Lucas Schmidt

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) HNO(3)) 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 O(2) (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) HNO(3)). 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 HNO(3), which is important to minimize the generation of laboratory residues.

Evaluation of a digestion procedure based on the use of diluted nitric acid solutions and H2O2 for the multielement determination of whole milk powder and bovine liver by ICP-based techniques

Many efforts have been made in order to reduce the amount of reagents and waste produced in analytical laboratories. However, suitable digestion efficiency must be considered, and depending on the sample preparation procedure, incomplete digestion can result in severe matrix effects during analysis by spectrometric techniques such as ICP OES and ICP-MS. In the present work a procedure based on the use of H2O2 was developed in order to minimize the consumption of HNO3 without decreasing the efficiency of digestion. Although H2O2 has been used combined with HNO3 for sample digestion, its role is still not completely elucidated even as its action combined with O2 in pressurized systems. The performance obtained using H2O2 was similar to that observed when adding O2 to the reaction vessel, driving the better understanding of the role of H2O2 in closed digestion procedures. Digestion using H2O2 allowed the use of HNO3 solutions as diluted as 1 mol L−1 to digest sample masses of biological materials as high as 500 mg. The proposed procedure allowed a reduction of up to 14 and 9.3-fold in the HNO3 amount normally used in whole milk powder and bovine liver digestions, respectively, without decreasing the digestion efficiency. Calcium, Cu, Fe, K, Mg, Mn, Mo, Na, and Zn were determined by ICP OES, while Cd, Co, and Pb were determined by ICP-MS. Using diluted HNO3 solution low blank values were obtained resulting in relatively lower limits of detection and relative standard deviations. The accuracy was evaluated by using certified reference materials of milk powder and bovine liver (agreement was better than 95% to certified values for all evaluated analytes).

Effect of simultaneous cooling on microwave-assisted wet digestion of biological samples with diluted nitric acid and O2 pressure

The present work evaluates the influence of vessel cooling simultaneously to microwave-assisted digestion performed in a closed system with diluted HNO3 under O2 pressure. The effect of outside air flow-rates (60-190 m(3) h(-1)) used for cooling of digestion vessels was evaluated. An improvement in digestion efficiency caused by the reduction of HNO3 partial pressure was observed when using higher air flow-rate (190 m(3) h(-1)), decreasing the residual carbon content for whole milk powder from 21.7 to 9.3% (lowest and highest air flow-rate, respectively). The use of high air flow-rate outside the digestion vessel resulted in a higher temperature gradient between liquid and gas phases inside the digestion vessel and improved the efficiency of sample digestion. Since a more pronounced temperature gradient was obtained, it contributed for increasing the condensation rate and thus allowed a reduction in the HNO3 partial pressure of the digestion vessel, which improved the regeneration of HNO3. An air flow-rate of 190 m(3) h(-1) was selected for digestion of animal fat, bovine liver, ground soybean, non fat milk powder, oregano leaves, potato starch and whole milk powder samples, and a standard reference material of apple leaves (NIST 1515), bovine liver (NIST 1577) and whole milk powder (NIST 8435) for further metals determination by inductively coupled plasma atomic emission spectroscopy (ICP-OES). Results were in agreement with certified values and no interferences caused by matrix effects during the determination step were observed.

A new method for chlorine determination in commercial pet food after decomposition by microwave-induced combustion

In this work, the feasibility of a new sample preparation method was demonstrated for commercial pet food digestion for the determination of chlorine by ion selective electrode (ISE). Pet food decomposition was performed by microwave-induced combustion (MIC) and up to 600 mg of sample was combusted in closed vessels using 20 bar of O2. Solutions of (NH4)2CO3 or ultrapure water were evaluated as absorbing mediums. Using the selected conditions, Cl recovery was higher than 96% and the relative standard deviation (RSD) was lower than 6%. Accuracy was evaluated using two certified reference materials with agreement better than 95%. Chlorine was also determined in digests by ion chromatography and results were in agreement better than 97%. For comparison of results, samples were also digested using the method recommended by Association of Official Analytical Chemists (AOAC) and results for Cl were in agreement with those obtained by the proposed method; however, the RSD was up to 16%. The limit of detection (LOD) for Cl by MIC and subsequent determination by ISE was 90 µg g−1, whereas the LOD for Cl by AOAC method was 3480 µg g−1. MIC allowed the digestion of commercial pet food samples with high efficiency (>98%) and was suitable for further Cl determination by ISE.

