Ashing Procedure Research Articles

Revisitation of mineralization modes for arsenic and selenium determinations in environmental samples

Mineralization procedures for arsenic and selenium analysis are usually limited to wet digestion methods owing to high volatility of these analytes. On the other hand, variable amounts of silicon in some types of samples imply elaborated mineralization procedures to liberate analytes which may be retained in an insoluble residue. Consequently, methods for such material generally include an hydrofluoric step followed by an evaporation to dryness. This type of mineralization is most easily accomplished using a dry ashing procedure. For plant analysis, a well validated and readily applicable dry ashing method is used for a long time in several laboratories but up today one could suppose that As and Se determinations cannot be performed after such a type of mineralization. Surprisingly, it has been observed that for plant samples these analytes are detected even after a calcination at 450°C. The general usefulness of a dry ashing method for analysis of all other analytes (main, minor and trace elements) incitates us to also verify As and Se recoveries. Results obtained in this work indicate clearly that plants of terrestrial origin may be mineralized using dry ashing procedure without As and Se losses. This statement was confirmed by analyses of several reference terrestrial plant samples (RMs) and laboratory control samples. Another confirmation was given by the direct graphite furnace analysis of the same plant samples but in slurried form (SS-ETAAS). As a direct consequence, As and Se analysis in terrestrial plants no more necessitates a separate preparation methodology. On the other hand, significant losses of As and Se were observed for aquatic plants, e.g. algaes. For the analysis of this type of samples, a separate wet digestion procedure remains unavoidable if the determination of As and Se has to be considered. Also some preparation procedures were tested for As and Se-analysis of soil and sediment reference samples. In these cases the wet digestion with a mixture of nitric, perchloric and hydrofluoric acids seems to remain the best alternative.

Comparison of sulphur retention by coal ash in different types of combustors

During coal combustion, a part of the total sulphur in coal will be retained as solid compounds in ash. In the present paper, the retention of sulphur by ash of typical Chinese coal used for electric power generation was correlated with the contents of alkaline elements relative to sulphur in the parent coal. Comparison was performed among the capabilities of sulphur retention by coal ashes in laboratory ashing procedure, pulverised coal-fired combustor and fluidised bed combustor (FBC). It was shown that calcium plays a dominant role in sulphur retention of laboratory-prepared ash and the sulphur retention percentage increases with the increase of Ca/S molar ratio of the parent coal, while the contributions of other elements are limited. The sulphur retention behaviour in pulverised coal-fired combustor is significantly different from that in laboratory ashing procedure. The contribution of calcium is reduced markedly, while the roles of the other alkaline elements are obviously enhanced. Using the literature data from foreign coals, it was found that the sulphur retention in FBC is comparable with that in laboratory ashing procedure. It was suggested from the results obtained in this study that the sulphur retention of laboratory-prepared ash can be applied to fluidised bed combustion because the behaviour of calcium is similar to that of lime in FBC.

Analysis of siliceous materials and coal by atomic absorption spectrophotometry with fusion for sample decomposition

The oxalic acid–lithium carbonate–lithium tetraborate (1:1:1) fusion agent proposed previously for cement analysis is applied to determine the composition of various siliceous samples including granite, clay, flyash, sand and silica fume. For samples with silica content higher than 70%, the sample size had to be reduced from 0.10 to 0.01 g and a recovery factor was needed in silica determination to account for the precipitation of silica during acid dissolution. Attempt has also been made to determine the major metallic elements in coal by fusion directly without the ashing procedure, and the results obtained were good. Further, the ash content of coal was proposed to be determined by establishing a relation between the ash content and the sum of the concentrations of four major oxides (i.e. the oxides of Si, Al, Fe and Ca), and the proposed method is accurate and is time saving compared with that determined by direct ashing at 800°C.

