Compensation of Ca and Na Interference Effects in Axially and Radially Viewed Inductively Coupled Plasmas

The linear relationship between Ca and Na interferences and energy potentials for a wide range of atomic and ionic lines is used to compensate for variable intensity suppression in robust axially and radially viewed inductively coupled plasmas (ICPs). In the axial configuration, intensity attenuation varied from 0 to 40% in the presence of 0.5% Ca and Na. In the radially viewed ICP, interferences were smaller, varying from 5 to 15% for 0.5% Na and from 10 to 30% for 0.5% Ca. Signal attenuation was broadly a linear function of the energies of the spectral lines. These linear functions were estimated by using spectral lines covering the energy range from Sb I 206.833 (5.98 eV), Y II 371.209 and Sc II 361.384 (9.9 eV), Sc II 255.237 (11.42 eV), and Be II 313.042 (13.28 eV) nm. These functions were then used as variable internal standards to compensate for Ca, Na, and mixed Ca–Na matrix effects. Whereas analyte recoveries without compensation varied from about 40 to 90%, they varied mostly from 100 ± 5% when these functions were used. In the presence of Ca and Na, the Mg II 280.270/Mg I 285.213 nm intensity ratio decreased from about 12 for an aqueous solution to 9 at 1.5 kW. Such values confirm that the radially and axially viewed ICPs were robust.     

Inductively-coupled plasma/atomic emission spectrometry of sulfur with a vacuum scanning spectrometer and its application to the analysis of coal liquefaction catalysts

Inductively-coupled plasma/atomic emission spectrometry with a high-resolution vacuum scanning monochromator is described for the determination of sulfur at 180.734 nm. The behavior of the signal-to-background ratio is investigated as functions of RF power, argon gas flow rate and observation height above the load coil. Under the operating conditions selected, the detection limit is 3 μg l^?1. The Se I 196.090-nm line is chosen as internal standard, because the S/Se line pair exhibited the least change with carrier gas flow rate and acid concentration of solution. Sulfur in NiMo and CoMo/ Al₂O₃ catalysts used for coal liquefaction is determined as S(II) and S(VI) species. The total amount of the species agreed well with the sulfur value obtained by the conventinal combustion method.     

Diagnostics in an air inductively coupled plasma

Radial temperature distributions in an air inductively coupled plasma discharge, operated at atmospheric pressure, are calculated from measurements of the absolute intensities of two atomic nitrogen lines (746.9 and 493.5 nm), the first negative band system of the nitrogen molecular ion at 391.4 nm, and the air continuum at 560.0 nm. The radial intensity distribution of the Mg I 285.2 and Mg II 279.6 nm lines are employed with the determined radial temperature distribution to calculate the radial electron number density throughout the normal analytical zone. The temperatures ranged from about 6000 to 10,000 K, and the electron number density varied from 5 × 10¹³ to 2 × 10¹⁶ cm^?3 in the regions above the induction coil where differences of less than 3 fold were observed between experimental and calculated Ca II to Ca I intensity ratios. On the basis of agreement among the measured temperatures and calcium ion-to-atom intensity ratios, the extent of local thermodynamic equilibrium is evaluated.     

Spatial profile measurement of electron number densities and analyte line intensities in an inductively coupled plasma

A versatile instrument for spatial profile measurement has been developed and applied to the measurement of electron number densities and analyte emission intensities in an inductively coupled plasma (ICP). A precise Y-Z stage on which the ICP source was mounted was set on a rail-based optical bench. By translating the ICP source with a precision of ± 0.01 mm, the H_β Stark broadening and analyte line intensities were measured with the use of a silicon intensified target (SIT) and a photomultiplier (PMT). Micro-computer assisted data acquisition allowed it to measure a large number of emission profiles in a short period. The ease of acquisition enabled to build up complete contour maps of electron number densities, Ca neutral atom (Ca I) and Ca ion (Ca II) line intensities, and intensity ratios of the Ca II and Ca I lines. The maximum electron number density was 4 × 10¹⁵ cm^?3 occurring low in the plasma and 5 mm off axis. In a contour map of the electron number densities a hollow region was found low in the plasma, and the distribution pattern looked like a deep “trench”. Along the central channel of the ICP, the peak position of Ca II emission occurred higher than that of Ca I emission, and the spatial distribution of Ca II emission was wider and taller than that of Ca I emission. It has been verified that Ca I is emitted mainly at the region where the electron number density is less than 1 × 10¹⁴ cm^?3.     

