- Email: [email protected]
Determination of Te in soldering tin using continuous flowing electrochemical hydride generation atomic fluorescence spectrometry
Spectrochimica Acta Part B 63 (2008) 710–713 Spectrochimica Acta Part B 63 (2008) 710–713
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Analytical note
Determination of Te in soldering tin using continuous flowing electrochemical hydride generation atomic fluorescence spectrometry Xianjuan Jiang, Wuer Gan ⁎, Suping Han, Youzhao He Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China
A R T I C L E
I N F O
Article history: Received 15 October 2007 Accepted 26 March 2008 Available online 7 April 2008 Keywords: Te Electrochemical hydride generation Atomic fluorescence spectroscopy
A B S T R A C T An electrochemical hydride generation system was developed for the detection of Te by coupling an electrochemical hydride generator with atomic fluorescence spectrometry. Since TeH2 is unstable and easily decomposes in solution, a reticular W filament cathode was used in the present system. The TeH2 generated on the cathode surface was effectively driven out by sweeping gas from the cathode chamber. In addition, a low temperature electrochemical cell (10 °C) was applied to reduce the decomposition of TeH2 in solution. The limit of detection (LOD) was 2.2 ng ml− 1 and the relative standard deviation (RSD) was 3.9% for nine consecutive measurements of standard solution. This method was successfully employed for determination of Te in soldering tin material. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The chemical hydride generation (CHG) of Te is a well-established sample introduction technique in atomic spectrometry when low detection limits are desired [1–5]. Although the CHG technique, which is sample introduction method based on the reaction of sodium tetrahydroborate with the acidic material that has many advantages and is widely used, there are some disadvantages listed in previous studies [6–8]. As a suitable alternative to the CHG technique for sample introduction, the electrolytic volatile generation of As, Se, Sb, Sn, Ge, Pb, Bi and Te was reported to be feasible as an electrolytic sample pretreatment method [9]. In the last fifteen years, electrochemical hydride generation (EC-HG) of H2Se, SbH3, AsH3, BiH3, GeH4 and SnH4 has been successfully coupled with a variety of spectrometric techniques such as atomic absorption spectrometry (AAS), graphite furnace atomic absorption spectrometry (GFAAS), atomic fluorescence spectrometry (AFS), atomic emission spectrometry (AES) and inductively coupled plasma-mass spectrometry (ICP-MS) [10]. Hydrides of Cd [11] and elemental Hg [12–14] have also been successfully generated by electrochemical methods, and the technique was also applied for analysis of water, medicines and biological samples [9,10] . However, we have not found any reports on the detection of Te by using EC-HG method as sample introduction method up to present. Trace amount of Te can affect the physical and mechanical properties of metals and metal alloys [14]. An accurate technique is therefore required for the determination of Te in such samples.
⁎ Corresponding author. Tel.: +86 551 3600021. E-mail address: [email protected] (W.E. Gan). 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.03.017
In this work, a simple and highly accurate method for the detection of Te in soldering tin material was created which coupled EC-HG with AFS. A three-dimensional reticular W filament electrode was used as the cathode. At low temperatures, the TeH2 generated on the reticular W filament cathode surface could be effectively driven out by sweeping gas in the cathode chamber and was then followed by AFS determination. 2. Experimental 2.1. Instrumentation A direct current (DC) constant current and voltage unit (Type DH1719A-3, Beijing Da Hua Wireless instrument Co, Beijing, China) operating at constant current mode was used. An AFS-230 doublechannel nondispersive atomic fluorescence spectrometer (Beijing Haiguang Instrument Co, Beijing, China) was employed throughout. A high intensity Te hollow cathode lamp (General Research Institute for Nonferrous Metals, Beijing, China) was used as a radiation source. Quartz tube (7 mm I.D. × 14 mm length) was used as atomizer. A hydrogen–argon–air entrained flame was maintained with the addition of auxiliary hydrogen. Two sequential gas liquid separators (GLS) were used for completely separating the gases from liquid as previously described [15]. A DG-0506 low constant temperature trough (Shi-Bo Bio-technique. Co. Shanghai, China) was employed. The capacity of the trough was 6 L. The adjustable temperature range was −5 °C to 100 °C and had a variability of 0.05 °C. A WFX-120 atomic absorption spectrometer with a WF-4C graphite furnace (Beijing Ruili Instrument Co. Beijing, China) was employed for the validation of the proposed method. The GF-AAS operating conditions were as follows: (1) The Te hollow cathode lamp was operated
X. Jiang et al. / Spectrochimica Acta Part B 63 (2008) 710–713
711
Fig. 1. Schematics of the electrochemical hydride generation atomic fluorescence spectrometry system, P: peristaltic pump; GLS: gas liquid separator; W: waste; AFS: atomic fluorescence spectrometry.