99/03497 The dissolution of titanium from an Australian bituminous coal with aqueous HF: Steel, K. M. et al. Proc. Annu. Int. Pittsburgh Coal Conf., [computer optical disk], 1998, 15, 1150–1160

The mode of occurrence of Ti in Gordonstone Thermal Coal, an Australian bituminous coal, is investigated and the dissolution behavior of the Ti from the coal with aqueous HF is presented. A low temperature ashing procedure at 350 C is performed on the coal to isolate the mineral matter. X-ray diffraction analysis of the mineral residue identifies anatase (TiO{sub 2}), amongst other compounds. Leaching experiments performed on the mineral residue with aqueous HF, however, indicate that not all of the Ti in the mineral sample displays similar dissolution characteristics to that of anatase. It is postulated that an amorphous hydrated titania (TiO{sub 2} x H{sub 2}O) occurs in the mineral residue produced at 350 C, along with anatase, and that this titania is highly reactive toward acid. It is further proposed that this hydrated titania is the dominant Ti bearing mineral in the coal, that it dehydrates to anatase during ashing and that anatase may not occur in the coal to a large extent. Leaching experiments performed on the coal are consistent with the notion that, while the titania is reactive toward aqueous HF, the particles of titania are very fine and highly disseminated throughout the coal structure.

The distribution and origin of trace elements in a UK coal; the importance of pyrite

Coal samples were obtained from a multigravity separator at Harworth (UK) coal-preparation plant. The minerals identified by X-ray diffraction in low-temperature ashes include kaolinite, illite, mixed-layer illite-smectite, chlorite, quartz, pyrite, apatite and gypsum. The latter is an artefact of the ashing procedure. Based on X-ray diffraction and major element analyses the coal samples show a preferential separation of quartz and pyrite. Samples were analysed for trace elements using energy dispersive, polarised XRF and data obtained for 25 trace elements with detection levels of ∼1 ppm for most elements and sub-ppm for some. One group of trace elements shows highly significant correlations within the group and with the pyritic S%. The concentrations (in ppmw) of these elements in pyrite were determined, per 1 wt% pyritic S, as: Mo 2, Se 0.6, Ni 8.2, As 21, Pb 6.1, Sb 0.4, Cu 13.8 and Zn 8.8. There are other sources for these elements in the coal, particularly for Ni, Cu and Zn, but pyrite is the main location. The concentration levels determined for pyrite may be applicable to pyrite in other coals because of a common process of enrichment from a seawater source. Another group of elements, consisting of Rb, V, Cr, Ga, Sr, Y, Zr, Nb, Sn, Ba, La, Ce and Th is thought to be contained within the detrital minerals. The only member of this group with a significant non-detrital origin is V, which is probably contained within the organic matter and in a low ash coal this would appear to be the major source. Germanium and Br are the two elements which have a dominant organic association.

A single plant tissue digestion for macronutrient analysis

A method was tested for determinations of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in one single digestion of plant tissue. Traditionally, for determinations of these five elements in plant tissue, at least two digestion or ashing procedures are used, totaling about 6 hours of tissue mineralization time. The method tested involved using a digesting medium of concentrated H2SO4 and a mixed catalyst of Na2SO4 and CuSO4. The total digestion time was about 60 minutes on a hot plate. After tissue digestion, determinations were by volumetric analysis (N), colorimetry (P), and atomic absorption (Ca, Mg) or emission spectrophotometry (K). Leaves of eight different species, including one standard tomato leaf sample and one internal quality‐control apple leaf sample were analyzed. High correlations occurred between the results obtained with the new procedure and with the standard digesting methods (furnace ashing and Kjeldahl digestion). The new method used much less reagents, electrical energy, and labor than standard methods. The new method appears acceptable for routine macronutrient analysis.

