Filter Furnace Research Articles

Direct Atomic Absorption Spectrometric Determination of Selenium in Biological Fluids by a Graphite Filter Furnace Atomizer with Carbon Thread

ABSTRACTA rapid and simple method was developed for the determination of selenium in biological fluids by direct electrothermal atomic absorption spectroscopy. The operation parameters of a commercial longitude heating furnace combined with a graphite filter furnace, graphite thread (tube-in-tube atomization system), and a Pd–Mg chemical modifier were proposed, and the corresponding analytical characteristics for the determination of Se at concentrations greater than or equal to 0.0025 mg/kg in blood, urine, saliva, and breast milk samples were established. Recovery values of Se with the proposed method varied within 93 to 107% for these matrices. Relative standard deviations for the analysis of certified material were also compatible; values of 5 and 7% were achieved for blood serum and urine, respectively. The relative standard deviation obtained by the proposed method did not exceed 8%, and the total time for Se determination was 5 to 7 min. The novelty of this technique is that it offers an effective tool for removing the interferences from 10 to 20 g/L organic and 1.0 to 1.5 g/L mineral components in the materials and for significantly reducing the background absorption of light caused by their presence.

Direct atomic absorption spectrometry determination of arsenic, cadmium, copper, manganese, lead and zinc in vegetable oil and fat samples with graphite filter furnace atomizer

The article presents the results of optimization of operation parameters, investigation of analytical characteristics and the abilities of a graphite filter-furnace (FF) atomizer for the direct electrothermal atomic absorption spectroscopy (ET AAS) determination of trace amounts of Mn, As, Pb, Cu, Cd and Zn in some vegetable oils and fats. The effect of pyrolysis and atomization temperatures of the graphite FF atomizer on atomic absorbance values of the listed elements at their evaporation from some organic solutions in the presence of a Pd-Mg chemical modifier (CM) was investigated. For the ET AAS determination of As, Pb, Cd and Zn with Pd-Mg CM, the temperature of the graphite FF atomizer for the pyrolysis step can be raised by 250–350°C. This mode allows to eliminate the background absorption, to increase the sensitivity of the elements to be analyzed and to enhance the total content of vegetable oils or fats in organic solutions up to 0.5gmL−1. The obtained limits of quantification for Mn, As, Pb, Cu, Cd and Zn were 0.002, 0.004, 0.004, 0.002, 0.0008, 0.0004mgkg−1, respectively. The relative standard deviation (RSD) varied between ∼3 and 8% and the time of one element determination did not exceed ∼3–5min. The reliability of the proposed method was checked using the reference method. A paired Student's t-test showed no significant difference between the results obtained by both methods on a 95% confidence level.

Graphite “Filter Furnace” Atomizer with Pd–Mg Chemical Modifier for Direct Analysis of Foods Using Electrothermal Atomic Absorption Spectrometry

This paper presents the results of optimization of operation parameters, investigation of analytical characteristics, and the capabilities of a graphite “filter furnaceˮ (FF) atomizer with a Pd–Mg chemical modifier (CM) for the direct determination of trace amounts of Pb, As, and Cd in foods by electrothermal atomic absorption spectrometry (ET AAS). The effects of heating parameters of the furnace on atomic absorbance values of Pb, As, and Cd were investigated during the pyrolysis and atomization steps including the carrying out of the investigation in the presence of mineral and organic macrocomponents of the analyzed materials. It is shown that the use of the graphite FF atomizer and Pd–Mg CM provides a ~2-fold increase in sensitivity of ET AAS determination of the listed elements in comparison with a conventional heated graphite furnace with a platform as well as completely or significantly eliminate matrix effects including background absorbance. The obtained limits of quantification for Pb, As, and Cd in foods were 0.0015, 0.002, and 0.0001 mg kg−1, respectively. The reliability of the proposed method was checked using standard methods.

Sulfur determination in coal using molecular absorption in graphite filter vaporizer

The vaporization of sulfur containing samples in graphite vaporizers for atomic absorption spectrometry is accompanied by modification of sulfur by carbon and, respectively, appearance at high temperature of structured molecular absorption in 200–210nm wavelength range. It has been proposed to employ the spectrum for direct determination of sulfur in coal; soundness of the suggestion is evaluated by analysis of coal slurry using low resolution CCD spectrometer with continuum light source coupled to platform or filter furnace vaporizers. For coal in platform furnace losses of the analyte at low temperature and strong spectral background from the coal matrix hinder the determination. Both negative effects are significantly reduced in filter furnace, in which sample vapor efficiently interacts with carbon when transferred through the heated graphite filter. The method is verified by analysis of coals with sulfur content within 0.13–1.5% (m/m) range. The use of coal certified reference material for sulfur analyte addition to coal slurry permitted determination with random error 5–12%. Absolute and relative detection limits for sulfur in coal are 0.16μg and 0.02mass%, respectively.