at a wavelength of 214.3 nm, (2) the signals were evaluated from the peak area yielding an integration time of 5 s, (3) the pyrolysis temperature was 1200 °C, (4) and the atomization temperature was 2600 °C. 2.2. Hydride generation system A schematic diagram of the analytical system is shown in Fig. 1. A homemade thin-layer electrochemical cell was used as the hydride generator as described in a previous publication [16]. The polytetrafluoroethylene (PTFE) cell consisted of an upper and lower chamber (both 100 × 15 × 5 mm, inner volumes of 7.5 ml). A cation ion exchange membrane (Huanyu Co., Beijing, China) was used to separate the gases generated in the anode and cathode chamber. A Pt foil anode was fixed on channel bottom of the anode chamber. The polished W wire was knitted into a multilayer, net-shaped cathode. The wire length was about 7 m, and the surface area was about 21.98 cm2. Sweeping gas (comprising carrier gas Ar and auxiliary gas H2) passed through the cathode chamber to purge the hydride from the surface of the cathode. Ar, H2 and the generated hydride were then separated by the first GLS and directed to the second one through a PTFE tube (0.8 mm I.D. × 100 mm length). Finally, these gases were transported to the atomizer of the AFS. 2.3. Reagents and materials All reagents were of the highest available purity, and were of at least analytical grade. Doubly deionized water (DDW) was solely used. The Te (IV) Stock solution (1.000 g l− 1) was prepared by dissolving Te powder in concentrated HNO3. Next, low-concentrated HCl was added to the solution. The Te (VI) stock solution was prepared by dissolving H2TeO4 (The Hui Hong reagent company, China). A series of standard solutions were prepared by stepwise dilution of stock solutions with 1.5 M H2SO4 just before use. Various concentrations of guaranteed reagents of HCl, H2SO4, HNO3 and H3PO4 were used to test the properties of the electrolytic solutions. A mixture of 50 g l− 1 thiourea and ascorbic acid solution was prepared by dissolving both thiourea and ascorbic in DDW. Analytical grade concentrated HBr was used to remove Sn from samples. Pd (NO3)2 at 1 g l− 1 was used as a chemical modifier when Te was detected by GFAAS. High-purity argon and hydrogen were used as the carrier gas and auxiliary gas, respectively. W wire (0.1 mm diameter), W foil, Pb foil and Pt foil (0.25 mm × 15 mm × 73.2 mm, 99.9%) were used as electrodes.
evaporated continuously until it was nearly dried. After being cooled to room temperature, 2 ml (1 + 1) H2SO4 and 10 ml (2 + 98) HCl was added and heated to eliminate Pb. Concentrated HBr (2.5 ml) and concentrated HCl (2.5 ml) were added to the beaker and heated to remove tin in the form of tin chloride or tin bromide. Finally, the sample solution was transferred to a 50 ml volumetric flask and brought up to volume with 1.5 M H2SO4 following the addition of 5 ml of 50 g l− 1 thiourea and ascorbic acid. The same reagents were chosen as the blank samples (controls), which were subjected to the same procedure as the sample. 2.5. Analytical procedures At first, the catholyte was pumped into a cross-tube at a flow rate of 3.4 ml min− 1. After being mixed with sweeping gas, the mixture of catholyte solution and gases was delivered to the cathode chamber. At the same time, the anolyte was pumped into the anode chamber at the same flow rate. After the anode and cathode chambers were filled, the flow rate was reduced to 1.8 ml min− 1 and electrolysis took place at 1.5 A. The supply time and sample volume consumed for a signal measurement were about 30 s and 8.4 ml, respectively. Oxygen produced in the anode chamber was driven out along with the anolyte and then diffused to the surroundings at the outlet of the transferring tube. The anolyte was reclaimed and recycled. Mixture of gases and solution from the cathode chamber were delivered to the two sequential GLS. The hydride-containing gas was then directed to the atomizer and finally detected by AFS. Peak area measurement with an integration time of 10 s was used for quantification. CHG-AFS was adopted for the validation of the proposed EC-HGAFS method. The sample solution was swept by 5% (v/v) HCl carrier
2.4. Sample preparation Samples were prepared according to GB/T 10574.13-2003 [17]. Sample aliquots of 200 mg were transferred to 50 ml beakers in which a mixture of 12 ml of concentrated HNO3 and 3 ml of concentrated HCl was added under gentle heating. The sample solution was then
Fig. 2. The effect of cathode materials on the atomic fluorescence intensity of 200 µg l− 1 Te (IV).
