Miniature Diffusion Flame Research Articles

Comparison of bismuth atomic lamps for a non-dispersive atomic fluorescence spectrometry

The performance of two types of radiation sources – an electrodeless discharge lamp (EDL) and a boosted discharge hollow cathode lamp (Superlamp) – for atomic fluorescence spectrometry (AFS) was studied and compared using bismuth as a model analyte at 222.8 and 223.1 nm fluorescence lines. Hydride generation was used for analyte introduction to a miniature diffusion flame where the generated bismuthane was atomized. An in-house assembled non-dispersive atomic fluorescence spectrometer was adjusted so that either of the sources could be employed. The optimization of the radiation sources at chosen 40 Hz modulation rate included a duty cycle and applied currents (primary and boost), in addition to proper focusing on the flame atomizer. With optimum operation parameters the sensitivity achieved with the EDL was around an order of magnitude higher than that with the Superlamp, in agreement with the higher radiation intensity measured by an optical power meter and resulting limits of detection of 1.5 pg and 11 pg, for the EDL and the Superlamp, respectively.

Ultra-sensitive speciation analysis of tellurium by manganese and iron assisted photochemical vapor generation coupled to ICP-MS/MS

Photochemical vapor generation (PVG) of Te4+ was undertaken with a simple reactor consisting of a polytetrafluoroethylene reaction coil wrapped around a low-pressure mercury tube lamp and using a flow-injection for sample delivery. The composition of a reaction medium, the influence of irradiation time and the effect of added sensitizers and interferents were investigated using high-resolution continuum source atomic absorption spectrometry and a miniature diffusion flame atomizer. A mixture of 5 M acetic acid and 3.5 M formic acid and sample flow rate of 4 mL min−1 permitting a 36 s irradiation time were found optimal for PVG of Te4+. The addition of 250 mg L−1 Mn2+ and 15 mg L−1 Fe2+ ions as sensitizers enhanced the overall PVG efficiency 2.75-fold to 50 ± 2%. In order to achieve higher sensitivity necessary for determination of Te in real environmental samples, PVG was coupled to inductively coupled plasma triple quadrupole mass spectrometer and detection was performed with O2 in the reaction cell utilizing a mass shift mode of measurement (m/z 128 → m/z 144) to ensure interference free ion detection. A limit of detection 1.3 ng L−1 and repeatability (RSD) 0.9% at 250 ng L−1 were achieved. This ultrasensitive methodology was validated for speciation analysis of Te in water samples of various matrix complexities (fresh water, well water, seawater and contaminated water). Since no response was observed from Te6+ under optimal PVG conditions, Te4+ was selectively determined by direct PVG. The sum of Te4+ and Te6+ was determined after pre-reduction of Te6+ in 6 M HCl (95 °C), evaporation to dryness and reconstitution in the reaction medium containing sensitizers. Very good accuracy was demonstrated by spiked recoveries for both Te4+ and total Te in water samples and also by total Te determination in fresh water Standard Reference Material NIST 1643f.

Efficient photochemical vapor generation of bismuth using a coiled Teflon reactor: Effect of metal sensitizers and analytical performance with flame-in-gas-shield atomizer and atomic fluorescence spectrometry

Photochemical vapor generation (PVG) of bismuth was accomplished in a simple flow–injection system with a standard Hg low-pressure tube lamp and a coiled Teflon reactor. The influence of a reaction medium flow rate (irradiation time) and composition was investigated using a miniature diffusion flame atomizer and high-resolution continuum source atomic absorption spectrometry. Combination of 40% (v/v) acetic acid, 1.25% (v/v) formic acid and a metal sensitizer at a flow rate of 3 mL min−1 (irradiation time of 90 s) was found optimal. The use of various metals as sensitizers was studied to initiate and enhance PVG substantially. The evident positive effect was found for four metal cations and the enhancement effect was in the following order: Cu2+<Cd2+<Fe2+ ≪ Co2+. At 50 mg L−1 Co2+ as the sensitizer, the overall PVG efficiency of 54 ± 2% was determined from relative comparison of sensitivities obtained with PVG and liquid nebulization coupled simultaneously to inductively coupled plasma mass spectrometry. In order to reach low limits of detection (LOD) at low operation cost, a coupling of PVG to atomic fluorescence spectrometry was tested and conditions of atomization in two flame atomizers were optimized. A blank-limited LOD of 12 ng L−1 was obtained with an advanced flame-in-gas-shield atomizer, which competes with PVG coupled to inductively coupled plasma mass spectrometry (5 ng L−1). Interferences from commonly used inorganic acids (HNO3, HCl and H2SO4) and other elements (As3+, Pb2+, Sb3+, Se4+ and Te4+) were also investigated and the accuracy was verified by analysis of water Standard Reference Material NIST 1643f.

