Capacitive-discharge-heated anisotropic pyrolytic graphite furnace used for atomic absorption spectrometry Part 2. Experimental Verification of Temperature Distribution of the Furnace Surface and of the Gas Phase

The predictions of a theoretical model, embodied in a computer program, describing the heating characteristics of the furnace surface and the gas phase of an anisotropic pyrolytic graphite furnace heated by the capacitive discharge technique are compared with the experimental results obtained by optical pyrometry and by two-line atomic absorption spectrometry, respectively. The surface temperature gradient around the circumference of the type 1 furnace and along the optical axis of the type 3 furnace are calculated and compared with the measured temperatures. The weighted-average of the theoretically predicted gas temperature is in reasonable agreement with the effective vapour temperature measured by two-line atomic absorption method. The heating rate of the furnace does not have a significant effect on the temperature distribution of either the furnace surface or the gas phase. The effect of the difference in the temperature distribution of the type 1 and type 3 furnaces on the atomic absorption signals is also discussed.     

Diffusion vapour transfer modelling for an end-capped atomizer. Part 1. Atomizer with closed injection port

For end-cap equipped transverse-heated graphite atomizers (THGA) with integrated contacts used for analytical atomic spectrometry, a model equation describing the diffusional losses of analyte atomic vapour through the tube ends was constructed. The model assumes that the atomic density distribution is stepwise linear along the tube axis and the absence of a sample injection hole. With a quartz tube system, providing controlled experimental conditions at room temperature, the time constant of the diffusion removal function (T_R) of mercury vapour was determined for various open and end-capped tube geometries. These results were also described by an empirical multiple regression equation with a residual standard deviation of 5%. The theoretically predicted T_R values, corrected with an empirical factor of 1.33, agreed well (correlation coefficient = 0.996) with the experimentally obtained T_R values for the endcapped quartz tubes. For the Perkin-Elmer THGA tubes, the diffusional transfer model was evaluated using the integrated atomic absorbance ratio between various end-capped and open tubes. This is meaningful because the signal ratio for graphite atomizers is closely equal to the corresponding T_R ratio. For recommended atomization temperatures the average deviation between these experimental signal ratios and the theoretically predicted ratios for the elements Ag, In, Cd, Co, Hg and Cu was 1–5% for various end-capped tube geometries. The results for the individual elements deviated more from the theoretically predicted ratios mainly because of small differences in the mean gas-phase temperature between open and end-capped tubes. For elements which tend to form molecules in the gas phase at low temperatures and for which the atomization efficiency is increased with the atomization temperature, the experimental ratios tended to be higher than the theoretically predicted values (In, Al, Se, Sn, As), whereas experimental ratios were slightly lower for other elements (Cd, Co, Cu).     

Channel-three decay in benzene a picosecond fluorescence investigation

Fluorescence quantum yields, decay times and spectra have been measured following excitation of benzene vapour above the channel-three threshold. Experiments have been carried out on the static gas and on molecules jet-cooled to a vibrational temperature of 160 K. The excitation source was a recently developed high-power, picosecond, narrow-bandwidth laser continuously tunable from 224 to 252 nm. Channel-three decay was observed for all vibrational levels, even those not excited via line-broadened transitions. In addition it was observed that fluorescence decay curves resulting from excitation just above the channel-three threshold were non-exponential. It is proposed that the cause of absorption line broadening — intramolecular vibrational redistribution — is not directly related to channel-three, which is postulated to be coupling to a state X, which could be an isomeric form of benzene or a hidden singlet electronic state.     

Simulation of emission molecular spectra by a semi-automatic programme package: the case of C2 and CN diatomic molecules emitting during laser ablation of a graphite target in nitrogen environment

Some emission spectra of diatomic molecules were calculated by a semi-automatic programme package in order to infer the rotational and vibrational temperatures in Boltzmann distribution by comparing them with the corresponding experimental ones. The calculation procedure was applied in the case of CN radical and C2 molecule whose optical emission spectra were recorded during pulsed excimer laser ablation of a graphite target in low-pressure nitrogen environment. Computed similar or dissimilar values of rotational and vibrational temperatures let to verify the existence or not of local thermodynamic equilibrium and to hypothesise the temporal range necessary to establish it in such experiments.     

Study of CN emission from a laser induced plasma of graphite in air

Emission of neutral N atoms and CN molecules has been studied in the plasma produced by focusing a 200 mJ Nd: YAG laser onto a graphite target in air. Emission of atomic fines appears at early times. The Stark broadening of these lines is strongly time dependent due to the evolution of electron density in the plasma. At later times the spectrum is dominated by the CN violet system B₂Σ-X₂Σ. In the range from 1 to 20 μs the population of B₂Σ rovibrational levels can be described by a temperature of about 8500 K.     