A New Microwave Acid Digestion Bomb Method for the Determination of Total Fluorine

A new microwave acid digestion method for total fluorine analysis was compared to the reliable reverse-extraction technique. The commercially available Parr bombs which are compatible with microwave heating were modified for this purpose. The Mann-Whitney statistical test did not show any significant differences (p > 0.05) in the determinations of total fluorine in various samples between the two above-mentioned methods. The microwave method also gave high fluorine recoveries (> 97%) when fluoride was added to different samples. The great advantage of the microwave acid digestion bomb method is that the digestion under pressure is so aggressive that only a few minutes is needed for complete digestion (also of covalently bonded fluorine), which reduces the time for fluorine analysis dramatically, while no loss of fluorine or contamination from extraneous sources could take place during the ashing procedure. The digestion solution was made up of 300 μl of concentrated nitric acid plus 537 μl of water. After digestion 675 μl of ∼ 8.5 M sodium hydroxide plus 643 μl of citrate/TISAB buffer was added resulting in an alkaline solution (pH ∼ 12) which was finally adjusted to a pH of ∼ 5.3 for fluoride determination.

Asbestos and non-asbestos fiber content in lungs of Korean subjects with no known occupational asbestos exposure history

To provide an estimation of asbestos exposure among normal Koreans and a criterion for determining occupational asbestos exposed diseases, pulmonary fiber contents of both asbestos and non-asbestos types were evaluated in 20 Korean subjects with no known occupational history of asbestos exposure. Pulmonary fiber contents were analyzed by transmission electron microscopy with energy dispersive X-ray analysis after using the low temperature ashing procedure. Chrysotile fiber (65.5%) was the major fiber type found in the lungs of Korean subjects. Asbestos fiber concentrations found in males and females were 0.30 × 10 6 fibers/g of dry lungs and 0.15 × 10 6 fibers/g of dry lungs, respectively, showing a geometric mean concentration of 0.26 × 10 6 fibers/g of dry lung tissue. Non-asbestos fiber contents found in males and females were 8.02 × 10 6 fibers/g of dry lungs and 7.85 × 10 6 fibers/g of dry lungs, respectively, with a geometric mean concentration of 7.98 × 10 6 fibers/g of dry lungs. There was no difference in asbestos content in lungs among various age groups. Comparing these results with Japanese male (2.11 × 10 6 fibers/g of dry lung) and female subjects (1.38 × 10 6 fibers/g of dry lung), Korean male and female subjects had much less asbestos in their lungs.

Critical discussion on the need for an efficient mineralization procedure for the analysis of plant material by atomic spectrometric methods

A wet digestion method for the mineralization of plant samples was developed. Among other usual means of attack (nitric acid and hydrogen peroxide) it also includes an hydrofluoric acid treatment followed by an evaporation to dryness in order to dissolve elements which may be retained by the insoluble silica residue. Already observed using dry ashing procedures, this particular problem associated with the plant matrix analysis is widely discussed. The proposed total wet digestion method may be applied to plant samples using both classical heating devices (sand-bath) or microwave open systems and its validity was confirmed by analyses by inductively-coupled plasma atomic emission spectrometry and electrothermal atomic absorption spectrometry of several reference plant materials for main, minor and trace elements. Unfortunately, the determination of mercury requires an alternative mineralization procedure because significant losses of Hg are observed during the evaporation to dryness. In this case a simple mineralization with sulphuric acid and hydrogen peroxide using classical or microwave heating devices allows volatilization losses of mercury to be overcome.

Ashing procedure for the determination of metals in petroleum fuels.

Ashing procedure for the determination of metals in petroleum fuels.

The determination of wear metals in used lubricating oils by flame atomic absorption spectrometry using sulphanilic acid as ashing agent

A simple and reliable ashing procedure is proposed for the preparation of used lubricating oil samples for the determination of calcium, magnesium, zinc, iron, chromium and nickel by flame atomic absorption spectrometry. Sulphanilic acid was added to oil samples and the mixture coked and the coke ashed at 550°C. The solutions of the ash were analysed by flame AAS for the metals. The release of calcium, zinc, iron and chromium was improved by the addition of sulphanilic acid to samples. The relative standard deviations of metal concentration results in the initial oil samples were 1.5% for Ca (1500 mg l −1 level), 0.3% for Mg (100 mg l −1 level), 3.1% for Zn (1500 mg l −1 level), 0.7% for Fe (500 mg l −1 level), 0.02% for Cr (50 mg l −1 level) and 0.002% for Ni (10 mg l −1 level). The optimum sample size for efficient metal release was 20 g while the optimum sulphanilic acid to oil ratio was 0.05 g per gram of oil for Zn and Cr and 0.10 g for Ca and Fe. Results obtained by this procedure were highly reproducible and comparable with those obtained for the same samples using standard procedures.

