A novel on-line preconcentration method for trace molybdenum determination by USN–ICP OES with biosorption on immobilized yeasts

A new system for on-line preconcentration of molybdenum by sorption on a minicolumn associated to inductively coupled plasma — optical emission spectrometry with ultrasonic nebulization was studied. It is based on the sorption of molybdenum on a column packed with immobilized baker's yeasts on controlled pore glass without further complexing reagent. The molybdenum preconcentrated by biosorption was subsequently eluted with hydrochloric acid. Considering a sample flow rate of 5.0 mL min^− 1, 10 mL of sample was preconcentrated in 2 min achieving a sensitive total enhancement factor of 480-fold, and the detection limit (3 s) obtained was 21 ng L^− 1. Additionally, the calculated precisions expressed as percent relative standard deviation (RSD%) was 1.9%.Satisfactory results were obtained for the determination of molybdenum in standard reference material NIST 1643e Trace Elements in Water and real water samples.     

Atomization Characteristic of Chromium on a Molybdenum Tube Atomizer by Electrothermal Atomic Absorption Spectrometry and Determination of Chromium in Biological Materials

Atomization characteristics of chromium have been studied by electrothermal atomic absorption spectrometry with a molybdenum tube atomizer. The appearance temperature (T_app) of chromium nitrate was 1300°C. TheT_appwas independent of the heating rate of atomizer, but the temperature at the peak of the Cr AA signal increased with the heating rate. A sensitive absorption chromium signal was obtained in pure argon purge gas. The chromium signal decreased as the ratio of hydrogen in the purge gas increased. The optimal gas flow rate was Ar 480 ml min⁻¹+ H₂20 ml min⁻¹because of the avoidance of oxidation of the atomizer. The absolute characteristic mass (the mass of element giving 0.0044 abs.) of chromium by the atomizer was 0.35 pg and the detection limit was 23 pg ml⁻¹(3S/N). The interferences caused by large amounts of interferents were evaluated. The addition of thiourea served to eliminate the severe interferences. The accuracies of the recommended method were considered almost satisfactory for the determination of chromium in biological materials, compared with the certified values of NIST materials. The recovery of spiked chromium in biological materials was in the range from 93.5 to 102%.     

Synthesis of amberlite XAD-2-PC resin for preconcentration and determination of trace elements in food samples by flame atomic absorption spectrometry

A new chelating sorbent has been developed using Amberlite XAD-2 resin anchored with pyrocatechol through –N=C– group. This sorbent, characterised by elemental analysis and infrared (IR) spectra, was used as packing for the minicolumn in an on-line system preconcentration system for cadmium, cobalt, copper and nickel determination. Metal ions were sorbed in the minicolumn, from which it could be eluted directly to the nebulizer–burner system of the flame atomic absorption spectrometer (FAAS). Elution of all metals from minicolumn can be made with 0.50 mol L^− 1 HCl or HNO₃. The enrichment factors obtained were 16 (Cd), 24 (Co), 15 (Cu) and 19 (Ni), for 60 s preconcentration time, and 39 (Cd), 69 (Co), 36 (Cu) and 41 (Ni), if used 180 s preconcentration time. Under the optimum conditions, the proposed procedure allowed the determination of cadmium, cobalt, copper and nickel with detection limits of 0.31, 0.32, 0.39 and 1.64 μg L^− 1, respectively, when used preconcentration periods of 180 s. The accuracy of the developed procedure was sufficient and evaluated by the analysis of the certified reference materials NIST 1515 apple leaves and NIST 1570a spinach leaves. The method was applied to the analysis of food samples (spinach, black tea and rice flour).     

Factorial design for multivariate optimization of preconcentration system for spectrophotometric phosphorus determination

The present paper proposes a preconcentration procedure for phosphorus determination by using ultraviolet-visible spectrophotometer. It is based on the formation of phosphomolybdate and its reduction to molybdenum blue. Phosphorus extraction as phosphomolybdenum blue complex was performed onto Amberlite XAD-4. The optimization step was carried out using two-level full factorial design. Three variables (resin amount, sample volume, flow rate) were regarded as factors in the optimization. The relative standard deviation was 2% at 0.08 μg mL⁻¹. The limit of detection was found to be 2.23 μg L⁻¹ (N = 15). The proposed solid-phase extraction procedure was applied to phosphorus in some fruit leaves, natural waters, and a standard reference material (SRM 1515 apple leaves).     

