Use of fractional factorial design for optimization of digestion procedures followed by multi-element determination of essential and non-essential elements in nuts using ICP-OES technique

Two digestion procedures have been tested on nut samples for application in the determination of essential (Cr, Cu, Fe, Mg, Mn, Zn) and non-essential (Al, Ba, Cd, Pb) elements by inductively coupled plasma-optical emission spectrometry (ICP-OES). These included wet digestions with HNO₃/H₂SO₄ and HNO₃/H₂SO₄/H₂O₂. The later one is recommended for better analytes recoveries (relative error < 11%). Two calibrations (aqueous standard and standard addition) procedures were studied and proved that standard addition was preferable for all analytes. Experimental designs for seven factors (HNO₃, H₂SO₄ and H₂O₂ volumes, digestion time, pre-digestion time, temperature of the hot plate and sample weight) were used for optimization of sample digestion procedures. For this purpose Plackett-Burman fractional factorial design, which involve eight experiments was adopted. The factors HNO₃ and H₂O₂ volume, and the digestion time were found to be the most important parameters. The instrumental conditions were also optimized (using peanut matrix rather than aqueous standard solutions) considering radio-frequency (rf) incident power, nebulizer argon gas flow rate and sample uptake flow rate. The analytical performance, such as limits of detection (LOD < 0.74 μg g⁻¹), precision of the overall procedures (relative standard deviation between 2.0 and 8.2%) and accuracy (relative errors between 0.4 and 11%) were assessed statistically to evaluate the developed analytical procedures. The good agreement between measured and certified values for all analytes (relative error <11%) with respect to IAEA-331 (spinach leaves) and IAEA-359 (cabbage) indicates that the developed analytical method is well suited for further studies on the fate of major elements in nuts and possibly similar matrices.     

Investigation of four digestion procedures for multi-element determination of toxic and nutrient elements in legumes by inductively coupled plasma-optical emission spectrometry

A simple and reliable multi-element procedure for determination of essential (Cr, Cu, Fe, Mg, Mn, Zn) and toxic (Al, Cd, Pb) elements in legumes by inductively coupled plasma-optical emission spectrometry (ICP-OES) was developed. In this contribution, four different digestion procedures were thoroughly investigated and accurately evaluated with respect to their affect on the analysis of legumes. These included wet digestion with HNO₃/H₂SO₄ and HNO₃/H₂SO₄/H₂O₂, and dry ashing with Mg(NO₃)₂ and Mg(NO₃)₂/HNO₃. Two calibrations (aqueous standard and standard addition) procedures were studied, and proved that standard addition was preferable for all analytes. ICP-OES operating parameters, such as radio-frequency (RF) incident power, sample uptake flow rate and nebulizer argon gas flow rate were optimized. The precision as repeatability, expressed as relative standard deviation (R.S.D.) for aqueous standard containing 250 μg l⁻¹ of each analyte was in the range1.5-8.0%. The accuracy, expressed as relative error was generally varied in the range of 0.5-10% for all analytes, while the quantification limits were lower than 2.5 μg g⁻¹. Although, acceptable results were obtained from all developed procedures, wet digestion method with HNO₃/H₂SO₄/H₂O₂ is recommended for better recovery. The good agreement between measured and certified concentrations with respect to IAEA-331 and IAEA-359 (CRM's supplied by IAEA, International Atomic Energy Agency) indicates that the developed analytical method is well suited for determination of toxic and nutrient elements in legumes and possibly similar matrices.     

Multivariate optimization of the synthesis and of the microwave dissolution of biomorphic silicon carbide ceramics

Biomorphic silicon carbide ceramics are a new class of materials that are used for various industrial applications owing to its attractive properties. The efficiency of the synthesis and the partly extreme properties of the biomorphic ceramic depend decisively on the synthesis parameters and on the impurities of the final ceramic. In the present article the synthesis as well as the decomposition of these materials is optimized using a multivariate methodology for the design of experiments. Three variables (initial amount of Si, infiltration temperature and reaction time) were considered as factors in the synthesis optimization and six variables (digestion time, ramp time, microwave power, volumes of concentrated HF, HNO₃ and H₂SO₄) in the microwave dissolution optimization. Interactions, between analytical factors and their optimised levels were investigated using full factorial, Plackett–Burman and central composite designs. The synthesis parameters that found higher percentage of SiC (quantified by FTIR) and the digestion procedure that found higher concentrations of metals (Co, Cr and Ni, determined by FI-ETAAS) were considered the optimum.     