微波等离子体炬光源基本特性的研究

用计算机化断层扫描成象技术研究了微波等离子体炬(MPT)放电的发射轮廓,证明该光源具有良好的对称性,并有一个有利于样品引入的中央通道,最佳分析区在炬管上方5～10 mm,用激光Thomson散射和Rayleigh散射技术测定了ArMPT和HeMPT放电的电子温度、电子密度和气体温度,证明MPT放电的电子温度很高而气体温度较低,是一种非热光源。其中的高能电子处于过布居状态,具有很高的激发能力。     

C2 and CN formation by optical pumping of CO/Ar and Co/N2/Ar mixtures at room temperature

A continuous wave carbon monoxide laser is used to excite the vibrational mode of CO in CO/Ar and CO/N₂/Ar mixtures flowing through a gas absorption cell. High steady-state excitation of the CO vibrational mode (0.3 eV/molecule) is achieved, while a translational—rotational temperature near 300 K is maintained by the steady flow of cold gas into the cell. These non-equilibrium conditions result in extreme vibration—vibration pumping, population high-lying vibrational quantum levels (to V = 42) of CO. N₂ can also be pumped by vibrational energy transfer from CO. Under these conditions, C₂ and CN molecules are formed, and are observed to fluoresce on various electronic band transitions, notably C₂ Swan (A ³Π_g—X ³Π_u) and CN violet (B ²Σ⁺—X²Σ⁺).     

Direkte Analyse fester Proben mit der induktiv gekoppelten Plasma-Emissionsspektrometrie bei hoher Leistung

Zusammenfassung Es wird eine Methode zur Einleitung von Probendampf in den Brenner des induktiv gekoppelten Plasmas bei hoher Leistung und dem Kühl-/Brenngas Stickstoff/Argon beschrieben. Der Dampf wird in einer einfachen Gasdurchflußkammer, die ein Elektrodensystem enthält, erzeugt. Bei metallischen Proben oder Preßlingen aus Cu-Pulver und Oxiden werden diese gegen eine Graphitelektrode geschaltet, während oxidisches Material in eine Graphit-elektrode mit Vorratsgefäß gegeben wird. Mit Hilfe eines Abreiß-oder Dauerbogens wird der Probendampf erzeugt und mit einem Ar-Trägergasstrom in das Brennersystem transportiert.Zahlreiche Elemente lassen sich direkt ohne spezielle Probenvorbereitung bestimmen. Da eine Verdünnung der Proben, z. B. durch einen Aufschluß oder eine Lösungstechnik, nicht nötig ist, kann die volle Empfindlichkeit der ICP-Methode für die analytische Aufgabe genutzt werden.

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Direct analysis of compact samples using the high power ICP emission spectrometry

Summary The introduction of sample vapour into the torch of the high power nitrogen/argon ICP system is described. The vapour can be produced by using a simple flow chamber containing an electrode gap. In case of metallic samples or briquetted mixtures of Cu powder and oxides a point-to-plane technique with a graphite counter electrode is used. Oxidic materials are put into a graphite cup-electrode. An interrupted arc is producing the vapour which is transported by the Ar carrier gas flow into the torch.Many elements can be determined directly without a special sample preparation. Because there is no necessity to dilute samples, by a fusion or dissolution procedure for example, the total sensitivity of the ICP method can be used for analytical problems.

     

The gas temperature inside graphite furnaces used for atomic absorption spectrometry

A simplified model for heat transfer is used to calculate the gas temperature inside graphite furnaces used for atomic absorption spectrometry. In the absence of a convective flow through the furnace, the gas temperature follows the wall temperature of the heated furnace to within a few degrees. When a significant flow of protective cold gas is forced through the furnace, the gas temperature varies greatly along the furnace and its average value can be several hundred degrees lower than the wall temperature. These predictions are confirmed by the vapour temperatures which were measured experimentally by means of the two-line absorption technique. The results also demonstrate that analyte species volatilized in the furnace have the same temperature as the inert gas. The advantages of preheating the protective gas are indicated.     

Relaxation of vibrational nonequilibrium distributions: the effect of multiphoton excitation on relaxation behavior

A theoretical study has been made on the non-stationary relaxation of highly vibrationally excited molecules in the presence of a source, producing these molecules. Initially, the nonequilibrium vibrational distribution is created using multiphoton absorption of laser radiation. An exact, analytical solution to the master equation is derived in terms of Meixner polynomials with regard to VV- and VT-processes. The time-dependent distribution of the system “molecule + field” is used to give explicit expressions for the mean number of photons distributed among vibrational and translational degrees of freedom and for the time dependence of average energy transferred per collision.     