The determination of molybdenum in water and biological samples by graphite furnace atomic spectrometry after polyurethane foam column separation and preconcentration

A system for molybdenum separation and enrichment aiming its determination in water and biological samples by graphite furnace atomic absorption spectrometry (GFAAS) is proposed. The procedure is based on the sorption of the molybdenum (VI) thiocyanate complex onto a mini-column packed with polyurethane foam (PUF). The elution is accomplished by a 3.0 mol l⁻¹ nitric acid solution. Flow variables were optimized and an enrichment factor of 10 as well as a limit of detection (LOD) (3 s) of 0.08 μg l⁻¹ in the sample solution were achieved. The coefficient of variation showed values of 3 and 2% for molybdenum solutions of 2.0 and 10.0 μg l⁻¹, respectively. The accuracy of the method was confirmed by the good concordance between found and certified values in the analysis of certified reference materials (CRMs) (CASS-3 Nearshore Seawater, NIST 1547 Peach Leaves, NIST 1515 Apple Leaves and NIST 1572 Citrus Leaves). The procedure was also applied for the molybdenum determination in mineral waters as well as in produced water samples. The results obtained for the mineral water samples compared well with those obtained by ICP-MS. Concerning the produced water samples, in spite of their large salinity, recoveries of 90 to 120% at the 1 μg l⁻¹ were observed.     

Ultra-trace determination of iodine in sediments and biological material using UV photochemical generation-inductively coupled plasma mass spectrometry

Several sample preparation techniques have been evaluated for the determination of iodine using UV-photochemical generation-quadrupole inductively coupled plasma mass spectrometry. Thermal decomposition of samples at 1000 °C followed by capture of the liberated iodine in dilute acetic acid permitted subsequent UV-photochemical generation of a volatile iodine species that serves to enhance sensitivity 25-fold over conventional solution nebulization, delivering reagent blank detection limits of 8.75 pg g^{–1 127}I and 0.075 pg g^–1 129I for solid samples (400 mg test mass). The methodology was validated through determination of total iodine in several Standard Reference Materials, including NIST 1572 Citrus leaves, NIST 1549 Non-fat milk powder, NIST 1566a Oyster tissue and NIST 2709 San Joaquin Soil. Liberation of iodine from samples and its collection as well as photochemical generation were quantitative, permitting calibration to be achieved using standards prepared in dilute acetic acid.     

Optimisation of focused-microwave assisted digestion procedure for Kjeldahl nitrogen determination in bean samples by factorial design and Doehlert design

In the present paper a focused-microwave Kjeldahl digestion procedure without metal catalyst for nitrogen determination in bean samples was developed. Temperature at which the decomposition plateau occurs, mass of potassium sulphate and either volume of sulphuric acid or hydrogen peroxide were optimised. Results of the two-level full factorial design (2⁴) based on an analysis of variance demonstrated that only the decomposition plateau temperature and the sulphuric acid volume were statistically significant. Optimal conditions for the digestion of bean samples were obtained by using Doehlert design. The modified digestion procedure of 0.25 g of bean samples has been performed in 27 min at optimised conditions. The accuracy of the developed procedure by the analysis of the two certified reference materials, peach leaves (NIST 1547) and apple leaves (NIST 1515). The t-test applied to the results revealed that they are in agreement (p > 0.05) with the certified values. The precision, expressed as relative standard deviation (R.S.D.) was of 0.96% for four successive Kjeldahl nitrogen determinations. In addition, interlaboratory exercises were performed with several bean samples in reference Brazilian food control laboratory.     

Application of Doehlert designs for optimisation of an on-line preconcentration system for copper determination by flame atomic absorption spectrometry

In the present paper, a system for on-line preconcentration and determination of copper by flame atomic absorption spectrometry (FAAS) was developed. It was based on solid phase extraction of copper(II) ions on a minicolumn of Amberlite XAD-2 loaded with 2-(2-thiazolylazo)-5-dimethylaminophenol (TAM). The optimisation process was carried out using Doehlert designs. Four variables (sampling flow rate, SR; elution flow rate, buffer concentration, BC; and pH) were regarded as factors in the optimisation. The parameter “sensitivity efficiency (SE)” proposed in this paper, and defined as the analytical signal obtained for an on-line enrichment system for a preconcentration time of 1 min was used as analytical response in the optimisation process. Using the established experimental conditions, the proposed on-line system allowed determination of copper with detection limit (3σ/S) of 0.23 μg l⁻¹, and a precision (repeatability), calculated as relative standard deviation (R.S.D.) of 3.9 and 3.7% for copper concentration of 5.00 and 20.00 μg l⁻¹, respectively. The preconcentration factor obtained is 62. The recovery achieved for copper determination in presence of several cations demonstrated that this has enough selectivity for analysis of food samples. The robustness of the proposed system was also evaluated. The accuracy was confirmed by analysis of the following certified reference materials (CRMs): Rice flour NIES 10a, Spinach leaves NIST 1570a, Apples leaves NIST 1515 and Orchard leaves NBS 1571. This procedure was applied for copper determination in natural food samples.     