Radiochemical separation methods for the determination of some toxic elements in biological reference materials

The present paper describes radiochemical separation procedures developed for the determination of the elements As, Cr, Hg, Sb and Se in biological reference materials. The methods chosen utilize ion exchange, retention on inorganic exchangers and solvent extraction techniques for the isolation of the elements of interest. These procedures are more rapid than previously used methods, such as distillation and precipitation. As to the sample dissolution, the following procedures were tested using radioactive tracers and reference materials: treatment of the materials in teflon bombs with a mixture of HNO₃+H₂SO₄ or HNO₃ only in a normal oven, and with HNO₃ in a microwave oven using specially designed digestion bombs.     

Dissolution of Al-Fe materials and analysis by inductively coupled plasma – atomic emission spectrometry

A technique was developed for the dissolution of Al-Fe materials containing difficult to dissolve Al₂O₃. The developed procedure uses HCl and HNO₃ for initial sample attack followed by digestion with a mixture of H₃PO₄ and H₂SO₄ at 200?°C. This procedure was employe to dissolve Al-Fe material samples before the determination of Al and Fe. Minor and trace elements (B, Cr, Cu, Mo, Si, Zr) were determined after dissolution in HCl and HNO₃. Results of a round robin study verified the procedure accuracy. The developed methods have the required accuracy and precision to be used as a quality control procedure for Al-Fe materials analysis.     

Comparison of Sample Digestion Procedures for the Determination of Arsenic in Bottom Sediment Using Hydride Generation AAS

Certified reference materials (JMS-2 and JMS-1 – Marine sediment, LKSD-1 Lake Sediment, and STSD-1 Stream Sediment) and bottom sediment were analysed for arsenic by hydride generation atomic absorption spectrometry (HG-AAS) after digestion by different methods (microwave digestion, digestion in aluminium block, dry digestion) and different combinations of acids (HNO₃, HCl, HClO₄, H₂SO₄). The study revealed that both wet and dry digestion can be used to digest the reference materials and bottom sediment. Exceptionally satisfactory results were produced by the application of aqua regia, HNO₃ + HCl + HClO₄, and HNO₃ + HCl mixtures. Addition of Mg(NO₃)₂ during dry digestion caused an increase in arsenic recovery in the reference materials and improved the accuracy of arsenic determination in the bottom sediments.     

A rapid ultrasound-assisted thiourea extraction method for the determination of inorganic and methyl mercury in biological and environmental samples by CVAAS

A rapid ultrasound-assisted extraction procedure for the determination of total mercury, inorganic and methyl mercury (MM) in various environmental matrices (animal tissues, samples of plant origin and coal fly ash) has been developed. The mercury contents were estimated by cold vapour atomic absorption spectrometry (CVAAS). Inorganic mercury (IM) was determined using SnCl₂ as reducing agent whereas total mercury was determined after oxidation of methyl mercury through UV irradiation. Operational parameters such as extractant composition (HNO₃ and thiourea), sonication time and sonication amplitude found to be different for different matrices and were optimized using IAEA-350 (Fish homogenate), IM and MM loaded moss and NIST-1633b (Coal fly ash) to get quantitative extraction of total mercury. The method was further validated through the analysis of additional certified reference materials (RM): NRCC-DORM2 (Dogfish muscle), NRCC-DOLT1 (Dogfish liver) and IAEA-336 (Lichen). Quantitative recovery of total Hg was achieved using mixtures of 5% HNO₃ and 0.02% thiourea, 10% HNO₃ and 0.02% thiourea, 20% HNO₃ and 0.2% thiourea for fish tissues, plant matrices and coal fly ash samples, respectively. The results obtained were in close agreement with certified values with an overall precision in the range of 5-15%. The proposed ultrasound-assisted extraction procedure significantly reduces the time required for sample treatment for the extraction of Hg species. The extracted mercury species are very stable even after 24 h of sonication. Closed microwave digestion was also used for comparison purposes. The proposed method was applied for the determination of Hg in field samples of lichens, mosses, coal fly ash and coal samples     

