A Thermo-Chemical Reactor for analytical atomic spectrometry

A novel atomization/vaporization system for analytical atomic spectrometry is developed. It consists of two electrically and thermally separated parts that can be heated separately. Unlike conventional electrothermal atomizers in which atomization occurs immediately above the vaporization site and at the same instant of time, the proposed system allows analyte atomization via an intermediate stage of fractional condensation as a two stage process: Vaporization → Condensation → Atomization. The condensation step is selective since vaporized matrix constituents are mainly non-condensable gases and leave the system by diffusion while analyte species are trapped on the cold surface of a condenser. This kind of sample distillation keeps all the advantages of traditional electrothermal atomization and allows significant reduction of matrix interferences. Integration into one design a vaporizer, condenser and atomizer gives much more flexibility for in situ sample treatment and thus the system is called a Thermo-Chemical Reactor (TCR). Details of the design, temperature measurements, vaporization-condensation-atomization mechanisms of various elements in variety of matrices are investigated in the TCR with spectral, temporal and spatial resolution. The ability of the TCR to significantly reduce interferences and to conduct sample pyrolysis at much higher temperatures as compared to conventional electrothermal atomizers is demonstrated. The analytical potential of the system is shown when atomic absorption determination of Cd and Pb in citrus leaves and milk powder without the use of any chemical modification. © 2008 Elsevier B.V. All rights reserved

Study of the optical systems of atomic-absorption spectrophotometers

This paper analyzes the factors that ensure that optical atomic-absorption spectrophotometers can give reliable information on the composition of a sample introduced into the atomization zone. It discusses how the parameters of the radiation source, the optical elements, the radiation detector, and the general layout of the optical system mutually affect the measurement results. Ways to improve the optical systems of atomic-absorption spectrometers are pointed out. Prospects of using position-sensitive linear radiation detectors in the devices are discussed. © 1996 The Optical Society of America

Comparative study of the effect of sample matrix on the atomic absorption of high-volatile elements in two-step and conventional atomizers

The space-time dynamics of absorbing atomic layers of cadmium and lead and molecular layers of zinc chloride in a commercially produced transverse-heated graphite atomizer and a newly developed two-step atomizer was studied. It was shown that the limiting temperature of cadmium pyrolysis in the two-step atomizer without the use of modifiers may be as high as 1000°C, whereas in the commercial analyzer is it not higher than 300°C. Levels of nonselective absorption due to sodium chloride were compared. It was found that, for a two-step atomizer, the maximum allowable mass of sodium chloride for which the background at lead and cadmium lines can be adequately compensated is 17-30 times higher than that for the commercial atomizer. The atomization of cadmium in the presence of sodium chloride was studied using time, space, and spectral resolution. It was shown that the effect of the chloride matrix in the two-step atomizer is suppressed because of sample fractionation and distillation in the course of its evaporation and condensation

Recording system with spatial resolution for atomic-absorption spectrophotometers

An alternative method is developed for recording atomic absorption with spatial resolution on the basis of a position-sensitive photodetector. The main requirements on the optical system, the source, the radiation detector, and the algorithms for processing and interpreting the results are formulated. A description is given of how the proposed method is put into practice on the basis of a commercial atomic-absorption spectrophotometer. The advantages of the new recording system are shown, using the atomization of biological specimens of complex composition as an example

Analytical measurement in electrothermal atomic absorption spectrometry - How correct is it?

The detection system for atomic absorption spectrometers based on the use of photomultiplier tubes (PMTs) is analysed from the point of view of its ability to provide accurate analytical information. It is shown that absorbance recorded by the system depends not only on the number of absorbing atoms but also on their distribution in the furnace volume. The typical non-uniformity of atomic distribution that occurs in graphite furnaces and its impact on the recorded signal are discussed. The cross-sectional distribution of the intensity of the radiation beam from a primary source was measured at different locations of the spectrometer for different source operating conditions. The distribution is rather non-uniform and can be described by the Gaussian function. An analysis of the joint effect of the radiation and analyte non-uniformity on the absorbance measured is given. The shape of the radiation beam cross-section changes from a circle to an ellipse with increasing lamp current. A new detection system based on the use of a solid-state detector (photodiode array, charge coupled device, charge injection device) instead of PMTs is proposed. The solid-state detector is located vertically along the monochromator exit slit and allows the detection of spatially resolved absorbances. It is shown that the analytical signal recorded by this new system is proportional to the number of absorbing atoms irrespective of the non-uniformities described above