712
X. Jiang et al. / Spectrochimica Acta Part B 63 (2008) 710–713
at a flow rate of 3.0 ml min− 1, and was merged with stream of 3.0 ml min− 1 2% (m/v) NaBH4 in 0.5% (m/v) NaOH. Then, the generated TeH2 was introduced to the quartz atomizer by argon carrier gas for AFS determination.
Table 1 Analytical results of Te in soldering tin samples Sample
3. Results and discussion 3.1. Cathode materials and configuration
1 2 3 a
Platinum was used as the anode because of its stability and inertness. As for the cathode, conventional materials including Pt foil, Pb foil and W foil were first considered. The results in Fig. 2 show that the atomic fluorescence intensities of Te under the aforementioned cathode materials are relative to the cathode material overpotential (Pt b W b Pb) as previously reported [18,19] . However, the sensitivity of the method is not satisfactory when using Pt foil, Pb foil or W foil as a cathode. For improving the sensitivity of the method, a reticular W filament was used as the cathode. The results (also in Fig. 2) showed that the atomic fluorescence intensity clearly increased by using a reticular W filament cathode. This increase was a result of the larger analyte access of the reticular W filament cathode. Furthermore, the bubbles formed on the reticular cathode surface were very small, which caused the generated hydride to be effectively driven off the surface of the cathode by the sweeping gas, and also caused a reduction of the decomposition of TeH2. As a result, the reticular W filament cathode was selected for this study. 3.2. Effect of sweeping gas According to our previous work [15], the gas stream (400 ml min− 1 Ar and 200 ml min− 1 H2) was employed for effectively transferring hydride to the AFS and for maintaining the hydrogen–argon–air entrained flame. In the present work, the gas stream also acts as sweeping gas to drive hydride out of cathode chamber. In order to evaluate the influence of the sweeping gas on atomic fluorescence intensity using W foil and a reticular W filament cathode, the gas stream (400 ml min− 1 Ar and 200 ml min− 1 H2) was divided into two portions. One portion, the sweeping gas, was directed into the inlet of the cathode chamber, and the other portion was directed into the cross-tube at the outlet of cathode chamber. In Fig. 3, it was found that the atomic fluorescence intensity greatly increased as the sweeping gas flow rate increased when the reticular W filament cathode was used. However, the atomic fluorescence intensity increased slightly when W foil was used as the cathode. The results indicated that the multilayer reticulated configuration of the W
b
CHG-AFS
GFAAS
Determined
ECHG-AFS Recoveryb
Determined
Determined
(μg g− 1)a
(%)
(μg g− 1)a
(μg g− 1)a
8.81 ± 0.31 12.57 ± 0.28 16.52 ± 0.54
99.6 103.8 104.4
8.88 ± 0.25 11.75 ± 0.74 15.80 ± 0.56
8.75 ± 0.24 11.78 ± 0.32 16.32 ± 0.21
Mean value ± standard deviation (n = 3). 10 μg ml− 1 Te standard solution spiked in the sample.
