Tungsten coil devices in atomic spectrometry: absorption, fluorescence, and emission.

In this work, tungsten coil (W-Coil) devices are used as atomizers for electrothermal atomization atomic absorption spectrometry (ETAAS), electrothermal atomization laser excited atomic fluorescence spectrometry (ETA-LEAFS), and electrothermal vaporization inductively coupled plasma atomic emission spectrometry (ETV-ICP-AES). For most cases in ETAAS and ETA-LEAFS, limits of detection (LODs) using the W-Coil are within a factor of ten of those observed with commercial graphite furnace systems. LOD for Cd by W-Coil AAS is 10 pg, while LODs for As, Se, Cr, Sb and Pb by W-Coil LEAFS are 950, 320, 1400, 330, and 160 fg, respectively. The compact W-Coil device makes it an ideal atomizer for portable atomic spectrometry instrumentation, especially when coupled with a miniature charge coupled device spectrometer. Alternatively, the atomizer can be used as an inexpensive, modular add-on to an existing commercial ICP-AES system; and the thermal separation of Pb with interference elements Al, Mn, and Fe is demonstrated.     

Comparison of electrothermal atomization diode laser Zeeman- and wavelength-modulated atomic absorption and coherent forward scattering spectrometry

Atomic absorption and coherent forward scattering spectrometry by using a near-infrared diode laser with and without Zeeman and wavelength modulation were carried out with graphite furnace electrothermal atomization. Analytical curves and limits of detection were compared. The magnetic field was modulated with 50 Hz, and the wavelength of the diode laser with 10 kHz. Coherent forward scattering was measured with crossed and slightly uncrossed polarizers. The results show that the detection limits of atomic absorption spectrometry are roughly the same as those of coherent forward scattering spectrometry with crossed polarizers. According to the theory with bright flicker noise limited laser sources the detection limits and linear ranges obtained with coherent forward scattering spectrometry with slightly uncrossed polarizers are significantly better than those obtained with crossed polarizers and with atomic absorption spectrometry. This is due to the fact that employing approaches of polarization spectroscopy reduce laser intensity fluctuations to their signal carried fractions.     

Comparative Studies on Chemical Modification by Diethyldithiocarbamate for ETV-ICP-OES and ETAAS Determination of Chromium and Nickel

The chemical modification of diethyldithiocarbamate (DDTC) in electrothermal vaporization inductively coupled plasma optical emission spectrometry (ETV-ICP-OES) and in electrothermal atomic absorption spectrometry (ETAAS) was comparatively investigated. The experimental results indicated that the formation of Cr- and Ni-DDTC chelates enhanced significantly the emission signals of Cr and Ni in ETV-ICP-OES, but decreased the absorption signal of Cr and Ni in ETAAS. The different role of DDTC in ETV-ICP-OES and ETAAS was attributed to the different functions of the graphite furnace in the two techniques. The graphite furnace was used as both a vaporizer and an atom-vessel for analytes in ETAAS, but only used as a vaporizer for the sample in ETV-ICP-OES. Thermal gravimetric analysis of Cr- and Ni-DDTC chelates and UV-Vis analysis of the sample vapor collected in CHCl₃ after vaporization of their chelates from the graphite furnace indicated that the analytes were vaporized and transported into ICP as their chelates. In addition, the vaporization mechanism of Cr and Ni was also briefly discussed.     

Ultrasensitive determination of gold in geogas by laser excited atomic fluorescence spectrometry

Ultratrace gold (Au) in geogas samples has been determined by means of laser excited atomic fluorescence spectrometry combined with graphite electrothermal atomization and time-gate technique. Gold atoms were excited from the ground state to the 6p²P_3/2 state by a pulsed laser beam with a wavelength of 242.8 nm. Fluorescence photons with a wavelength of 312.3 nm were measured by a photon-counting unit. The time-gate technique was used to reduce the background radiation caused by the furnace. This method has proved to be highly sensitive with minimal background interference. Eighty-two geogas samples were analysed and the Au contents obtained were in the range of 0.002–0.182 ng l⁻¹. The study of Au concentration of geogas in soil is of great interest in prospecting gold deposits.     

Determination of vanadium in water by electrothermal atomisation atomic absorption spectrometry after extraction with 8-hydroxyquinoline in isobutyl methyl ketone

A sensitive method for the determination of vanadium in water by electrothermal atomisation atomic absorption spectrometry (ETAAS) is described. The vanadium is chelated with 8-hydroxyquinoline in isobutyl methyl ketone and determined by ETAAS after pre-heating the pyrolytic graphite coated graphite tube of a graphite furnace atomiser before injection. The effects of the pH and amount of reagent required for the extraction were studied. The precision, accuracy and interferences of the method were also investigated. The proposed method allows concentrations of vanadium of 0.16 microgram l-1 to be detected.     

