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Ultrasensitive determination of gold in geogas by laser excited atomic fluorescence spectrometry1
Spectrochimica Acta Part B 53 (1998) 1421–1425
Ultrasensitive determination of gold in geogas by laser excited atomic fluorescence spectrometry1 Ma Wan-yun*, Xue Meng, Zhang Jun, Chen Die-yan Department of Modern Applied Physics, Centre of Atomic and Molecular Sciences, Tsinghua University, Beijing 100084, People’s Republic of China Received 7 December 1997; accepted 25 April 1998
Abstract Ultratrace gold (Au) in geogas samples has been determined by means of laser excited atomic fluorescence spectrometry combined with graphite electrothermal atomization and time-gate technique. Gold atoms were excited from the ground state to the 6p 2P 3/2 state by a pulsed laser beam with a wavelength of 242.8 nm. Fluorescence photons with a wavelength of 312.3 nm were measured by a photon-counting unit. The time-gate technique was used to reduce the background radiation caused by the furnace. This method has proved to be highly sensitive with minimal background interference. Eighty-two geogas samples were analysed and the Au contents obtained were in the range of 0.002–0.182 ng l −1. The study of Au concentration of geogas in soil is of great interest in prospecting gold deposits. q 1997 Elsevier Science B.V. All rights reserved Keywords: Au; Geogas; Graphite furnace; Time-gate; Laser excited atomic fluorescence spectrometry
1. Introduction A new efficient and low-cost method for prospecting gold deposits is to analyse the concentration of Au in geogas [1–3]. Wang et al. [4] developed a new rapid method for dynamic collection of geogas from soil over a buried deposit. This method has proved to be satisfactory. However, determining Au in such samples is very difficult, because the sample quantity is very small and the concentration of Au is very low, estimated roughly lower than 0.1 ng l −1 (l −1 means per litre geogas). Using conventional analytical methods for determining the Au content in these natural objects, preliminary chemical enrichment must be * Corresponding author. Tel.: 0086 10 62788938; fax: 0086 10 62781598; e-mail: [email protected] 1 This paper was published in the Special Issue from the BCEIA Conference in Shanghai, China, in October 1997.
used. The complexity of the multistage chemical treatment of samples makes such a preconcentration technique labour-intensive and not very reliable. The most severe problem is that up to now there is almost no suitable preliminary chemical enrichment method for the Au in geogas, because of the very small quantity. In our previous work the gold content in geogas has been determined by resonance ionization mass spectrometry (RIMS) with high sensitivity and high selectivity [5]. The detection limit of 0.003 ng ml −1 for Au has been obtained. But RIMS instrumentation is complicated and expensive, it uses three laser beams and high vacuum. A simple and sensitive analysis method is important for analysing a large number of samples. Laser excited atomic fluorescence spectrometry (LEAFS) has been proved to be an extremely powerful tool for detecting trace amounts of many elements,
0584-8547/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 8 )0 0 15 6 -6
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M. A. Wan-yun et al./Spectrochimica Acta Part B 53 (1998) 1421–1425
and it has been widely used in geochemistry, chemical exploration and many other fields [6–8]. LEAFS combined with graphite furnace electrothermal atomization and time-gate technique is characterized by a high elemental selectivity, a high sensitivity with minimal background interference and a large linear dynamic range. Other advantages of furnace-LEAFS are the small sample volumes needed for an analysis and both liquid and solid samples can be analysed directly, as well as little or no preliminary chemical treatment and preconcentration steps are required. The present paper first reports on the ultrasensitive determination of Au content in geogas samples, based on graphite electrothermal atomization of a substance followed by LEAFS with time-gate technique. The aim of our work is to develop a simple and ultrasensitive analysis method for Au in geogas, and to test if this method can be used in the search for gold deposits.
