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Optimization of electrochemical deposition of noble metals for permanent modification in graphite furnace atomic absorption spectrometry1
Spectrochimica Acta Part B 53 (1998) 1057–1062
Optimization of electrochemical deposition of noble metals for permanent modification in graphite furnace atomic absorption spectrometry1 Ewa Bulska*, Karina Liebert-Ilkowska, Adam Hulanicki Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warszawa, Poland Received 12 January 1998; accepted 30 March 1998
Abstract Palladium, rhodium and iridium were investigated as chemical modifiers following their electrodeposition on the graphite surface. Conditions for electrodeposition were optimized to find the best analytical performance. Cadmium was used as a test element in this study. Using modified tubes, maximum pyrolysis temperatures were 8008C with Ir as modifier, 9008C with Pd, 10008C with Pd + Rh, and 11008C with Rh alone. The modified tubes exhibit a long-term performance, which indicates an advantage in use of the electrodeposition of noble metals as permanent modification. q 1998 Elsevier Science B.V. All rights reserved Keywords: Electrothermal atomic absorption spectrometry; Electrodeposition; Permanent modifier; Cadmium
1. Introduction The use of modifiers has become an important part of the analytical procedure in graphite furnace atomic absorption spectrometry (GFAAS). In particular, with the stabilized temperature platform furnace (STPF) concept, the modifiers are essential in order to obtain interference-free measurement conditions [1]. A large number of chemical compounds have been described as chemical modifiers in recent years [2]. Also, different procedures for introduction of modifiers into the furnace were investigated by different authors. The modifier may be introduced together with a sample solution or dosed separately into the furnace [3]. This can be accomplished by chemical or thermal * Corresponding author. 1 This paper was published in the special issue to honour Professor C.L. Chakrabarti.
reduction of the metal, by sputtering [4] or electrodeposition [5] onto the inner surface of the graphite furnace. It has been shown that electrodeposited palladium exhibits a long lifetime, up to 500 firings, for the use in determination of Hg [6] or Se [3] without loss of sensitivity. There are many examples of the application of palladium or other noble metals for modification purposes. Welz et al. [7] introduced a mixed palladium/ magnesium nitrate modifier, which was sufficient for stabilization of many elements [8]. By mixing palladium with other metals, it was possible to achieve better thermal stabilization of analytes. Mandjukov and Tsalev [9] described the use of palladium alone or mixed with cerium. For 24 elements, among them Cd, Cu, Fe, Pb and Zn, maximum thermal stabilization was achieved with a modifier consisting of 4 mg of Pd and 20 mg of Ce. Also, mixtures of palladium with vanadium
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 12 6 -8
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E. Bulska et al./Spectrochimica Acta Part B 53 (1998) 1057–1062
(20 mgV + 4 mg Pd) [10] or with tungsten (20 mgW + 4 mg Pd) [11] were successfully used for improving thermal stabilization of several volatile metals. In the last few years, increased attention has been paid to the use of noble metals, in particular, palladium, iridium [12] and rhodium [13]. Therefore, the aim of this work is to investigate the performance of these metals after their electrodeposition onto the graphite surface for modification purposes.
2. Experimental 2.1. Instrumentation An atomic absorption spectrometer, Analyst 300, equipped with an HGA-800 graphite furnace and AS-72 autosampler (Perkin-Elmer, Germany) was used throughout this work. Hollow cathode lamps for Cd (Perkin-Elmer, Germany), Pd and Ir (Photron, Australia) and Rh (Beckman, Australia) were used at wavelengths of 228.8 nm, 247.6 nm, 264.0 nm and 343.5 nm, respectively. Pyrolytic graphite coated tubes were used for all experiments. 2.2. Reagents The following analytical grade reagents were used: ultrapure HCl and HNO 3 (J.T. Baker, Netherlands), PdCl 2 (Merck, Germany), RhCl 3 and IrCl 3 (KochLight, USA). Solutions containing 2 mg l −1 of each metal were prepared for electrodeposition purposes. A 1000 mg l −1 standard solution of cadmium was prepared from Titrisol (Merck, Germany). Working solutions of 2 mg l −1 were prepared daily by appropriate dilution. Aliquots of 20 ml were added to the modified graphite furnace. Deionized water was obtained from a Milli-Q system (Millipore, USA). Sample manipulation and preparation of solutions was conducted in a laminar flow box (Bleymehl Reinraumtechnik, Germany). 2.3. Procedure The electrodeposition of Pd, Ir and Rh was performed using a home-made electroplating unit [5] consisting of a constant current source, a Pt
anode and a holder for the graphite tube acting as a cathode. Before electrodeposition, each graphite tube was heated five times to 26508C with a cooling step in between. The tube was then wrapped with a Teflon band, the appropriate solution of the modifier was poured into the tube and the platinum wire electrode carefully inserted. In order to control the progress of the electrodeposition process, 20 ml aliquots were sampled every 30 min and the concentration of respective metals was determined by GFAAS after appropriate dilution. After finishing the electrodeposition cycle, the tube was washed with a stream of doubly distilled water and dried at room temperature. The tube was then heated in the HGA unit up to 20008C for 4 s, after which it was ready for further investigation.