Development of electrothermal atomic absorption spectrometry in 2005–2016

The review covers publications of 2005–2016 on achievements in the development of electrothermal atomic absorption spectrometry (ETAAS). The main directions in the authors’ opinion are revealed, i.e., (1) improvements of the method and equipment and (2) studies of thermochemical processes in a graphite furnace. In the first group, the authors consider high- and low-resolution continuum source atomic absorption spectrometry, diode laser atomic absorption spectrometry, new designs of electrothermal atomizers, and new devices for ETAAS. Studies of mechanisms of element atomization, formation of analytical signals, and action of chemical modifiers belong to the second group.     

Some observations on electrothermal vaporisation for sample introduction into the inductively coupled plasma

The development and analytical utility of electrothermal vaporisation techniques employing a graphite rod for sample introduction into the inductively coupled plasma (ICP) are assessed. In most instances detection limits are superior to those obtained with nebulisation based systems, and are comparable to those obtained with graphite furnace atomic absorption spectrometry. A model is derived for the sample injection process. Additionally the major interference and alteration of the plasma excitation phenomena ensuing from the sample introduction of a solvent free aerosol are discussed in order to assess the analytical potential of the technique for routine μl volume sample introduction in ICP spectrometry. The capability for simultaneous multi-element analyses is maintained with the electrothermal vaporisation technique.     

Test of the generalized standard addition method and linearization algorithms for the direct,simultaneous, multi-element analysis of solid samples by electrothermal atomic absorption spectrometry

The application of a multi-element electrothermal atomic absorption spectrometry (ETAAS) instrument with line sources and Zeeman-effect background correction to the direct, simultaneous determination of Ag, Cd, Cr and Pb in solid reference materials is described. Due to the occurrence of interference effects, the generalized standard addition method (GSAM) was required for calibration purposes. In order to extend the dynamic range for analytes present at high concentrations, linearization algorithms were tested. The combination of the GSAM and extension of the linear range, when necessary, generally yielded acceptable analytical results for the suite of elements studied, and may provide a useful approach to calibration in the direct analysis of solid samples by multi-element ETAAS. However, while linearization yielded good results for Ag and Cd in solid reference materials, it remains to be demonstrated that acceptable performance can be obtained for other elements in real samples.     

Graphite filter atomizer in atomic absorption spectrometry

Graphite filter atomizers (GFA) for electrothermal atomic absorption spectrometry (ETAAS) show substantial advantages over commonly employed electrothermal vaporizers and atomizers, tube and platform furnaces, for direct determination of high and medium volatility elements in matrices associated with strong spectral and chemical interferences. Two factors provide lower limits of detection and shorter determination cycles with the GFA: the vaporization area in the GFA is separated from the absorption volume by a porous graphite partition; the sample is distributed over a large surface of a collector in the vaporization area. These factors convert the GFA into an efficient chemical reactor. The research concerning the GFA concept, technique and analytical methodology, carried out mainly in the author's laboratory in Russia and South Africa, is reviewed. Examples of analytical applications of the GFA in AAS for analysis of organic liquids and slurries, bio-samples and food products are given. Future prospects for the GFA are discussed in connection with analyses by fast multi-element AAS.     

The graphite filter furnace: a new atomization concept for atomic spectroscopy

A new atomization concept for electrothermal atomic absorption spectroscopy (ETAAS) is proposed and discussed. The basic idea is to form within part of an atomizer a large surface over which the collection of fine particles of the sample is to be distributed, and to constrain the vapor exit from that part to the analytical zone. For the amounts of analyte which are in the range of ordinary ETAAS, these techniques do not affect the character of atomic vapor release because the surface area for the analyte is smaller than that of the vapor passage. However, when there is an excess of matrix, this technique creates the conditions for matrix vapor saturation which impedes the evaporation. This in turn restricts the concentration of matrix vapor in the analytical zone and, respectively, limits spectral background and gas-phase chemical interferences. The concept is realized in the design of a graphite filter furnace. The principle of operation of the atomizer and practical methods to obtain analytical advantages are discussed and compared, using experimental data.     

Preconcentration and separation of cadmium by use of cobalt(III) hexamethylenedithiocarbamate as a collector prior to its determination by atomic absorption spectrometry

Co(III) hexamethylenedithiocarbamate has been applied as a collector in colloid flotation preconcentration of Cd from water prior to electrothermal atomic absorption spectrometry (ETAAS). All experimental parameters necessary for successful flotation have been studied and optimized. The ETAAS results were compared with those obtained by inductive coupled plasma-atomic emission spectrometry (ICP–AES). The ETAAS detection limit was found to be 0.003 μg L^–1 Cd.     