Dissolution of Al-Fe materials and analysis by inductively coupled plasma – atomic emission spectrometry

A technique was developed for the dissolution of Al-Fe materials containing difficult to dissolve Al₂O₃. The developed procedure uses HCl and HNO₃ for initial sample attack followed by digestion with a mixture of H₃PO₄ and H₂SO₄ at 200 °C. This procedure was employe to dissolve Al-Fe material samples before the determination of Al and Fe. Minor and trace elements (B, Cr, Cu, Mo, Si, Zr) were determined after dissolution in HCl and HNO₃. Results of a round robin study verified the procedure accuracy. The developed methods have the required accuracy and precision to be used as a quality control procedure for Al-Fe materials analysis. Received: 9 February 1998 / Revised: 1 April 1998 / Accepted: 4 April 1998     

Determination of arsenic,selenium and mercury in an estuarine sediment standard reference material using flow injection and atomic absorption spectrometry

A flow-injection analysis atomic absorption spectrometric (FIA-AAS) method was developed for the determination of trace amounts of arsenic, selenium and mercury in a proposed estuarine sediment standard reference material (SRM 1646a). The samples were prepared in two manners: a) A wet digestion procedure with HNO₃, H₂SO₄, and HClO₄ using a reflux column and b) A microwave-oven digestion procedure utilizing HNO₃, H₂SO₄, and HCl for As and Se, and HNO₃ for Hg. Microwave-oven digestion provides results comparable to those found by reflux column digestion and reduces the sample preparation time by a factor of 10. The proposed method employing the microwave-oven digestion procedure coupled with FIA-AAS for As and Se, and FIA-CVAAS for Hg, has detection limits of 0.15 ng As/ml, O.17 ng Se/ml and 0.15 ng Hg/ml.On leave from the Defense Metallurgical Research Laboratory, Hyderabad, India     

Critical comparison of wet and dry digestion procedures for trace metal analysis of meat and fish tissues

The efficiency of four methods of digestion was evaluated for trace metal analysis of pork meat and carp fish tissues. Two methods of dry and two methods of wet ashing were compared in terms of calculated variances. Mixtures of HCl+HNO₃ were applied for wet ashing of the samples at 100 °C, while dry ashing with or without H₂SO₄ at 450 °C were the alternative methods. The digests were subsequently analysed for Pb, Cd, Cu and Zn by graphite furnace atomic absorption spectrometry. Analysis of variance and Student's t-test were performed separately for meat and fish analytical results. Wet digestion with a (1+1) mixture of HCl+HNO₃ has given better recovery and repeatability for almost all metals than a (9+1) mixture of HCl+HNO₃. Also between the dry ashing methods, the use of H₂SO₄ has given better results than ashing of the tissues without H₂SO₄.     

Assessment of three digestion procedures for Zn contents in Pakistani soil by flame atomic absorption spectrometry

Evaluation of three different digestion procedures for accurate determination of elemental concentration in soils was undertaken. The digestion procedures, two leaching and a total dissolution processes were compared for twenty-one soil samples. The soil standard reference materials (SRMs), IAEA Soil-5 and IAEA Soil-7 were analysed for quality control purposes. Zinc (Zn) was analysed using flame atomic absorption spectrometry (FAAS). Precise analysis was accomplished in the SRM and soil samples, which was better than 4.7% for leaching and total dissolution procedure. Compared with the elemental concentration in soil samples, HF–HClO₄ procedure achieved greater accuracy, where as HNO₃–H₂O₂ and HNO₃–H₂SO₄–HCl procedures were comparable with slight variation in a few samples.     