3. Results and discussion The way in which the modifier is introduced into the graphite furnace is an important factor influencing its performance during the atomization process. Depending on the procedure used, long-term performance may be achieved [3,6]. Cadmium is well known to suffer losses during the pyrolysis step at temperatures above 3008C when no modifier is present [14]. The volatility of cadmium can be reduced by the addition of modifiers and, in this study, the effectiveness of the electrodeposited modifiers was examined. 3.1. Optimization of the electrodeposition process During preliminary electrodeposition experiments, the composition of the galvanic bath was typical for the usual galvanostatic process [5]. However, this requires the use of several compounds, difficult to obtain in sufficient purity [15]. Therefore, the aim of this work was to verify whether the one-component solution consisting of the respective salt of Pd, Ir or Rh only, could be used for the modification purpose. A palladium bath was prepared from PdCl 2 dissolved in 2 mol l −1 HCl; rhodium from RhCl 3 dissolved in 2 mol l −1 HCl; and iridium from IrCl 3 dissolved in 0.5 mol l −1 HCl. A further important factor is the total mass of deposited metal, which may influence the
E. Bulska et al./Spectrochimica Acta Part B 53 (1998) 1057–1062
performance of modification. It was found [3] that, in the case of selenium determination, the optimum amount of electrodeposited palladium in one tube was about 500 mg. In this respect, the electrodeposition process was optimized for Pd, Rh, the Pd + Rh mixture and Ir. For all investigated modifiers, the electrodeposition process was monitored by the determination of the concentration of the respective metal left in the solution during the electrolysis. The temperature of the solution and the current applied were optimized in order to achieve stable modification and the best analytical performance for determination of cadmium. The temperature, within the range from room temperature up to 708C, has no significant influence on the deposition rate. Further experiments were done with a constant temperature of 308C. The influence of the current density on the electrodeposition of Pd from a solution containing 4 mg of Pd is shown in the Fig. 1. With a current of 1 mA, the concentration of palladium in the solution slowly
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decreased over 2.5 h; almost 2 h are required to deposit about 500 mg of palladium. With larger currents, the rate of the process increases. It was found that the best analytical performance, evaluated from the precision of 40 replicate atomization cycles, was obtained using a current of 3 mA. This means that the required mass of palladium is deposited within 40 min. Larger currents result in decreased precision of cadmium determination. The results for iridium as modifier were very similar to those obtained for palladium. In the case of rhodium, a current of 1 mA was not sufficient to achieve effective electrodeposition of the metal (Fig. 2). Using a current of 10 mA, electrodeposition of 800 mg of rhodium was achieved in 1 h. It is well known that a mixture of palladium with a less volatile element may result in better thermal stabilization of the analyte. Therefore, it was of interest to check whether an electrodeposited mixture of palladium and rhodium could form an effective permanent modifier on the graphite surface. For this
Fig. 1. Mass of deposited metal as a function of electrolysis time for different currents: l, 1 mA; A, 3 mA; =, 10 mA; × , 30 mA. Total amount of Pd in solution is 4 mg.
