Microwave digestion of ancient peat and determination of Pb by voltammetry

Different acid compositions (HNO₃, H₂O₂, HF and HClO₄) and low pressure microwave digestion were applied to the microwave assisted dissolution of ancient peats. The digests were evaluated with respect to the decomposition of the inorganic and organic fraction of peats and to an optimised determination of Pb by differential pulse anodic stripping voltammetry (DPASV). Addition of hydrofluoric acid was necessary to achieve a complete dissolution of the resistant inorganic phase of the peat and to obtain the total Pb concentration. Addition of perchloric acid decreased the concentration of dissolved organic carbon significantly, enabling reliable voltammetric signals for Pb determinations with satisfying peak shape, baseline, sensitivity, and precision. The limit of detection for Pb in peats was 1 ng/mL in the acid digest, equivalent to 0.2 μg/g in solid peat. This allowed the determination of low, pre-industrial concentrations of Pb in peat bogs from various locations (Europe and SE Asia). A chelation column used to isolate the metals from the complex matrix to further improve the detection limits failed to provide good recoveries for the peats and plants. Received: 12 June 1998 / Revised: 24 July 1998 / Accepted: 28 July 1998     

Optimized microwave preparation procedure for the elemental analysis of aquatic sediment

 This paper summarizes several key points in applying the microwave preparation technique to the elemental analysis of aquatic sediments and reports systematic experiments in searching for an optimal microwave preparation procedure for element analysis in sediment samples. The determination of the elements Cu, Pb and Cd in a standard reference aquatic sediment sample (CRM 280, COMEUR) was achieved by first digesting the samples in a microwave oven equipped with PFA advanced composite vessels, followed by AAS measurement. The influence of microwave power, digestion time, various dissolution reagents and the HF removing conditions was studied. It has been shown that for a 0.1 g sediment sample the optimal microwave preparation conditions are: 4–5ml HNO₃/HF/H₂O₂ as solvent, digesting time 30 min with 100% microwave power and evaporating the residual acid within 8 min in an open vessel at 80 °C. The element recovery rates with AAS measurement can reach up to 92.4–100.6%. Received: 23 July 1996/Revised: 23 September 1996/Accepted: 25 September 1996     

Optimization of microwave‐assisted extraction for six inorganic and organic arsenic species in chicken tissues using response surface methodology

Response surface methodology was applied to optimize the parameters for microwave‐assisted extraction of six major inorganic and organic arsenic species (As(III), As(V), dimethyl arsenic acid, monomethyl arsenic acid, p‐arsanilic acid, and roxarsone) from chicken tissues, followed by detection using a high‐performance liquid chromatography with inductively coupled mass spectrometry detection method, which allows the simultaneous analysis of both inorganic and organic arsenic species in the extract in a single run. Effects of extraction medium, solution pH, liquid‐to‐solid ratio, and the temperature and time of microwave‐assisted extraction on the extraction of the targeted arsenic species were studied. The optimum microwave‐assisted extraction conditions were: 100 mg of chicken tissue, extracted by 5 mL of 22% v/v methanol, 90 mmol/L (NH₄)₂HPO₄, and 0.07% v/v trifluoroacetic acid (with pH adjusted to 10.0 by ammonium hydroxide solution), ramping for 10 min to 71°C, and holding for 11 min. The method has good extraction performance for total arsenic in the spiked and nonspiked chicken tissues (104.0 ± 13.8% and 91.6 ± 7.8%, respectively), except for the ones with arsenic contents close to the quantitation limits. Limits of quantitation (S/N = 10) for As(III), As(V), dimethyl arsenic acid, monomethyl arsenic acid, p‐arsanilic acid, and roxarsone in chicken tissues using this method were 0.012, 0.058, 0.039, 0.061, 0.102, and 0.240 mg/kg (dry weight), respectively.     

High-pressure microwave dissolution of ceramics prior to trace metal determinations by microwave induced plasma atomic emission spectrometry

A commercial laboratory microwave acid digestion system was evaluated for the acid dissolution of ceramic powders (Al₂O₃, AlN, BN and Si₃N₄) prior to the determination of their trace element content by microwave induced plasma atomic emission spectrometry. Newly designed vessels, capable of withstanding internal pressures of over 110 bar, provide rapid and satisfactory results for sample dissolution. Sample preparation time was approximately 30 min (including the subsequent cooling time and preparation of the final solution). Results from conventional stainless-steel acid digestion vessel (Teflon bomb) dissolution are compared with the microwave bomb results of microwave plasma atomic emission spectrometry.     