Comparative study of bulk and partial digestion methods for airborne PM10-bound elements in a high mineral dust urban site in Constantine,Algeria

When high mineral loads in atmospheric particulate matter (PM) are present, particular attention should be paid to the selection of appropriate acidic digestion protocols for wet chemical analysis. We report on a comparative study of elemental recovery yields from five different pre-analytical acid digestion procedures for mineral-rich urban background PM₁₀ samples collected in the city of Constantine (Northeastern Algeria). Five reference materials (NIST 1633b, UPM 1648, NAT-7, SO-2 and SO-4) were also digested according to the same protocols. The selected acidic digestion/extraction procedures are widely used for PM chemical analysis and comprise P1 (HNO₃/HF/HCl), P2 (HCl/HNO₃), P3 (HCl/H₂O₂/HNO₃), P4 (HNO₃/HF/HClO₄) and P5 (HNO₃/H₂O₂); the latter assisted with microwave digestion. Elemental recovery yields were compared for major and trace elements typically determined in PM for source apportionment analysis and the results evidenced large differences. For most elements, the bulk extraction procedures (requiring the use of HF) allowed a full elemental recovery, particularly for elements that are associated with aluminium silicate species and oxides that are resistant to mild acid attack. In contrast, in the extraction protocols without HF low recovery yields were obtained for elements such as Al, Ti, Zr, Sc and other aluminium silicate-related elements in PM₁₀ samples with high mineral dust load. We highlight that the European standard digestion method EN-14902:2005 should be applied specifically for the metals for which this method was developed, but caution should be taken when the analysis of other elements in PM is required, especially in urban areas where road and vehicle wear dust is likely to be a major component of ambient PM. When using wet chemistry analysis for PM source apportionment studies, we strongly recommend HF bulk dissolution of samples to ensure the reliability of the geochemical information when coupled with an appropriate analytical tool.     

Determination of metal concentrations in lichen samples by inductively coupled plasma atomic emission spectroscopy technique after applying different digestion procedures

Three digestion procedures have been tested on lichen samples for application in the determination of major, minor and trace elements (Al, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, V and Zn) in lichen samples collected in Aegean Region of Turkey by inductively coupled plasma atomic emission spectrometer (ICP-AES). The acid mixture of concentrated HNO₃, H₂O₂ and HF were used. The instrument was optimized using lichen matrix considering RF power, nebulizer pressure, auxiliary flow rate and pump rate. The accuracy of the overall analyses was first estimated by analysis of two certified reference materials. Good agreement between measured and reference values were found for almost all elements. As the second way of determining the accuracy, results obtained from independent analytical techniques (ICP-AES and instrumental neutron activation analysis (INAA)) were compared for all elements by analyzing real samples. Correlation coefficients of two techniques for the elements ranged between 0.70 (Mg) and 0.96 (Fe). Among the three digestion systems, namely microwave, open vessel and acid bomb, microwave digestion system gave the best recovery results. The method detection limit (MDL) was computed using reagent blanks of microwave digestion system since it provides cleaner sample preparation. Detection limit is adequate for all elements to determine the elements in lichen samples. The precision was assessed from the replicate analyses of reagent blanks of microwave digestion system and was found to be less than 1.5% relative standard deviation (R.S.D.).     

Determination of arsenic, antimony, bismuth, selenium and tin in biological and environmental samples by continuous flow hydride generation ICP-AES without gas-liquid separator

Summary A simple continuous flow hydride generation system without conventional gas-liquid phase separator was developed for the determination of As, Se, Sb, Bi and Sn in biological and environmental samples by sequential ICP-AES with 1.5 kW power. The interchange of operating mode from normal solution nebulization to hydride generation or vice versa can easily be done without interrupting the plasma in this system.Two digestion methods were compared, i.e. the pressurized digestion with HNO₃ in a closed vessel and HNO₃/HClO₄/H₂SO₄ acid digestion in an open system. It was found that further treatment is necessary after normal pressurized digestion for As determination in marine samples, e.g. mussel. Interferences, especially by copper and nickel were examined and completely eliminated up to 10 g/ml by using a mixed reductant (3% NaBH₄ and 2% KI) with lower flow rate as well as the sample solution media of 30% HCl (v/v) and 20% HNO₃ (v/v). The effect of KI on the elemental oxidation states of As, Se and Sb is discussed. The accuracy of the method was validated by the analysis of a number of biological and environmental SRM's of NIST, BCR and NIES. Most results were in agreement with the certified values or reference values. The detection limits for these elements were in the range of 0.x ng/ml.On leave from Shanghai Institute of Metallurgy, Academia Sinica, Shanghai, China 200050     