2. Experimental 2.1. Apparatus The schematic diagram of the apparatus is shown in Fig. 1. A 20 ml sample solution was pipetted into a graphite crucible and was heated by an alternating current. When the temperature was raised to about 15008C a neutral atomic beam of Au was obtained. The pulsed laser beam with a wavelength of 242.8 nm intersected the atomic beam perpendicularly to excite Au atoms from the ground state 6s 2S 1/2 to the
intermediate state 6p 2P 3/2. Then fluorescence photons with a wavelength of 312.3 nm of Au atoms were collected at 908 relative to the laser beam. Passing through a diaphragm and a monochromator (Model WDG30, Beijing Optical Instrumental Factory, Beijing, China), the fluorescence photons were detected by a fast response photo-multiplier tube (PMT, Model 212UH, Hamamatsu, Japan, high voltage 1200 V). The signal was amplified 10 times by a fast amplifier (AMP, home-made). Then the current pulse signal from AMP was coincided with the delayed timegate signal in synchronism with the laser pulse. The width of the time-gate was 200 ns. The background emission from the furnace was almost eliminated by the time-gate technique, because only the current pulse signal in synchronism with the laser pulse can be integrated into a charge, and then was converted to a digital signal by a charge-to-digital converter (QDC, home-made). Finally the data were acquired and processed automatically by a microcomputer. The laser beam was produced by a dye laser (Model FL3002EC, Lambda Physik, Germany) pumped by an excimer laser (Model EMG 202 MSC, Lambda Physik, Germany). The excimer laser (XeCl) can produce laser pulses of 400 mJ energy with 308 nm wavelength. The laser pulse width is 28 ns. The repetition rate of the excimer laser can be varied in the range of 1–150 Hz. In our experiments, the repetition rate selected was 20 Hz. The linewidth of the dye laser was 0.2 cm −1. The atomizer was a home-made graphite rod crucible (shown in Fig. 2) heated by a power supply (Model WF-1, Beijing Second Optical Instrumental Factory,
Fig. 1. Schematic diagram of the experimental setup.
M. A. Wan-yun et al./Spectrochimica Acta Part B 53 (1998) 1421–1425
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to re-dissolve AuCl 3 before analysis. Twenty ml of the sample solution was pipetted into a graphite crucible for each measurement. 3. Results and discussion 3.1. Calibration
Fig. 2. Detailed drawing of the graphite rod crucible (unit mm).
Beijing, China). The graphite rod was purged with argon gas at a flow rate of 0.5 l min −1. The electrodes of the atomizer were cooled by water. 2.2. Procedure of samples The samples were taken from the surface of the surrounding area of a known gold deposit in Shandong Province, China, which is concealed deep underground. For the large-scale area survey of giant gold deposits, the average sampling density was about one sample per 800 km 2. For every sample, 5 l geogas in soil were extracted by a gas sampling device [4] from a hole 60–70 cm deep underground and Au atoms were captured by a piece of polyurethane foam. The chemical treatment of the samples was very simple: the polyurethane foam was ashed at 4508C in a ceramic crucible and then dissolved in 1 ml pure aqua-regia to form AuCl 3. After drying, the sample can be preserved for at least several weeks. One ml 10% aqua-regia was added again in the ceramic crucible
The calibration curves were made by measuring the AuCl 3 standard solutions with the concentration series of 0.0625, 0.125, 0.25, 0.5, 1.0, 2.0 and 3.0 ng ml −1 before and after determining the geogas samples every day because of a lack of a standard geogas sample. Aqueous AuCl 3 solutions with different concentrations were obtained by dilution of the AuCl 3 standard solutions with de-ionized water. The acidity of aqueous AuCl 3 solutions was 10%. The calibration curve given in Fig. 3 shows excellent linearity. 3.2. Detection limit, precision and accuracy The AuCl 3 standard solution with concentration of 0.0625 ng ml −1 and the blank sample of de-ionized water, both with 10% aqua-regia, were used for determining the detection limit of Au. By taking 10 measurements for each sample and using the criterion of 3j, where j is the standard deviation of the blank sample, the detection limit of Au obtained in the AuCl 3 standard solution was 0.005 ng ml −1, a little higher than that by the RIMS method. For the 1 ng ml −1 AuCl 3 standard solution, the analysis precision was about 6% in 15 measurements. The standard
Fig. 3. The calibration curve for aqueous AuCl 3 standard solutions.
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M. A. Wan-yun et al./Spectrochimica Acta Part B 53 (1998) 1421–1425
Fig. 4. The Au content of geogas in soil (ng l −1).
geochemical sample of GAu-5 was selected to test the accuracy of this method. The mean value of Au in GAu-5 was 3.2 ng g −1 in 10 measurements using this method, quite coincident with the standard value of 3.3 6 0.2 ng g −1, given by the Chinese National Standard Institute. The accuracy was about 3%. 3.3. Geogas samples In our experiment, 82 geogas samples were analysed. Each sample was measured three times and the mean values are shown in Fig. 4. The concentrations of Au in these 82 geogas samples were in the range of 0.002–0.182 ng l −1. The precision is about 30% for the Au content in geogas at levels as low as 0.04 ng l −1. It seems that some places with a Au content higher than 0.1 ng l −1 have the possibility of having gold deposits.
operation by comparison with the RIMS method, providing for rapid measurements with little or no sample preparation and allowing the use of a very small quantity of sample. These merits are very important in ultratrace element analysis for hard-to-obtain samples and greatly improve the speed of the analysis. It is demonstrated that furnace-LEAFS with time-gate technique will promote the development of exploring the concealed deposits and has a wide range of applications in many fields.