Closed-vessel microwave digestion technique for lichens and leaves prior to determination of trace elements (Pb,Zn, Cu) and stable Pb isotope ratios

A reliable and robust procedure using closed-vessel microwave digestion of lichens and leaves for precise and accurate determination of trace elements (Pb, Zn and Cu) and stable Pb isotope ratios is presented. The method was developed using certified reference material CRM 482 Pseudovernia furfurea (Lichens), NIST 1515 (Apple Leaves) and NIST 1547 (Peach Leaves) and tested on lichens from a mining site in Russia. A mixture of 3?mL of HNO₃, 3?mL of H₂O₂, 2?mL of H₂O and 0.8?mL of HF ensured complete sample dissolution with 100?±?5% recovery for Pb, Zn and Cu at a maximum temperature of 210°C and pressure of 350?psi. The amount of HF and microwave pressure significantly influenced Pb, Zn and Cu recovery. Comparison between EMMA-XRF and ICP-AES showed a good correlation between Pb, Zn and Cu concentrations. Using the newly developed digestion method, Pb isotopes in lichens from the mining site were determined with an internal precision better than 0.02%.     

The effect of sample preparation on metal determination in soil by FAAS

Different methods for the determination of several metals in soils by flame atomic absorption spectrometry (FAAS) were investigated. Different procedures for total dissolution of soil: I – HF+HClO₄, H₃BO₃, HCl digestion (conventional heating), II – HF+HClO₄, H₃BO₃ digestion followed by fusion with LiBO₂ (conventional heating) and III – HF+HCl+HNO₃, H₃BO₃ digestion (microwave heating), as well as a leaching procedure with HNO₃+HClO₄, HCl were tested and compared. For quality assessment, the certified reference material S-1 soil was used. For most of the investigated metals, the best accuracy and precision were achieved when the procedure I or III were used. The developed procedure was applied to the analysis of soil samples from crude oil refinery and dump of petroleum origin wastes regions. Received: 22 Dezember 1997 / Revised: 9 February 1998 / Accepted: 12 February 1998     

Determination of trace and platinum-group elements in high ionic-strength volcanic fluids by sector-field inductively coupled plasma mass spectrometry (ICP-MS)

The accurate measurement of ultra-trace concentrations of rare metals and platinum group elements in volcanic fluids is complicated by interferences, complex matrices, and preferential element partitioning. We analyzed condensed, high-temperature magmatic fluids collected from Kudryavy volcano (Kurile Islands, Russia) for Be, B, Rb, Sr, Mo, Ru, Rh, Pd, Cd, W, Re, Pt, Pb, Bi, and U using ICP-MS. The samples had three different matrices: 5 mol L^–1 sodium hydroxide (NaOH, pH = 11), gas condensates of sulfuric acid (H₂SO₄, pH = 0), and solid elemental sulfur. Interferences and suppression/enhancement effects were investigated using standards in concentrated NaOH and H₂SO₄ solutions to determine adequate dilution for sample analyses, which then required very low levels of element detection (< 1 μg L^–1 for most elements). Depending on the field sampling technique of volcanic gas vents, our results show significant differences in concentrations of some trace elements due to the precipitation of solid sulfur during gas condensation and variations in element volatility as a function of temperature. Received: 1 March 1998 / Revised: 1 July 1998 / Accepted: 7 July 1998     

Application of co-electroosmotic capillary electrophoresis for the determination of inorganic anions and carboxylic acids in soil and plant extract with direct UV detection

Summary Indirect UV detection in capillary zone electrophoresis (CZE) is frequently used for the determination of inorganic anions and carboxylic acids. However, there are few reports on direct UV detection of these solutes in real samples. This paper describes the use of direct UV detection of inorganic anions and organic acids in environmental samples using co-electroosmotic capillary zone electrophoresis (co-CZE) at 185 nm. The best separation and detection of the solutes was achieved using a fused silica capillary with an electrolyte containing 25 mM phosphate, 0.5 mM tetradecyltrimethylammonium bromide (TTAB) and 15% acetonitrile (v/v) at pH 6.0. Four common inorganic anions (Cl⁻, NO₂ ⁻, NO₃ ⁻ and SO₄ ²⁻) and 11 organic acids (oxalic, formic, fumaric, tartaric, malonic, malic, citric, succinic, maleic, acetic, and lactic acid), were determined simultaneously in 15 min. Linear calibration plots for the test solutes were obtained in the range 0.02–0.5 mM with detection limits ranging from 1–9 μM depending on the analyte. The proposed method was successfully used to determine inorganic anions and carboxylic acids in soil and plant tissue extracts with direct injection of the sample.     