Relaxation behavior of rovibrationally excited H2 in a rarefied expansion

The evolution of the rotational and vibrational distributions of molecular hydrogen in a hydrogen plasma expansion is measured using laser induced fluorescence in the vacuum-UV range. The evolution of the distributions along the expansion axis shows the relaxation of the molecular hydrogen from the high temperature in the upstream region to the low ambient temperature in the downstream region. During the relaxation, the vibrational distribution, which has been recorded up to v = 6, is almost frozen in the expansion and resembles a Boltzmann distribution at T approximately 2200 K. However, the rotational distributions, which have been recorded up to J = 17 in v = 2 and up to J = 11 in v = 3, cannot be described with a single Boltzmann distribution. In the course of the expansion, the lower rotational levels (J < 5) adapt quickly to the ambient temperature ( approximately 500 K), while the distribution of the higher rotational levels (J > 7) is measured to be frozen in the expansion at a temperature between 2000 and 2500 K. A model based on rotation-translation energy transfer is used to describe the evolution of the rotational distribution of vibrational level v = 2 in the plasma expansion. The behavior of the low rotational levels (J < 5) is described satisfactory. However, the densities of the higher rotational levels decay faster than predicted.     

Optical and X-ray emission spectroscopy of high-power laser-induced dielectric breakdown in molecular gases and their mixtures

Large-scale plasma was created in molecular gases (CO, CO2, N2, H2O) and their mixtures by high-power laser-induced dielectric breakdown (LIDB). Compositions of the mixtures used are those suggested for the early earth's atmosphere of neutral and/or mildly reducing character. Time-integrated optical spectra emitted from the laser spark have been measured and analyzed. The spectra of the plasma generated in the CO-containing mixtures are dominated by emission of both C2 and CN radicals. A vibrational temperature of approximately 10(4) K was determined according to an intensity distribution in a vibronic structure of the CN (B2Sigma(+)u-X2Sigma(+)g) violet band. For comparison, the NH3-CH4-H2-H2O mixture has been irradiated as a model of the strongly reducing version of the early earth's atmosphere. In this mixture, excited CN seems to be significantly less abundant than C2. The LIDB experiments were in the molecular gases carried out not only in the static cell but also using a large, double stream pulse jet (gas puff target) placed in the vacuum interaction chamber. The obtained soft X-ray emission spectra indicate the presence of highly charged atomic ions in the hot core of high-power laser sparks.     

Physikalische und emissionsspektroskopische Untersuchungen am Lichtbogen unter Helium-Drücken bis zu 50 at

This study deals with the application of the d.c. arc as a spectrochemical source of light at high helium pressures, viz. power reactor conditions, for identifying spherical graphitic fuel elements which had been deliberately contaminated with defined impurities. Tipped tungsten tacks and graphite electrodes of cylindrical or spherical shape were used: the tacks served as cathodes, the graphite electrodes as anodes. The He-gas pressure during discharge was varied between 1 atm and 50 atm. The runs at high He-pressure show that the arc changes at pressures > 30 atm into a form of discharge governed by convection. At 10 atm however the arc was still found stabilised by the electrodes. At pressures > 30 atm we got distinct evidence for vaporisation. The change in characteristics of the gas type arc into a vapour arc is marked by an increase in operating voltage at currents greater than 20 amps. Not unexpectedly the operating voltage increases rapidly with increasing gas pressure during discharge. The lines of the spectrum are broadening proportionally to the gas pressure. A homogeneous magnetic field, simultaneously applied along the axis of the arc, centralises and stabilises the arc discharge and reduces the material transport from the anode. With increasing magnetical induction the line broadening reduces. The good thermal conductivity of the He and the increasing heat convection with rising pressure both result in rapid increase of power and radiation density with the consequence of higher temperature. Temperature measurements with two pairs of Mg II lines using a 20 amps arc discharge at He pressure of 40 atm yielded about 13000 K. In an example for a pair of ion lines in the Co/Ni element combination a satisfactory spectral-analytical calibration curve was obtained at helium pressure. It is possible to increase the reproducibility and narrow down the lines of the spectrum with the consequence of greater accuracy of the resulting analytical curve if a homogeneous magnetic field is applied.     

