Direct determination of vanadium in high saline produced waters from offshore petroleum exploration by electrothermal atomic absorption spectrometry

The present work reports the development of a methodology for the direct determination of vanadium in high saline waters derived from offshore petroleum exploration employing electrothermal atomic absorption spectrometry. Such waters, usually called produced waters, present complex composition containing various organic and inorganic substances. In order to attain best conditions (highest sensitivity besides lowest background) for the methodology, studies about the effects of several variables (evaluation of pyrolysis and atomization temperatures, type of chemical modifier, concentration of modifier and pyrolysis time) and the convenient calibration strategy were performed. Best conditions were reached with the addition of 10 μg of NH₄H₂PO₄ as chemical modifier employing pyrolysis (during 10 s) and atomization temperatures of 1500 and 2700 °C, respectively. Obtained results indicated that, in this kind of sample, vanadium can be determined by standard addition method or employing an external calibration approach with standard solutions prepared in 0.8 mol l⁻¹ NaCl medium. In order to evaluate possible matrix interferences, a recovery test was performed with five spiked samples of produced waters. The limit of detection, limit of quantification and relative standard deviation in 0.8 mol l⁻¹ NaCl medium were also calculated and the derived values were 1.9 μg l⁻¹, 6.3 μg l⁻¹ and 5.6% (at 10 μg l⁻¹ level), respectively.     

Direct determination of lead in produced waters from petroleum exploration by electrothermal atomic absorption spectrometry X-ray fluorescence using Ir-W permanent modifier combined with hydrofluoric acid

The present work reports the development of a methodology for the direct determination of lead in high saline waters derived from petroleum exploration employing electrothermal atomic absorption spectrometry with permanent Ir-W and HF as modifiers. These waters, so-called produced waters, have complex composition containing several types of organic and inorganic substances. In order to attain best conditions (highest analytical signal besides lowest background) for the methodology studies about the effect of several variables and the convenient calibration strategy were performed. Also, the efficiency of other modification approaches was evaluated. At best conditions, pyrolysis and atomization temperature were 800 and 2200 °C, respectively, when the modifiers cited above were utilized. Obtained results indicate that, in this kind of sample, lead can be determined by standard addition method or employing external calibration with standard solutions prepared in 0.8 mol l⁻¹ NaCl medium. In order to evaluate the accuracy of the procedure, a recovery test was performed with six spiked samples of produced waters. The detection limit, quantification limit and the relative standard deviation in 0.8 mol l⁻¹ NaCl were also calculated and the values are 1.5 μg l⁻¹, 5.0 μg l⁻¹ and 5.0% (at 10 μg l⁻¹ level), respectively.     

Direct determination of manganese in produced waters from petroleum exploration by Electrothermal Atomic Absorption Spectrometry using Ir-W as permanent modifier

This present work reports the development and evaluation of a method for the direct determination of manganese in waters extracted during petroleum exploitation by Electrothermal Atomic Absorption Spectrometry (ET AAS) using Ir-W as permanent modifier. These waters, usually called produced waters, contain a wide range of organic and inorganic substances and are characterized by their high salinity. In order to achieve suitable experimental conditions for the method application, studies about the effect of operational variables (chemical modifier, pyrolysis and atomization temperatures) were performed, as well as the establishment of convenient calibration strategy. The best results were verified when the temperatures of pyrolysis and atomization were 1000 °C and 2300 °C, respectively, and using Ir-W as permanent modifier. The results showed that manganese can be determined by the standard addition method or employing external calibration with standard solutions prepared in the same salinity of the samples (with NaCl). Three real samples with salinities varying between 74 and 84‰ were successfully analyzed by the developed procedure. The limits of detection and quantification were 0.24 and 0.80 μg L⁻¹, respectively, in purified water, and 0.34 and 1.1 μg L⁻¹, respectively, in 0.4 mol L⁻¹ NaCl medium (approximately 23‰ salinity).     

Evaluation of zirconium as a permanent chemical modifier using synchrotron radiation and imaging techniques for lithium determination in sediment slurry samples by ET AAS

In the present paper, lithium was determined in river sediment using slurry sampling and electrothermal atomic absorption spectrometry (ET AAS) after L’vov platform coating with zirconium (as a permanent chemical modifier). The performance of this modifier and its distribution on the L’vov platform after different heating cycles were evaluated using synchrotron radiation X-ray fluorescence (SRXRF) and imaging scanning electron microscopy (SEM) techniques. The analytical conditions for lithium determination in river sediment slurries were also investigated and the best conditions were obtained employing 1300 and 2300 °C for pyrolysis and atomization temperatures, respectively. In addition, 100 mg of sediment samples were prepared using 4.0 mol l⁻¹ HNO₃. The Zr-coating permitted lithium determination with good precision and accuracy after 480 heating cycles using the same platform for slurry samples. The sediment samples were collected from five different points of the Cachoeira river, São Paulo, Brazil. The detection and quantification limits were, respectively, 0.07 and 0.23 μg l⁻¹.     