Matrix modification by Cu(NO3)2 for the determination of gold by electrothermal atomic absorption spectrometry with a molybdenum tube atomizer

Matrix modification by copper nitrate in electrothermal atomic absorption Spectrometry (ETAAS) of gold with a molybdenum tube atomizer has been investigated. The addition of copper nitrate served to eliminate the interferences from 10⁴–10⁵-fold concentrations of foreign elements at the 890 °C pyrolysis temperature. The absolute characteristic mass (giving 0.0044 absorbance) of gold in the presence of copper nitrate with the Mo atomizer was 0.26 pg and the detection limit was 38 pg/ml. These values were several times better than those obtained with graphite atomizers. The recovery of spiked gold in biological materials was in the range 96–106%. A sensitive and accurate ETAAS method was developed for complex matrix samples.     

Effect of simultaneous cooling on microwave-assisted wet digestion of biological samples with diluted nitric acid and O2 pressure

The present work evaluates the influence of vessel cooling simultaneously to microwave-assisted digestion performed in a closed system with diluted HNO₃ under O₂ pressure. The effect of outside air flow-rates (60–190 m³ h⁻¹) used for cooling of digestion vessels was evaluated. An improvement in digestion efficiency caused by the reduction of HNO₃ partial pressure was observed when using higher air flow-rate (190 m³ h⁻¹), decreasing the residual carbon content for whole milk powder from 21.7 to 9.3% (lowest and highest air flow-rate, respectively). The use of high air flow-rate outside the digestion vessel resulted in a higher temperature gradient between liquid and gas phases inside the digestion vessel and improved the efficiency of sample digestion. Since a more pronounced temperature gradient was obtained, it contributed for increasing the condensation rate and thus allowed a reduction in the HNO₃ partial pressure of the digestion vessel, which improved the regeneration of HNO₃. An air flow-rate of 190 m³ h⁻¹ was selected for digestion of animal fat, bovine liver, ground soybean, non fat milk powder, oregano leaves, potato starch and whole milk powder samples, and a standard reference material of apple leaves (NIST 1515), bovine liver (NIST 1577) and whole milk powder (NIST 8435) for further metals determination by inductively coupled plasma atomic emission spectroscopy (ICP-OES). Results were in agreement with certified values and no interferences caused by matrix effects during the determination step were observed.     

Some observations on the use of a new matrix modification electrothermal atomization-atomic absorption spectrophotometric method for the determination of micro- and sub-microgram amounts of molybdenum in human teeth

A very sensitive and selective electrothermal atomization-atomic absorption Spectrophotometric (ETA-AAS) matrix-modified method for the determination of micro- and submicrogram amounts (0–25 μ g⁻¹ molybdenum) in whole human one-rooted teeth has been developed. Hydrazine sulfate, (NH₂)₂ · H₂SO₄, which has been used as a matrix modifying reagent (MMR) is found to be very influential in removing matrix interference effects such as calcium which is present in hydroxy apatite (the main mineral constituent of tooth), at pH 2.0–2.2. Beer's law is obeyed over the range 0–1.5 ng molybdenum 5/μl injected solution. The absolute sensitivity and detection limit of the method are respectively 6.46 and 1.32 pg molybdenum/5 μl injected sample solution. The recovery percentage and RSD% are also determined. Compared to the neutron activation method, the proposed method is rapid, more available, and less expensive and requires no grinding of the tooth with metals, mortar, or mills. It is more sensitive and simpler than flame techniques. Background correction is not necessary. No separation or preconcentration of molybdenum is required. The method has been applied for the determination of molybdenum in teeth taken from representative districts of Baghdad.     