Comparison of microwave-assisted acid leaching techniques for the determination of heavy metals in sediments, soils, and sludges

Microwave-assisted EPA method 3051 for nitric acid leaching of environmentally key elements from sediments, soils, and sludges was tested, and the influence of leaching temperature and time on element recovery for an estuarine sediment (CRM 277) was investigated. The extraction efficiencies for four certified reference materials applying EPA method 3051, an optimized nitric acid procedure, and an aqua regia (HCl/HNO₃ 3:1) procedure were compared. Digestions were carried out in a high-pressure microwave system offering simultaneous temperature and pressure control for all digestion vessels employed. Eight elements (Cd, Co, Cr, Cu, Hg, Ni, Pb, and Zn) were determined by ICP-AES and ICP-MS. Extraction efficiency strongly depended on the applied leaching parameters and varied for certain elements among different materials when a nitric acid procedure was applied. In general, element recoveries obtained from the aqua regia procedure were superior to those obtained from nitric acid procedures and showed good agreement with the 95% confidence interval of the certified value for most of the elements investigated. Received: 27 March 1998 / Revised: 29 June 1998 / Accepted: 3 July 1998     

Optimised method of coal digestion for trace metal determination by atomic absorption spectroscopy

Understanding the transport of trace elements through a coal-fired power plant requires reliable analytical methods for these elements in all the ingoing and outgoing mass streams. Coal and different kinds of ashes comprise the most abundant mass streams in such a plant. As a continuation of our previous work, we have optimised a digestion method for the AAS determination of heavy metals in coal samples. It has become evident that complete dissolution of metals in coal samples and accurate results in subsequent analysis can be obtained by means of applying pressurised digestion under microwave heating. The combination of HNO₃ (conc.) and HF (conc.) in the volume ratio of 50 : 1 attacked the sample well enough, and good recoveries for all the metals studied were obtained. Surprisingly good results were obtained also when HNO₃ alone was used as the digestion acid. Received: 18 June 1998 / Revised: 23 September 1998 / Accepted: 24 September 1998     

Determination of mercury species using thermal desorption analysis in AAS

Analytical aspects of the determination of inorganic mercury (Hg) species by thermal desorption followed by atomic absorption spectrometry (AAS) detection were investigated in this work. Characteristic Hg release curves of the following species were observed: Hg⁰, HgCl₂, HgO, HgSO₄, HgS, and the Hg bound to humic acids. Particular attention was dedicated to the thermal stability and change of bond of Hg⁰ in the following matrices: sand, kaolinite, granite, peat, power plant ash, and soil. The bond of elemental Hg in environmental materials was described on basis of this experiment. Contaminated soil samples from two locations in the Czech Republic were investigated by thermal desorption analysis. Afterwards, the contents of volatile and plant-available Hg in the studied samples were determined. The determination of Hg⁰ using the thermal method was related to the results of liquid sequential extraction. The development of Hg speciation and the stability of Hg were assessed on basis of the data obtained. Thus, the analytical procedure used is a suitable tool for the study of inorganic Hg species in contaminated soils.     

210Pb and210Po analysis in sediments and soils by microwave acid digestion

A microwave acid digestion method prior to the determination of²¹⁰Pb and²¹⁰Po in sediments and soils is described. It involves an acid (HNO₃, HCl, HF and H₃BO₃ mixture) digestion with microwave heating in closed vessels at high pressures. Analyses carried out for various reference materials showed that the results were statistically equal to certified values and reproducibility was also assured. The advantage of the microwave technique compared to the traditional leaching procedures is that the solid materials are completely dissolved and, therefore, ca. 100% efficiency is achieved in the extraction of²¹⁰Po and²¹⁰Pb, even though a fraction is associated to the silica net. Moreover, time of analysis is drastically reduced, as are the risks associated to vapour inhalation and material corrosion.     