Quantitation of sulphur in xylene with an inductively-coupled plasma photodiode-array spectrometer

The application of an inductively-coupled plasma spectrometer with a photodiode-array detection system to the direct quantitation of total sulphur in xylene is described. The emission lines utilized are the near-infrared atomic sulphur (S I) lines at 921.29, 922.81, and 923.75 nm. Structure in the background emission spectrum in this region is attributed to the Philips molecular C₂ band. The best spatial zone to observe is at 12 mm above the load coil. The detection limit for sulphur at the 921.29-nm line is 100 mg l^?1.     

Preconcentration of trace manganese from natural waters by complexation with dithiocarbamate and adsorption onto C18-solid phase extraction column for neutron activation analysis

A method was developed for the preconcentration and separation of trace manganese from natural water samples by complexation with dithiocarbamate followed by adsorption onto C₁₈-solid phase extraction column prior to irradiation. The Mn recovery was better than 99.8% without inteference from iron(III) at 5 mg^.l^-1, copper(II), zinc(II), aluminum(III) and cobalt(II) at 0.5 mg^.l^-1 and sodium(I), potassium(I), magnesium(II) and calcium(II) at 1 mg^.l^-1. The separation factor was 100 and the detection limit was 0.01 μg^.l^-1 with good precision and accuracy with a relative error lower than 3%. The method was applied to the determination of Mn in tap, well, river and treated water samples. This revised version was published online in August 2006 with corrections to the Cover Date.     

Analytical characterization of sample entraining multi-electrode plasma sources for atomic emission spectroscopy–II. Interelement effects and excitation temperature measurements

Interelement effects are reported for a vertical 4-electrode plasma source. The magnitude and direction of the interelement effects are influenced by the experimental conditions. For a 30-mm plasma length, the interference effect of either phosphate or aluminum on calcium emission is insignificant over the entire vertical region of the plasma observed. Ionization interference effects on Ca(II) emission produced by an easily ionized element (EIE) are in general agreement with observations in an inductively coupled plasma (ICP). Nine Fe(I) lines were used to determine the excitation temperature from the slope of a Boltzmann plot. The excitation temperature in the center of the 25-mm plasma at 6mm above the quartz tube is 5800 ± 600 K. As the plasma length increases from 25 to 30 mm, the maximum excitation temperature in the sample aerosol channel increases by 2000 K and changes spatially. The electron number density increases by a factor of 10. Under the assumption of local thermal equilibrium (LTE), the electron number density obtained from the Saha-Eggert equation is about 1.0 × 10¹⁵ cm⁻³ for a 30-mm plasma. As the sample carrier gas flow rate decreases from 0.85 to 0.711/min, the maximum excitation temperature for a 30-mm plasma increases by 1000 K.     

Alpha activation of calcium and its possible use for analysis

This method is based upon the measurement of 3.95-hr⁴³Sc which is formed during α-activation from⁴⁰Ca, the most abundant (96.8%) isotope of calcium. The excitation function for the⁴⁰Ca(α, p)⁴³Sc reaction was determined and the maximum yield of⁴³Sc (about 10⁷ cpm per mg of calcium for a 1-hr irradiation at a beam current of 1 μA) was obtained at an irradiation energy of 14 MeV. The interference free sensitivity of the method at this energy was found to be 8.5·10^?12 g, for a 1-hr irradiation at a beam current of 10 μA. The elements most likely to interfere with the determination are potassium and scandium. The extent of this interference was investigated as a function of irradiation energy and methods to eliminate or subtract the activity formed from these elements are discussed. An attempt was made to determine non-destructively the calcium content of very pure silicon and aluminium and upper limits for the concentration of calcium in these samples were set at 0.27 ppb and 6.9 ppb, respectively. Magnesium, thulium oxide and yttrium oxide samples of known calcium content were also analysed.     

Trace Analysis of Lead and Copper Ions in Fish Tissue Using Paste Electrodes

Abstract

DNA was immobilized onto a carbon nanotube surface through cyclic voltammetry, in which paste electrode (PE) was subjected to lead and copper trace ion analysis. Optimized conditions for square‐wave stripping voltammetry were then searched. The results indicated three other linear working ranges—3–21 mg l^?1, 2–16 µg l^?1, and 3–17 ng l^?1 Pb(II) Cu(II)—within an accumulation time of 190 s in 0.1‐M ammonium phosphate electrolyte solutions of pH 10.0. At the optimized conditions, the detection limit (S/N) was pegged at 0.4 ng l^?1 (1.93×10^?12 M Pb(II) and 6.29×10^?12 M Cu(II)). And the relative standard deviation at 10 mg l^?1 Pb(II) and Cu(II) was a 0.074 and 0.069% precision, in 15 measurements. The method can be applied to assays of fish tissue.     