Three-dimensional time-dependent computer modeling of the electrothermal atomizers for analytical spectrometry

© 2015 Elsevier B.V. All rights reserved. A full three-dimensional nonstationary numerical model of graphite electrothermal atomizers of various types is developed. The model is based on solution of a heat equation within solid walls of the atomizer with a radiative heat transfer and numerical solution of a full set of Navier-Stokes equations with an energy equation for a gas. Governing equations for the behavior of a discrete phase, i.e., atomic particles suspended in a gas (including gas-phase processes of evaporation and condensation), are derived from the formal equations molecular kinetics by numerical solution of the Hertz-Langmuir equation. The following atomizers test the model: a Varian standard heated electrothermal vaporizer (ETV), a Perkin Elmer standard THGA transversely heated graphite tube with integrated platform (THGA), and the original double-stage tube-helix atomizer (DSTHA). The experimental verification of computer calculations is carried out by a method of shadow spectral visualization of the spatial distributions of atomic and molecular vapors in an analytical space of an atomizer

Dynamics of the Spatial Distribution of Atomic and Molecular Absorbing Layers in the Electrothermal Vaporization and Electrostatic Precipitation of an Analyte in an Atomizer

An experimental device was described for atomic absorption analysis with the electrothermal vaporization of the initial sample followed by the condensation of vaporization products and the electrostatic precipitation of the resulting aerosol in the secondary atomizer. Working conditions ensuring the maximum transfer of the sample to the atomizer were determined. The dynamics of the spatial distribution of the absorbing atomic and molecular layers was studied for atomization in a graphite furnace after the direct sample injection and electrostatic precipitation. The contribution of some physicochemical processes to the formation of the structure of cadmium atomic layers was assessed for different methods of sample injection into the atomizer. It was shown that an additional vaporization-condensation step significantly decreases the level of nonselective absorption and smoothes its gradients

Two-Stage Atomizer for Electrothermal Atomic Absorption Spectrometry: Dynamics of the Spatial Temperature Distribution

A two-stage atomizer for atomic absorption spectrometry is proposed. Its distinctive feature is the introduction of an extra stage of the fractional condensation of analyte atoms and the carrying out of the analytical cycle by a vaporization-condensation-atomization scheme. A special computer-driven power supply unit allows the heating of the upper and lower parts of the graphite furnace to be controlled independently. The dynamics of the temperature of the inner surface of the furnace for various temperature programs is studied. Using aqueous solutions of lead as an example, it is shown that one can control the processes of condensation-revaporization of elements to be determined proceeding in the atomizer volume

Spatially resolved atomic absorption analysis

Previous research carried out in our laboratories has shown that all the key parameters of electrothermal atomic absorption spectrometry, gas phase temperature, intensity of the probing beam and number density of absorbing species, are generally highly non-uniform over the absorption volume. Further, it was shown that, when using conventional detection systems based on a photomultiplier tube or a photodiode that can only detect radiation spatially integrated over their working area, absorbance measurements are subject to photometric errors when the absorbing layer is spatially non-uniform. This error is eliminated when using spatially-resolved detection of transmitted intensities with a linear solid state detector (photodiode array, linear CCD). The photometric error of the conventional detection systems does result in an analytical error, if analyte distributions in the absorption volume produced from an aqueous standard solution and the unknown sample are different. Such a differing distribution could be created under the influence of the sample matrix on the analyte gas phase distribution. An atomic absorption spectrometer is described in the paper that allows spatially and temporally resolved detection of both specific and non-specific absorbances. The effect of sample matrix on the analyte gas phase distribution is investigated when atomizing some environmental samples and, for the first time, the results of spatially-resolved atomic absorption determination of cadmium and lead in these samples are presented. It is shown that the influence of the matrix on the analyte distribution is significant, resulting in a significant analytical error. By avoiding such errors, the benefits of atomic absorption analysis with spatial resolution over conventional AAS are directly demonstrated

Spatiotemporal visualization of the dynamics of absorbing layers in a two-stage atomizer

This paper describes an experimental apparatus for studying the dynamics of the variation of the spatial distribution of the absorbing layers in a two-stage atomizer. An optical system with a telecentric beam, using a diode laser as the radiation source, is proposed and analyzed. The spatiotemporal resolution and the contribution of refraction effects to the resulting absorption pattern are estimated. The dynamics of the evaporation and condensation processes are visualized in a two-stage atomizer of Cd atoms and NaCl molecules