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Fig. 2. Mass of deposited metal as a function of electrolysis time for different currents: l, × , 3 mA; A, =, 10 mA. Solid lines, deposition of Rh from one-component solution containing 2 mg Rh; Dashed lines, deposition of Rh ( × ) and Pd (=) from solution containing 2 mg of each metal.
purpose, a solution containing 2 mg of Pd and 2 mg of Rh was used (Fig. 2). Taking into account the results obtained for rhodium, a current of 10 mA was applied; after 30 min, similar amounts of both elements were deposited. 3.2. Influence of modification on cadmium determination In order to compare the effectiveness of different electrodeposited modifiers, pyrolysis studies were performed for the determination of cadmium. In each study, the pretreatment temperature varied
from 3008C to 13008C with a 10 s ramp time and a hold-time at the pyrolysis temperature of 30 s. Without modifier, cadmium losses occur above 3008C. The presence of electrodeposited modifiers provides increased thermal stability of cadmium (Fig. 3). All investigated modifiers (Pd, Ir, Rh and Pd + Rh) were used after their electrodeposition on the graphite surface as well as after thermal reduction prior to each firing. In all cases, similar stabilization was achieved, whether the modifier was electrodeposited on the graphite surface or injected before each atomization cycle and thermally pretreated. The maximum pretreatment temperature which can be
Fig. 3. Effect of pyrolysis temperature on the integrated absorbance of cadmium (C Cd = 0.22 ng l -1) in the presence of electrodeposited modifiers: A, no modifier; S, 650 mg of Ir; =, 550 mg of Pd; × , 450 mg of Rh; *, 450 mg of Pd + 450 mg of Rh.
E. Bulska et al./Spectrochimica Acta Part B 53 (1998) 1057–1062
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Table 1 Maximum pyrolysis temperature for cadmium (C Cd = 2 ng ml −1) in the presence of noble metals
3.3. Influence of chloride on the permanent performance of noble metals
Modifier
In order to examine the permanent performance of the investigated modifiers, the determination of Cd in an aqueous solution containing chloride at a concentration of about 30 mg l −1 as NaCl in 0.1% (v/v) HCl was performed. This was of special importance with respect to iridium and rhodium, as it is well known that both metals react with chloride. The formation of a chloride could potentially result in loss of the modifier. The long-term investigations ensure that all investigated noble metals keep their performance even in the presence of chloride containing matrix. When using the maximum pyrolysis temperature determined for each modifier (see Table 1), no loss of the sensitivity was found for several (more than 60) atomization cycles. This means that the presence of chloride does not influence the long-term performance of electrodeposited noble metals.
Ir Pd Pd + Rh Rh
Maximum pyrolysis temperature (8C) Reduced prior to each firing
Electrodeposited
900 900 1000 1000
600 900 1000 1100
used with each noble metal is summarized in Table 1. The best stabilization can be achieved with rhodium; a mixture of palladium and rhodium allows better thermal stabilization of cadmium than does palladium alone. The advantage of electrodeposition is the permanent performance of the modifier for several atomization cycles. The long-term performance of the modification were tested in detail for palladium. During 30 min at 3 mA, 600 mg of palladium was deposited on the graphite tube. With an atomization temperature of 20008C, 520 atomization cycles could be performed without significant losses of Cd during thermal stabilization (Table 2). When a simple thermal pretreatment procedure was used, 20 ml of a solution containing 1 mg ml −1 Pd must be introduced before each atomization cycle. This requires use of 10.3 mg of Pd for 520 atomization cycles. Thermal reduction takes at least 50 s, thus 520 atomization cycles require an additional 7 h of operation of the spectrometer as compared to use of the electrodeposited permanent modifier.
Table 2 Cadmium signal stability and precision in the presence of electrodeposited palladium. Pyrolysis temperature 8008C. The firings were chosen at random Firing number
Mean absorbance(s)
RSD (%)
15–55 96–145 180–246 295–346 390–443 490–520
0.199 0.205 0.209 0.196 0.210 0.198
2.4 2.9 1.8 1.5 2.1 2.4
4. Conclusion For the first time, detailed studies of the conditions for noble metal electrodeposition onto the graphite surface of the atomizer have been performed. The procedure is simplified in comparison to that used for a typical galvanic process, the one-component solutions consisting of only the chloride salt. It was also shown that a mixed modifier of Pd + Rh can be deposited on the surface which has good stabilization performance for cadmium. All investigated noble metals stabilized cadmium to similar temperatures whether they are thermally reduced before each atomization cycle or electrodeposited onto the inner surface of the graphite tube. The advantage of electrodeposition, however, is that it is ‘permanent’, meaning that once deposited the modifier can be used for several hundred atomization cycles, saving reagents as well as shortening the analyses time.
Acknowledgements This research was carried out in the frame of the BST 562/2/97 grant.
