Edson I. Muller

Determination of toxic elements in coal by ICP-MS after digestion using microwave-induced combustion.

A microwave-induced combustion (MIC) procedure was applied for coal digestion for subsequent determination of As, Cd and Pb by inductively coupled plasma mass spectrometry (ICP-MS) and Hg using cold vapor (CV) generation coupled to ICP-MS. Pellets of coal (500 mg) were combusted using 20 bar of oxygen and ammonium nitrate as aid for ignition. The use of nitric acid as absorbing solution (1.7, 3.5, 5.0, 7.0 and 14 mol L(-1)) was evaluated. For coal samples with higher ash content, better results were found using 7.0 mol L(-1) HNO(3) and an additional reflux step of 5 min after combustion step. For coal samples with ash content lower than 8%, 5.0 mol L(-1) nitric acid was suitable to the absorption of all analytes. Accuracy was evaluated using certified reference material (CRM) of coal and spikes. Agreement with certified values and recoveries was better than 95 and 97%, respectively, for all the analytes. For comparison of results, a procedure recommended by the American Society of Testing and Materials (ASTM) was used. Additionally, a conventional microwave-assisted digestion (MAD) in pressurized vessels was also performed. Using ASTM procedure, analyte losses were observed and a relatively long time was necessary for digestion (>6h). By comparison with MAD procedure, higher sample mass can be digested using MIC allowing better limits of detection. Additionally, the use of concentrated acids was not necessary that is an important aspect in order to obtain low blank levels and lower limits of detection, respectively. The residual carbon content in digests obtained by MAD and MIC was about 15% and <1%, respectively, showing the better digestion efficiency of MIC procedure. Using MIC it was possible to digest completely and simultaneously up to eight samples in only 25 min with relatively lower generation of laboratory effluents.

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.

Bromine and iodine determination in active pharmaceutical ingredients by ICP-MS

A method based on microwave-induced combustion (MIC) was applied for digestion of active pharmaceutical ingredients (APIs) and subsequent determination of bromine and iodine by inductively coupled plasma mass spectrometry (ICP-MS). Ten APIs including amoxicillin, atenolol, clavulanic acid, clonazepan, diltiazem, haloperidol, imipramine, nimesulide, propranolol and sodium diclofenac were decomposed by MIC. Combustion of 500 mg of each API was possible in less than 30 s using 20 bars of oxygen as initial pressure. A single and diluted solution (50 mmol L−1 (NH4)2CO3) was used for the absorption of both analytes and a reflux step of 5 min was applied to improve analyte recoveries. Final digests were suitable to Br and I determination by ICP-MS. Accuracy was evaluated using certified reference materials and agreement better than 95 and 97% for Br and I was obtained, respectively. Results were also compared with those obtained by ion chromatography (IC). The carbon content in digests obtained after decomposition was lower than 500 mg L−1 avoiding interferences in the determination step for both techniques. With the use of MIC, up to eight samples could be processed simultaneously and only diluted solutions are required, minimizing reagent consumption and waste generation. The limits of detection for Br and I by ICP-MS were 0.02 and 0.001 μg g−1, respectively, that were considered suitable for the determination of these elements in the investigated active pharmaceutical ingredients.

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

(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.

Simultaneous determination of sulphamethoxazole and trimethoprim in powder mixtures by attenuated total reflection-Fourier transform infrared and multivariate calibration.

A partial least-squares calibration (PLS) procedure in combination with infrared spectroscopy has been developed for simultaneous determination of sulphamethoxazole (SMZ) and trimethoprim (TMP) in raw material powder mixtures used for manufacturing commercial pharmaceutical products. Multivariate calibration modeling procedures, interval partial least squares (iPLS) and synergy partial least squares (siPLS), were applied to select a spectral range that provided the lowest prediction error in comparison to the full-spectrum model. The experimental matrix was constructed using 49 synthetic samples and 15 commercial samples. The considered concentration ranges were 400-900 mg g(-1) SMZ and 80-240 mg g(-1) TMP. Spectral data were recorded between 650 and 4000 cm(-1) with a 4 cm(-1) resolution by Fourier transform infrared spectroscopy coupled with attenuated total reflectance (ATR-FTIR) accessory. The proposed procedure was compared with conventional procedure by high performance liquid chromatography (HPLC) using 15 commercial samples containing SMZ and TMP. The results showed that PLS regression model combined to ATR-FTIR is a relatively simple, rapid and accurate procedure that could be applied to the simultaneous determination of SMZ and TMP in routine quality control of powder mixtures. A root mean square error of prediction (RMSEP) of 13.18 mg g(-1) for SMZ and 6.03 mg g(-1) for TMP was obtained after selection of better intervals by siPLS. Using the proposed procedure it is possible to analyze each sample in less than 3 min considering two replicates (excluding the grinding step). Accuracy was checked by comparison to HPLC method and agreement better than 98.8% was achieved.

Determination of Trace Elements in Fluoropolymers after Microwave-Induced Combustion

An effective approach to the digestion of fluoropolymers for the determination of Ag, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, and Ni impurities has been developed using microwave-induced combustion (MIC) in closed quartz vessels pressurized with oxygen. Samples that were examined included the following: polytetrafluorethylene (PTFE); polytetrafluoroethylene with an additional modifier, perfluoropropylvinylether (PTFE-TFM); and fluorinated ethylene propylene (FEP). A quartz device was used as a sample holder, and the influence of the absorber solution was evaluated. Determination of trace elements was performed by inductively coupled plasma-optical emission and mass spectrometry. Neutron activation analysis (NAA) was used for validation purposes. Results were also compared to those obtained using microwave-assisted acid extraction in high-pressure closed systems. Dilute nitric acid (5 mol L(-1)), which was selected as the absorbing medium, was used to reflux the sample for 5 min after the combustion. Using these conditions, agreement for all analytes was better than 98% when compared to values determined by NAA. The residual carbon content in the digests was lower than 1%, illustrating the high efficiency of the method. Up to 8 samples could be digested within 30 min using MIC, providing a suitable throughput, taking into account the inertness of such samples.