filament cathode enabled the hydride to be easily driven out from the cathode chamber. Consequently, the gas stream (400 ml min− 1 Ar and 200 ml min− 1 H2) was totally introduced into the inlet of cathode chamber. 3.3. Operating parameters The entire electrochemical cell employed in our work was immersed into a constant temperature trough for increased sensitivity. The results demonstrated that the atomic fluorescence intensity increased along with decreasing temperature from 40 °C to 10 °C, and then leveled off. Thus, the constant temperature was set at 10 °C for the study. In this work, 0.5 M H2SO4 was selected as the anolyte. The effects of different concentrations of catholytes consisting of HCl, H2SO4, H3PO4 and HNO3 on the atomic fluorescence intensity of 200 µg l− 1 Te (IV) were studied. The results indicated that 1.5 M H2SO4 was the optimum catholyte in the present work. The atomic fluorescence intensity of Te (IV) increased along with a sample flow rate that varied from 1 ml min− 1 up to 1.8 ml min− 1. The intensity then decreased with a further increase in sample flow rate. A constant sample flow rate of 1.8 ml min− 1 was chosen. An electrolysis current of 1.5 A was used throughout so that maximum atomic fluorescence could be obtained. The corresponding current density was about 68.24 mA cm− 2. The influence of the transferring tube length (from the first gas liquid separator to the second) on atomic fluorescence intensity was studied. The atomic fluorescence intensity decreased slowly as the tube length varied from 100 to 400 mm due to TeH2 decomposition in the transferring process. As a result, a 100 mm length transferring tube was used. No discernible signal was observed when the Te (VI) solution was used in this experiment. In this experiment, 5 g l− 1 ascorbic acid and 5 g l− 1 thiourea was found to be suitable for the reduction of Te (VI) to Te (IV) and selected as reducing agent. The ascorbic acid helped to avoid the influence of matrix with oxidation as well as to reduce Te (VI) to Te (IV). Because excessive ascorbic acid can reduce Te to elemental form, the maximum concentration of the ascorbic acid should not exceed 6 g l− 1. 3.4. Effect of coexisting ions
Fig. 3. The effect of flow rate of sweeping gas on 200 µg l− 1 Te (IV).
In this work, 200 μg l− 1 Te (IV) solution possibly containing coexisting ions was analyzed. Four microgrammes per milliliter Bi (III), 2 μg ml− 1 Cu (II), 5 μg ml− 1 Fe (II), 2 μg ml− 1 Zn (II), 2 μg ml− 1 Cd (II) and 1 μg ml− 1 Ni (II) have no significant influence on the atomic fluorescence intensity of Te (IV). Hydride-forming elements such as As (III) (1 μg ml− 1) and Sb (III) (1 μg ml− 1) also do not interfere with the detection of Te. The matrix interference was also taken into account, as the soldering tin sample is a type of Pb–Sn alloy. It was found that 0.2 μg ml− 1 Pb (II) and 1 μg ml− 1 Sn (IV) would not interfere with the determination of the Te (IV). The concentrations of Pb (II) and Sn (IV) in sample solutions were determined by GF-AAS and were less than
X. Jiang et al. / Spectrochimica Acta Part B 63 (2008) 710–713
0.2 and 1 μg ml− 1, respectively. As a result, the interference of matrix elements was neglected in this work. 3.5. Analytical figures of merit A regression line between fluorescence signal response (If) and the concentration (C) are expressed by If = −72.66 + 3.14C with the regression coefficients R of 0.9990. The Te limit of detection for the sample blank solution is 2.2 ng ml− 1 (3σ). The relative standard deviation (RSD) was 3.9% for nine consecutive measurements of 200 μg l− 1 Te (IV) standard solution. The calibration curve was linear up to 800 µg l− 1. 3.6. Method validation and analytical results of samples Three types of soldering tin material produced according to GB/ T3131-2001 were analyzed using the present method. These results are listed in Table 1. The sample blank experiment was conducted for the same time with blank values less than 0.67 μg g− 1. In order to validate the developed method, standard solutions were spiked into the actual sample as shown by Table 1. Recovery values varied from 99.9% to 104.4%. Further validation of this method was carried out by comparison of the data found by ECHG-AFS with those obtained by GFAAS and HGAFS. The results are also listed in Table 1. The statistical method used was the paired t-test (95% confidence level), which shows that the results of EC-HG-AFS are statistically equivalent to that of CHG-AFS and GF-AAS. 4. Conclusions In the present work, a reticular W filament cathode EC-HG system operating at low temperatures was fabricated for the determination of trace amounts of Te. The developed EC-HG system could effectively drive out the generated TeH2 by sweeping gas from the cathode chamber and reduce TeH2 decomposition in the transferring procedure. Coupled with AFS, the proposed method was successfully used for the determination of Te in soldering tin material. The precision and accuracy of the method were satisfactory. This method offers the advantages of avoiding the use of sodium tetrahydroborate and NaOH and only requires the use of acids, which can easily be obtained at ultra pure levels, thereby greatly decreasing the potential risk of reagent contamination as well as material costs. It should be noted that detection limit obtained by the EC-HG method is not as low that of CHG as mentioned in the literature [20,21] . Acknowledgement This work is supported by the National Nature Science Foundation of PR China (No. 20675074). References [1] F. Laborda, E. Bolea, J.R. Castillo, Tubular electrolytic hydride generator for continuous and flow injection sample introduction in atomic absorption spectrometry, J. Anal. At. Spectrom. 15 (2000) 103–107.