Comparison of direct sampling and emulsion analysis using a filter furnace for the determination of lead in crude oil by graphite furnace atomic absorption spectrometry

The determination of trace elements in crude oil is difficult due to the complex nature of the sample and the various different chemical forms in which the metals can occur. The advantage of graphite furnace atomic absorption spectrometry is that only a minimum of sample pretreatment is required. In this work two techniques have been compared to establish a fast and reliable method for lead determination in crude oil. In the first one the crude oil samples were weighed directly onto solid sampling (SS) platforms and introduced into the graphite tube for analysis. In the second one the samples were prepared as oil-in-water emulsions and analyzed in a filter furnace (FF). Twenty μL of a mixture of 0.5 mg L − 1 Pd + 0.3 mg L − 1 Mg + Triton X-100 has been used as the modifier, and calibration against aqueous solutions has been used for both methods. The sensitivity obtained with the FF was more than a factor of two better than that with SS; however, as a larger sample mass could be introduced in the latter case, so that the limits of detection for both techniques were 0.004 mg kg − 1 . Seven crude oil samples were analyzed using the two procedures, and all results were in agreement at a 95% confidence level using a paired Student's t-test. For validation purposes, three crude oil samples have been mineralized using an open-vessel acid digestion, and the results were in agreement with those found with direct sampling and with emulsion sampling using FF according to ANOVA test. Both methods are simple, fast and reliable, being appropriated for routine analysis; however, the direct method using SS technology should be preferred because of its simplicity, speed and commercial availability.

Determination of Cd and Pb in fuel ethanol by filter furnace electrothermal atomic absorption spectrometry

A method was developed for quantification of Cd and Pb in ethanol fuel by filter furnace atomic absorption spectrometry. Filter furnace was used to eliminate the need for chemical modification, to stabilize volatile analytes and to allow the application of short pyrolysis step. The determinations in samples were carried out against calibration solutions prepared in ethanol. Recovery tests were made in seven commercial ethanol fuel samples with values between 90 and 120%. Limits of detection were 0.1 µg L-1 for Cd and 0.3 µg L-1 for Pb. Certified water samples (APS 1071, APS 1033, NIST 1643d, NIST 1640) were also used to evaluate accuracy and recoveries from 86.8% to115% were obtained.

Chemically assisted release of transition metals in graphite vaporizers for atomic spectrometry

The processes associated with the vaporization of microgram samples and modifiers in a graphite tube ET AAS were investigated by the example of transition metals. The vapor absorption spectra and vaporization behavior of μg-amounts Cd, Zn, Cu, Ag, Au, Ni, Co, Fe, Mn and Cr were studied using the UV spectrometer with CCD detector, coupled with a continuum radiation source. The pyrocoated, Ta or W lined tubes, with Ar or He as internal gases, and filter furnace were employed in the comparative experiments. It was found that the kinetics of atomic vapor release changed depending on the specific metal–substrate–gas combination; fast vaporization at the beginning was followed by slower ‘tailing.’ The absorption continuum, overlapped by black body radiation at longer wavelengths, accompanied the fast vaporization mode for all metals, except Cd and Zn. The highest intensity of the continuum was observed in the pyrocoated tube with Ar. For Cu and Ag the molecular bands overlapped the absorption continuum; the continuum and bands were suppressed in the filter furnace. It is concluded that the exothermal interaction of sample vapor with the material of the tube causes the energy evolution in the gas phase. The emitted heat is dispersed near the tube wall in the protective gas and partially transferred back to the surface of the sample, thus facilitating the vaporization. The increased vapor flow causes over-saturation and gas-phase condensation in the absorption volume at some distance from the wall, where the gas temperature is not affected by the reaction. The condensation is accompanied by the release of phase transition energy via black body radiation and atomic emission. The particles of condensate and molecular clusters cause the scattering of light and molecular absorption; slow decomposition of the products of the sample vapor–substrate reaction produces the ‘tailing’ of atomic absorption signal. The interaction of graphite with metal vapor or oxygen, formed in the decomposition of metal oxide, is the most probable source of chemical energy, which facilitates the vaporization. Intensity of the process depends on chemical properties of the sample and substrate and efficiency of mass and heat transfer by the protective gas. The discussed mechanism of chemically assisted vapor release signifies the energy exchange between all participants of the vaporization process in ET AAS including the matrix, modifier, purge gas and analyte. The finding contributes in the ET AAS theory regarding the mechanisms of vaporization and mass transfer in the presence of matrix and modifiers.

