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A continuum-source single-detector resonance-monochromator for atomic-absorption spectrometry
0039-g 140/79/0301-0249102.00/0
Tulunt.. Vol. 26. pp. 249-250 0 Pergamon Press Ltd 1979. Printed in Great Britain
A CONTINUUM-SOURCE SINGLE-DETECTOR MONOCHROMATOR FOR ATOMIC-ABSORPTION
RESONANCESPECTROMETRYT
JAMES BOWER, JOHN BRADSHAWand JAMS WINEFORDNER~ Department of Chemistry, University of Florida, Gainesville, FL 32611, U.S.A. (Received 9 May 1978. Accepted 8 June 1978) Summary-The detector, for atomic-absorption spectrometry, consists of a furnace into which a .constant concentration of the element to be analysed is atomized. Resonance radiation is excited by the light from a xenon-arc lamp which traverses the burner flame. The resonance radiation passes through a tunable grating filter and is measured with a photomultiplier.
Resonance monochromators for atomic-absorption spectrometry (AAS) have been an attractive proposition since Russell and Walsh’ observed resonance radiation from a hollow-cathode lamp in 1959. Experimental use of resonance monochromators as detectors for AAS has centred around the use of specific atomic-line sources (electrodeless discharge lamps, EDLs; and hollow-cathode lamps, HCLs), and individual element-sputtering, thermal, or flame atom-cloud generators.z4 The present work involves the use of a continuum source and an atmosphericpressure flow-through Molnar and Winefordner type? furnace for atom generation. Preliminary detection limits for Cu and Mg are included as well as a discussion of sensitivity. The advantages realized by use of a resonance monochromator as a detector for AAS are manifold. They include simplicity of design, ease of use, right-angle geometry and elimination of the monochromator generally used to isolate the line of interesi. These advantages were realized in a commercial instrument in the late 1960s. The major reason for the lack of acceptance of this instrument was the necessity to use a different source and detector for each element, thus incurring quite a large expense for routine operation. The present work concentrates on the use of a singlesource, single-detector system, with an inexpensive tunable grating filter (low-resolution, large aperture). The basis for the continuum-source resonance detector is evident from the plot of log IF (atomic-fluorescence intensity) us. log analyte concentration (Fig. 1). which exhibits a plateau, as predicted by Hooymayer@ and by Winefordner et a/.’ The plateau region of this curve should result in freedom from the low-frequency noise produced in the nebulization-atomization process. Changes in the analyte concentration produced by fluctuating gas fiows and flame conditions should result in no I, fluctuation, because the derivative of IF with respect to the analyte concentration is zero,
making the signal limited’by analyte shot noise (or by background flicker noise in a few cases).
To realize the advantages of the resonance monochromator, the design of the atom cell must be carefully considered. The most important characteristics are high temperature and an inert or reducing atmosphere to produce good atomization of a wide range of analyte species, and low background to yield low emission noise. Molnar and Winefordner’s furnace designs provides these advantages and was used with minor modifications. The furnace power-supply used provides an owratina temperature of ilOO (m&&l bi optical pyiometerr ai a power dissipation of 1.9 kW. The fIow-aas is argon. .+ . with knough hydrogen to support a diffusion flame. A low flowrate of methane is used to provide longer carbon-furnace tube-life. System design (Fig. 2) included the following considerations. The source was chosen for output power, with reasonable stability. Standard optics for atomic fluorescence were chosen, with the exception of a collimated section for the atomic-absorption flame. A slit mask immediately followed the second lens to provide some semblance of the geometry of the three-slot burner used with a standard Perkin-Elmer chamber for the AA measurements. Collection of the resonance radiation is through one lens to a 0.1-m tunable grating filter (monochromator) with a 40-A bandpass; the grating filter is used to reduce the amount of stray room-light striking the photomultiplier and to minimize background and scatter. The detector is supplied with desolvated salt particles at a low gas flow-rate by an ultrasonic nebulizer system, providing a residence time consistent with the detector requirements.
Fig. 1. Log atomic-fluorescence intensity us. log analyte concentration.
Fig. 2. Experimental system: A, 300-W Eimac continuum Xe lamp; B, focusing lens; C, iris diaphragm; D, chopper; E, collimating lens; F, slit mask; G, 3-slot burner; H, focusing lens; I, flow-through furnace; J, collection lens;. K, tunable grating filter.