713
[2] A. Matsumoto, T. Shiozaki, T. Nakahara, Simultaneous determination of bismuth and tellurium in steels by high power nitrogen microwave induced plasma atomic emission spectrometry coupled with the hydride generation technique, Anal. Bioanal. Chem. 379 (2004) 90–95. [3] P. Cava. Montesinos, M.L. Cervera, A. Pastor, M.D.L. Guardia, Hydride generation atomic fluorescence spectrometric determination of ultratraces of selenium and tellurium in cow milk, Anal. Chim. Acta 481 (2003) 291–300. [4] Y.L. Chen, S.J. Jiang, Determination of tellurium in a nickel-based alloy by flow injection vapor generation inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 15 (2000) 1578–1582. [5] M. Grotti, C. Lagomarsino, R. Frache, Multivariate study in chemical vapor generation for simultaneous determination of arsenic, antimony, bismuth, germanium, tin, selenium, tellurium and mercury by inductively coupled plasma optical emission spectrometry, J. Anal. At. Spectrom. 20 (2005) 1365–1373. [6] X. Li, J. Jia, Z.H. Wang, Speciation of inorganic arsenic by electrochemical hydride generation atomic absorption spectrometry, Anal. Chim. Acta 560 (2006) 153–158. [7] J. Liang, Q.Q. Wang, B.L. Huang, Electrochemical vapor generation of selenium species after online photolysis and reduction by UV-irradiation under nano TiO2 photocatalysis and its application to selenium speciation by HPLC coupled with atomic fluorescence spectrometry, Anal. Bioanal. Chem. 381 (2005) 366–372. [8] B. Özmen, F.M. Matysik, N.H. Bings, J.A.C. Broekaert, Optimization and evaluation of different chemical and electrochemical hydride generation systems for the determination of arsenic by microwave plasma torch atomic emission spectrometry, Spectrochim. Acta Part B 59 (2004) 941–950. [9] E. Denkhaus, A. Golloch, X.M. Guo, B. Huang, Electrolytic hydride generation (EC-HG) — a sample introduction system with some special features, J. Anal. At. Spectrom. 16 (2001) 870–878. [10] F. Laborda, E. Bolea, J.R. Castillo, Electrochemical hydride generation as a sampleintroduction technique in atomic spectrometry: fundamentals, interferences, and applications, Anal. Bioanal. Chem. 388 (2007) 743–751. [11] M.H. Arbab-Zavar, M. Chamsaz, A. Youssefi, M. Aliakbari, Electrochemical hydride generation atomic absorption spectrometry for determination of cadmium, Anal. Chim. Acta 546 (2005) 126–132. [12] X. Li, Z.H. Wang, Determination of mercury by intermittent flow electrochemical cold vapor generation coupled to atomic fluorescence spectrometry, Anal. Chim. Acta 588 (2007) 179–183. [13] M.H. Arbab-Zavar, G.H. Rounaghi, M. Chamsaz, M. Masrournia, Determination of Mecurry(II) ion by electrochemical cold vapor generation atomic absorption spectrometry, Anal. Sci. 19 (2003) 743–746. [14] V. Červený, P. Rychlovský, J. Netolická, J. Šíma, Electrochemical generation of mercury cold vapor and its in-situ trapping in gold-covered graphite tube atomizers, Spectrochim. Acta Part B 62 (2007) 317–323. [15] W.B. Zhang, W.E. Gan, X.Q. Lin, Electrochemical hydride generation atomic fluorescence spectrometry for the simultaneous determination of arsenic and antimony in Chinese medicine samples, Anal. Chim. Acta 539 (2005) 335–340. [16] W.B. Zhang, W.E. Gan, X.Q. Lin, Development of a new electrochemical hydride generator with W wire cathode for the determination of As and Sb by atomic fluorescence spectrometry, Talanta 68 (2006) 1316–1321. [17] Methods for chemical analysis of tin–lead solders—Determination of copper, iron, cadmium, silver, gold, arsenic, zinc, aluminum, bismuth, phosphorous content. GB/T 10574.13-2003. [18] Y.H. Lin, X.R. Wang, D.X. Yuan, P.Y. Yang, B.L. Huang, Z.X. Zhuang, Flow injectionelectrochemical hydride generation technique for atomic absorption spectrometry, J. Anal. Atom. Spectrom. 