Determination of iron in seawater by electrothermal atomic absorption spectrometry and atomic fluorescence spectrometry: A comparative study

Two methods available for direct determination of total Fe in seawater at low concentration level have been examined: electrothermal atomization atomic absorption spectrometry (ETAAS) and electrothermal atomization laser excited atomic fluorescence spectrometry (ETA-LEAFS). In a first part, we have optimized experimental conditions of ETAAS (electrothermal program, matrix chemical modification) for the determination of Fe in seawater by minimizing the chemical interference effects and the magnitude of the simultaneous background absorption signal. By using the best experimental conditions, a detection limit of 80 ng L⁻¹ (20 μL, 3σ) for total Fe concentration was obtained by ETAAS. Using similar experimental conditions (electrothermal program, chemical modification), we have optimized experimental conditions for the determination of Fe by LEAFS. The selected experimental conditions for ETA-LEAFS: excitation wavelength (296.69 nm), noise attenuation and adequate background correction led to a detection limit (3σ) of 3 ng L⁻¹ (i.e. 54 pM) for total Fe concentration with the use a 20 μL seawater sample. For the two methods, concentration values obtained for the analysis of Fe in a NASS-5 (0.2 μg L⁻¹) seawater sample were in good agreement with the certified values.     

Separation preconcentration method for platinum and rhodium from environmental samples using a chelating resin

A method of determining trace levels of platinum and rhodium in different samples was investigated. The method involves separation and preconcentration of the platinum and rhodium from the matrix by flow injection (FI) on-line coupled with electrothermal atomic absorption spectrometry (ETAAS) with Zeeman effect background correction. Platinum and rhodium were adsorbed on a microcolumn packed with 1,5-bis(di-2-pyridyl)methylene thiocarbohydrazide immobilized on silica gel (DPTH-gel). The sorbed metals were directly eluted with nitric acid into the graphite furnace and determined by AAS.     

On the possibilities of high-resolution continuum source graphite furnace atomic absorption spectrometry for the simultaneous or sequential monitoring of multiple atomic lines

This paper explores the potential of commercially available high-resolution continuum source graphite furnace atomic absorption spectrometry instrumentation for the simultaneous or sequential monitoring of various atomic lines, in an attempt to highlight the analytical advantages that can be derived from this strategy. In particular, it is demonstrated how i) the monitoring of multiplets may allow for the simple expansion of the linear range, as shown for the measurement of Ni using the triplet located in the vicinity of 234.6 nm; ii) the use of a suitable internal standard may permit improving the precision and help in correcting for matrix-effects, as proved for the monitoring of Ni in different biological samples; iii) direct and multi-element analysis of solid samples may be feasible on some occasions, either by monitoring various atomic lines that are sufficiently close (truly simultaneous monitoring, as demonstrated in the determination of Co, Fe and Ni in NIST 1566a Oyster tissue) or, alternatively, by opting for a selective and sequential atomization of the elements of interest during every single replicate. Determination of Cd and Ni in BCR 679 White cabbage is attempted using both approaches, which permits confirming that both methods can offer very similar and satisfactory results. However, it is important to stress that the second approach provides more flexibility, since analysis is no longer limited to those elements that show very close atomic lines (closer than 0.3 nm in the ultraviolet region) with a sensitivity ratio similar to the concentration ratio of the analytes in the samples investigated.     

Studies of critical factors in the determination of arsenic in Standard Reference Materials of Marine origin by ETAAS: ;NMKL* interlaboratory study

A study to determine factors which are known to influence the electrothermal atomic absorption (ETAAS) determination of As has been performed. The study has been carried out using five sample solutions of marine Standard Reference Materials distributed to four participating laboratories. Uncoated and pyrolytically coated graphite tubes with L'vov platform and Ni and Pd/Mg as chemical modifiers have been tested. No differences in results have been obtained between AAS instruments equipped with Zeeman correction or deuterium arc background correction. Small differences in concentration levels of arsenic as well as in characteristic mass were found when chemical modifiers were compared. Pd/Mg will be recommended in order to avoid a contamination of the graphite furnace with nickel. The characteristic mass was improved by using pyrolytically coated graphite tubes with the L'vov platform compared with uncoated graphite tubes with the L'vov platform. In the interlaboratory study, the standard addition procedure will be recommended.     

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.     

Pyrolytic and non-pyrolytic boron nitride platforms in electrothermal AAS for the determination of cadmium in sea and estuarine water without chemical modification.