An on-line cloud point extraction system for flame atomic absorption spectrometric determination of trace manganese in food samples

The development of an on-line preconcentration system with cloud point extraction for the determination of manganese is described. The system was used to determine manganese levels in food samples using flame atomic absorption spectrometry (FAAS). All steps of the cloud point extraction procedure were performed on-line, from the mixing of reagents to detection. The manganese ions are complexed in a mixture of the reagent 2-[2′-(6-methyl-benzothiazolylazo)]-4-bromophenol (Me-BTABr) and Triton X-114. The components are retained on a minicolumn and then desorbed with eluent acid to subsequent detection of manganese by FAAS. Under the optimized conditions, the method presented a detection limit of 0.7 μg L^− 1 and an enrichment factor of 17 to a volume of 3000 μL. The sampling frequency was 30 h^− 1. The accuracy of the method was tested by evaluating the amount of Mn in certified reference materials (apple leaves NIST 1515 and spinach leaves NIST 1570a). The proposed procedure was applied to food samples (shrimp powder, flaxseed flour, wheat flour, soy flour and oat), and the results agreed with those obtained by the determination of Mn in foods by atomic absorption spectrometry with electrothermal atomization (ETAAS).     

Sensitive and selective determination of molybdenum by flow injection chemiluminescence method combined with controlled potential electrolysis technique

A sensitive and selective flow injection chemiluminescence (CL) method combined with controlled potential electrolysis technique was described for the determination of molybdenum. The method is based on the chemiluminescence reaction of luminol with unstable molybdenum(III) in alkaline solution. The molybdenum(III) was on-line reduced from molybdenum(VI) via controlled potential electrolysis technique using a homemade flow-through carbon electrolytic cell at the potential of −0.6 V (versus Ag/AgCl). The method allows the determination of molybdenum in the 5.0×10⁻¹⁰ to 5.0×10⁻⁷ g ml⁻¹ range with a limit of detection (3σ) of 5×10⁻¹¹ g ml⁻¹ molybdenum. The relative standard deviation is 2.6% for the 1.0×10⁻⁹ g ml⁻¹ molybdenum solution in 11 repeated measurements. This method was successfully applied to the determination of molybdenum in water samples.     

Selective Determination of Nickel in Biological Samples by High Performance Liquid Chromatography

A highly selective method for the determination of trace amounts of nickel(II) by high performance liquid chromatography was developed. 2-[(2-Hydroxyphenyl)azo]-4,5-diphenylimidazole (HAI) was used for pre-column derivatization of nickel(II) in reversed-phase chromatographic separation followed by spectrophotometric detection. In the presence of nickel(II), iron(III), cobalt(II), copper(II), cadmium(II), zinc(II), manganese(II), aluminum(III) and vanadium(V), only nickel(II) chelate with HAI gave a resolved peak in chromatograms with a C8-bonded reversed phase column and a 45% (w/w) acetonitrile-water mobile phase containing 1.0 × 10⁻⁴ mol kg⁻¹ ethylenediaminetetraacetic acid and 5.0 × 10⁻³ mol kg⁻¹ sodium acetate (pH 7.5). The nickel(II) chelate was detected spectrophotometrically at 585 nm. When 100 µL of a test solution was injected, the calibration graph was linear up to 240 pg for nickel(II), and the detection limit defined as three times the standard deviation of the reagent blank was 0.8 pg at 0.001 absorbance unit full scale. The proposed method was applied to the analysis of rice, tea leaves and mussels.     

Low level determination of thallium in biological and environmental reference materials by RNAA using several counting methods

A new RNAA procedure was developed capable of low level determination of thallium in biological and environmental samples. After high fluence neutron irradiation in a nuclear reactor, wet ashing of samples and T1(I) separation by solvent extraction with sodium diethyldithiocarbamate at pH 13, several types of counting were employed to compare their detection limits and to utilize the self-validation principle of NAA. The following measurement modes were used: High efficiency counting of -rays of²⁰²T1 and Hg X-rays produced on decay of²⁰⁴T1 using a well-type HPGe detector, combined ^– ray and ^–-counting of²⁰⁴T1 with the aid of a HPGe planar detector, and liquid scintillation counting and counting of Cerenkov radiation of ^–-particles of²⁰⁴T1. The lowest detection limit of 0.034 ng of T1 was achieved on liquid scintillation counting of²⁰⁴T1. The method was applied for the analysis of biological NIST SRMs 1515, 1573a, 1577b and environmental NIST SRM 1633a. Good agreement was found between the thallium certified value in SRM 1633a and values determined in this work by all counting modes. For SRM 1573a, results in agreement were obtained by two counting modes, while counting of Hg X-rays of²⁰⁴T1 was only used for SRMs 1515 and 1577b.     