石墨炉原子吸收光谱法测定蚕蛹中Cr、Se量

建立了石墨炉原子吸收光谱法测定蚕蛹中Cr、Se的方法,为蚕蛹新资源食品的开发及明确其营养价值提供科学数据。利用石墨炉原子吸收光谱法,偏振塞曼效应扣除背景,石墨炉程序升温方式进行Cr、Se的原子化,检测峰值吸收。加入硝酸镍、吐温X-100为基体改进剂,在体积分数0.5%HNO3介质中对桑蚕蛹中Cr、Se进行测定。方法的精密度:4.0%(Cr),3.1%(Se)。加标回收率:96.8%~105.3%(Cr),92.1%~108.8%(Se)。Cr、Se的线性范围和检出限分别为:0~10μg/L(Cr),0~40μg/L(Se);LOD=1.22μg/L(Cr),1.86μg/L(Se)。建立的分析方法适用于蚕蛹中Cr、Se的测定。     

Speciation of chromium in wastewater and sludge by extraction with liquid anion exchanger Amberlite LA-2 and electrothermal atomic absorption spectrometry

A simple, integrated method for the speciation of chromium in wastewater and sewage sludge was developed, utilising liquid anion exchange by Amberlite LA-2 (LAES) and final determination by electrothermal atomic absorption spectrometry (ETAAS). Samples were filtered through a 0.45 μm membrane filter and chromium species were determined in filtered water samples and in sludge on the filters. In the former case (filtrate), total Cr was determined directly by ETAAS, while for the determination of Cr(VI) the filtrate was buffered to pH 6.4, extracted with LAES and Cr(VI) was determined in the organic extract. Cr(III) was determined by the difference. In the latter case (filter), the filters were leached with an alkaline buffer solution (pH 12.7) and the supernatant was subjected to the same extraction procedure. For the determination of total leachable Cr, the filters were subjected to acid leaching with dilute HNO₃ (pH 1) and the supernatant was subjected to ETAAS, after appropriate dilution with water. Then, Cr(III) was determined by the difference. The limits of detection (LOD) were 0.39 and 0.45 μg l⁻¹ for total Cr and Cr(VI), respectively, in the dissolved phase and 2.10 and 0.87 ng g⁻¹ for total Cr and Cr(VI) in the suspended solids. The recoveries of total Cr and Cr(VI) in filtrated wastewater samples and filters were quantitative, ranged from 93 to 106%. The effect of time and temperature of sonication and suspended solids concentration on total Cr and Cr(VI) recovery was studied. No significant difference in recoveries was obtained for sonication temperatures between 30 and 70 °C. However, sonication time equal to or higher than 30 min and concentration of suspended solids equal to or less than 30 mg significantly improved Cr recovery. The ETAAS program for the determination of Cr(VI) in Amberlite/MIBK extract was carefully optimised in the absence of a chemical modifier to avoid memory effects. The developed analytical method was applied for the determination of chromium species in wastewater and suspended solids of a municipal and a lab-scale wastewater treatment plant.     

Evaluation of different permanent modifiers for the determination of arsenic in environmental samples by electrothermal atomic absorption spectrometry

Single noble metal permanent modifiers such as, Rh, Ir, and Ru, as well as mixed tungsten plus noble metal (W-Rh, W-Ru, W-Ir) permanent modifiers thermally deposited on the integrated platform of transversally heated graphite atomizer were employed for the determination of arsenic in sludges, soils, sediments, coals, ashes and waters by electrothermal atomic absorption spectrometry. Microwave digests of solid samples and water samples were employed for obtaining the analytical characteristics of the methods with different permanent modifiers. The performance of the modifiers for arsenic determination in the real samples depended strongly on the type of permanent modifier chosen. The single noble metal (Rh, Ir and Ru) permanent modifiers were suitable for the analyte determinations in simpler matrices such as waters (recoveries of certified values 95-105%), but the analyte recoveries of certified values in sludges, soils, sediments, coals, and ashes were always lower than 90%. On the other hand, for the determination of arsenic, using W-Rh, W-Ru, and W-Ir permanent modifiers presented recoveries of certified values within 95-105% for all the samples. Long-term stability curves obtained for the determination of arsenic in environmental samples with different permanent modifiers (Rh, Ir, Ru, W-Rh, W-Ir, W-Ru) showed that the improvement in the tube lifetime depends on the tungsten deposit onto the platform. The tungsten plus noble metal permanent modifier presents a tube lifetime of at least 35% longer when compared with single permanent modifier. The results for the determination of As employing different permanent modifiers in the samples were in agreement with the certified reference materials, since no statistical differences were found after applying the paired t-test at the 95% confidence level.     

