ca, Cd, Co, Cr, Fe, La, Mg, Mo, Ni, Pb and Zn) were determined in CNTs by inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS, except for Al, Ca, Fe and Mg). Method parameters were evaluated, as the mass of CNT (25-300 mg), the temperature (220-270 °C) and the time (35-75 min) of irradiation program. The accuracy was evaluated by using a certified reference material (CRM) of CNT and also by comparison of the results with those obtained using neutron activation analysis (NAA) and high resolution continuum source graphite furnace atomic absorption spectrometry with direct solid sampling (DSS-HR-CS-GF AAS). Quantitative recoveries for all elements were obtained using 275 mg of CNTs, 6 mL of 14.4 mol L^-1 HNO₃ and 0.5 mL of 30% H₂O₂ with an irradiation program of 65 min (35 min at 270 °C). No statistical difference was observed between the results obtained after the decomposition of CNTs by MAWD-SRC with those obtained by NAA and DSS-HR-CS-GF AAS. No difference was also observed for the results using the proposed method and the values for the CRM of CNT. The use of MAWD-SRC showed good performance for CNTs digestion using relatively high sample mass (up to 275 mg), contributing to low limits of quantification (LOQs) and overcoming the current limitations of sample preparation. To the best knowledge of the authors, this work reports the highest sample mass feasible to be decomposed using wet digestion for CNTs among the methods proposed in literature.

Comparative study for carcinogenicity of 7 different multi-wall carbon nanotubes with different physicochemical characteristics by a single intraperitoneal injection in male Fischer 344 rats

The present study comparatively examined carcinogenicity of 7 different multi-wall carbon nanotubes (MWCNTs) with different physicochemical characteristics. Physicochemical characteristics of MWCNTs (referred to as M-, N-, WL-, SD1-, WS-, SD2- and T-CNTs in the present study) were determined using scanning electron and light microscopes and a collision type inductively coupled plasma mass spectrometer. Male Fischer 344 rats (10 weeks old, 15 animals per group) were administered MWCNTs at a single intraperitoneal dose of 1 mg/kg body weight, and sacrificed up to 52 weeks after the commencement. Fibers of M-, N-, WL- and SD1-CNTs were straight and acicular in shape, and contained few agglomerates. They were relatively long (38-59% of fibers were longer than 5 μm) and thick (33% to more than 70% of fibers were thicker than 60 nm). All of these 4 MWCNTs induced mesotheliomas at absolute incidences of 100%. Fibers of WS-, SD2- and T-CNTs were curled and tightly tangled to form frequent agglomerates. They were relatively short and thin (more than 90% of measured fibers were thinner than 50 nm). WS- CNT did not induce mesothelioma, and only one of 15 rat given SD2- or T-CNT developed tumor. Any correlations existed between the metal content and neither the size or form of fibers, nor the carcinogenicity. It is thus indicated that the physicochemical characteristics of MWCNTs are critical for their carcinogenicity. The straight and acicular shape without frequent agglomerates, and the relatively long and thick size, but not the iron content, may be critical factors. The present data can contribute to the risk management, practical use and social acceptance of MWCNTs.

The application of microwave digestion in decomposing some refractory ore samples with solid fusion agent

It is known that most of the refractory ore are the basis of national economy and widely applied in various fields, however, the complexity of the chemical composition and the diversity of the crystallinity in the mineral phases make the sample pre-treatment of refractory ore still remains a challenge. In this work, the complete decomposition of the refractory ore sample can be achieved just by exposing the solid fusion agent and the refractory ore sample in the microwave irradiation environment for a few minutes, and induced by a drop of water. A digestion time of 15 min for 3.0 g solid fusion agent mixture of sodium peroxide/sodium carbonate (Na₂O₂/Na₂CO₃) in a corundum crucible via microwave heating is sufficient to decompose 0.1 g refractory ore sample. An excellent microwave digestion solid agent should meet the following conditions, a good decomposition ability, an outstanding ability of absorbing microwave energy and converting it into heat quickly, a higher melting point than the decomposing temperature of the ore sample. In the research, the induction effect of water plays an important role for the microwave digestion. The energy which is released by the reaction of water and the solid fusion agent (Na₂O₂) is the key to decompose refractory ore samples with solid fusion agent, which replenished the total energy required for the microwave digestion and made the microwave digestion completed successfully. This microwave digestion technique has good reproducibility and precision, RSD % for Mo, Fe, Ti, Cr and W in the refractory ore samples were all better than 6, except RSD % for Be of about 8 because of the influence of matrix-effect. Meanwhile, the analysis results of the elements in the refractory ore samples provided by the microwave digestion technique were all in good agreement with the analysis results provided by the traditional fusion method except for Cr in the mixture ore samples. In the study, the non-linear dependence of the electromagnetic and thermal properties of the solid fusion agent on temperature under microwave irradiation and the selective heating of microwave are fully applied in this simple microwave technique. Comparing to the traditional fusion decomposition method, this microwave digestion technique is a simple, economical, fast and energy-saving sample pre-treatment technique.