Study of ionic-to-atomic line intensity ratios for two axial viewing-based inductively coupled plasma atomic emission spectrometers

Using two axially-viewed inductively coupled plasma (ICP) systems that exhibited different behaviors to matrix effects, the sensitivity of the Mg II 280.270 nm/ Mg I 285.213 nm line intensity ratio to the ICP operating conditions and to matrix effects was compared to that observed for alternative ionic-to-atomic line intensity ratios such as the Cd II 226.502 nm/Cd I 228.802 nm, Cr II 267.716 nm/Cr I 357.868 nm, Ni II 231.604 nm/Ni I 232.138 nm, Pb II 220.353 nm/Pb I 217.000 nm, and Zn II 206.200 nm/Zn I 213.857 nm ratios. Both robust and non-robust conditions were used. Some lines behaved differently, in particular the Mg I and Cr I lines, not only as a function of the matrix, but also as a function of the ICP system. The Mg II/Mg I ratio was found to remain a good compromise to follow changes in plasma conditions. The use of several ionic-to-atomic line ratios confirmed that axial viewing leads to matrix effects that are particularly sensitive for atomic lines. The effects cannot be totally suppressed, even under robust conditions, and regardless of the ICP system. An alternative to minimize matrix effects was the use of a buffer such as Cs at 10 g l⁻¹.     

Measurement of calcium and titanium from 47sc by instrumental neutron activation analysis

A procedure is outlined for measuring calcium and titanium from ⁴⁷Sc by instrumental neutron activation analysis. Precise calcium measurements obtained from ⁴⁷Sc are compared with measurements based on ⁴⁷Ca and ⁴⁹Ca. The method is particularly suitable for low levels (ca. 1%) of calcium.     

The determination of sulphate in natural waters by inductively-coupled plasma emission spectrometry

Sulphate is determined simultaneously with other constituents by using inductively-coupled plasma emission spectrometry; the intensity of the 180.73-nm sulphur line is monitored. At a forward power of 1100 W, under compromise conditions, a 3 s detection limit of 0.08 mg l^-1 sulphate and a precision of 0.8% RSD at the 200 mg l^-1 sulphate level were obtained. A small spectral interference from calcium is overcome by software corrections, and good agreement is demonstrated between the proposed method and spectrophotometric sulphate measurements for a variety of natural waters including seawater.     

Differential-pulse anodic stripping voltammetry of mercury with gold-film micro-electrodes

Cylindrical gold film micro-electrodes are easily produced by plasma-sputtering of gold onto carbon fiber electrodes. The micro-electrodes produced were found to maintain their cylindrical geometry indefinitely, unlike gold wire electrodes of similar dimensions. Application of these electrodes in differential-pulse anodic stripping voltammetry provides a method for quantifying trace levels of mercury(II). Up to 100 μg l^?1 Hg(II) the area of the mercury stripping peak varied linearly with mercury concentration; the detection limit was 3.7 μg l^?1. With more than 100 μg l^?1 Hg(II) a new mercury stripping peak grows in at less positive potentials; its peak height is linear with Hg(II) concentration.     

Determination of lead and nickel in animal bone by microwave-induced plasma atomic emission spectrometry with sample introduction by electrothermal vaporization

Sulphuric/nitric acid digestion of the bone precedes introduction of the diluted digest into a helium microwave-induced plasma via a tantalum electrothermal vaporizer. Atomic emission is measured at the Ni I 352.454-nm and Pb I 405.781-nm lines. Detection limits (aqueous solution) are 1.1 × 10^?10 g Ni and 8 × 10^?11 g Pb. The technique was applied to the analysis of IAEA H-5 animal bone reference material.     

A spatial study of electron density and analyte emission in a d.c. argon plasma

Spatially resolved electron density measurements have been performed on a three-electrode d.c. plasma using a linear photodiode array based spectrometer. The electron density values measured are between 1× 10¹⁵ and 1 × 10¹⁶ cm^?3 depending on spatial position. The spatial distribution of Ca I (422.7 nm) and Ca II (393.4 nm) emission has also been measured and the Ca II-Ca I emission intensity ratio evaluated. Using the n_e values measured, an analagous LTE ratio has been calculated and this has been compared to the experimental values. Measured ratios are found to be from 28 to 100 times less than LTE ratios. Some possible sources leading to these infrathermal ratios are discussed.     