Evaluation of nitrates as igniters for microwave-induced combustion: understanding the mechanism of ignition

Recent studies have reported the use of microwave-induced combustion (MIC) for digestion of several kinds of matrices. In spite of several applications of MIC, relatively few information is available regarding the mechanism of ignition. In this work, a systematic study related to the role of NH4NO3 solution and other nitrates for the ignition step in the MIC system was performed. In this sense, aqueous solutions of Ca(NO3)2, KNO3, Mg(NO3)2, NaNO3 and NH4NO3 were evaluated. It was observed that the ignition is dependent on nitrate concentration and microwave power and seems to be related to the oxidation of organic matter by NO3−, which releases enough energy for starting a chain reaction leading to combustion. Additionally, a special action promoted by microwaves without using nitrate solutions was not observed. According to the results, all the evaluated nitrate solutions can be used as igniters with microwave power of 750 W or higher. It was also possible to use nitrate solutions as diluted as 1 mol L−1 and relatively short time was required for ignition of filter paper (below 10 s). Furthermore, the use of higher microwave power allowed a more reproducible ignition.

Arsenic speciation in seafood by LC-ICP-MS/MS: method development and influence of culinary treatment

Arsenic speciation in seafood after several culinary treatments was performed and AsB, As(III), DMA, MMA and As(V) species were determined by liquid chromatography hyphenated to triple-quadrupole inductively coupled plasma mass spectrometry (LC-ICP-MS/MS) using O2 as the reaction gas for the conversion of 75As to 75As16O. The influence of culinary treatments (boiling, frying and sauteing) with or without the addition of spices (salt, lemon juice and garlic) on the As species in blacktip shark and Asian tiger shrimp was investigated. Arsenic species were extracted by using a 30 mmol L−1 HNO3 solution. Ammonium phosphate (10 mmol L−1) was used as the mobile phase. The influence of pH and the addition of 1% (v/v) methanol were investigated. Oil, water, butter and the spices used during cooking were analysed to perform a close mass balance of the total As. The speciation method was also employed for a certified reference material (CRM, DORM-3), and the accuracy was evaluated by statistical comparison between the certified value and the total As concentration determined by ICP-MS after acid digestion and also by a comparison of the sum of As species with the total As. It was demonstrated that the culinary treatments practically did not influence the stability of As species in uncooked seafood. On the other hand, significant analyte losses (from 15 up to 45%) were observed for boiled seafood. The speciation analysis method presented accuracy and robustness for both raw seafood and seafood after the culinary treatments. The limits of quantification were 4, 21, 4, 9 and 18 ng g−1 for AsB, As(III), DMA, MMA and As(V), respectively. This study allowed the determination of As species in seafood after culinary treatments, thus offering additional information about the behaviour of species during cooking.

Desorption electrospray ionization mass spectrometry (DESI-MS) applied to the speciation of arsenic compounds from fern leaves.

The different chemical forms of arsenic compounds, including inorganic and organic species, present distinct environmental impacts and toxicities. Desorption electrospray ionization mass spectrometry (DESI-MS) is an excellent technique for in situ analysis, as it operates under atmospheric pressure and room temperature and is conducted with no/minimal sample pretreatment. Aimed at expanding its scope, DESI-MS is applied herein for the quick and reliable detection of inorganic (arsenate—As(V): AsO43− and arsenite—As(III): AsO2−) and organic (dimethylarsinic acid—DMA: (CH3)2AsO(OH) and disodium methyl arsonate hexahydrate: CH3AsO3·2Na·6H2O) arsenic compounds in fern leaves. Operational conditions of DESI-MS were optimized with DMA standard deposited on paper surfaces to improve ionization efficiency and detection limits. Mass spectra data for all arsenic species were acquired in both the positive and negative ion modes. The positive ion mode was shown to be useful in detecting both the organic and inorganic arsenic compounds. The negative ion mode was shown only to be useful in detecting As(V) species. Moreover, MS/MS spectra were recorded to confirm the identity of each arsenic compound by the characteristic fragmentation profiles. Optimized conditions of DESI-MS were applied to the analysis of fern leaves. LC-ICP-MS was employed to confirm the results obtained by DESI-MS and to quantify the arsenic species in fern leaves. The results confirmed the applicability of DESI-MS in detecting arsenic compounds in complex matrices.