Generation of tellurium hydride and its atomization in a dielectric barrier discharge for atomic absorption spectrometry

Atomization of tellurium hydride in a dielectric barrier discharge (DBD) atomizer was investigated for the first time. First, the conditions of hydride generation were optimized with the use of a miniature diffusion flame atomizer and high-resolution continuum source atomic absorption spectrometry. Generation efficiency of more than 90% was determined from a comparison of the response to solution nebulization with inductively coupled plasma mass spectrometry. Subsequently, atomization of tellurium hydride in a plane-parallel configuration of the DBD atomizer was optimized. Argon was found as the best carrier gas under a flow rate of 75 mL min−1 while the peak-to-peak high voltage was 18 kV. A limit of detection of 56 ng L−1 (3σ, n = 15) and repeatability (RSD) of 2.8% at 5 μg L−1 were reached. The accuracy and applicability of this methodology was verified by analysis of water Standard Reference Material NIST 1643f and two model samples of tap and stream water. Laser-induced fluorescence was employed to investigate a spatial distribution of free Te atoms and to estimate atomization efficiency in the DBD. Significant fractions of free Te atoms were identified all over the length of the optical tube of the DBD atomizer covered by the electrodes and atomization efficiency of up to 100% was quantified. A feasibility of tellurium hydride in-situ collection in the DBD atomizer was studied and collection efficiency of around 50% was found promising for further studies.

An open-source tool for predictive simulation of diffusion flames in analytical chemistry

This work presents an open-source computational model of gas flow and hydrogen combustion in a miniature diffusion flame atomizer.

Flame-in-gas-shield and miniature diffusion flame hydride atomizers for atomic fluorescence spectrometry: optimization and comparison

A detailed optimization of relevant experimental parameters of two hydride atomizers for atomic fluorescence spectrometry: flame-in-gas-shield atomizer with a two-channel shielding unit and a standard atomizer for atomic fluorescence spectrometry, miniature diffusion flame, was performed. Arsine, generated by the reaction with NaBH4 in a flow injection arrangement, was chosen as the model hydride. Analytical characteristics of both the atomizers (sensitivity, noise, limits of detection) were compared. Under optimum conditions sensitivity obtained with flame-in-gas-shield atomizer was approximately twice higher than with miniature diffusion flame. The additional advantage of flame-in-gas-shield atomizer is significantly lower flame emission resulting in a better signal to noise ratio. The resulting arsenic limits of detection for miniature diffusion flame and flame-in-gas-shield atomizer were 3.8ngl−1 and 1.0ngl−1, respectively.

Speciation analysis of arsenic by selective hydride generation-cryotrapping-atomic fluorescence spectrometry with flame-in-gas-shield atomizer: achieving extremely low detection limits with inexpensive instrumentation.

This work describes the method of a selective hydride generation-cryotrapping (HG-CT) coupled to an extremely sensitive but simple in-house assembled and designed atomic fluorescence spectrometry (AFS) instrument for determination of toxicologically important As species. Here, an advanced flame-in-gas-shield atomizer (FIGS) was interfaced to HG-CT and its performance was compared to a standard miniature diffusion flame (MDF) atomizer. A significant improvement both in sensitivity and baseline noise was found that was reflected in improved (4 times) limits of detection (LODs). The yielded LODs with the FIGS atomizer were 0.44, 0.74, 0.15, 0.17 and 0.67 ng L–1 for arsenite, total inorganic, mono-, dimethylated As and trimethylarsine oxide, respectively. Moreover, the sensitivities with FIGS and MDF were equal for all As species, allowing for the possibility of single species standardization with arsenate standard for accurate quantification of all other As species. The accuracy of HG-CT-AFS with FIGS was verified by speciation analysis in two samples of bottled drinking water and certified reference materials, NRC CASS-5 (nearshore seawater) and SLRS-5 (river water) that contain traces of methylated As species. As speciation was in agreement with results previously reported and sums of all quantified species corresponded with the certified total As. The feasibility of HG-CT-AFS with FIGS was also demonstrated by the speciation analysis in microsamples of exfoliated bladder epithelial cells isolated from human urine. The results for the sums of trivalent and pentavalent As species corresponded well with the reference results obtained by HG-CT-ICPMS (inductively coupled plasma mass spectrometry).

