A Fast Microwave-Assisted Procedure for Loss on Drying Determination in Saccharides

Um procedimento rápido para determinação de perda por dessecação (LOD) foi desenvolvido empregando aquecimento por radiação micro-ondas. Amostras de sacarídeos foram utilizadas e diferentes parâmetros foram avaliados, tais como a posição da amostra na cavidade do micro-ondas, massa de amostra e tempo de irradiação. Massas de amostra de 1 g e tempo de irradiação entre 15 e 25 min foram suficientes para a determinação da perda por dessecação de todos sacarídeos. Os resultados obtidos para perda por dessecação assistida por micro-ondas (MALOD) foram comparados com os resultados obtidos por LOD convencional em estufa e não apresentaram diferença significativa. O tempo de análise foi reduzido de 2,4 a 15 vezes, quando comparado ao sistema convencional de LOD e o desvio padrão relativo das medidas foi inferior a 1%. Até 16 amostras podem ser processadas simultaneamente, tornando o procedimento MALOD apropriado para análise de rotina. A fast procedure for loss on drying (LOD) determination was developed using microwave radiation. Samples of commercial saccharides were dried and the influence of sample position inside the microwave cavity, sample mass and irradiation time were evaluated. Sample mass of 1 g and irradiation time between 15 to 25 min were enough to LOD determination for all saccharides. Results obtained using the proposed microwave-assisted loss on drying (MALOD) procedure were compared with those obtained by conventional LOD determination using an oven and no statistical difference was found among results of these techniques. Using MALOD procedure the relative standard deviation was below 1%. The time for analysis was reduced from 2.4 to 15 times when compared to conventional LOD determination and up to 16 samples could be simultaneously processed making MALOD procedure suitable for routine analysis. Keywords: loss on drying, saccharides, microwave, food and pharmaceutical samples Introduction Loss on drying (LOD) determination is a parameter often evaluated to access the quality of products. Several applications of LOD can be found in industry, in special for food and pharmaceutical industries, which currently use LOD to determine the amount of volatile matter (in general, water) that is driven off under specific conditions. 1-3 The LOD determination is relatively simple to be performed and results with relatively good precision can be obtained resulting in a widespread use. For example, in the 30 th edition of The United States Pharmacopeia more than one thousand monographs recommended the use of LOD in their tests that makes it a parameter that must be routinely evaluated in current quality control. 14,15 Several papers dealing with microwave radiation for pharmaceuticals drying are available in the literature. Therefore, in this work the application of microwaves for LOD determination is proposed using simple and inexpensive instrumentation. Saccharide samples of pharmaceutical grade were chosen as examples in order to evaluate the feasibility of the proposed procedure and results were compared with those obtained by LOD using conventional heating in an oven described in The United States Pharmacopeia. Experimental Samples Commercially available saccharide samples of pharmaceutical grade (potato starch, maize starch, guar, agar, microcrystalline cellulose and hypromellose) were used in this work. Samples were maintained in their original package before LOD determinations in an environment with controlled temperature (22 ± 2 °C) and humidity (less than 50% of air relative humidity). All samples had particle size lower than 150 mm. Instrumentation Microwave-assisted loss on drying (MALOD) determination was performed using a domestic microwave oven (model BMK38ABBNA, 38 L, 2450 MHz, Brastemp, Brazil), with 950 W of nominal power. In order to allow a continuous microwave irradiation of samples and prevent damages to magnetron a polyethylene coil (1 m length and 5 mm i.d.) was fitted inside the cavity in the opposite side of the wave-guide. Water was passed within the coil in a constant flow rate (700 mL min -1 ). The polyethylene coil was passed through the microwave oven using the holes originally designed for air circulation in order to avoid changes in the metallic cavity cover and minimize the risk of microwave losses. For safety reasons, a microwave spill detector was periodically used during the experiments in order to check eventual microwave loss (model LT-2D, 2450 MHz, maximum limit of 5 mW cm -2 , Milestone S.R.L., Sorisole, Italy). For conventional LOD determination a drying oven (model 400/2ND, Nova Ética, Brazil) was used. Samples were dried in a weighing bottle of 25 mm of internal diameter and volume of 30 mL. Samples were accurately weighed using an analytical balance (model AY 220, max. 220 g, 0.1 mg of resolution, Shimadzu, Kyoto, Japan). Determination of microwave power output and distribution of microwave radiation within the oven cavity Ultrapure water (Milli-Q, 18.2 MW cm) was used for both determination of microwave power output and distribution of microwave radiation experiments. The power output of magnetron (real power) was indirectly determined by measuring the increase of temperature of water after microwave irradiation. In this work 1,000 g of water was heated at full power for 2 min and the power output of the microwave oven was evaluated using a general relationship where the power output (P), in Watts, was calculated according to equation 1: P = k cp m ΔT/t, where k is the conversion factor (from thermal chemical calories s -1 to Watts, 4.184 J cal ), m is the sample mass (g), ΔT is the temperature change (°C) after microwave heating, and t is the time of irradiation (s). 