Method development for the determination of lead in wine using electrothermal atomic absorption spectrometry comparing platform and filter furnace atomizers and different chemical modifiers

A method has been developed for the determination of lead in wine by electrothermal atomic absorption spectrometry without any sample preparation and calibration against aqueous standards, using 7.5 μg Pd as a chemical modifier. The results obtained for seven wines using the proposed method and an acid digestion procedure did not show any significant difference using a Student's t-test. Atomization in a transversally heated filter atomizer (THFA) was compared with atomization in a conventional transversally heated platform furnace. The former provided a 2.6-fold higher sensitivity, improving the characteristic mass from 34 to 12 pg and a 1.6-fold better limit of detection (0.3 μg L⁻¹ compared to 0.5 μg L⁻¹) for aqueous solutions using the same injection volume of 20 μL. However, the average precision, expressed as the relative standard deviation for the determination of lead in wine under routine conditions was improved from 4.6% with platform atomization to 0.6% in the THFA. The lead content found in seven arbitrarily chosen white and red wines, five from Brazil, one from Chile and one from Spain, ranged from 6 to 60 μg L⁻¹ Pb with an average content of 11.4 μg L⁻¹ Pb for the wines from South America.     

Titanium as a chemical modifier for the determination of cobalt in marine sediments

The determination of cobalt in marine sediments by electrothermal atomic absorption spectrometry was studied using no modifier and magnesium and titanium as modifiers. Titanium is one of the major sediment constituents, which widely affects the cobalt determination and it was studied as a chemical modifier since it was the only concomitant that increased the cobalt signal in the concentration range usually found in sediments. The performance of Mg and Ti as chemical modifiers was compared relative to maximum pyrolysis and atomization temperatures, linear calibration range, sensitivity and matrix effects. The pyrolysis curves showed that the analyte could be stabilized up to 1400 °C when either Ti or Mg(NO₃)₂ was present, while only 1000 °C could be used in the absence of a modifier. The optimum atomization temperature was 2500 °C in all cases. Analytical curves were compared using no modifier, 5 μg Ti and 100 μg Mg(NO₃)₂ as modifiers, and the linear range found was up to approximately 4 ng Co whether a modifier was used or not. With Ti as a chemical modifier, analytical curves for cobalt in aqueous solution and in a synthetic matrix resulted in the same sensitivity (m₀=55 pg), whereas the use of Mg led to characteristic mass values of 59 and 72 pg in aqueous solution and in a synthetic matrix, respectively, showing some matrix effect. The detection limits (3σ, n=10) were 0.4 μg g⁻¹ using no modifier and 0.3 μg g⁻¹ with Ti as a modifier in the original matrix. A reference estuarine sediment NIST 1646 with a non-certified content of 10.5 μg g⁻¹ Co was analyzed and the found value of 10.9±2.4 μg g⁻¹, (n=3), using Ti as chemical modifier and calibration against aqueous standards, was in good agreement with the recommended value.     

On chromium direct ETAAS determination in serum and urine

A simple, accurate and reliable method for direct electrothermal atomic absorption spectrometric (ETAAS) determination of chromium in serum and urine samples without any preliminary sample pretreatment is described. Instrumental parameters are optimized in order to define: the most suitable atomizer, optimal temperature program and efficient modifier. An appropriate quantification method is proposed taking into account a matrix interference study. Pyrocoated graphite tubes and wall atomization, pretreatment temperature of 700 °C, atomization temperature of 2600 °C, hydrogen peroxide as modifier and standard addition calibration are recommended. The accuracy of the method proposed for Cr determination in serum and urine was confirmed by comparative analysis of parallel samples after wet or dry ashing as well as by the analysis of two certified reference materials: Serum, Clin Rep 1 and Lypochek Urine, level 1. The detection and determination limits achieved for both matrices are 0.08 μg/L and 0.15 μg/L respectively. The relative standard deviation varied between 15 and 18 % for the chromium content in the samples in the range 0.08–0.2 μg/L and between 4 and 7 % for the chromium content in the range 0.2–2.0 μg/L for both matrices.      