Acknowledgements The authors would like to thank Professors X. J. Xie and X. Q. Wang of the Geophysical and Geochemical Exploration Institute for providing the geogas samples and for many helpful discussions during the experiment.
4. Conclusion References The furnace-LEAFS with time-gate technique is an extremely efficient method for the determination of ultratrace concentrations of elements. Significant attributes include excellent sensitivity with minimal background interference, simple device and easy
[1] X.J. Xie, Search for giant deposits by new opinion and technology, Scientific Chinese 5 (1995) 15–16. [2] T.X. Ren, Y.H. Liu, M.Q. Wang, Nanometer science and hidden mineral resources, Chinese Science and Technology Review 8 (1995) 18–19.
M. A. Wan-yun et al./Spectrochimica Acta Part B 53 (1998) 1421–1425 [3] R.W. Klusman, Soil Gas and Related Methods for Natural Resource Exploration, Wiley, New York, 1993. [4] X.Q. Wang, X.J. Xie, Y.X. Lu, Dynamic collection of geogas and its preliminary application in search for concealed deposits, Chinese Geophysical and Geochemical Exploration 19 (1995) 161–171. [5] W.Y. Ma, Q. Hui, M. Xue, W.X. Ji, D.Y. Chen, Determination of the gold content in geogas by resonance ionization mass spectrometry, J. Analyt. Atom. Spectrom. 12 (1997) 57–59.
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[6] S. Sjostrom, P. Mauchien, Laser atomic spectroscopic techniques— the analytical performance for trace element analysis of solid and liquid samples, Spectrochimica Acta Rev. 15 (1993) 153–180. [7] V. Cheam, J. Lechner, R. Desrosiers, J. Azcue, F. Rosa, A. Mudroch, LEAFS determination and concentration of metals in great lakes ecosystem, Fresenius J. Analyt. Chem. 355 (1996) 336–339. [8] Y. Oki, K. Furukawa, M. Maeda, Extremely sensitive Na detection in pure water by laser ablation atomic fluorescence spectroscopy, Opt. Commun. 133 (1997) 123–128.
Ultrasensitive determination of gold in geogas by laser excited atomic fluorescence spectrometry1 Ma Wan-yun*, Xue Meng, Zhang Jun, Chen Die-yan Department of Modern Applied Physics, Centre of Atomic and Molecular Sciences, Tsinghua University, Beijing 100084, People’s Republic of China Received 7 December 1997; accepted 25 April 1998
Abstract Ultratrace gold (Au) in geogas samples has been determined by means of laser excited atomic fluorescence spectrometry combined with graphite electrothermal atomization and time-gate technique. Gold atoms were excited from the ground state to the 6p 2P 3/2 state by a pulsed laser beam with a wavelength of 242.8 nm. Fluorescence photons with a wavelength of 312.3 nm were measured by a photon-counting unit. The time-gate technique was used to reduce the background radiation caused by the furnace. This method has proved to be highly sensitive with minimal background interference. Eighty-two geogas samples were analysed and the Au contents obtained were in the range of 0.002–0.182 ng l −1. The study of Au concentration of geogas in soil is of great interest in prospecting gold deposits. q 1997 Elsevier Science B.V. All rights reserved Keywords: Au; Geogas; Graphite furnace; Time-gate; Laser excited atomic fluorescence spectrometry
1. Introduction A new efficient and low-cost method for prospecting gold deposits is to analyse the concentration of Au in geogas [1–3]. Wang et al. [4] developed a new rapid method for dynamic collection of geogas from soil over a buried deposit. This method has proved to be satisfactory. However, determining Au in such samples is very difficult, because the sample quantity is very small and the concentration of Au is very low, estimated roughly lower than 0.1 ng l −1 (l −1 means per litre geogas). Using conventional analytical methods for determining the Au content in these natural objects, preliminary chemical enrichment must be * Corresponding author. Tel.: 0086 10 62788938; fax: 0086 10 62781598; e-mail: [email protected] 1 This paper was published in the Special Issue from the BCEIA Conference in Shanghai, China, in October 1997.