Determination of total iodine in biological material by alkaline ashing and column-switching ion-pair liquid chromatography

A method for the determination of total iodine in biological material has been developed. The method combines alkaline ashing with a selective and sensitive column-switching ion-pair HPLC technique. The ashing procedure which converts the organically bound iodine to iodide is miniaturised and requires only about 100 mg of sample. The first column of the column-switching system is polymer based and can therefore withstand the alkaline pH obtained after ashing. On the analytical column the iodide is separated as an ion-pair with tetrabutyl ammonium hydrogensulphate. The method has been applied to samples from whole blood, urine, liver, lung, carcass, and a sample throughput of at least 50 samples per day can be achieved. Validation studies by spiking experiments showed the precision to be better than 10% R.S.D. for all matrices with recoveries in the range 87–97%. The method has been applied for samples with an iodine content in the range 0.07–1060 μg/g.

Comparison of sample digestion procedures for the determination of arsenic in certified marine samples using the FI-HG-AAS-technique

High-pressure digestion and a closed-vessel microwave heated system, both employing a mixture of nitric acid and hydrogen peroxide as digesting agent, were tested for decomposing the certified samples of BCR 278 mussel tissue (Mytilus edulis) and of BCR 422 cod muscle to determine arsenic by use of FI-HG-AAS. While the microwave system is insufficient to mineralize arsenic in marine samples (arsenic recoveries of 13±10% in BCR 278, 2±1% in BCR 422; n=4), high-pressure ashing at 300 °C results in recovery percentages of 56±15% (n=4) in mussel tissue (BCR 278) and of 25±10% (n=4) in cod muscle (BCR 422). A dry ashing procedure is given as a reference digestion, yielding complete recoveries of arsenic for both materials. The nitrite interference arising during measurement can be entirely overcome by using an amino sulfuric acid concentration of about 350 mmol/L in the solutions for measurement.

Determination of Iodine in Food-related Certified Reference Materials Using Wet Ashing and Detection by Inductively Coupled Plasma Mass Spectrometry

E. H. Larsen and M. B. Ludwigsen, J. Anal. At. Spectrom., 1997, 12, 435 DOI: 10.1039/A607581I

Blood boron concentrations in pregnant rats fed boric acid throughout gestation

Timed-mated Sprague-Dawley rats (28 to 32/group) were exposed to boric acid (BA) in the diet from Gestational Day (GD) 0 to 20. Dietary concentrations of added BA (0%, 0.025%, 0.050%, 0.075%, 0.100%, or 0.200%) yielded average daily intakes equivalent to 0, 3, 6,10, 13, or 25 mg boron/kg body weight/d. Dams and their fetuses were evaluated for evidence of maternal or developmental toxicity, as reported previously. At termination on GD 20, maternal whole blood was collected in heparinized Vacutainer ® tubes, stored frozen (−20°C), and subsequently prepared by a high-temperature alkaline ashing procedure for analysis of boron by inductively coupled plasma optical emission spectrometry. Increasing dietary concentrations of BA were positively associated with whole blood boron concentrations in confirmed pregnant rats, specifically 0.229 ±0.143, 0.564 ± 0.211, 0.975 ± 0.261, 1.27 ± 0.298, 1.53 ± 0.546, or 2.82 ± 0.987 μg boron/g whole blood (mean ±SD) for the control through high-dose groups. Maternal blood boron concentrations were positively correlated with indices of maternal dietary intake of boron and with embryo/fetal toxicity observed at 0.100% and 0.200% BA in the diet reported previously. Thus, blood boron concentrations of 1.27 ± 0.298 and 1.53 ± 0.546 μg boron/g were associated with the no-observed-adverse-effect level (10 mg boron/kg/d) and lowest-observedadverse-effect level (13 mg boron/kg/d) for developmental toxicity reported previously.