Determination of vanadium in mussels by electrothermal atomic absorption spectrometry without chemical modifiers

A method was developed for the quantitative determination of total vanadium concentration in mussels via electrothermal atomic absorption spectrometry (ETAAS). After the microwave digestion of the samples, a program using temperatures of 1600 °C and 2600 °C for ashing and atomization respectively, without any matrix modifiers, allowed us to obtain results that were satisfactory since they agreed closely with certified reference material values. The detection limit was 0.03 mg kg^–1 (dry weight), indicating that the method is suitable for the analysis of mussel samples. This determination was compared with matrix modifiers that have been reported previously. The method was applied to various cultivated and wild mussels from the Galician coast, yielding levels below 1 mg kg^–1 (wet weight).     

Determination of cadmium, chromium and lead in marine sediment slurry samples by electrothermal atomic absorption spectrometry using permanent modifiers

A procedure for the determination of cadmium, chromium, and lead in marine sediment slurries by electrothermal atomic absorption spectrometry is proposed. Slurry was prepared by mixing 10 mg of ground sample with particle size smaller than 50 μm completed to the weight of 1.0 g with a 3% nitric acid and 10% hydrogen peroxide solution. The slurry was maintained homogeneous with an aquarium air pump. For cadmium, the best results were obtained using iridium permanent with optimum pyrolysis and atomization temperatures of 400 and 1300 °C, respectively, a characteristic mass, m_o (1% absorption), of 2.3 pg (recommended 1 pg). Without modifier use, zirconium, ruthenium, and rhodium m_o were 3.4, 4.1, 4.6, and 4.8 pg, respectively. For chromium, the most sensitive condition was obtained with zirconium permanent with optimum pyrolysis and atomization temperatures of 1500 and 2500 °C, m_o of 6.6 pg (recommended 5.5 pg); and without modifier use, rhodium, iridium, and ruthenium m_o were 5.3, 8.8, 8.8, and 8.9 pg, respectively. For lead, the best modifier was also zirconium, m_o of 8.3 pg for the optimum pyrolysis and atomization temperatures of 600 and 1400 °C, respectively, (recommended m_o of 9.0 pg). For iridium, ruthenium, without modifier, and rhodium, m_o were 14.7, 15.5, 16.5, and 16.5 pg, respectively. For all the modifiers selected in each case, the peaks were symmetrical with r² higher than 0.99. Being analyzed (n = 10), two marine sediment reference materials (PACS-2 and MESS-2 from NRCC), the determined values, μg l⁻¹, and certified values in brackets, were 2.17 ± 0.05 (2.11 ± 0.15) and 0.25 ± 0.03 (0.24 ± 0.01) for cadmium in PACS-2 and MESS-2, respectively. For chromium in PACS-2 and MESS-2 the values were 94.7 ± 5.6 (90.7 ± 4.6) and 102.3 ± 10.7 (106 ± 8), respectively. Finally, for lead in PACS-2 and MESS-2, the results obtained were 184 ± 7 (183 ± 8) and of 25.2 ± 0.40 (21.9 ± 1.2), respectively. For cadmium and lead in both samples and chromium in PACS-2, calibration was accomplished with aqueous calibration curves. For chromium in MESS-2, only with the standard addition technique results were in agreement with the certified ones. The limits of detection (k = 3, n = 10) obtained with the diluents were 0.1, 3.4, and 3.6 μg l⁻¹ for cadmium, chromium, and lead, respectively.     

Comparison of different permanent chemical modifiers for lead determination in Orujo spirits by electrothermal atomic absorption spectrometry