The analytical performance of pyrolytic and non-pyrolytic boron nitride (PBN and NBN) platforms, attached to a commercially available graphite tube furnace, in electrothermal atomic absorption spectrometry (ETAAS) for Cd was studied. Although the tolerable pyrolysis temperature was 300 degrees C with the conventional pyrolytic graphite platform, it increased to 600 and 950 degrees C with the PBN and NBN platforms, respectively. The lifetime of the ceramic platform was 500 firings. The NBN platform provided an enhanced sensitivity with a better reproducibility than others. Using the NBN platform allowed the LOD, based on the variability of the blank (3sigma), to be 0.1 microg l(-1) within a seawater matrix (20,000 mg NaCl l(-1)) and a constant sensitivity in the range 0-30,000 mg NaCl l(-1). Good recovery in the range of 90-105% was observed for Cd (2.0 microg l(-1)) spiked into sea, estuarine and river water samples using the recommended procedure. This work proposes that using the NBN platform allows the direct monitoring and control of contaminated water for Cd by ETAAS without any chemical modification.     

Solid sampling by electrothermal vaporization in combination with electrostatic particle deposition for electrothermal atomization multi-element analysis

A novel combination of electrothermal sample vaporization from one furnace and electrostatic deposition of the aerosol on a L'vov platform in a second graphite furnace used for subsequent electrothermal atomization multi-element analysis is described. The aerosol generated by vaporization of liquid as well as solid primary samples is transported to the graphite furnace by an Ar gas flow and is piped into the furnace through the dosing hole via a glass capillary mounted on the autosampler arm of a continuum source coherent forward scattering spectrometer. The deposition on the graphite platform is obtained electrostatically by a corona-like discharge. The near quantitative deposition of the produced and transported aerosol allows optimal direct determination of the transport efficiencies by comparing the signals obtained by measuring liquid samples directly with the spectrometer with signals obtained with samples transferred with electrothermal vaporization and electrostatic deposition. Over all transfer efficiencies up to 30% are observed with liquid primary samples. Results obtained with solid sampling of BCR CRM 189 Wholemeal flour are in good agreement with the certified values.     

Evaluation of electrodeposited tungsten chemical modifier for direct determination of chromium in urine by ETAAS

The direct determination of chromium in urine by electrothermal atomic absorption spectrometry (ETAAS) using graphite tubes modified with tungsten is proposed. Modification of the graphite is made by tungsten electrodeposition over the whole surface atomizer followed by carbide formation by heating the tube inside its own furnace. For tungsten electrocoating, the graphite tube and a platinum electrode were connected to a power supply as cathode and anode, respectively, and immersed in a solution containing 2 mg of W in 0.1% v/v HNO₃. Then, 5 V was applied between the electrodes during 20 min for tungsten electrodeposition over the whole atomizer. A SpectrAA 220 Varian atomic absorption spectrometer equipped with a deuterium background corrector was used throughout. Undiluted urine (20 μl) was delivered over the tungsten-treated tube and the chromium-integrated absorbance was measured after applying a suitable heating program with maximum pyrolysis at 1300 °C and atomization at 2500 °C. With electrodeposited tungsten modifier, the tube lifetime increased up to four times when compared to previous published methods for Cr determination in urine by ETAAS, reaching 800 firings. Method detection limit (3 S.D.) was 0.10 μg l⁻¹, based on 10 integrated absorbance measurements of a urine sample with low Cr concentration. Two reference materials of urines (SRM 2670) from National Institute of Standards and Technology (NIST) were analyzed for method validation. For additional validation, results obtained from eight human urine samples were also analyzed in a spectrometer with Zeeman effect background correction.     

Diffusion of molecular vapors through heated graphite

The reasons for the elimination of interferences in a filter furnace used as an atomizer in electrothermal atomic absorption spectroscopy (ETAAS), are examined theoretically and experimentally. The results of observations of stepped or multipeak background absorption signals for NaCl, NaI, KCl, KBr, KI, CaCl₂ and MgSO₄, which are characteristic for each matrix and different types of graphite used, leads to hypotheses about the formation of molecule-graphite intercalation composites. These are treated as stable compounds of different stoichiometry having substantial specific enthalpy of formation. The validity of the idea is confirmed with the measurements of the rate of NaCl vapor release through a graphite filter as a function of temperature, and comparison with the literature data for similar compounds of alkali metals. The effect of implantation of molecular particles into a crystalline lattice leads to reduction of their diffusion rate through graphite, which in turn provides time-resolved background and atomic absorption signals. Further investigations and development of the idea to understand the analytical effect are proposed.