Determination of sulfur in metal sulfides by bromine water-CCl4 oxidative dissolution and modified EDTA titration

The sulfur content in metal sulfides can be determined after oxidative dissolution with bromine water-CCl₄ at 50 °C by modified EDTA titration. The sulfides were quantitatively oxidized to SO₄ ^2– without evolving H₂S, and the metal cations were removed by cation exchange on an H-type column. Sulfate was precipitated as BaSO₄, and the remaining Ba²⁺ was subsequently titrated with EDTA at pH = 10. The standard deviation for the determined sulfur in the metal sulfides, some of which can be hardly dissolved in usual mineral acid media, is about a few %. The method can be carried out with sample amounts containing as little as 1.5–2 mg sulfur. Received: 31 July 1997 / Revised: 5 January 1998 / Accepted: 9 January 1998     

Determination of picomole amounts of formaldehyde in air and bio-fluids based on its enhanced myoglobin-luminol chemiluminescence reaction

Abstract

The aim of this study was to assess the performance of selected destruction methods for the determination of heavy metals (Cd, Cu, Pb and Zn) in woodlouse (Oniscus asellus). A vigorous total analysis involving microwave destruction with HF, HCl and HNO₃ (method 1) was used as a reference method. Consistently low values for the dry ashing method may indicate incomplete dissolution of the elements and/or losses through volatilisation. Method 3 (concentrated HNO₃) that frequently is used in literature, produced erroneous values for Cd, Cu and Pb. Results were consistent with the microwave digestion, provided H₂O₂ was used during digestion (method 4, HNO₃/H₂O₂). Method 5 (HNO₃/HCIO₄, one destruction step) yielded low recoveries when only one destruction step was applied. Applying two destruction steps (method 6) resulted in values consistent with the microwave method, but was at the expense of reproducibility and rendered the method more lengthy and laborious. Because of the very good performance combined with speed and simplicity, destruction with HNO₃/H₂O₂ (method 4) emerged as the most convenient method.     

A method for rapid determination of arsenic species in vegetables using microwave‐assisted extraction followed by detection with HPLC hyphenated to inductively coupled plasma‐mass spectrometry

Driven by the significant need for characterization of the chemical speciation of arsenic in food, this work developed a method for rapid determination of four common arsenic species, namely, arsenite, arsenate, monomethyl arsenic acid, and dimethyl arsenic acid, in vegetables using microwave‐assisted extraction, followed by detection with high‐performance liquid chromatography hyphenated to inductively coupled plasma‐mass spectrometry. Initial screening results showed that microwave‐assisted extraction using 1% HNO₃ exhibited the highest overall efficiencies for all arsenic species without causing significant degradation of the organic ones. With the aid of response surface methodology, the optimum conditions established for extraction of arsenic species from vegetables were: 500 mg of freeze‐dried vegetable sample, extracted by closed vessel microwave‐assisted extraction using 10 mL of 2% v/v HNO₃ at 90°C for 17 min. Application of the method in the analysis of 24 market vegetable samples indicates that the extraction efficiencies for total arsenic species were in the range of 91.4–106%. Arsenite and arsenate were found to be the predominant arsenic species in the vegetables, which suggests that vegetable consumption could be an important route of inorganic arsenic exposure for the population with a heavy vegetable diet in arsenic polluted regions.     

Determination of sulfur in metal sulfides by bromine water-CCl4 oxidative dissolution and modified EDTA titration

The sulfur content in metal sulfides can be determined after oxidative dissolution with bromine water-CCl₄ at 50 °C by modified EDTA titration. The sulfides were quantitatively oxidized to SO₄ ^2– without evolving H₂S, and the metal cations were removed by cation exchange on an H-type column. Sulfate was precipitated as BaSO₄, and the remaining Ba²⁺ was subsequently titrated with EDTA at pH = 10. The standard deviation for the determined sulfur in the metal sulfides, some of which can be hardly dissolved in usual mineral acid media, is about a few %. The method can be carried out with sample amounts containing as little as 1.5–2 mg sulfur. Received: 31 July 1997 / Revised: 5 January 1998 / Accepted: 9 January 1998     

Determination of lead in waters and sediments from the wetland Tablas de Daimiel National Park (Spain)