used. The complexity of the multistage chemical treatment of samples makes such a preconcentration technique labour-intensive and not very reliable. The most severe problem is that up to now there is almost no suitable preliminary chemical enrichment method for the Au in geogas, because of the very small quantity. In our previous work the gold content in geogas has been determined by resonance ionization mass spectrometry (RIMS) with high sensitivity and high selectivity [5]. The detection limit of 0.003 ng ml −1 for Au has been obtained. But RIMS instrumentation is complicated and expensive, it uses three laser beams and high vacuum. A simple and sensitive analysis method is important for analysing a large number of samples. Laser excited atomic fluorescence spectrometry (LEAFS) has been proved to be an extremely powerful tool for detecting trace amounts of many elements,
0584-8547/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 8 )0 0 15 6 -6
1422
M. A. Wan-yun et al./Spectrochimica Acta Part B 53 (1998) 1421–1425
and it has been widely used in geochemistry, chemical exploration and many other fields [6–8]. LEAFS combined with graphite furnace electrothermal atomization and time-gate technique is characterized by a high elemental selectivity, a high sensitivity with minimal background interference and a large linear dynamic range. Other advantages of furnace-LEAFS are the small sample volumes needed for an analysis and both liquid and solid samples can be analysed directly, as well as little or no preliminary chemical treatment and preconcentration steps are required. The present paper first reports on the ultrasensitive determination of Au content in geogas samples, based on graphite electrothermal atomization of a substance followed by LEAFS with time-gate technique. The aim of our work is to develop a simple and ultrasensitive analysis method for Au in geogas, and to test if this method can be used in the search for gold deposits.
2. Experimental 2.1. Apparatus The schematic diagram of the apparatus is shown in Fig. 1. A 20 ml sample solution was pipetted into a graphite crucible and was heated by an alternating current. When the temperature was raised to about 15008C a neutral atomic beam of Au was obtained. The pulsed laser beam with a wavelength of 242.8 nm intersected the atomic beam perpendicularly to excite Au atoms from the ground state 6s 2S 1/2 to the
intermediate state 6p 2P 3/2. Then fluorescence photons with a wavelength of 312.3 nm of Au atoms were collected at 908 relative to the laser beam. Passing through a diaphragm and a monochromator (Model WDG30, Beijing Optical Instrumental Factory, Beijing, China), the fluorescence photons were detected by a fast response photo-multiplier tube (PMT, Model 212UH, Hamamatsu, Japan, high voltage 1200 V). The signal was amplified 10 times by a fast amplifier (AMP, home-made). Then the current pulse signal from AMP was coincided with the delayed timegate signal in synchronism with the laser pulse. The width of the time-gate was 200 ns. The background emission from the furnace was almost eliminated by the time-gate technique, because only the current pulse signal in synchronism with the laser pulse can be integrated into a charge, and then was converted to a digital signal by a charge-to-digital converter (QDC, home-made). Finally the data were acquired and processed automatically by a microcomputer. The laser beam was produced by a dye laser (Model FL3002EC, Lambda Physik, Germany) pumped by an excimer laser (Model EMG 202 MSC, Lambda Physik, Germany). The excimer laser (XeCl) can produce laser pulses of 400 mJ energy with 308 nm wavelength. The laser pulse width is 28 ns. The repetition rate of the excimer laser can be varied in the range of 1–150 Hz. In our experiments, the repetition rate selected was 20 Hz. The linewidth of the dye laser was 0.2 cm −1. The atomizer was a home-made graphite rod crucible (shown in Fig. 2) heated by a power supply (Model WF-1, Beijing Second Optical Instrumental Factory,
Fig. 1. Schematic diagram of the experimental setup.
M. A. Wan-yun et al./Spectrochimica Acta Part B 53 (1998) 1421–1425
1423
to re-dissolve AuCl 3 before analysis. Twenty ml of the sample solution was pipetted into a graphite crucible for each measurement. 3. Results and discussion 3.1. Calibration
Fig. 2. Detailed drawing of the graphite rod crucible (unit mm).