Fuel Analysis by Filter Furnace Electrothermal Atomic Absorption Spectrometry

Abstract The determination of Mn, Fe, Co, and Zn was performed in gasoline, jet, and diesel fuel samples by electrothermal atomic absorption spectrometry using the Transverse Heated Filter Atomizer (THFA). Thermal conditions were experimentally defined for the investigated elements. The elements were analyzed without the addition of chemical modifiers, using organometallic standards for the calibration. Gasoline samples were analyzed directly, while jet and diesel fuel samples were diluted 1+3 with n-heptane. The following LODs were obtained using 40 µL injections: 0.08 µg/L Mn, 0.35 µg/L Fe, 0.54 µg/L Co, and 0.06 µg/L Zn. The limits of determination, calculated for diesel and jet fuel samples, were 1.2 µg/kg Mn, 4.9 µg/kg Fe, 7.6 µg/kg Co, and 0.8 µg/kg Zn. The corresponding limits of quantitation for gasoline samples were about four times higher.

Atomic and molecular spectra of vapors evolved in graphite furnace. Part 7: Alkaline metal sulfates and sulfides

Thermal behavior and vapor absorption spectra of Li, Na and K sulfates and Li, Na sulfides in graphite vaporizers for atomic absorption spectrometry are investigated using a UV spectrometer with a charge-coupled device (CCD) detector. Fifty spectra in the wavelength range 190–380 nm are collected during the vaporization of 1 μg and 10 μg of metal as sulfide and sulfate, respectively, simultaneously to the increase of vaporizer temperature from 700 to 2700 K during 10 s. The behavior of transient spectra is compared for the pyrocoated tube and filter furnace. It is found that Na and K sulfates in the pyrocoated tube evolve, first, metal bound sulfur–oxygen compounds, while Li sulfate emanates S 2 vapor. For all substances studied, the vaporization involves active interaction with graphite, suggestively with formation of graphite–metal and graphite–sulfur intercalation compounds. Those compounds retained metal and sulfur until above 2000 K. Both vapor dissociation and decomposition of intercalation compounds are accompanied by appearance in gas phase of CS molecules. In the filter furnace at high temperature the structured spectrum between 190 and 210 nm is characteristic for all sulfur containing compounds. It is suggestively attributed to C n S m ( n> m) molecule. The samples of Li and Na sulfides do not retain initial chemical composition after dissolving, sampling from solution, drying and pyrolysis at 700 K. Their vapor spectra show the presence of sodium sulfate (or sulfite) and lithium oxide in the respective dry residues. The diffuse absorption bands at 190–230 nm with maxima at 200–210 nm precede or appear together with molecular spectra. The phenomenon is suggestively assigned to the light scattering on specific molecular clusters.

Atomic and molecular spectra of vapors evolved in graphite furnace. Part 6: Sulfur

The vaporization of micrograms of sulfur, sulfuric acid, Cu, Hg, Ni and Zn sulfides and Ca sulfate in graphite electrothermal vaporizers (ETV) for atomic absorption spectrometry is investigated using UV spectrometer with charge-coupled device (CCD) detector and deuterium light source. Fifty vapor absorption spectra (187–380 nm) are acquired during 10-s temperature ramp of the ETV from 500 to 2300 K or from 700 to 2700 K. The spectral patterns reflect vapor evolution and composition during the vaporization pulse. A pyrocoated graphite tube is used as ETV, and a filter furnace is also employed to highlight spectral patterns that indicate the interaction of sample vapor with graphite. The vapor spectra having similar features and, therefore, characteristic for common vapor components are discussed. Two types of spectra between 240 and 320 nm are attributed to S 2 (structured spectrum) and SO/SO 2/SO 3 (diffuse spectrum) species according to their behavior in the tube and filter furnace. The spectrum observed at 250–270 nm correlates with that of CS. It is also observed that sulfur and all sulfur compounds except HgS induce structured spectra approximately 200 nm if an excess of carbon in contact with sulfur vapor is provided. It is suggested that the spectrum belongs to molecule C n S m (with n> m), formed in sample vapor if experimental conditions are favorable. Diffuse bands at 187–230 nm precede or accompany the appearance of molecular spectra. Light scattering by molecular clusters is suggested to explain the phenomenon.