Electrothermal atomic absorption spectrometry (ETAAS) has been used for lead control in waters and sediments of the wetland Tablas de Daimiel National Park. The lead determination in the wetland water is hindered by an enhanced salt content, so matrix interference was evaluated. The use of stabilized temperature platform atomization technique, Zeeman background correction and palladium nitrate as matrix modifier were found advisable. For the sediments a closed-vessel microwave dissolution method has been proposed using a mixture of HCl-HNO₃-HF and a heating time of 90 s. With subsequent ETAAS using platform atomization and Zeeman background correction, no chemical modification was necessary. Using these conditions, the interferences were completely removed for both waters and sediments and the calibration curve in ultrapure water (1% nitric acid) was linear up to 30 μg/L. The detection limits for waters and sediments were 0.95 μg/L and 0.78 μg/g, respectively. The accuracy of the proposed method for sediments was validated by analyzing certified reference materials (obtained values were within the certified ones) and spiked wetland sediments (mean recovery of 99.0%). Received: 11 October 1998 / Revised: 3 December 1998 / Accepted: 11 March 1999     

Determination of lead in waters and sediments from the wetland Tablas de Daimiel National Park (Spain)

Electrothermal atomic absorption spectrometry (ETAAS) has been used for lead control in waters and sediments of the wetland Tablas de Daimiel National Park. The lead determination in the wetland water is hindered by an enhanced salt content, so matrix interference was evaluated. The use of stabilized temperature platform atomization technique, Zeeman background correction and palladium nitrate as matrix modifier were found advisable. For the sediments a closed-vessel microwave dissolution method has been proposed using a mixture of HCl-HNO₃-HF and a heating time of 90 s. With subsequent ETAAS using platform atomization and Zeeman background correction, no chemical modification was necessary. Using these conditions, the interferences were completely removed for both waters and sediments and the calibration curve in ultrapure water (1% nitric acid) was linear up to 30 μg/L. The detection limits for waters and sediments were 0.95 μg/L and 0.78 μg/g, respectively. The accuracy of the proposed method for sediments was validated by analyzing certified reference materials (obtained values were within the certified ones) and spiked wetland sediments (mean recovery of 99.0%). Received: 11 October 1998 / Revised: 3 December 1998 / Accepted: 11 March 1999     

Vergleichende atomabsorptionsspektrometrische untersuchungen zur elementspurenbestimmung in Urin

When using a direct determination procedure with graphite-furnace a.a.s. (e.t.a.a.s.), it is sufficient to make an addition of nitric acid in order to arrive at the optimal reduction of the spectral background. A “matrix modifier” (NH₄NO₃, (NH₄)₂HPO₄) produces a background which often cannot be compensated completely. Detection limits of the direct determination technique are: Cd 0.1, Co 8, Cu 4, Ni 5, Pb 2 and Tl 3 (μg l^?1). A similar power of detection can be achieved as with flame-a.a.s. due to a preceding preconcentration step (trace adsorption on highly dispersed silicic acid). After preconcentration and determination with e.t.a.a.s., the detection limits are: Cd 0.002, Co 0.1, Cu 0.05, Ni 0.09, Pb 0.09 and Tl 0.06 (μg l^?1). The trace concentrations in urine of healthy persons were found to be: Cd 0.2–0.8, Co ? 0.1, Cu 4–10, Ni 1–3, Pb 6–10 and Tl 0.7–1.3 (μg l^?1). Direct e.t.-a.a.s. is, therefore, found to be suitable for the determination of Cd, Cu and Pb. For the determination of Co, Ni and Tl concentrations, a preconcentration is required. Cobalt was not found in any of the urine samples at the limit of detection of 0.1 μg l^?1.     

Evaluation of extraction/digestion techniques used to determine lead isotopic composition in forest soils

Lead isotopic studies in soils provide an efficient tool for tracing the sources of lead pollution. Five different extraction/digestion techniques (0.05 M EDTA, 0.5 M HNO₃, 2 M HNO₃, aqua regia, total digestion) were used for lead isotopic composition (²⁰⁶Pb/²⁰⁷Pb) determination in three forest soil profiles with different kinds of prevailing Pb contamination (unpolluted area, smelting area and vicinity of a motorway). The results obtained showed that all extraction/digestion methods used for the determination of ²⁰⁶Pb/²⁰⁷Pb ratios in surface horizons containing high organic matter contents gave statistically identical values (according to the Tukey test). In mineral soil horizons, differences between the individual extraction/digestion methods could be observed (the lowest ²⁰⁶Pb/²⁰⁷Pb ratios were obtained from EDTA extracts, corresponding to weakly bound anthropogenic lead, and the highest ²⁰⁶Pb/²⁰⁷Pb ratios were obtained from total digestion). The combination of total digestion and EDTA extraction (labile lead fraction) seems to be the optimal combination for ²⁰⁶Pb/²⁰⁷Pb ratio determination and optimal result interpretation.