7 (1992) 287–291. [19] E. Denkhaus, F. Beck, P. Bueschler, R. Gerhard, A. Golloch, Electrolytic hydride generation atomic absorption spectrometry for the determination of antimony, arsenic, selenium, and tin – mechanistic aspects and figures of merit, Fresenius' J. Anal. Chem. 370 (2001) 735–743. [20] İ Menemenlioğlu, D. Korkmaz, O.Y. Ataman, Determination of antimony by using a quartz atom trap and electrochemical hydride generation atomic absorption spectrometry, Spectrochim. Acta Part B 62 (2007) 40–47. [21] J. Šíma, P. Rychlovský, Electrochemical selenium hydride generation with in situ trapping in graphite tube atomizers, Spectrochim. Acta Part B 58 (2003) 919–930.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Analytical note
Determination of Te in soldering tin using continuous flowing electrochemical hydride generation atomic fluorescence spectrometry Xianjuan Jiang, Wuer Gan ⁎, Suping Han, Youzhao He Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China
A R T I C L E
I N F O
Article history: Received 15 October 2007 Accepted 26 March 2008 Available online 7 April 2008 Keywords: Te Electrochemical hydride generation Atomic fluorescence spectroscopy
A B S T R A C T An electrochemical hydride generation system was developed for the detection of Te by coupling an electrochemical hydride generator with atomic fluorescence spectrometry. Since TeH2 is unstable and easily decomposes in solution, a reticular W filament cathode was used in the present system. The TeH2 generated on the cathode surface was effectively driven out by sweeping gas from the cathode chamber. In addition, a low temperature electrochemical cell (10 °C) was applied to reduce the decomposition of TeH2 in solution. The limit of detection (LOD) was 2.2 ng ml− 1 and the relative standard deviation (RSD) was 3.9% for nine consecutive measurements of standard solution. This method was successfully employed for determination of Te in soldering tin material. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The chemical hydride generation (CHG) of Te is a well-established sample introduction technique in atomic spectrometry when low detection limits are desired [1–5]. Although the CHG technique, which is sample introduction method based on the reaction of sodium tetrahydroborate with the acidic material that has many advantages and is widely used, there are some disadvantages listed in previous studies [6–8]. As a suitable alternative to the CHG technique for sample introduction, the electrolytic volatile generation of As, Se, Sb, Sn, Ge, Pb, Bi and Te was reported to be feasible as an electrolytic sample pretreatment method [9]. In the last fifteen years, electrochemical hydride generation (EC-HG) of H2Se, SbH3, AsH3, BiH3, GeH4 and SnH4 has been successfully coupled with a variety of spectrometric techniques such as atomic absorption spectrometry (AAS), graphite furnace atomic absorption spectrometry (GFAAS), atomic fluorescence spectrometry (AFS), atomic emission spectrometry (AES) and inductively coupled plasma-mass spectrometry (ICP-MS) [10]. Hydrides of Cd [11] and elemental Hg [12–14] have also been successfully generated by electrochemical methods, and the technique was also applied for analysis of water, medicines and biological samples [9,10] . However, we have not found any reports on the detection of Te by using EC-HG method as sample introduction method up to present. Trace amount of Te can affect the physical and mechanical properties of metals and metal alloys [14]. An accurate technique is therefore required for the determination of Te in such samples.