Total sulfur determination in residues of crude oil distillation using FT-IR/ATR and variable selection methods.

Total sulfur concentration was determined in atmospheric residue (AR) and vacuum residue (VR) samples obtained from petroleum distillation process by Fourier transform infrared spectroscopy with attenuated total reflectance (FT-IR/ATR) in association with chemometric methods. Calibration and prediction set consisted of 40 and 20 samples, respectively. Calibration models were developed using two variable selection models: interval partial least squares (iPLS) and synergy interval partial least squares (siPLS). Different treatments and pre-processing steps were also evaluated for the development of models. The pre-treatment based on multiplicative scatter correction (MSC) and the mean centered data were selected for models construction. The use of siPLS as variable selection method provided a model with root mean square error of prediction (RMSEP) values significantly better than those obtained by PLS model using all variables. The best model was obtained using siPLS algorithm with spectra divided in 20 intervals and combinations of 3 intervals (911-824, 823-736 and 737-650 cm(-1)). This model produced a RMSECV of 400 mg kg(-1) S and RMSEP of 420 mg kg(-1) S, showing a correlation coefficient of 0.990.

Study and determination of elemental impurities by ICP-MS in active pharmaceutical ingredients using single reaction chamber digestion in compliance with USP requirements

In this work a method for active pharmaceutical ingredients (APIs) digestion using the single reaction chamber (SRC-UltraWave™) system was proposed following the new recommendations of United States Pharmacopeia (USP). Levodope (LEVO), primaquine diphosphate (PRIM), propranolol hydrochloride (PROP) and sulfamethoxazole (SULF) were used to evaluate the digestion efficiency of the proposed method. A comparison of digestion efficiency was performed by measuring the carbon content and residual acidity in digests obtained using SRC and in digests obtained using conventional microwave-assisted digestion system (Multiwave(TM)). Three digestion solutions (concentrated HNO3, aqua regia or inverse aqua regia) were evaluated for digestion of APIs. The determination of Cd, Ir, Mn, Mo, Ni, Os, Pb, Pd, Pt, Rh, Ru was performed using inductively coupled plasma mass spectrometry (ICP-MS) in standard mode. Dynamic reaction cell (DRC) mode was used for the determination of (51)V, (52)Cr, (53)Cr, (63)Cu and (65)Cu in order to solve polyatomic ion interferences. Arsenic and Hg were determined using chemical vapor generation coupled to ICP-MS (FI-CVG-ICP-MS). Masses of 500mg of APIs were efficiently digested in a SRC-UltraWave™ system using only HNO3 and allowing a carbon content lower than 250mg L(-1) in final digests. Inverse aqua regia was suitable for digestion of sample masses up to 250mg allowing the determination of Ir, Pd, Pt, Rh and Ru. By using HNO3 or inverse aqua regia, suitable recoveries were obtained (between 91 and 109%) for all analytes (exception for Os). Limits of quantification were in agreement with USP requirements and they ranged from 0.001 to 0.015µg g(-1) for all elemental impurities (exception for Os). The proposed method was suitable for elemental impurities determination in APIs and it can be used in routine analysis for quality control in pharmaceutical industries.

Feasibility of microwave-induced combustion for trace element determination in Engraulis anchoita by ICP-MS.

A method based on microwave-induced combustion (MIC) was developed for fish (Engraulis anchoita) digestion and subsequent determination of As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Se, and Zn by inductively coupled plasma mass spectrometry (ICP-MS). A reflux step (5 min) was applied to improve absorption and recovery of analytes. Nitric acid was investigated as absorbing solution and suitable results were achieved using 5 mol L⁻¹ HNO₃. Microwave-assisted digestion in closed vessels using concentrated HNO₃ was also evaluated for comparison of results. Both sample preparation methods were considered suitable for sample digestion but MIC was preferable not only because diluted HNO3 can be used as absorbing solution but also because it provides higher efficiency of digestion and also better limits of detection. Accuracy was evaluated by the analysis of certified reference materials (DORM-2 and TORT-2) after MIC digestion with subsequent determination by ICP-MS. Agreement with certified values was better than 94%.

Attenuated total reflectance with Fourier transform infrared spectroscopy (ATR/FTIR) and different PLS Algorithms for simultaneous determination of clavulanic acid and amoxicillin in powder pharmaceutical formulation

A method for simultaneous determination of clavulanic acid (CA) and amoxicillin (AMO) was developed using Fourier transform mid infrared technique coupled with attenuated total reflectance (ATR/FTIR) accessory. 27 samples were used as calibration set and 8 samples were used for prediction set. Calibration models were developed using partial least squares (PLS), interval PLS (iPLS), synergy PLS (siPLS) and backward PLS (biPLS). Multiplicative scatter correction and the mean centering were used and produced the best models. Relative standard error of prediction (RSEP) of 3.8% for CA and 5.1% for AMO were obtained using biPLS algorithm for ATR/FTIR data. Results obtained by the proposed methodology were compared with those using high performance liquid chromatography (HPLC) and no significant differences were obtained. The proposed method using ATR/FTIR combined to multivariate analysis methods was suitable for the simultaneous determination of CA and AMO in commercial pharmaceutical products.