24 A Fast Microwave-Assisted Procedure for Loss on Drying Determination in Saccharides J. Braz. Chem. Soc. 378 In order to evaluate the influence of sample position inside the microwave cavity on the heating of samples, twenty one glass beakers (25 mm diameter and 20 mL capacity) each one containing 15 g of ultrapure water were symmetrically positioned on the turntable of the microwave cavity. The oven was operated at maximum power for 60 s of irradiation. The temperature increase of water was measured with a thermocouple device (digital thermometer, model AF0806, Incoterm, Brazil). This procedure was performed by measuring the temperature, each run, of four beakers positioned at the same distance from the center. Further, for all the beakers, water was replaced by a new amount of cold water and the same procedure was performed for beakers positioned in other distances from the center. This procedure was repeated up to the water temperature has been determined in all the 20 beakers. Therefore, microwave distribution inside the oven was determined using this procedure. This procedure was repeated four-times and the mean absorbed power for each position was calculated according to equation 1. LOD and MALOD determination The LOD determination in conventional oven was performed by introduction of 1 g of each sample in a weighing bottle previously dried under the recommended conditions (130 °C for potato and maize starch and 100 to 105 °C for the other samples) up to constant mass. For MALOD determination the influence of microwave irradiation time and sample amount were evaluated from 1 to 30 min and from 0.5 to 2.0 g of saccharide samples, respectively. Samples were dried in weighing bottles, which were previously prepared under the same conditions described for LOD determination in an oven. The kinetics of drying for the MALOD procedure was evaluated by means of relative dielectric loss factor determination. 14 For this study, 4.0 ± 0.1 g of sample were dried in the microwave oven at the maximum power for 30 s. The temperature increase was determined using a thermocouple device with digital display for both dried and non dried samples. Each experiment was repeated four times. Results and Discussion Determination of microwave power output and distribution of microwave radiation within the oven cavity Considering that domestic microwave ovens are not originally designed for analytical purposes the evaluation of total irradiated microwave power and microwave distribution inside the cavity was necessary. Influence of microwave irradiation time for MALOD As can be seen in 14,27,33 Thus, an indication of the relative magnitude of the dielectric characteristics may be obtained by measuring the temperature variation induced when the sample was irradiated with microwaves. In the present work, the drying profile of saccharides could be considered as a function of relative dielectric loss factor, as shown in 14 During the first falling-rate period, a higher amount of free water was present. Therefore, in view of the higher dielectric loss factor of water, microwaves could be selectively absorbed by these molecules thus facilitating a high drying-rate. During the drying process, the solid content becomes more significant and therefore the solid sample begins to absorb a greater proportion of microwave energy. Therefore, the initial amount of water present in the samples had great influence on the microwave drying profile of saccharides samples. The necessary time to achieve a constant mass was 15 min for maize starch, guar, hypromellose and microcrystalline cellulose, and 20 to 25 min for potato starch and agar, respectively as shown in Influence of sample mass The influence of sample mass on MALOD determination was evaluated using the previously selected time of irradiation observed for each saccharyde (before section). With the increase of mass the loss on A Fast Microwave-Assisted Procedure for Loss on Drying Determination in Saccharides J. Braz. Chem. Soc. 380 drying was constant as shown in 2 Therefore, it was not considered a limitation for LOD determination using the proposed procedure. Comparison of proposed MALOD with LOD An advantage of the drying oven process is related to the fact that this simple technique can be carried out in practically every analytical laboratory. In spite that this technique provides reproducible results, the LOD using an oven cannot be regarded as a rapid determination method due to the excessive time for determination. It could reduce the throughput for routine analysis. The recommended time for LOD determination using an oven described in pharmacopeias ranges from 90 min (e.g. maize and potato starch) to 300 min (e.g. guar and agar samples). 2 Using the proposed MALOD procedure, the time for drying all the samples was between 15 and 25 min resulting in a time reduction up to 15 times (for guar sample) as can be seen in Furthermore, the results obtained with proposed procedure were in agreement (t-Test, 95% of confidence level) to those obtained by conventional LOD determination using an oven as shown in Conclusions The proposed MALOD procedure provided results comparable to conventional LOD for evaluated samples in a faster way. For some saccharides, carbonized spots were observed when the time was increased beyond 20 min (microcrystalline cellulose) and with masses higher than 1.5 g (guar and microcrystalline cellulose) or 2.0 g (potato starch, maize starch, agar and hypromellose). In addition, the relative standard deviations for MALOD were considered suitable for routine LOD determination. Reduced time for analysis can be cited as the mainly advantage of MALOD when compared to conventional LOD determination. Therefore, MALOD procedure can be recommended as an alternative method to LOD determination in saccharides samples. To the best of our knowledge, the present work describes the first application of microwaves for LOD determination in pharmaceutical products related to the specifications of pharmacopeias