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E. Bulska et al./Spectrochimica Acta Part B 53 (1998) 1057–1062
References [1] W. Slavin, Graphite Furnace Atomic Absorption Spectrometry — a Source Book, Perkin-Elmer, Norwalk, CT 1984. [2] D.L. Tsalev, V.I. Slaveykova, P.B. Mandjukov, Chemical modification in graphite furnace atomic absorption spectrometry, Spectrochim. Acta Rev. 13 (1990) 225–274. [3] E. Bulska, K. Pyrzyn´ska, Comparison of chemical modifiers for the determination of selenium by electrothermal atomicabsorption spectrometry, Spectrochim. Acta Part B 52 (1997) 1283–1291. [4] C.J. Rademeyer, B. Radziuk, N. Romanova, N.P. Skangset, A. Skogstad, Y. Thomassen, Permanent iridium modifier for electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 10 (1995) 739. [5] E. Bulska, W. Jedral, Application of palladium- and rhodiumplating of the graphite furnace in electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 10 (1995) 49–53. [6] B. Bulska, W. Kandler, A. Hulanicki, Noble metals as permanent modifiers for the determination of mercury by electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 51 (1996) 1263–1270. [7] B. Welz, G. Schlemmer, J.R. Mudakavi, Palladium nitrate– magnesium nitrate modifier for electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 7 (1992) 1257. [8] C. Bendicho, M.T.C. Loos-Vollebregt, The influence of pyrolysis and matrix modifiers for analysis of glass materials
[9]
[10]
[11]
[12]
[13]
[14]
[15]
by GFAAS using slurry sample introduction, Spectrochim. Acta Part B 45 (1990) 679. P.B. Mandjukov, D.L. Tsalev, Study of cerium (IV) and cerium (IV)–palladium (II) as chemicals modifiers in electrothermal atomization-atomic absorption spectrometry, Microchim. J. 42 (1990) 339. D.L. Tsalev, T.A. Dimitrov, P.B. Mandjukov, Study of vanadium (V) as a chemical modifier in electrothermal atomization atomic absorption spectrometry, J. Anal. At. Spectrom. 5 (1990) 189. V.I. Slavekova, D.L. Tsalev, Study of some tungstencontaining chemical modifiers in graphite-furnace atomic absorption spectrometry, Anal. Lett. 23 (1990) 1921. I.L. Shuttler, M. Feuerstein, G. Schlemmer, Long-term stability of a mixed palladium–iridium trapping reagent for in situ hydride trapping with a graphite electrothermal atomizer, J. Anal. At. Spectrom. 7 (1992) 1299. Z.N. Ni, Z. Rao, M. Li, Minimization of phosphate intererence in the direct determination of arsenic in urine by electrothermal atomic absorption spectrometry, Anal. Chim. Acta 334 (1996) 177–182. E. Bulska, Z. Grobenski, G. Schlemmer, In situ removal of contamination from a palladium–magnesium chemical modifier within an electrothermal atomiser for the determination of cadmium in serum, J. Anal. At. Spectrom. 5 (1990) 203–204. E. Bulska, K. Pyrzyn´ska, The application of soild sorbents for the purification of aluminum contaminated chemicals used as modifiers in electrothermal atomic absorption spectrometry, Fresenius J. Anal. Chem. 355 (1996) 672–675.
Optimization of electrochemical deposition of noble metals for permanent modification in graphite furnace atomic absorption spectrometry1 Ewa Bulska*, Karina Liebert-Ilkowska, Adam Hulanicki Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warszawa, Poland Received 12 January 1998; accepted 30 March 1998
Abstract Palladium, rhodium and iridium were investigated as chemical modifiers following their electrodeposition on the graphite surface. Conditions for electrodeposition were optimized to find the best analytical performance. Cadmium was used as a test element in this study. Using modified tubes, maximum pyrolysis temperatures were 8008C with Ir as modifier, 9008C with Pd, 10008C with Pd + Rh, and 11008C with Rh alone. The modified tubes exhibit a long-term performance, which indicates an advantage in use of the electrodeposition of noble metals as permanent modification. q 1998 Elsevier Science B.V. All rights reserved Keywords: Electrothermal atomic absorption spectrometry; Electrodeposition; Permanent modifier; Cadmium
1. Introduction The use of modifiers has become an important part of the analytical procedure in graphite furnace atomic absorption spectrometry (GFAAS). In particular, with the stabilized temperature platform furnace (STPF) concept, the modifiers are essential in order to obtain interference-free measurement conditions [1]. A large number of chemical compounds have been described as chemical modifiers in recent years [2]. Also, different procedures for introduction of modifiers into the furnace were investigated by different authors. The modifier may be introduced together with a sample solution or dosed separately into the furnace [3]. This can be accomplished by chemical or thermal * Corresponding author. 1 This paper was published in the special issue to honour Professor C.L. Chakrabarti.