Sintering characteristics of low-rank coal ashes

The electrical resistance and compressive strength were measured to gain a better understanding of the sintering characteristics of low-rank coal ashes involved in deposit formation in combustion systems. Low-rank coal ashes were prepared by the standard ASTM ashing procedures at 750°C and then separated into three different particle size fractions. The sinter point determined by the electrical resistance method decreased with decreasing particle size at three different particle size fractions of each coal ash. The compressive strength lest was made as a function of temperature in the range 750–950°C. At a given sintering temperature, strength of the sintered ash was inversely proportional to particle size. For any given particle size of each coal ash, the strength increased with increasing sintering temperature. X-ray diffraction of the sintered coal ashes showed that, as sintering temperature increased, there was an inverse relationship between sinter strength and the amount of anhydrite in the sintered ash, and a direct relationship between strength and the amount of hauyne.

Quantitative analysis of organic wastes: Effects of sample preparation in the determination of metals

Abstract One of the most important analytical problems is the sample preparation to measure the total concentration of elements. Ashing procedure and microwave preparation are two important methods used. Ashing procedure is based in the mineralization of the sample in a furnace for 6 to 8 hours at 400 to 500°C followed by acid dissolution. Microwave digestion is based in an acid attack of the sample favoured by electrothermal radiation for some minutes. Organic residues are often used as soil amendments in agriculture. Some of them like sewage sludge and urban residues have heavy metals in their composition. It is important to measure them in order to establish their adequate use and minimize the potential environmental pollution. In this work, cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) were analyzed in six residues from agriculture (almonds residue), cities (urban refuses from Madrid and Gandia‐Valencia), and three sewage sludge from a tou...

Preconcentration of trace elements in biological materials by dry ashing for TTPIXE analysis A study of matrix effects

Trace elements in biological materials were preconcentrated by dry ashing at 550°C in order to improve the detection limits for thick-target PIXE analyses. The analytical procedure was calibrated with equally prepared ashes of certified biological standard reference materials (SRMs). The matrix effects were studied both theoretically and experimentally. A drawback of the ashing procedure is that volatile elements such as halogenides and sulfur can be partly lost depending on the composition of the material studied. Thick-target PIXE combined with dry ashing is a sensitive and reliable technique for the determination of elements with atomic number > 20 in biological materials with a low ash content.

Determination of metallic elements in crude oil-water emulsions by flame AAS

A method is presented for the determination of V, Ni, Fe, Mg and Na in highly stable crude oil-water emulsions, of which Orimulsion®, a new fuel product from the Venezuelan oil industry, is a typical example. The method is based on a dry ashing procedure for sample preparation which includes a dehydration process in the presence of ethyl alcohol, and determination of elements by flame atomic absorption spectrometry. Interference studies for major components in the matrix have been made using a factorial experimental design. Recoveries for the elements range from 95.8 to 104.3% with relative standard deviations lower than ±4%.

SHORT COMMUNICATION: A comparison of plant tissue digestion methods for nitrogen and phosphorus analyses

Four variations of the Kjeldahl digestion were compared with a H2SO4–H2O2 digestion for N and P analysis, and a microwve digestion method and a dry ashing procedure for P analysis. One Kjeldahl variant using a Na2SO4–Se catalyst was found to be suitable for multi-element analysis while doubling the per diem output of digestions compared with the H2SO4–H2O2 method, provided that P was analyzed by the molybdivanadate (MV) procedure. Key words: Kjeldahl digestion, nitrogen, phosphorus, plant tissue