Comparison of sample digestion techniques for the determination of trace and residual catalyst metal content in single-wall carbon nanotubes by inductively coupled plasma mass spectrometry

A single-wall carbon nanotube material produced by laser ablation of renewable biochar in the presence of Ni and Co catalyst was characterized for residual catalyst (Co and Ni) as well as trace metal impurity content (Fe, Mo, Cr, Pb and Hg) by isotope dilution ICP-MS following sample digestion. Several matrix destruction procedures were evaluated, including a multi-step microwave-assisted acid digestion, dry ashing at 450 C and microwave-induced combustion with oxygen. Results were benchmarked against those derived from neutron activation analysis and also supported by solid sampling continuum source GF-AAS for several of the elements. Although laborious to execute, the multi-step microwave-assisted acid digestion proved to be most reliable for recovery of the majority of the analytes, although content of Cr remained biased low for each approach, likely due to its presence as refractory carbide.Peer reviewed: YesNRC publication: Ye

Ultrasound-Assisted Demineralization Process of Sugarcane Straw and Its Influence on the Further Biomass Conversion

Lignocellulosic materials have been considered as an alternative source from which liquid biofuel and fine chemicals can be produced with a moderate environmental impact. However, they can be contaminated with metals, soil, and ash, owing incrustation and corrosion of industrial reactors and pipelines. In this work, the use of ultrasound energy was applied for the removal of metals and nonmetals (Ba, Ca, Mg, Mn, P, S, Si, and Sr) from sugarcane straw. Ultrasound-assisted demineralization (UAD) experiments were carried out in ultrasonic baths in several frequencies (from 25 up to 130 kHz). The following experimental conditions were evaluated: demineralization solution (HNO3, HCl, H2SO4, H2O2, and H2O), H2O2 concentration (from 5 to 30% v v−1), extraction temperature (from 30 to 70 °C), sonication time (from 5 to 45 min), and ultrasound amplitude (from 10 to 70%). Better demineralization efficiencies (66%) were obtained employing an ultrasound bath operating at 25 kHz for 30 min, ultrasound amplitude of 60%, and using a diluted H2O2 solution (15% v v−1) at 70 °C. When the obtained results were compared with those obtained by mechanical stirring (MS, 500 rpm), it was observed that the use of ultrasound energy increased the demineralization efficiency up to 16%. Furthermore, acid hydrolysis was performed to evaluate the influence of US and mechanical stirring in fermentable sugars’ production. The total sugars’ yield (glucose, xylose, and arabinose) increased around 55% for both systems (US and MS). To prove the applicability of the proposed process, some experiments for scaling up were performed using several reaction loads (0.5 to 3 L). An attempt for scaling the proposed process up was well succeeded up to a 3 L load. Therefore, the proposed ultrasound-assisted procedure can be considered as a suitable alternative for high-efficiency demineralization from sugarcane straw

Bromine and chlorine determination in cigarette tobacco using microwave-induced combustion and inductively coupled plasma optical emission spectrometry

A combustão iniciada com micro-ondas (MIC) foi aplicada para decomposição de amostras de tabaco de cigarro e subsequente determinação de bromo e cloro por espectrometria de emissão óptica com plasma indutivamente acoplado (ICP OES). Massas de amostra de até 500 mg foram decompostas em frascos fechados e pressurizados com 20 bar de oxigênio. A combustão foi completada em menos de 30 s e os analitos foram absorvidos em solução diluída de (NH4)2CO3. A exatidão foi avaliada usando materiais de referência certificados e mediante a determinação utilizando ICP-MS. A concordância foi melhor do que 98% usando 50 mmol L-1 de (NH ) CO 4 2 3 como solução absorvedora e 5 min de refluxo. A temperatura durante a combustão foi superior a 1400 °C e o conteúdo de carbono residual nos digeridos após MIC foi menor que 1%. Até oito amostras podem ser decompostas simultaneamente. Limites de quantificação utilizando MIC e determinação por ICP OES foram de 12 e 6 µg g-1 para Br e Cl, respectivamente.The microwave-induced combustion (MIC) was applied for cigarette tobacco samples digestion and further determination of bromine and chlorine by inductively coupled plasma optical emission spectrometry (ICP OES). Samples masses up to 500 mg were combusted in closed vessels using 20 bar of oxygen. Combustion was complete in less than 30 s and analytes were absorbed in diluted (NH4)2CO3 solution. Accuracy was evaluated using certified reference materials with similar matrix composition and comparison with results obtained using ICP-MS. The agreement was better than 98% using 50 mmol L-1 (NH4)2CO3 as absorbing solution and 5 min of reflux. Temperature during combustion was higher than 1400 °C and the residual carbon content in digest obtained after MIC was lower than 1%. Up to eight samples could be processed simultaneously and a single absorbing solution was suitable for both Br and Cl. Limit of quantification by MIC and further ICP OES determination was 12 and 6 μg g-1 for Br and Cl, respectively