Evaluation of high-pressure microwave digestion methods for hydride generation atomic absorption spectrometric determination of total selenium and arsenic in sediment

Five closed-vessel microwave digestion methods were compared for the accurate determination of arsenic and selenium in NIST SRM 1645 River Sediment by flow-injection hydride-generation atomic absorption spectrometric methods. The digestion methods using five different acid mixtures (HNO₃/ H₂SO₄, HNO₃/HCl0₄, HNO₃/HCl, HNO₃/HCl/HF, HNO₃/H₂SO₄/HClO₄) were all found to be reliable for the determination of the analytes. Taking into consideration the safety and suitability for the analysis of other metals, the methods based on the use ofaqua regia are recommended for closed vessel microwave digestion with pressure control. Using the quick digestion program, the presence of up to 10% organic content in soil samples did not adversely affect the closed vessel digestion and did not cause the loss of volatile analytes. After digestion, opening the vessel under an inner pressure of below 345 kPa (50 psi) had no effect on the accuracy of the results. The recommended digestion methods (HNO₃/HCl and HNO₃/ HCl/HF) for the reliable determination of arsenic and selenium in different sediment samples were demonstrated. The calculated detection limits (3 _b ) were less than 0.030 g/g and 0.033 g/g for arsenic and selenium, respectively. All analytical results for arsenic and selenium in SRM 1645 River sediment, NRCC BCSS-1 Marine Sediment and NIES CRM Pond Sediment were within or near the certified and reported ranges, with the exception of selenium in NIES CRM No. 2 Pond Sediment.     

Determination of 36 elements in plant reference materials with different Si contents by inductively coupled plasma mass spectrometry: comparison of microwave digestions assisted by three types of digestion mixtures

Closed-vessel microwave digestion of nine standard reference plant materials (NIST, BCR, IAEA) and a laboratory standard of plant material with different Si contents assisted by HNO₃ + H₂O₂ (procedure A), HNO₃ + H₂O₂ + HF + H₃BO₃ (procedure B) and HNO₃ + H₂O₂ + HBF₄ (procedure C) were used to determine the recovery of 36 elements by ICP-MS: Ag, Al, As, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, In, La, Li, Mn, Mo, Nd, Ni, Pb, Pr, Rb, Sb, Se, Sn, Sr, Th, Tl, U, V, W, Y, Zn. Additions of HF + H₃BO₃ and HBF₄ in procedures B and C exceeded by 10% (B1, C1) and 100% (B2, C2) the equivalent concentrations of Si in the samples determined by ICP-OES. Most recoveries of certified elements (e.g., Al^*, Cu, Mo^*, Rb^*, Sb^*, Th) decreased significantly (^*p ≤ 0.05) with increasing Si content in plant reference materials digested by procedure A, while the recoveries from procedures B and C decreased insignificantly only for Mo and Sb. Digestions B and C gave significantly higher recoveries of Al, Sb, W and REEs, which were tighter to the reference values of these elements. A similar effect was found for Cu, Fe, Li, Ni, Sn, Th, Tl, V, Zn, Ba, Rb and Sr recoveries in samples with Si contents exceeding 2000 μg g⁻¹. If the Si content in plant samples is less than 10 mg g⁻¹, digestion of 0.5 g of plant samples through 0.05 mL of HF and 0.5 mL of 4% H₃BO₃ or 0.1 mL of HBF₄ is recommended to get satisfactory results for most of the elements. For materials with Si content exceeding 10 mg g⁻¹ the weight of the sample for digestion should be reduced to 0.25 g. However, the operation of potential interferences should be taken into account and eliminated through correction equations and adequate dilution of the samples.     

Selective extraction of Hg(II) from aqueous mineral acid solution containing iodide ions using 2-benzylpyridine in benzene

A method for the selective extraction of mercury has been developed. The extraction of Hg(II) by 2-benzylpyridine (BPy) in benzene from dilute mineral acid solution containing iodide ions has been investigated, and variables such as concentration of acids, iodide and the extractant have been optimized. The optimum conditions for the extraction of Hg(II) by 0.1M BPy/benzene are: 0.01M (HCl, HNO₃, H₂SO₄)+0.01M KI. The distribution coefficients and separation factors of 19 elements relative to Hg(II), have been reported. Effect of anions such as ascorbate, acetate, citrate, oxalate and thiosulfate has also been studied. The method developed could find useful applications in selective extraction of small amounts of mercury from environmental samples.     