Flame-in-gas-shield miniature flame hydride atomizers for ultra trace element determination by chemical vapor generation atomic fluorescence spectrometry

Flame-in-gas shield miniature hydride atomizers (FIGS) have been investigated and evaluated in view of their alternative use to miniature diffusion flame hydride atomizer (MDF) to determination of hydride forming elements by atomic fluorescence spectrometry (AFS). Chemical vapour generation (CVG) by aqueous phase derivatization by NaBH 4 in a continuous flow generator (CF) was employed for the generation of volatile hydrides of As, Sb, Bi, Se, Te and Sn. A dispersive AFS apparatus using electrodeless discharge lamps (EDL) as the excitation sources has been employed for both spectra acquisition and analytical determinations. The characteristics of FIGS in terms of background emission spectra, most intense AF spectral lines and limits of detection were compared with those of most popular MDF. FIGS presents a lower background emission with respect to MDF, allowing also the control of the molecular fluorescence of OH radicals in the determination of bismuth. Limits of detection for FIGS compare very well with to those obtained by MDF giving improvement factor of 5.5, 4.4, 3.6, 3.6, 0.7 an 0.5 for Bi, As, Se, Son, Te and Sb. Accuracy of FIGS has proven by determination of arsenic and antimony in seawater (NASS-5) and river water (SRLS-4) certified reference materials and bismuth in unalloyed copper (CuV 398, CuVI 399) standard reference materials by dispersive CVG-AFS.

Organic solvents as interferents in arsenic determination by hydride generation atomic absorption spectrometry with flame atomization

Interference effects of various organic solvents miscible with water on arsenic determination by hydride generation atomic absorption spectrometry have been studied. Arsine was chemically generated in continuous flow hydride generation system and atomized by using a flame atomizer able to operate in two modes: miniature diffusion flame and flame-in-flame. The effects of experimental variables and atomization mode were investigated: tetrahydroborate and hydrochloric acid concentrations, argon, hydrogen and oxygen supply rates for the microflame, and the distance from the atomization region to the observation zone. The nature of the species formed in the flame due to the pyrolysis of organic solvent vapors entering the flame volume together with arsine is discussed. The observed signal depression in the presence of organic solvents has been mainly attributed to the atomization interference due to heterogeneous gas–solid reaction between the free arsenic atoms and finely dispersed carbon particles formed by carbon radicals recombination. The best tolerance to interferences was obtained by using flame-in-flame atomization (5–10 ml min − 1 of oxygen flow rate), together with higher argon and hydrogen supply rates and elevated observation heights.

Continuous flow hydride generation-atomic fluorescence spectrometric determination and speciation of arsenic in wine

Methods for the atomic fluorescence spectrometric (AFS) determination of total arsenic and arsenic species in wines based on continuous flow hydride generation (HG) with atomization in miniature diffusion flame (MDF) are described. For hydride-forming arsenic, l-cysteine is used as reagent for pre-reduction and complexation of arsenite, arsenate, monomethylarsonate and dimethylarsinate. Concentrations of hydrochloric acid and tetrahydroborate are optimized in order to minimize interference by ethanol. Procedure permits determination of the sum of these four species in 5–10-fold diluted samples with limit of detection (LOD) 0.3 and 0.6 μg l − 1 As in white and red wines, respectively, with precision between 2% and 8% RSD at As levels within 0.5–10 μg l − 1 . Selective arsine generation from different reaction media is used for non-chromatographic determination of arsenic species in wines: citrate buffer at pH 5.1 for As(III); 0.2 mol l − 1 acetic acid for arsenite + dimethylarsinate (DMA); 8 mol l − 1 HCl for total inorganic arsenic [As(III) + As(V)]; and monomethylarsonate (MMA) calculated by difference. Calibration with aqueous and ethanol-matched standard solutions of As(III) is used for 10- and 5-fold diluted samples, respectively. The LODs are 0.4 μg l − 1 for As(III) and 0.3 μg l − 1 for the other three As species and precision is within 4–8% RSDs. Arsenic species in wine were also determined by coupling of ion chromatographic separation on an anion exchange column and HG-flame AFS detection. Methods were validated by means of recovery studies and comparative analyses by HG-AFS and electrothermal atomic absorption spectrometry after microwave digestion. The LODs were 0.12, 0.27, 0.15 and 0.13 μg l − 1 (as As) and RSDs were 2–6%, 5–9%, 3–7% and 2–5% for As(III), As(V), MMA and DMA arsenic species, respectively. Bottled red and white wines from Bulgaria, Republic of Macedonia and Italy were analyzed by non-chromatographic and chromatographic procedures and the As(III), arsenite, has been confirmed as major arsenic species.

Surfactant assisted chemical vapour generation of silver for AAS and ICP-OES: a mechanistic study

A chemical vapour generation (CVG) procedure for the production of volatile species of Ag was developed and studied using AAS and ICP-OES detection. A characteristic mass of 0.12 ng was achieved using a miniature diffusion flame atomizer-AAS system, whereas a 38-fold higher sensitivity (as cps per ng of introduced analyte) compared with conventional liquid nebulization was obtained with ICP-OES. The overall efficiency of introduction of Ag to the plasma was verified to be over 20%. More than one order of magnitude enhancement of efficiency was obtained by the addition of a surfactant (Triton X-100 in combination with an antifoaming agent), the effect of which, however, cannot be ascribed to the formation of organized media. Experiments concerning the mechanism of the CVG, transport and memory effect are discussed: Ag CVG is not a simple one-step process as is the case for CVG of selenium, but rather a two-step procedure. Transport properties of the volatile Ag species resemble more the transport of the liquid aerosol than a simple gaseous species such as H2Se. Possible alternatives concerning the identity of the volatile metal species are presented.