Different analytical methods for the determination of lead in Orujo spirits by electrothermal atomic absorption spectrometry (ETAAS) were developed using permanent modifiers (W, Ir, Ru, W-Ir and W-Ru) thermally deposited on platforms inserted in pyrolitic graphite tubes and Pd-Mg(NO₃)₂ conventional modifier mixture. In all cases, the Pb determination was performed without any sample pretreatment or preconcentration steps. The comparison between the chemical modifiers employed has been made in terms of pyrolysis and atomization temperatures, characteristic masses, detection limits, and atomization and background signal shapes. The limits of detection obtained were 0.375, 0.387, 0.109, 0.251 and 0.267 ng mL⁻¹ for W, Ir, Ru, W-Ir and W-Ru, respectively and 0.710 ng mL⁻¹ for Pd-Mg(NO₃)₂. The characteristic masses were 14.1, 11.2, 5.6, 8.3 and 9.3 pg for W, Ir, Ru, W-Ir and W-Ru, respectively and 22.2 pg for Pd-Mg(NO₃)₂. For all the developed procedures using the different modification systems, the relative standard deviations (<10%) and the analytical recoveries (95-103%) were acceptable. The more suitable methods for Pb determination in distillate spirits were those using permanent modifiers in contrast with classical Pd-Mg(NO₃)₂. The best analytical performance was achieved for W, Ir and W-Ir methods, which were applied to lead determination in Orujo spirit samples from Galicia (NW Spain). The Pb concentrations found in the analyzed samples were comprised in the range (<LOD to 1.5 μg mL⁻¹).     

Selenium determination by electrothermal atomic absorption spectrometry in petroleum refinery aqueous streams containing volatile organic compounds

An electrothermal atomic absorption spectrometry (ETAAS) with polarized Zeeman background correction was used for determining selenium in petroleum refinery aqueous streams containing large amounts of volatile unknown organic compounds. Some parameters that might affect the measurement were investigated such as the amount of matrix modifier added, the temperature program and the calibration mode employed. Obtained results indicate that, in this kind of sample, selenium must be determined by standard addition procedure with a careful control of the dry step temperature and ramp pattern. Also, the results show that 2.5 μg of Pd must be added as matrix modifier to stabilize the analyte in the range of 2-20 ng Se. In order to evaluate the accuracy of the procedure, selenium was determined in 18 samples by ETAAS and hydride generation atomic absorption spectrometry (HGAAS) (as reference methodology). In both techniques the results agreed well.     

Chromium determination in pharmaceutical grade barium sulfate by solid sampling electrothermal atomic absorption spectrometry with Zeeman-effect background correction

A procedure for chromium (Cr) determination in pharmaceutical grade barium sulfate by direct solid sampling electrothermal atomic absorption spectrometry (DSS-ET AAS) with Zeeman-effect background correction was developed. Operational conditions for the proposed procedure and the use of citric acid, ammonium phosphate, palladium and magnesium nitrate as chemical modifiers were evaluated. Pyrolysis and atomization temperatures were set at 1500 and 2400 °C, respectively and the use of matrix modifiers did not improve these conditions. Graphite platform presented high degradation rate, but minima changes were observed in the sensitivity or signal profile. Samples (0.3-1 mg) were weighted and introduced into the furnace using a manual solid sampling system. The linear concentration range of the calibration curve was from 100 to 1800 pg (R² > 0.995). The characteristic mass was 7.7 pg and the limit of detection was 2.4 pg. Chromium concentration in commercial samples ranged from 0.45 to 1.06 μg g⁻¹ and these results were confirmed by standard addition method. The mean reproducibility was 12% (n = 20 in a 3-day period) and repeatability was less than 9%. Results obtained using inductively coupled plasma optical emission spectrometry and conventional electrothermal atomic absorption spectrometry after extraction with HNO₃ were around 20% lower than those obtained by the proposed procedure. It was assumed that the low results were due to incomplete extraction even using hard conditions related to temperature and pressure. The proposed procedure by DSS-ET AAS provided some advantages related to recommended pharmacopoeias methodology, as lower risks of contamination and analyte losses, higher specificity, accuracy and sensitivity, no toxic or unstable reagents are required, and calibration with aqueous standards was feasible.     

Slurry-based procedures to determine chromium, nickel and vanadium in complex matrices by ETAAS

Two slurry-based ETAAS methodologies are compared to determine directly Cr, Ni and V in coal fly ash, soils and sediments: current slurry analysis (USS) and ‘slurry extraction’ (SE). Slurries for USS-ETAAS were prepared in 0.5% HNO₃. HF and HNO₃ were evaluated as chemical modifiers. HF was the extractant for the SE-ETAAS approach.A unique slurry-based procedure to measure Cr, Ni and V could not be established. Cr concentrations up to 100 μg g^{− 1} were accurately determined by USS (0.5% HNO₃; λ = 429.0 nm; LOD = 0.35 μg g^{− 1}) whereas higher contents required the SE procedure (sample grinding, 30% HF; LOD = 0.02 μg g^{− 1}). Both methods were appropriate to determine Ni (LOD = 0.11 and 0.08 μg g^{− 1}, for USS and SE, respectively). V was satisfactorily quantified only with the USS approach (LOD = 0.80 μg g^{− 1}).     