Use of stirred tanks for studying matrix effects caused by inorganic acids,easily ionized elements and organic solvents in inductively coupled plasma atomic emission spectrometry

A stirred tank was used for the first time to elucidate the mechanism responsible for inductively coupled plasma atomic emission spectroscopy (ICP-AES) matrix effects caused by inorganic, acids and easily ionized elements (EIEs), as well as organic, ethanol and acetic acid, compounds. In order to gradually increase the matrix concentration, a matrix solution was introduced inside a stirred container (tank) initially filled with an aqueous multielement standard. PolyTetraFluoroEthylene (PTFE) tubing was used to deliver the resulting solution to the liquid sample introduction system. Matrix concentration ranged from 0 to 2 mol l^− 1 in the case of inorganic acids (i.e., nitric, sulfuric, hydrochloric and a mixture of them), from 0 to about 2500 mg l^− 1 for EIEs (i.e., sodium, calcium and mixtures of both) and from 0% to 15%, w/w for organic compounds. Up to 40–50 different solutions were prepared and measured in a period of time shorter than 6–7 min. This investigation was carried out in terms of emission intensity and tertiary aerosols characteristics. The experimental setup used in the present work allowed to thoroughly study the effect of matrix concentration on analytical signal. Generally speaking, the experiments concerning tertiary aerosol characterization revealed that, in the case of inorganic acids and EIEs, the mechanism responsible for changes in aerosol characteristics was the droplet fission. In contrast, for organic matrices it was found that the interference was caused by a change in both aerosol transport and plasma thermal characteristics. The extent of the interferences caused by organic as well as inorganic compounds was compared for a set of 14 emission lines through a wide range of matrix concentrations. With a stirred tank, it is possible to choose an efficient internal standard for any given matrix composition. The time required to complete this procedure was shorter than 7 min.     

Sample preparation for the determination of radiocalcium by accelerator mass spectrometry

An accelerator mass spectrometric (AMS) technique has been developed for determining ⁴¹Ca/⁴⁰Ca ratios at terrestrial levels of 10^?16?10^?14. Calcium is extracted as calcium oxide from bone and rock samples. The calcium oxide is converted into calcium metal and subsequently into calcium hydride in a two-step reduction process. From these calcium hydride samples 5–10 μA of a negative-ion beam of CaH^?₃ is produced. A new gas-ionization detector and a high-resolution velocity detector allow discrimination against ⁴¹K and the stable isotopes of Ca, resulting in background levels of less than 6 × 10^?16. Hence it is possible (for the first time without pre-enrichment) to study the distribution of ⁴¹Ca in the terrestrial environment. The AMS method for ⁴¹Ca determination is applicable to samples of less than 1 g.     

Preconcentration and determination of calcium and magnesium in brine with flow-injection systems

On-line preconcentration on a chelating resin (Dowex A-1) and elution with 0.1 M hydorchloric acid is followed by spectrophotometry based on the metal complexes formed with 1- (2-hydroxy-4-diethylamino-1-phenylazo)-2-hydroxynaphthalene-3,6-disulfonic acid. The total concentration of calcium and magnesium is determined; in a second sample, calcium is masked with a ligand buffer containing excess of barium(II) and EGTA, and magnesium is determined. The calcium concentration is measured by difference. Magnesium (1–30 μg l^?1 and calcium (8– 10 μg l^?1) in 2.5 M sodium chloride can be determined. Calcium and magnesium in analytical reagent-grade sodium chloride and potassium chloride and primary standard sodium chloride are aslo determined. The method based on the exchange between calcium ions and Mg(EDTA) is proposed to enchance the sensitivity for calcium.     

Flow potentiometric and constant-current stripping analysis for silver(I) with carbon- and platinum-fibre electrodes

At concentrations above 50 μg l^?1, silver(I) is determined in nitric acid medium by means of potentiostatic deposition onto a platinum-fibre electrode and subsequent constant-current stripping in the sample or potentiometric stripping in a potassium permanganate medium. Interference from copper(II) is reduced by a pulsed potential procedure whereby copper deposited onto the fibre electrode is reoxidized intermittently. At concentrations below 50 μg l^?1, silver(I) is determined by using a mercury-coated carbon-fibre electrode and constant-current stripping in acetonitrile containing 0.20 M perchloric acid. Potentiostatic deposition for 30 min yielded a detection limit of 0.24 μg l^?1 silver(I) at the 3σ level.     

Flow-injection techniques for the removal of stable-compound interferences in flame atomic absorption spectrometry of calcium

The effects of phosphate and aluminum on calcium in an air/acetylene flame were studied in a flow-injection system. The depressive effect of phosphate was eliminated by operating the nebulizer under starvation conditions with air compensation. Pulse dampers were incorporated into the flow line and the instrument operating conditions were optimized for minimum interference. The depressive effect of aluminum could not be removed in this way. To achieve this, a two-line manifold was used in which the calcium was precipitated as oxalate, retained on a membrane filter and redissolved in hydrochloric acid. Up ot 200 mg l^?1 aluminum could be tolerated with a precision of 2% for 10 mg l^t?1 calcium.