Effect of magnesium nitrate vaporization on gas temperature in the graphite furnace

The vaporization of magnesium nitrate was observed in longitudinally-heated graphite atomizers, using pyrocoated and Ta-lined tubes and filter furnace, Ar or He as purge gas and 10–200-μg samples. A charge coupled device (CCD) spectrometer and atomic absorption spectrometer were employed to follow the evolution of absorption spectra (200–400 nm), light scattering and emission. Molecular bands of NO and NO 2 were observed below 1000°C. Magnesium atomic absorption at 285.2 nm appeared at approximately 1500°C in all types of furnaces. The intensity and shape of Mg atomization peak indicated a faster vapor release in pyrocoated than in Ta-lined tubes. Light scattering occurred only in the pyrocoated tube with Ar purge gas. At 1500–1800°C it was observed together with Mg absorption using either gas-flow or gas-stop mode. At 2200–2400°C the scattering was persistent with gas-stop mode. Light scattering at low temperature showed maximum intensity near the center of the tube axis. Magnesium emission at 382.9, 383.2 and 383.8 nm was observed simultaneously with Mg absorption only in the pyrocoated tube, using Ar or He purge gas. The emission lines were identified as Mg 3P°– 3D triplet having 3.24 eV excitation energy. The emitting species were distributed close to the furnace wall. The emitting layer was thinner in He than in Ar. The experimental data show that a radial thermal gradient occurs in the cross section of the pyrocoated tube contemporaneously to the vaporization of MgO. This behavior is attributed to the reaction of the sample vapor with the graphite on the tube wall. The estimated variation of temperature within the cross section of the tube reaches more than 300–400°C for 10 μg of magnesium nitrate sampled. The increase of gas temperature above the sample originates a corresponding increase of the vaporization rate. Fast vaporization and thermal gradient together cause the spatial condensation of sample vapor that induces the light scattering.

Determination of Lead in Whole Blood by Filter Furnace Laser-Excited Atomic Fluorescence Spectrometry

Filter furnace laser-excited atomic fluorescence spectroscopy was used for the determination of lead in whole blood. The use of a Katskov filter has enabled the direct comparison between lead fluorescence signals obtained with blood standards and water standards. Only a simple dilution of the blood wasrequired. This direct correlation has allowed the interpolation of the lead signal of blood samples using a lead water standard calibration curve. NIST certified blood samples were analyzed using the method with excellentresults.

Characterization of the vapor-phase molecular and atomic absorption from sea water matrices in electrothermal atomic absorption spectrometry

Vaporization of sea water was investigated by molecular and atomic absorption spectrometry. Using the combination of an ultraviolet spectrometer with multi-channel detector and an electrothermal atomizer, vapor-phase absorption was monitored in the wavelength region between 200 and 400 nm. The effects of the vaporization temperature, thermal pretreatment, internal gas flow and use of nitric acid as matrix modifier on the composition of the vapor phase were investigated using tube, platform and filter furnaces. Moreover, solutions of the main components of sea water (chlorides and sulfates of sodium, magnesium and calcium) were vaporized to clarify the contribution of each salt to the overall absorption. The addition of nitric acid to sea water led to partial suppression of the intense NaCl bands and to the appearance of oxygen containing species. Total suppression of chlorides could not be attained even with high concentration of nitric acid (more than 8% m/m). Molecular bands attributed to NaCl, NaO x , MgCl, CaCl, SO x and NO species were observed at different stages of the vaporization process. In the filter furnace, the bands of the oxygen containing molecules were less intense due to the reaction with carbon. An intense spectral continuum, attributed to light scattering, was decreased with the filter furnace relative to that observed for the tube and platform furnaces with the gas stop mode of operation.