Binary modifiers for the determination of zinc in pure copper and nickel-based alloy by longitudinal Zeeman electrothermal atomic absorption spectrometry

The determination of zinc in pure copper and nickel-based alloy was successfully accomplished with a longitudinal Zeeman-effect correction and end-capped transversely heated graphite atomizer. Since aqua regia (an acid mixture of nitric acid and hydrochloric acid, 1:3, v/v) was used as the dissolving reagent, volatile ZnCl₂ was formed. Consequently, less Zn was found in the sample. EDTA could improve the atomic absorption profiles. Binary modifiers, EDTA + Pd(NO₃)₂ and EDTA + Mg(NO₃)₂, were effective for eliminating the chloride interference and the spectral interference from Cu I 213.853 nm. The experimental results obtained with and without the modifiers were compared. Increase of 200 °C in the pyrolysis temperature resulted from the addition of binary modifiers for both pure copper and nickel-based alloys. For pure copper, the atomization temperature increased from 1400 to 1600 °C whereas the atomization temperature increased from 1100 to 1600 °C for nickel-based alloys. The analytical performance of the proposed method was evaluated. Zinc contents in the pure copper and nickel-based alloy standards determined with both binary modifiers agreed closely with the certified values. The recovery ranged from 93 ± 2 to 104 ± 6% at 95% confidence level. The detection limits obtained by the binary modifiers of EDTA + Pd(NO₃)₂ and EDTA + Mg(NO₃)₂ were 0.77 and 0.31 pg, respectively.     

Determination of nickel, chromium and cobalt in wheat flour using slurry sampling electrothermal atomic absorption spectrometry

The slurry technique was applied to the determination of Ni, Cr and Co in wheat flour by electrothermal atomic absorption spectrometry (ETAAS). The influence of the graphite furnace temperature programme was optimized. Optimum sensitivity was obtained by using a mixture of 15% HNO₃–10% H₂O₂ as suspended medium for a 3% w/v slurry in the determination of Ni; lower concentrations of HNO₃ were necessary for the determination of Co and Cr (viz. 5 and 10%). The precision of direct analyses of the slurries was improved by using mechanical agitation between measurements; thus, the RSD of the measurements was ca. 5% for repeatability. The direct slurry sampling (SS) technique is suitable for the determination of Ni and Cr in wheat flour samples at levels of 150–450 and 30–72 ng g⁻¹, respectively, as it provides results similar to those obtained by ashing the sample. However, the typically low level of Co in these samples precluded its determination by the proposed method (the study was made in an SRM spiked wholemeal flour), at least in those samples that were contaminated with elevated concentrations of the metal (viz. more than 90 ng of Co per g of flour). The method provides a relative standard deviation of 6, 8, and 4% for Ni, Cr, and Co, respectively.     

The use of solid-phase extraction and direct injection of a copolymer sorbent as slurry into the graphite furnace prior to determination of cadmium by ETAAS

Abstract

Several aquatic heterogeneous humic-solute fractions with known molecular-size ranges were fractionated further by coupled size-exclusion chromatography (SEC) and polyacrylamide gel electrophoresis (PAGE). The results of SEC-PAGE, obtained for several humic-solute fractions, corresponded reasonably well to the known molecular-size distributions of the dissolved organic matter (DOM). Despite the limited resolving power PAGE appears to offer an alternative coarse method, beside SEC, for fractionation of the heterogeneous humic matter (HM).     

Use of sodium tungstate as a permanent chemical modifier for slurry sampling electrothermal atomic absorption spectrometric determination of indium in soils

A number of chemical modifiers have been assessed for the direct determination of indium in soils using electrothermal atomic absorption spectrometry and slurry sampling. The best results were obtained when the graphite atomizer was impregnated with sodium tungstate, which acts as a permanent chemical modifier. Slurries were prepared by suspending 100 mg sample in a solution containing 1% (v/v) concentrated nitric acid and 10% (v/v) concentrated hydrofluoric acid and then 15-microL aliquots were directly introduced into the atomizer. Standard indium solutions prepared in the suspension medium in the range 4-80 microg L(-1) indium were used for calibration. The relative standard deviation for ten consecutive measurements of a 40 microg L(-1) indium solution was 2.8%. The limit of detection in soils was 0.1 microg g(-1). The reliability of the procedures was confirmed by analysing two standard reference materials and by using an alternative procedure.