EXPERIMENTAL
250
SHORT
COMMUNICATlONS
Table 1. Furnace operating characteristics Gas flow-rates, I./min
Ar 0.43 H2 1.10 CH4 0.083
Power 1.9 kW (235 A at -8 V) Temperature 2100°C Residence time 4 msec
The furnace operating characteristics are given in Table I. All measurements are made under the same furnace: atom-cell conditions. The furnace tubes are high-temperature graphite precoated with a layer of pyrolytic graphite as described by Clyburn et a/.* The operating temperature should be kept high in order to maintain the pyrolytic coating, thereby increasing tube-life.
been of the order of Cochran and Hieftje’s result from droplet modulation AAS, 0.073 pg/ml,’ but was about four times higher. A possible explanation is that the increased line-width due to operation on the plateau of Fig. 1 caused the sensitivity loss. However, the sensitivity was better than that indicated by Cochran and Hieftje for conventional AAS with a continuum source (0.66 &ml). Several extensions are possible for a system of the type described, e.g., simultaneous multielement AAS by use of either interference filters or a direct reader system. Magnesium was measured with a filter-type system and a comparable detection limit was obtained. For simultaneous analysis, a multielement mixture would be introduced into the atom cell, producing resonance radiation at the characteristic wavelength of each element. An experiment of this type has led to usable resonance signals for Cu. Ca, Mg. Cr and Ni, observed by scanning the spectrum, while keeping all atomization conditions the same as listed in Table I. Other possible uses of the present system include isotope detection, and determination of elements for which HCLs or EDLs give poor performance.
REFERENCES
RESULTS Preliminary high detection limits of 0.1 ppm for both Cu and Mg indicated a problem with system noise. This was traced to two sources, a faulty regulation card in the lamp power-supply causing major noise problems in the detector output, and flame wander in the furnace. Use of the 900-W xenon short-arc lamp eliminated much of the lamp noise. (Unfortunately, the power supply failed before significant experimentation could be completed.) Preliminary experiments with a quartz sheath have shown the possibility of reduction in flame-wander noise. In addition, a new furnace has been built to give isokinetic sheathing. A sensitivity for copper (IUPAC characteristic concentration) of 0.27 pg/ml was somewhat less than that of linesource atomic-absorption measurements. It should have
1.
B. J. Russell and A. Walsh, Spectrochim. Acta, 1959, 15, 883. 2. J. V. Sullivan and A. Walsh, Appl. Optics, 1968, 7, 1271. 3. E. F. Palermo and S. R. Crouch, Anal. Chem., 1973, 45, 1594. 4. A. Walsh, Analyst,
1975, 100, 764. 5. C. J. Molnar and J. D. Winefordner, Anal. C&m., 1974, 46, 1419. 6. H. P. Hooymayers, Specrrochim. Acta, 1968, 23B. 567. 7. J. D. Winefordner, V. Svoboda and F. F. Browner,
Appl. Spectry., 1972, 26, 505. 8. S. A. Clyburn, T. Kantor and C. Veillon, Anal. Chent., 1974, 46, 2213. 9. R. L. Cochran and G. M. Hieftje, ibid., 1977, 49, 98.
Tulunt.. Vol. 26. pp. 249-250 0 Pergamon Press Ltd 1979. Printed in Great Britain
A CONTINUUM-SOURCE SINGLE-DETECTOR MONOCHROMATOR FOR ATOMIC-ABSORPTION
RESONANCESPECTROMETRYT
JAMES BOWER, JOHN BRADSHAWand JAMS WINEFORDNER~ Department of Chemistry, University of Florida, Gainesville, FL 32611, U.S.A. (Received 9 May 1978. Accepted 8 June 1978) Summary-The detector, for atomic-absorption spectrometry, consists of a furnace into which a .constant concentration of the element to be analysed is atomized. Resonance radiation is excited by the light from a xenon-arc lamp which traverses the burner flame. The resonance radiation passes through a tunable grating filter and is measured with a photomultiplier.
Resonance monochromators for atomic-absorption spectrometry (AAS) have been an attractive proposition since Russell and Walsh’ observed resonance radiation from a hollow-cathode lamp in 1959. Experimental use of resonance monochromators as detectors for AAS has centred around the use of specific atomic-line sources (electrodeless discharge lamps, EDLs; and hollow-cathode lamps, HCLs), and individual element-sputtering, thermal, or flame atom-cloud generators.z4 The present work involves the use of a continuum source and an atmosphericpressure flow-through Molnar and Winefordner type? furnace for atom generation. Preliminary detection limits for Cu and Mg are included as well as a discussion of sensitivity. The advantages realized by use of a resonance monochromator as a detector for AAS are manifold. They include simplicity of design, ease of use, right-angle geometry and elimination of the monochromator generally used to isolate the line of interesi. These advantages were realized in a commercial instrument in the late 1960s. The major reason for the lack of acceptance of this instrument was the necessity to use a different source and detector for each element, thus incurring quite a large expense for routine operation. The present work concentrates on the use of a singlesource, single-detector system, with an inexpensive tunable grating filter (low-resolution, large aperture). The basis for the continuum-source resonance detector is evident from the plot of log IF (atomic-fluorescence intensity) us. log analyte concentration (Fig. 1). which exhibits a plateau, as predicted by Hooymayer@ and by Winefordner et a/.’ The plateau region of this curve should result in freedom from the low-frequency noise produced in the nebulization-atomization process. Changes in the analyte concentration produced by fluctuating gas fiows and flame conditions should result in no I, fluctuation, because the derivative of IF with respect to the analyte concentration is zero,
making the signal limited’by analyte shot noise (or by background flicker noise in a few cases).