reduction of the metal, by sputtering [4] or electrodeposition [5] onto the inner surface of the graphite furnace. It has been shown that electrodeposited palladium exhibits a long lifetime, up to 500 firings, for the use in determination of Hg [6] or Se [3] without loss of sensitivity. There are many examples of the application of palladium or other noble metals for modification purposes. Welz et al. [7] introduced a mixed palladium/ magnesium nitrate modifier, which was sufficient for stabilization of many elements [8]. By mixing palladium with other metals, it was possible to achieve better thermal stabilization of analytes. Mandjukov and Tsalev [9] described the use of palladium alone or mixed with cerium. For 24 elements, among them Cd, Cu, Fe, Pb and Zn, maximum thermal stabilization was achieved with a modifier consisting of 4 mg of Pd and 20 mg of Ce. Also, mixtures of palladium with vanadium
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 12 6 -8
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E. Bulska et al./Spectrochimica Acta Part B 53 (1998) 1057–1062
(20 mgV + 4 mg Pd) [10] or with tungsten (20 mgW + 4 mg Pd) [11] were successfully used for improving thermal stabilization of several volatile metals. In the last few years, increased attention has been paid to the use of noble metals, in particular, palladium, iridium [12] and rhodium [13]. Therefore, the aim of this work is to investigate the performance of these metals after their electrodeposition onto the graphite surface for modification purposes.
2. Experimental 2.1. Instrumentation An atomic absorption spectrometer, Analyst 300, equipped with an HGA-800 graphite furnace and AS-72 autosampler (Perkin-Elmer, Germany) was used throughout this work. Hollow cathode lamps for Cd (Perkin-Elmer, Germany), Pd and Ir (Photron, Australia) and Rh (Beckman, Australia) were used at wavelengths of 228.8 nm, 247.6 nm, 264.0 nm and 343.5 nm, respectively. Pyrolytic graphite coated tubes were used for all experiments. 2.2. Reagents The following analytical grade reagents were used: ultrapure HCl and HNO 3 (J.T. Baker, Netherlands), PdCl 2 (Merck, Germany), RhCl 3 and IrCl 3 (KochLight, USA). Solutions containing 2 mg l −1 of each metal were prepared for electrodeposition purposes. A 1000 mg l −1 standard solution of cadmium was prepared from Titrisol (Merck, Germany). Working solutions of 2 mg l −1 were prepared daily by appropriate dilution. Aliquots of 20 ml were added to the modified graphite furnace. Deionized water was obtained from a Milli-Q system (Millipore, USA). Sample manipulation and preparation of solutions was conducted in a laminar flow box (Bleymehl Reinraumtechnik, Germany). 2.3. Procedure The electrodeposition of Pd, Ir and Rh was performed using a home-made electroplating unit [5] consisting of a constant current source, a Pt
anode and a holder for the graphite tube acting as a cathode. Before electrodeposition, each graphite tube was heated five times to 26508C with a cooling step in between. The tube was then wrapped with a Teflon band, the appropriate solution of the modifier was poured into the tube and the platinum wire electrode carefully inserted. In order to control the progress of the electrodeposition process, 20 ml aliquots were sampled every 30 min and the concentration of respective metals was determined by GFAAS after appropriate dilution. After finishing the electrodeposition cycle, the tube was washed with a stream of doubly distilled water and dried at room temperature. The tube was then heated in the HGA unit up to 20008C for 4 s, after which it was ready for further investigation.