A feasible method for As speciation in several types of seafood by LC-ICP-MS/MS

A method for arsenic speciation in shark, shrimp, squid, oyster and scallop using liquid chromatography coupled to inductively coupled plasma triple quadrupole mass spectrometry (LC-ICP-MS/MS) was proposed. Suitable sensitivity and selectivity by LC-ICP-MS/MS were obtained using 10 mmol L-1 (NH4)2HPO4 diluted in 1% methanol (pH 8.65) as mobile phase. Recoveries from 90 to 104% for arsenobetaine (AsB), arsenite [As(III)], dimethylarsinic acid (DMA), monomethylarsonic acid (MMA) and arsenate [As(V)] were obtained for all samples. A certificated reference material was also analyzed and the sum of As species was in agreement with the total As concentration. Limits of quantification (LOQ) for AsB, As(III), DMA, MMA, and As(V) were 6, 30, 6, 12 and 26 ng g-1, respectively. Higher concentration of AsB was found in all seafood, while As(III) and DMA were found only in oyster. Arsenate was found in squid and scallops, and MMA was below the LOQ in all samples.

Microwave-induced combustion: towards a robust and predictable sample preparation method

Sample preparation methods based on combustion for further element determination in carbon-based samples are normally performed using an excess of O2 in order to ensure the complete oxidation of the organic matrix. However, experimental conditions such as O2 pressure and sample mass are commonly selected based on empirical knowledge. In the present work, in order to provide a guide for planning combustion reactions using microwave-induced combustion, a mathematical model was proposed based on the sample composition to calculate the amount of O2 required for each matrix. A critical study was performed using selected matrices such as graphite (C), anthracene (C14H10), polypropylene (C3H6)n, microcrystalline cellulose (C6H10O5)n, saccharose (C12H22O11), dibenzothiophene (C12H8S), carbazole (C12H9N), carbamazepine (C15H12N2O), and methionine (C5H11NO2S) in order to demonstrate the feasibility of the proposed model. The obtained results showed that our prediction model allowed for more efficient digestion of graphite, hydrocarbons, and methionine (compounds presenting O, N, and S) using only 10 ± 2% of O2 excess. For samples more difficult to burn, such as dibenzothiophene and carbazole, a relatively higher excess of O2 was required (30%). Digestion efficiency was evaluated by determining the residual carbon content (RCC) in the final solutions, which was always lower than 0.15%, even for sample masses as high as about 1500 mg digested in relatively small vessels (78 ± 1 mL). The proposed model allowed faster development of combustion methods for digesting the greatest possible sample mass, resulting in improvements in the limits of quantification in a robust and safe way. The costs related to such analyses could be reduced and improvements in analytical operations could be highlighted as important features by using the prediction model for combustion processes assisted by microwave radiation.

Maxwell–Wagner Effect Applied to Microwave-Induced Self-Ignition: A Novel Approach for Carbon-Based Materials

A new method for analytical applications based on the Maxwell-Wagner effect is proposed. Considering the interaction of carbonaceous materials with an electromagnetic field in the microwave frequency range, a very fast heating is observed due to interfacial polarization that results in localized microplasma formation. Such effect was evaluated in this work using a monomode microwave system, and temperature was recorded using an infrared camera. For analytical applications, a closed reactor under oxygen pressure was evaluated. The combination of high temperature and oxidant atmosphere resulted in a very effective self-ignition reaction of sample, allowing its use as sample preparation procedure for further elemental analysis. After optimization, a high sample mass (up to 600 mg of coal and graphite) was efficiently digested using only 4 mol L-1 HNO3 as absorbing solution. Several elements (Ba, Ca, Fe, K, Li, Mg, Na, and Zn) were determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Accuracy was evaluated by using a certified reference material (NIST 1632b). Blanks were negligible, and only a diluted solution was required for analytes absorption preventing residue generation and making the proposed method in agreement with green chemistry recommendations. The feasibility of the proposed method for hard-to-digest materials, the minimization of reagent consumption, and the possibility of multi elemental analysis with lower blanks and better limits of detection can be considered as the main advantages of this method.