Characterization and use of the new NIST rapid pneumatic tube irradiation facility

Recently a new rapid pneumatic tube facility was inserted into a long unused location in the NIST 20 MW nuclear reactor. This facility was designed and constructed specifically for rapid INAA using short lived activation products. Included is a computer controlled console which uses fast sensors to accurately measure the irradiation capsule flight time, and a loss-free counting system connected to a 32% efficient PHGe detector with a transistor reset preamplifier. Measurement of travel, times from end-of-irradiation to detector were 473±8 ms. Measurement of the thermal neutron fluence rate was 5.0·10¹³ n·cm⁻²·s⁻¹. The other three pneumatic tubes in the NIST reactor have transfer times of 3 to 15 seconds, and no timing capability more accurate than human response. This new facility substantially improves our ability to accurately determine activation products with half-lives from 1 to 100 seconds. Characterization information reported on this new irradiation facility includes absolute fluence measurements, fluence rate variations within the capsule and variations with time, and determination of analytical sensitivities for fluorine-20 selenium-77m, and silver-110g.     

Analysis of non-organic elements in plant foliage using polarised X-ray fluorescence spectrometry

An X-ray fluorescence (XRF) method for the rapid and non-destructive analysis of 30 non-organic elements in plant leaves over five orders of magnitude concentration from several percentage of dry weight to sub-milligram per kilogram, is described. There is a growing need for a simple method of monitoring non-organic trace elements in plant material, especially those which accumulate in soils with application of fertilisers, both inorganic and biosolids. Earlier attempts to use XRF for the analysis of plant material suffered from sensitivity loss due to the high background of organic matrices caused by scattering of the X-ray source. We overcome this by using polarised X-ray sources in a Cartesian geometrical arrangement with sample and energy dispersive detector (EDPXRF), a configuration which can achieve an order of magnitude reduction in background compared with unpolarised sources. Further sensitivity gains are made using a high power tube as a source of primary X-rays and for some analytes employing secondary targets for near-monochromatic excitation. Sample preparation is simple involving only pulverising and briquetting of dried samples. Accuracy is evaluated by comparing data obtained for the suite of NIST plant leaf samples (NIST 1515 apple leaves, NIST 1547 peach leaves, NIST 1570a spinach leaves, NIST 1573a tomato leaves and NIST 1575a pine needles) and tobacco leaf standards CTA-OTL-1 and CTA-VTL-2. The data compare well over the whole concentration range and the method provides a rapid analytical tool for monitoring potentially toxic trace elements in all types of foliage with adequate sensitivity for many purposes.     

Determination of silver in biological reference materials by neutron activation analysis

Silver in selected, predominantly biological, reference materials (NIST SRM 1515, 1547, 1549, 1566a, 1571, 1577b, 2704, CTA-OTL-1, and Bowen’s Kale) was determined using neutron activation analysis (NAA) in two different analytical modes: instrumental NAA with epithermal neutrons (ENAA), and NAA with radiochemical separation (RNAA). The ENAA mode was based on long-time 5-hour irradiation of samples in a special Cd lined box with counting after 8-month decay. The RNAA procedure consisted in 20-hour irradiation of samples, their decomposition/dissolution by alkaline-oxidative fusion, and precipitation of AgCl including several purification steps. Both methods provided Ag contents in the analyzed reference materials consistent with certified and/or literature values down to the ng·g⁻¹ level.     

Extractive spectrophotometric determination of molybdenum(V) in molybdenum steels

An extraction spectrophotometric method has been developed for the determination of traces of molybdenum present in molybdenum steels which is based on the extraction of the orange-red molybdenum-thiocyanate-acetonethiosemicarbazone complex into chloroform from hydrochloric acid medium. The complex has an absorption maximum at 472 nm with a molar absorptivity of 1.9 × 10⁴ liters mol⁻¹ cm⁻¹. Beer's law is valid over the concentration range 0.1–9.5 ppm of molybdenum with an optimum concentration range of 0.4–9 ppm. The equilibrium shift method indicates 1:4:2 composition for molybdenumthiocyanate-acetonethiosemicarbazone complex. The effect of acidity, reagent concentrations, temperature, and interferences from various ions are reported.     

A micro-XRF spectrometer based on a rotating anode generator and capillary optics

The analytical characteristics of a laboratory-scale micro-X-ray fluorescence spectrometer, based on a rotating anode X-ray generator and capillary optics, are described. Usually, a microbeam 15 μm in diameter, derived from a copper or molybdenum anode operated at 45 kV, is used for sample irradiation. Elemental yields around the 1 count s⁻¹(μg cm⁻²)⁻¹ level are obtained, corresponding to absolute detection limits for thin samples in the 0.05–1 pg range and to relative MDL levels of 3–10 ppm for thick organic samples. The use of the instrument for studying (trace) element migration in Roman glass and for the nondestructive analysis of decorations on Japanese 18th century porcelain vases is described.