3. Results and discussion The way in which the modifier is introduced into the graphite furnace is an important factor influencing its performance during the atomization process. Depending on the procedure used, long-term performance may be achieved [3,6]. Cadmium is well known to suffer losses during the pyrolysis step at temperatures above 3008C when no modifier is present [14]. The volatility of cadmium can be reduced by the addition of modifiers and, in this study, the effectiveness of the electrodeposited modifiers was examined. 3.1. Optimization of the electrodeposition process During preliminary electrodeposition experiments, the composition of the galvanic bath was typical for the usual galvanostatic process [5]. However, this requires the use of several compounds, difficult to obtain in sufficient purity [15]. Therefore, the aim of this work was to verify whether the one-component solution consisting of the respective salt of Pd, Ir or Rh only, could be used for the modification purpose. A palladium bath was prepared from PdCl 2 dissolved in 2 mol l −1 HCl; rhodium from RhCl 3 dissolved in 2 mol l −1 HCl; and iridium from IrCl 3 dissolved in 0.5 mol l −1 HCl. A further important factor is the total mass of deposited metal, which may influence the
E. Bulska et al./Spectrochimica Acta Part B 53 (1998) 1057–1062
performance of modification. It was found [3] that, in the case of selenium determination, the optimum amount of electrodeposited palladium in one tube was about 500 mg. In this respect, the electrodeposition process was optimized for Pd, Rh, the Pd + Rh mixture and Ir. For all investigated modifiers, the electrodeposition process was monitored by the determination of the concentration of the respective metal left in the solution during the electrolysis. The temperature of the solution and the current applied were optimized in order to achieve stable modification and the best analytical performance for determination of cadmium. The temperature, within the range from room temperature up to 708C, has no significant influence on the deposition rate. Further experiments were done with a constant temperature of 308C. The influence of the current density on the electrodeposition of Pd from a solution containing 4 mg of Pd is shown in the Fig. 1. With a current of 1 mA, the concentration of palladium in the solution slowly
1059
decreased over 2.5 h; almost 2 h are required to deposit about 500 mg of palladium. With larger currents, the rate of the process increases. It was found that the best analytical performance, evaluated from the precision of 40 replicate atomization cycles, was obtained using a current of 3 mA. This means that the required mass of palladium is deposited within 40 min. Larger currents result in decreased precision of cadmium determination. The results for iridium as modifier were very similar to those obtained for palladium. In the case of rhodium, a current of 1 mA was not sufficient to achieve effective electrodeposition of the metal (Fig. 2). Using a current of 10 mA, electrodeposition of 800 mg of rhodium was achieved in 1 h. It is well known that a mixture of palladium with a less volatile element may result in better thermal stabilization of the analyte. Therefore, it was of interest to check whether an electrodeposited mixture of palladium and rhodium could form an effective permanent modifier on the graphite surface. For this
Fig. 1. Mass of deposited metal as a function of electrolysis time for different currents: l, 1 mA; A, 3 mA; =, 10 mA; × , 30 mA. Total amount of Pd in solution is 4 mg.
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E. Bulska et al./Spectrochimica Acta Part B 53 (1998) 1057–1062
Fig. 2. Mass of deposited metal as a function of electrolysis time for different currents: l, × , 3 mA; A, =, 10 mA. Solid lines, deposition of Rh from one-component solution containing 2 mg Rh; Dashed lines, deposition of Rh ( × ) and Pd (=) from solution containing 2 mg of each metal.
purpose, a solution containing 2 mg of Pd and 2 mg of Rh was used (Fig. 2). Taking into account the results obtained for rhodium, a current of 10 mA was applied; after 30 min, similar amounts of both elements were deposited. 3.2. Influence of modification on cadmium determination In order to compare the effectiveness of different electrodeposited modifiers, pyrolysis studies were performed for the determination of cadmium. In each study, the pretreatment temperature varied
from 3008C to 13008C with a 10 s ramp time and a hold-time at the pyrolysis temperature of 30 s. Without modifier, cadmium losses occur above 3008C. The presence of electrodeposited modifiers provides increased thermal stability of cadmium (Fig. 3). All investigated modifiers (Pd, Ir, Rh and Pd + Rh) were used after their electrodeposition on the graphite surface as well as after thermal reduction prior to each firing. In all cases, similar stabilization was achieved, whether the modifier was electrodeposited on the graphite surface or injected before each atomization cycle and thermally pretreated. The maximum pretreatment temperature which can be
Fig. 3. Effect of pyrolysis temperature on the integrated absorbance of cadmium (C Cd = 0.22 ng l -1) in the presence of electrodeposited modifiers: A, no modifier; S, 650 mg of Ir; =, 550 mg of Pd; × , 450 mg of Rh; *, 450 mg of Pd + 450 mg of Rh.