The relation of double peaks, observed in quartz hydride atomizers, to the fate of free analyte atoms in the determination of arsenic and selenium by atomic absorption spectrometry

The mechanism at the origin of double peaks formation in quartz hydride atomizers were investigated by continuous flow hydride generation atomic absorption spectrometry. Arsenic and selenium were used as model analytes. The effect of atomization mode (flame-in-gas-shield (FIGS), miniature diffusion flame and double flame (DF)) and some experimental parameters as oxygen supply rate for microflame and the distance from atomization to free atoms detection point, were investigated on the shape of both analytical signals and calibration graphs. Rollover of calibration graphs and double peak formation are strictly related each to the other and could be observed only in FIGS atomizer mode under some particular conditions. A mechanism based on incomplete atomization of hydrides cannot explain the collected experimental evidences because the microflame of FIGS is able to produce quantitative atomization of large amount of hydrides even at supply rate of oxygen close to extinction threshold of microflame. The heterogeneous gas–solid reactions between finely dispersed particles, formed by free atom recombination, and the free atoms in the gaseous phase are at the origin of double peak formation.

Selenium hydride atomization, fate of free atoms and spectroscopic temperature in miniature diffusion flame atomizer studied by atomic absorption spectrometry

The mechanism of hydride atomization and the fate of free atoms was investigated in the miniature diffusion flame. Selenium hydride was used as a model for the other hydrides. Mercury vapor was employed as an analyte to study physical processes, such as macroscopic movements and free atom diffusion, controlling the distribution of free analyte atoms in the observation volume, separately from chemical reactions of the free atoms. Free atoms were detected by atomic absorption spectrometry. Spectroscopic temperature measurements based on atomic absorption at 196.1 and 204.0nm Se lines were used to determine the temperature distribution. The spatial temperature distribution was highly inhomogeneous ranging from 150°C to 1300°C. The whole flame volume is actually a cloud of hydrogen radicals maintaining analyte in the free atom state since hydrogen radicals formed in outer zone of the flame diffuse to its cooler inner parts.

Argon shielded, highly fuel-rich, hydrogen–oxygen diffusion microflame—a new hydride atomizer

A new hydride atomizer for atomic fluorescence (AFS) and possibly also for atomic absorption spectrometry (AAS) is described. Named flame-in-gas-shield atomizer, it is actually an argon-shielded highly fuel-rich hydrogen–oxygen diffusion microflame. Its sensitivity and blank noise the for AFS detection are compared with those of a miniature diffusion flame. Selenium hydride is employed as a model for other analytically important hydrides. The influence of gas flow rates on AFS and AAS signals at various observation heights is determined. Analyte free atoms are removed from the observation volume by chemical reactions with oxygen penetrating from ambient atmosphere. No significant quenching effect due to interaction of excited Se atomic levels with nitrogen or hydrogen was found. The AFS sensitivity of the flame-in-gas-shield atomizer is at least two times higher and its inherent noise is substantially lower than with the miniature diffusion flame. This offers the possibility to improve limits of detection in AFS more than by one order of magnitude.

Interferences in hydride atomization studied by atomic absorption and atomic fluorescence spectrometry

A hydride atomizer able to operate in the flame-in-tube mode and in the miniature diffusion flame mode was used to investigate interferences of arsenic in selenium atomization. A twin-channel continuous flow hydride generator was utilized to eliminate liquid phase interferences. Both atomic absorption and atomic fluorescence detectors (EDL sources) were employed. The miniature diffusion flame can tolerate interferent concentrations up to 70 μg ml −1. The magnitude of interferences in the flame-in-tube atomizer is controlled by the distance between the atomization and detection zones. The best tolerance to interferents, comparable with that in the miniature diffusion flame, was obtained for the minimum distance of the zones. The figures were deteriorated by two orders of magnitude when increasing the distance between the observation and the atomization zones to 50 mm. Also a curvature and rollover of calibration graphs was observed when increasing the distance. The presence of the interferent enhanced substantially the curvature and rollover, so that the magnitude of observed interferences was dependent on the analyte concentration. All the observed interferences and the calibration graph curvature are due to the decay of free analyte atoms by reactions in the free space. The nature of the species formed is discussed. No significant depletion of hydrogen radicals was observed. As demonstrated by measurements in the miniature diffusion flame, the species formed can be reatomized by interaction with hydrogen radicals with an efficiency better than 90%.