Assessment the Exposure Level of Rare Earth Elements in Workers Producing Cerium, Lanthanum Oxide Ultrafine and Nanoparticles

In order to assess occupational exposure level of 15 rare earth elements (REEs) and identify the associated influence, we used inductively coupled plasma mass spectrometry (ICP-MS) based on closed-vessel microwave-assisted wet digestion procedure to determinate the concentration of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu in urinary samples obtained from workers producing ultrafine and nanoparticles containing cerium and lanthanum oxide. The results suggest that La and Ce were the primary component, together accounting for 97 % of total REEs in workers. The urinary levels of La, and Ce among the workers (6.36, 15.32 μg.g^-1 creatinine, respectively) were significantly enriched compared to those levels measured in the control subjects (1.52, 4.04 μg.g^-1 creatinine, respectively) (p < 0.05). This study simultaneously identified the associated individual factors, the results indicate that the concentrations in over 5 years group (11.64 ± 10.93 for La, 27.83 ± 24.38 for Ce) were significantly elevated compared to 1-5 years group (2.58 ± 1.51 for La, 6.87 ± 3.90 for Ce) (p < 0.05). Compared the urinary levels of La and Ce at the separation and packaging locations (9.10 ± 9.51 for La, 22.29 ± 21.01 for Ce) with the other locations (2.85 ± 0.98 for La, 6.37 ± 2.12 for Ce), the results show urinary concentrations were significantly higher in workers at separation and packaging locations (p < 0.01). Inter-individual variation in levels of La and Ce in urine is the result of multi-factorial comprehensive action. Further researches should focus on the multiple factors contributing to the REEs levels of the occupationally exposed workers.

Determination of As, Cr, Mo, Sb, Se and V in agricultural soil samples by inductively coupled plasma optical emission spectrometry after simple and rapid solvent extraction using choline chloride-oxalic acid deep eutectic solvent

A rapid, simple and green ultrasound-assisted extraction method using deep eutectic solvents (DES) for extraction of As, Cr, Mo, Sb, Se and V in soil samples, has been developed. Choline chloride-oxalic acid based DES was used as a solvent. The target analytes were subsequently quantified using inductively coupled plasma optical emission spectrometer (ICP OES). The parameters that affect the extraction of the target analytes was optimized using standard reference material of San Joaquin soil (SRM 2709a). In the optimization step, a two-level full factorial experimental design was used. The factors under investigation include extraction time, sample mass and acid concentration. Under optimized conditions, limits of detection (LOD) and limits of quantification (LOQ) ranged from 0.009 to 0.1 and 0.03-0.3µgg^-1, respectively. The repeatability (n=20) estimated in terms of relative standard deviation (%RSD) ranged from 0.9% to 3.7%. The accuracy of the proposed method was carried out using SRM 2709a. The obtained and certified/ indicative values were statistically in good agreement at 95% confidence level. The proposed method applied for quantification of As, Cr, Mo, Sb, Se and V in real soil samples. For comparison, the analytes of interest were also determined using a conventional acid digestion method. According to the paired t-test, the analytical results were not significant differences at 95% confidence level. The method was found to be accurate, precise and environmentally friendly.

Elemental Quantification and Residues Characterization of Wet Digested Certified and Commercial Carbon Materials

Inductively coupled plasma optical emission spectroscopy (ICP-OES) is a common, relatively low cost, and straightforward analytical technique for the study of trace quantities of metals in solid materials, but its applicability to nanocarbons (e.g., graphene and nanotubes) has suffered from the lack of efficient digestion steps and certified reference materials (CRM). Here, various commercial and certified graphitic carbon materials were subjected to a "two-step" microwave-assisted acid digestion procedure, and the concentrations of up to 18 elements were analyzed by ICP-OES. With one exception (Sm), successful quantification of all certified elements in the two reference nanocarbons studied was achieved, hence validating the sample preparation approach used. The applicability of our "two-step" protocol was further confirmed for a commercial single-walled carbon nanotube sample. However, the digestion was markedly incomplete for all other commercial materials tested. Where possible, the digestion residues of the carbon materials analyzed (CRM included) were characterized to understand the structural changes that take place and how this may explain the challenge of disintegrating graphitic carbon. In this respect, it was found that solid state nuclear magnetic resonance holds considerable promise as a nonlocalized, easily interpretable, and reliable tool to access the efficient disintegration of these materials.