Statistical mixture design development of digestion methods for Oyster tissue using inductively coupled plasma optical emission spectrometry for the determination of metallic ions

The quantitative determination of chemical elements in organic or biological samples is an important analytical problem. Normally the elements to be determined in the organic matrix must be transformed into a simple inorganic form. A digestion method by heating on a block digestor has been developed for the determination of Al, As, Ba, Ca, Cd, Co, Cu, Fe, Mg, Mn, V and Zn in Oyster tissue by ICP OES. A simplex centroid statistical mixture design has been used to study the effects of changing HNO₃, HCl and H₂O₂ reagent proportions on the digestion of these samples. Response surface and principal component analyses show that the species Ca, Cd, Cu, Fe, Mg, Mn and Zn have very similar analytical tendencies under this experiment. By means of mixture modeling maximum recoveries for these ions were predicted using 19%, 18% and 63% of the HCl, HNO₃ and H₂O₂ pseudocomponent mixtures, respectively. This corresponds to 21.4%, 30.8% and 47.8% of the HCl, HNO₃ and H₂O₂ commercial solutions. Furthermore the As, Co and V ions present large recoveries for these mixtures as well. The Al and Ba ion recoveries are seen to be independent of the mixture proportions. The analysis of Oyster tissue reference material (SRM 1566b - NIST) under optimized conditions at the selected wavelengths resulted in ion recoveries between 90% and 100%.     

Decomposition of organoarsenic compounds for total arsenic determination in marine organisms by the hydride generation technique

The conditions necessary for the complete decomposition of six organic arsenic compounds, namely methylarsonic acid (MMAA), dimethylarsinic acid (DMAA), trimethylarsine oxide, tetramethylarsonium iodide, arsenocholine bromide (AsC) and arsenobetaine (AB), were investigated. The degree of decomposition of the arsenic compounds was monitored using a hydride generation (HYD) technique, because the response from this system depends strongly on the chemical species of arsenic, with inorganic arsenic (the expected product from these decomposition experiments) giving a much more intense HYD signal than the organic arsenic compounds. The arsenic compounds were decomposed by heating them with three types of acid mixture, namely HNO₃? HClO₄, HNO₃? HClO₄? HF, or HNO₃? HClO₄? H₂SO₄. Both MMAA and DMAA were decomposed completely using any of the mixed acids at a decomposition temperature of 200 °C or higher. The HNO₃? HClO₄? H₂SO₄ mixture was the most effective for decomposing AsC and AB, which are the most difficult compounds among all types of organic arsenic compound to decompose and render inorganic. The complete decomposition of AB was only achieved, however, when the temperature was 320 °C or higher, and the sample was evaporated to dryness. When the residue from this treatment was examined by high‐performance liquid chromatography combined with inductively coupled plasma atomic emission spectrometry, all of the arsenic was found to be present as arsenic(V). The optimized conditions (HNO₃? HClO₄? H₂SO₄ at 320 °C) for decomposing AB were then used to determine the total amount of arsenic in marine organisms known to contain AB. Copyright © 2005 John Wiley & Sons, Ltd.     

Multivariate technique for optimization of digestion procedure by focussed microwave system for determination of Mn, Zn and Fe in food samples using FAAS

This article describes the development by response surface methodology (RSM) of a procedure for iron, zinc and manganese determination by flame atomic absorption spectrometry (FAAS) in food samples after digestion employing a focussed microwave system. A Doehlert matrix was used to find optimal conditions for the procedure through response surface study. Three variables (irradiation power and time and composition of oxidant solution—HNO₃ + H₂O₂) were regarded as factors in the optimization study. The working conditions were established as a compromise between optimum values found for each analyte taking into consideration the robustness of the procedure. These values were 12 min, 260 W and 42% (v/v) for irradiation time, irradiation power and percent of H₂O₂ in solution, respectively. The accuracy of the optimized procedure was evaluated by analysis of certified reference materials and by comparison with a well-established closed vessel microwave dissolution methodology.