E. Bulska et al./Spectrochimica Acta Part B 53 (1998) 1057–1062
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Table 1 Maximum pyrolysis temperature for cadmium (C Cd = 2 ng ml −1) in the presence of noble metals
3.3. Influence of chloride on the permanent performance of noble metals
Modifier
In order to examine the permanent performance of the investigated modifiers, the determination of Cd in an aqueous solution containing chloride at a concentration of about 30 mg l −1 as NaCl in 0.1% (v/v) HCl was performed. This was of special importance with respect to iridium and rhodium, as it is well known that both metals react with chloride. The formation of a chloride could potentially result in loss of the modifier. The long-term investigations ensure that all investigated noble metals keep their performance even in the presence of chloride containing matrix. When using the maximum pyrolysis temperature determined for each modifier (see Table 1), no loss of the sensitivity was found for several (more than 60) atomization cycles. This means that the presence of chloride does not influence the long-term performance of electrodeposited noble metals.
Ir Pd Pd + Rh Rh
Maximum pyrolysis temperature (8C) Reduced prior to each firing
Electrodeposited
900 900 1000 1000
600 900 1000 1100
used with each noble metal is summarized in Table 1. The best stabilization can be achieved with rhodium; a mixture of palladium and rhodium allows better thermal stabilization of cadmium than does palladium alone. The advantage of electrodeposition is the permanent performance of the modifier for several atomization cycles. The long-term performance of the modification were tested in detail for palladium. During 30 min at 3 mA, 600 mg of palladium was deposited on the graphite tube. With an atomization temperature of 20008C, 520 atomization cycles could be performed without significant losses of Cd during thermal stabilization (Table 2). When a simple thermal pretreatment procedure was used, 20 ml of a solution containing 1 mg ml −1 Pd must be introduced before each atomization cycle. This requires use of 10.3 mg of Pd for 520 atomization cycles. Thermal reduction takes at least 50 s, thus 520 atomization cycles require an additional 7 h of operation of the spectrometer as compared to use of the electrodeposited permanent modifier.
Table 2 Cadmium signal stability and precision in the presence of electrodeposited palladium. Pyrolysis temperature 8008C. The firings were chosen at random Firing number
Mean absorbance(s)
RSD (%)
15–55 96–145 180–246 295–346 390–443 490–520
0.199 0.205 0.209 0.196 0.210 0.198
2.4 2.9 1.8 1.5 2.1 2.4
4. Conclusion For the first time, detailed studies of the conditions for noble metal electrodeposition onto the graphite surface of the atomizer have been performed. The procedure is simplified in comparison to that used for a typical galvanic process, the one-component solutions consisting of only the chloride salt. It was also shown that a mixed modifier of Pd + Rh can be deposited on the surface which has good stabilization performance for cadmium. All investigated noble metals stabilized cadmium to similar temperatures whether they are thermally reduced before each atomization cycle or electrodeposited onto the inner surface of the graphite tube. The advantage of electrodeposition, however, is that it is ‘permanent’, meaning that once deposited the modifier can be used for several hundred atomization cycles, saving reagents as well as shortening the analyses time.
Acknowledgements This research was carried out in the frame of the BST 562/2/97 grant.
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E. Bulska et al./Spectrochimica Acta Part B 53 (1998) 1057–1062
References [1] W. Slavin, Graphite Furnace Atomic Absorption Spectrometry — a Source Book, Perkin-Elmer, Norwalk, CT 1984. [2] D.L. Tsalev, V.I. Slaveykova, P.B. Mandjukov, Chemical modification in graphite furnace atomic absorption spectrometry, Spectrochim. Acta Rev. 13 (1990) 225–274. [3] E. Bulska, K. Pyrzyn´ska, Comparison of chemical modifiers for the determination of selenium by electrothermal atomicabsorption spectrometry, Spectrochim. Acta Part B 52 (1997) 1283–1291. [4] C.J. Rademeyer, B. Radziuk, N. Romanova, N.P. Skangset, A. Skogstad, Y. Thomassen, Permanent iridium modifier for electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 10 (1995) 739. [5] E. Bulska, W. Jedral, Application of palladium- and rhodiumplating of the graphite furnace in electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 10 (1995) 49–53. [6] B. Bulska, W. Kandler, A. Hulanicki, Noble metals as permanent modifiers for the determination of mercury by electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 51 (1996) 1263–1270. [7] B. Welz, G. Schlemmer, J.R. Mudakavi, Palladium nitrate– magnesium nitrate modifier for electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 7 (1992) 1257. [8] C. Bendicho, M.T.C. Loos-Vollebregt, The influence of pyrolysis and matrix modifiers for analysis of glass materials
[9]
[10]
[11]
[12]
[13]
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