To realize the advantages of the resonance monochromator, the design of the atom cell must be carefully considered. The most important characteristics are high temperature and an inert or reducing atmosphere to produce good atomization of a wide range of analyte species, and low background to yield low emission noise. Molnar and Winefordner’s furnace designs provides these advantages and was used with minor modifications. The furnace power-supply used provides an owratina temperature of ilOO (m&&l bi optical pyiometerr ai a power dissipation of 1.9 kW. The fIow-aas is argon. .+ . with knough hydrogen to support a diffusion flame. A low flowrate of methane is used to provide longer carbon-furnace tube-life. System design (Fig. 2) included the following considerations. The source was chosen for output power, with reasonable stability. Standard optics for atomic fluorescence were chosen, with the exception of a collimated section for the atomic-absorption flame. A slit mask immediately followed the second lens to provide some semblance of the geometry of the three-slot burner used with a standard Perkin-Elmer chamber for the AA measurements. Collection of the resonance radiation is through one lens to a 0.1-m tunable grating filter (monochromator) with a 40-A bandpass; the grating filter is used to reduce the amount of stray room-light striking the photomultiplier and to minimize background and scatter. The detector is supplied with desolvated salt particles at a low gas flow-rate by an ultrasonic nebulizer system, providing a residence time consistent with the detector requirements.
Fig. 1. Log atomic-fluorescence intensity us. log analyte concentration.
Fig. 2. Experimental system: A, 300-W Eimac continuum Xe lamp; B, focusing lens; C, iris diaphragm; D, chopper; E, collimating lens; F, slit mask; G, 3-slot burner; H, focusing lens; I, flow-through furnace; J, collection lens;. K, tunable grating filter.
EXPERIMENTAL
250
SHORT
COMMUNICATlONS
Table 1. Furnace operating characteristics Gas flow-rates, I./min
Ar 0.43 H2 1.10 CH4 0.083
Power 1.9 kW (235 A at -8 V) Temperature 2100°C Residence time 4 msec
The furnace operating characteristics are given in Table I. All measurements are made under the same furnace: atom-cell conditions. The furnace tubes are high-temperature graphite precoated with a layer of pyrolytic graphite as described by Clyburn et a/.* The operating temperature should be kept high in order to maintain the pyrolytic coating, thereby increasing tube-life.
been of the order of Cochran and Hieftje’s result from droplet modulation AAS, 0.073 pg/ml,’ but was about four times higher. A possible explanation is that the increased line-width due to operation on the plateau of Fig. 1 caused the sensitivity loss. However, the sensitivity was better than that indicated by Cochran and Hieftje for conventional AAS with a continuum source (0.66 &ml). Several extensions are possible for a system of the type described, e.g., simultaneous multielement AAS by use of either interference filters or a direct reader system. Magnesium was measured with a filter-type system and a comparable detection limit was obtained. For simultaneous analysis, a multielement mixture would be introduced into the atom cell, producing resonance radiation at the characteristic wavelength of each element. An experiment of this type has led to usable resonance signals for Cu. Ca, Mg. Cr and Ni, observed by scanning the spectrum, while keeping all atomization conditions the same as listed in Table I. Other possible uses of the present system include isotope detection, and determination of elements for which HCLs or EDLs give poor performance.
REFERENCES
RESULTS Preliminary high detection limits of 0.1 ppm for both Cu and Mg indicated a problem with system noise. This was traced to two sources, a faulty regulation card in the lamp power-supply causing major noise problems in the detector output, and flame wander in the furnace. Use of the 900-W xenon short-arc lamp eliminated much of the lamp noise. (Unfortunately, the power supply failed before significant experimentation could be completed.) Preliminary experiments with a quartz sheath have shown the possibility of reduction in flame-wander noise. In addition, a new furnace has been built to give isokinetic sheathing. A sensitivity for copper (IUPAC characteristic concentration) of 0.27 pg/ml was somewhat less than that of linesource atomic-absorption measurements. It should have
1.
B. J. Russell and A. Walsh, Spectrochim. Acta, 1959, 15, 883. 2. J. V. Sullivan and A. Walsh, Appl. Optics, 1968, 7, 1271. 3. E. F. Palermo and S. R. Crouch, Anal. Chem., 1973, 45, 1594. 4. A. Walsh, Analyst,
1975, 100, 764. 5. C. J. Molnar and J. D. Winefordner, Anal. C&m., 1974, 46, 1419. 6. H. P. Hooymayers, Specrrochim. Acta, 1968, 23B. 567. 7. J. D. Winefordner, V. Svoboda and F. F. Browner,
Appl. Spectry., 1972, 26, 505. 8. S. A. Clyburn, T. Kantor and C. Veillon, Anal. Chent., 1974, 46, 2213. 9. R. L. Cochran and G. M. Hieftje, ibid., 1977, 49, 98.