Beijing, China). The graphite rod was purged with argon gas at a flow rate of 0.5 l min −1. The electrodes of the atomizer were cooled by water. 2.2. Procedure of samples The samples were taken from the surface of the surrounding area of a known gold deposit in Shandong Province, China, which is concealed deep underground. For the large-scale area survey of giant gold deposits, the average sampling density was about one sample per 800 km 2. For every sample, 5 l geogas in soil were extracted by a gas sampling device [4] from a hole 60–70 cm deep underground and Au atoms were captured by a piece of polyurethane foam. The chemical treatment of the samples was very simple: the polyurethane foam was ashed at 4508C in a ceramic crucible and then dissolved in 1 ml pure aqua-regia to form AuCl 3. After drying, the sample can be preserved for at least several weeks. One ml 10% aqua-regia was added again in the ceramic crucible
The calibration curves were made by measuring the AuCl 3 standard solutions with the concentration series of 0.0625, 0.125, 0.25, 0.5, 1.0, 2.0 and 3.0 ng ml −1 before and after determining the geogas samples every day because of a lack of a standard geogas sample. Aqueous AuCl 3 solutions with different concentrations were obtained by dilution of the AuCl 3 standard solutions with de-ionized water. The acidity of aqueous AuCl 3 solutions was 10%. The calibration curve given in Fig. 3 shows excellent linearity. 3.2. Detection limit, precision and accuracy The AuCl 3 standard solution with concentration of 0.0625 ng ml −1 and the blank sample of de-ionized water, both with 10% aqua-regia, were used for determining the detection limit of Au. By taking 10 measurements for each sample and using the criterion of 3j, where j is the standard deviation of the blank sample, the detection limit of Au obtained in the AuCl 3 standard solution was 0.005 ng ml −1, a little higher than that by the RIMS method. For the 1 ng ml −1 AuCl 3 standard solution, the analysis precision was about 6% in 15 measurements. The standard
Fig. 3. The calibration curve for aqueous AuCl 3 standard solutions.
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M. A. Wan-yun et al./Spectrochimica Acta Part B 53 (1998) 1421–1425
Fig. 4. The Au content of geogas in soil (ng l −1).
geochemical sample of GAu-5 was selected to test the accuracy of this method. The mean value of Au in GAu-5 was 3.2 ng g −1 in 10 measurements using this method, quite coincident with the standard value of 3.3 6 0.2 ng g −1, given by the Chinese National Standard Institute. The accuracy was about 3%. 3.3. Geogas samples In our experiment, 82 geogas samples were analysed. Each sample was measured three times and the mean values are shown in Fig. 4. The concentrations of Au in these 82 geogas samples were in the range of 0.002–0.182 ng l −1. The precision is about 30% for the Au content in geogas at levels as low as 0.04 ng l −1. It seems that some places with a Au content higher than 0.1 ng l −1 have the possibility of having gold deposits.
operation by comparison with the RIMS method, providing for rapid measurements with little or no sample preparation and allowing the use of a very small quantity of sample. These merits are very important in ultratrace element analysis for hard-to-obtain samples and greatly improve the speed of the analysis. It is demonstrated that furnace-LEAFS with time-gate technique will promote the development of exploring the concealed deposits and has a wide range of applications in many fields.
Acknowledgements The authors would like to thank Professors X. J. Xie and X. Q. Wang of the Geophysical and Geochemical Exploration Institute for providing the geogas samples and for many helpful discussions during the experiment.
4. Conclusion References The furnace-LEAFS with time-gate technique is an extremely efficient method for the determination of ultratrace concentrations of elements. Significant attributes include excellent sensitivity with minimal background interference, simple device and easy
[1] X.J. Xie, Search for giant deposits by new opinion and technology, Scientific Chinese 5 (1995) 15–16. [2] T.X. Ren, Y.H. Liu, M.Q. Wang, Nanometer science and hidden mineral resources, Chinese Science and Technology Review 8 (1995) 18–19.
M. A. Wan-yun et al./Spectrochimica Acta Part B 53 (1998) 1421–1425 [3] R.W. Klusman, Soil Gas and Related Methods for Natural Resource Exploration, Wiley, New York, 1993. [4] X.Q. Wang, X.J. Xie, Y.X. Lu, Dynamic collection of geogas and its preliminary application in search for concealed deposits, Chinese Geophysical and Geochemical Exploration 19 (1995) 161–171. [5] W.Y. Ma, Q. Hui, M. Xue, W.X. Ji, D.Y. Chen, Determination of the gold content in geogas by resonance ionization mass spectrometry, J. Analyt. Atom. Spectrom. 12 (1997) 57–59.
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[6] S. Sjostrom, P. Mauchien, Laser atomic spectroscopic techniques— the analytical performance for trace element analysis of solid and liquid samples, Spectrochimica Acta Rev. 15 (1993) 153–180. [7] V. Cheam, J. Lechner, R. Desrosiers, J. Azcue, F. Rosa, A. Mudroch, LEAFS determination and concentration of metals in great lakes ecosystem, Fresenius J. Analyt. Chem. 355 (1996) 336–339. [8] Y. Oki, K. Furukawa, M. Maeda, Extremely sensitive Na detection in pure water by laser ablation atomic fluorescence spectroscopy, Opt. Commun. 133 (1997) 123–128.