The possibility of using femtosecond IR spectroscopy to determine the composition of molecules in the gas phase

The possibility of using one method of femtosecond laser spectroscopy, IR pump-probe, is considered for analyzing the molecular composition of gas. The measured intramolecular relaxation time of the vibrational energy is proposed as an individual characteristic of molecules. A number of compounds of different classes having the molecular chromophore group C=O are investigated. It is shown that the characteristic relaxation times of the corresponding analytical signals in the resonant exciting of C=O vibrations are significantly different for different molecules (ten or more), making it possible to identify molecules in mixtures by means of temporal selection. The linear dependence of the analytical signal on the vapor pressure of the investigated compounds and the energy fluence of the exciting radiation is revealed. It is shown that the analytical signal is not dependent on the pressure of the buffer gas (nitrogen) up to 1 atm.     

Influence of graphite furnace tube design on vapour temperatures and chemical interferences in ETA-AAS

A two-line atomic absorption method for determination of lead was used for calculation of the temperatures experienced by analyte atoms in the gas phase after wall atomization with modified Philips SP-9 graphite tubes. For each tube, the influence of the temperature gradient on the vapour phase temperature and chemical interferences experienced by Cd, Mn and Pb in ETA-AAS was investigated. A higher vapour temperature and lower chemical interference by chlorides were observed when the tube temperature gradient was reversed through a reduction in the wall thickness towards the ends of the tube.     

Temperature Distribution in a Pilot Plasma Tundish: Comparison Between Plasma Torch and Graphite Electrode Systems

Arc, bath, and refractory wall temperatures are measured in a pilot transferred-arc plasma furnace by atomic emission spectroscopy (AES) and multiwavelength pyrometry. Argon plasma torch and graphite electrode with nitrogen as plasma gas are both examined and compared using the steel bath as anode. With argon, a two-slope characteristic curve is measured for arc temperature, which ranges from 9000 to 25,000 K. Another trend is observed with nitrogen for temperatures in the range 8000–12,000 K. In this latter case, the bath temperature is very sensitive to arc length: more than 100 K increase results in arc length rise from 50 to 150 mm. Experimental data shows the variation of heat transfer efficiency between the two configurations, which is supported by results about surface emissivity in the spectral domain 1–15 m.     

Vector and scalar correlations in the photodissociation of NCCN

The CN photofragments from the photodissociation of NCCN at 193 nm have been measured by high-resolution transient absorption spectroscopy. Doppler-broadened profiles of isolated rotational lines in the 2-0 and 3–1 vibrational bands of the CN A---X transition were observed under collisionless conditions with a tunable, single-frequency Ti:sapphire ring laser. Analysis of the Dopple profiles reveals a vector correlation between the translation and rotation of CN photoproducts, with the angular momentum of the high rotational states increasingly perpendicular to the recoil velocity. After correction for vector correlations, the laboratory-frame scalar speed distribution of state-selected photoproducts can be determined. The mean squared laboratory velocity is directly related to the average internal energy of coincident CN fragments. The wings of the Doppler profiles indicate that the available energy for a pair of ground state CN photoproducts following 193 nm dissociation of NCCN at 295 K is 5300±150 cm⁻¹, which includes the average vibrational energy of the parent molecules selected by the photolysis laser. Phase space theory with an optimized available energy of 5300 cm⁻¹ produces laboratory speed distributions that are in qualitatively reasonable agreement with the kinetic energy measurements, but overestimate the total internal energy of the photofragments. The measurements are good enough to warrant comparison with more sophisticated models of unimolecular decomposition.     

Energy distributions of CO produced in an acetone-containing discharge

Tunable infrared diode laser absorption (TDLAS) and Fourier transform infrared absorption spectroscopies (FTIR) have been utilized to characterize the translational, rotational and vibrational distributions of CO in an acetone/argon DC plasma at total pressures ranging from 4 to 5 Torr and currents of 0.1–0.3 A. A broad vibrational distribution of CO was observed with gradually decreasing intensities from the fundamental band to v=12←11. When nitrogen was added to the plasma, the distribution is narrower, due to the efficient energy transfer between CO and N₂ molecules. The measured translational temperature in such plasmas ranged from 400–550 K. The rotational distribution can generally be fitted to a Boltzmann distribution within each vibrational level although the rotational temperature is highest for the lowest vibrational quantum number.     

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.     

Vibrational excitation of SF6 scattering from graphite

We have observed vibrationally excited sulfur hexafluoride molecules in direct inelastic scattering from hot graphite surfaces. The vibrational temperature for the scattered flux has been determined by probing the effect of internal temperature on electron-induced fragmentation observed in mass spectra. The vibrational excitation depends on incident translational energy, E_tr, and a maximum temperature increase of 50 K is reached in direct scattering at E_tr = 2.5 eV. No effect of surface temperature has been observed at 950–1400 K. Inelastic angular distributions are reproduced by a collision complex model, and the experimental results are related to existing models for vibrational excitation.