Transverse heated filter atomizer for electrothermal atomic absorption spectrometry

The filter furnace atomization concept was applied for the transverse heated atomizer. A graphite filter with graphite fiber reeled onto it was inserted into the tube of the standard transverse heated graphite atomizer (THGA) in the place of the platform. Automatic plugging of the sampling hole was applied during the atomization stage. The performance of the filter atomizer (THFA), compared with the THGA, was tested for the determination of Ag, As, Au, Bi, Cd, Cu, Ga, In, Mn, Pb, Sb, Se and Tl. The analytical performances of the THFA displayed some advantages in comparison with the THGA. The sampling volume varied in the range of 5–90 μl, while drying time for any volume was less than half of that used for the THGA. Owing to the reduced diameter of the analytical zone (2 mm) along the filter axis, a sensitivity improvement was observed for all elements, 1.3–2.8-fold without plugging and 4.3–4.8-fold for Bi, Cd, Pb and Tl with plugging of the dosing hole. An increased peak width (by two to five times for the elements tested) limited the determination of less-volatile metals. The intensity of light decreased by 20–30% in comparison with the THGA. Taking into account the sensitivity, sampling volume, light loss and signal width, the calculated gain in relative detection limit is substantial (about 2.5–7 times) only for volatile elements when the plugging is applied. The pyrolysis temperatures for Ag, As, Au, Cd, Cu and Se in the THFA without addition of modifier were by 200–600°C higher than in the THGA using Pd/Mg modifier. The lifetime of THFA tubes was similar to that of THGA tubes.

Adaptation of the filter furnace atomizer for atomic absorption determination of less volatile metals

The filter furnace atomizer (FF), which has already been proposed for electrothermal atomic absorption spectrometry, provides enhanced sensitivity, a reduction of interferences and reduced analytical time. The main disadvantage of the FF is the rapid destruction of the filter at the atomization temperatures of less volatile or carbide-forming elements. In the current work, radiative heat exchange between the components of the FF was considered to be predominant at high temperature. The thermal behaviour of the FF was therefore examined at fast heating rate as a function of its configuration, the type of graphite used for the components, and the voltage applied. The optimal combination of electrical resistivity, relative dimensions and mass of the FF components was determined for the HGA-500 atomizer. The improvements resulted in a significantly increased lifetime of the FF in the multiple determination of Al, As, Co, Cr, Cu and Ni. The filter withstood 150 firings at 2600°C (hold time 15 s) in the determination of Cr. The theoretical approach can be applied to different types of atomizers.

Design, operation and analytical characteristics of the filter furnace, a new atomizer for electrothermal atomic absorption spectrometry

The design of a filter furnace (FF), a new atomizer for electrothermal atomic absorption spectrometry (ETAAS) introduced earlier, has been optimized for applications in conventional analytical instrumentation. The factors controlling the analytical characteristics of the FF, i.e. the general geometry of the FF, the configuration of the dosing hole, pyrocoating, the spatial distribution of the shield gas around the FF, the amount of thread, and the porosity of the filter graphite are examined. The analytical characteristics of the improved design of the FF have been investigated and compared with those of tube and platform furnaces. The determination of high- and medium-volatility elements is evaluated in the presence of an excess of volatile matrix. The new FF has a number of advantages: the potential to set the sampling volume in the range 10–100 μl while maintaining short drying times for any volume; sensitivity levels comparable with those attainable by the stabilized temperature platform furnace (STPF) technique, without the use of matrix modifier; and significant reductions in background and chemical interference. The speed and accuracy of the method is shown in the determination of various elements in the presence of halides, seawater, nitric acid, whole blood and other chemicals. The main disadvantage of the method is that the FF components are destroyed at the temperatures required for the determination of low-volatility and carbide-forming elements.

An alternative to the popular atomization concept for electrothermal atomic absorption spectrometry

When chemical modification of the sample in electrothermal atomic absorption spectrometry (ETAAS) using a tubeor platform furnace atomizer is carried out, the smooth surface of the pyrographite substrate does not provide favorable conditions for the effective interaction of reagents and temperature resolved release of products. This goal can be reached by means of spatial separation of the reaction and evaporation from the analytical zone, narrowing of the vapor passage, supplying the reaction volume with a substrate of large surface area and using graphite as a selective adsorbent. The proposal is embodied in the design of the filter furnace atomizer. Examples of radical improvement of analytical characteristics of the ETAAS method are provided in original papers using the filter furnace to verify the necessity of alteration of basic concept for ETAAS atomization.

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.

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, CaCl 2 and MgSO 4, 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.