⁎ Corresponding author. Tel.: +86 551 3600021. E-mail address: [email protected] (W.E. Gan). 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.03.017
In this work, a simple and highly accurate method for the detection of Te in soldering tin material was created which coupled EC-HG with AFS. A three-dimensional reticular W filament electrode was used as the cathode. At low temperatures, the TeH2 generated on the reticular W filament cathode surface could be effectively driven out by sweeping gas in the cathode chamber and was then followed by AFS determination. 2. Experimental 2.1. Instrumentation A direct current (DC) constant current and voltage unit (Type DH1719A-3, Beijing Da Hua Wireless instrument Co, Beijing, China) operating at constant current mode was used. An AFS-230 doublechannel nondispersive atomic fluorescence spectrometer (Beijing Haiguang Instrument Co, Beijing, China) was employed throughout. A high intensity Te hollow cathode lamp (General Research Institute for Nonferrous Metals, Beijing, China) was used as a radiation source. Quartz tube (7 mm I.D. × 14 mm length) was used as atomizer. A hydrogen–argon–air entrained flame was maintained with the addition of auxiliary hydrogen. Two sequential gas liquid separators (GLS) were used for completely separating the gases from liquid as previously described [15]. A DG-0506 low constant temperature trough (Shi-Bo Bio-technique. Co. Shanghai, China) was employed. The capacity of the trough was 6 L. The adjustable temperature range was −5 °C to 100 °C and had a variability of 0.05 °C. A WFX-120 atomic absorption spectrometer with a WF-4C graphite furnace (Beijing Ruili Instrument Co. Beijing, China) was employed for the validation of the proposed method. The GF-AAS operating conditions were as follows: (1) The Te hollow cathode lamp was operated
X. Jiang et al. / Spectrochimica Acta Part B 63 (2008) 710–713
711
Fig. 1. Schematics of the electrochemical hydride generation atomic fluorescence spectrometry system, P: peristaltic pump; GLS: gas liquid separator; W: waste; AFS: atomic fluorescence spectrometry.
at a wavelength of 214.3 nm, (2) the signals were evaluated from the peak area yielding an integration time of 5 s, (3) the pyrolysis temperature was 1200 °C, (4) and the atomization temperature was 2600 °C. 2.2. Hydride generation system A schematic diagram of the analytical system is shown in Fig. 1. A homemade thin-layer electrochemical cell was used as the hydride generator as described in a previous publication [16]. The polytetrafluoroethylene (PTFE) cell consisted of an upper and lower chamber (both 100 × 15 × 5 mm, inner volumes of 7.5 ml). A cation ion exchange membrane (Huanyu Co., Beijing, China) was used to separate the gases generated in the anode and cathode chamber. A Pt foil anode was fixed on channel bottom of the anode chamber. The polished W wire was knitted into a multilayer, net-shaped cathode. The wire length was about 7 m, and the surface area was about 21.98 cm2. Sweeping gas (comprising carrier gas Ar and auxiliary gas H2) passed through the cathode chamber to purge the hydride from the surface of the cathode. Ar, H2 and the generated hydride were then separated by the first GLS and directed to the second one through a PTFE tube (0.8 mm I.D. × 100 mm length). Finally, these gases were transported to the atomizer of the AFS. 2.3. Reagents and materials All reagents were of the highest available purity, and were of at least analytical grade. Doubly deionized water (DDW) was solely used. The Te (IV) Stock solution (1.000 g l− 1) was prepared by dissolving Te powder in concentrated HNO3. Next, low-concentrated HCl was added to the solution. The Te (VI) stock solution was prepared by dissolving H2TeO4 (The Hui Hong reagent company, China). A series of standard solutions were prepared by stepwise dilution of stock solutions with 1.5 M H2SO4 just before use. Various concentrations of guaranteed reagents of HCl, H2SO4, HNO3 and H3PO4 were used to test the properties of the electrolytic solutions. A mixture of 50 g l− 1 thiourea and ascorbic acid solution was prepared by dissolving both thiourea and ascorbic in DDW. Analytical grade concentrated HBr was used to remove Sn from samples. Pd (NO3)2 at 1 g l− 1 was used as a chemical modifier when Te was detected by GFAAS. High-purity argon and hydrogen were used as the carrier gas and auxiliary gas, respectively. W wire (0.1 mm diameter), W foil, Pb foil and Pt foil (0.25 mm × 15 mm × 73.2 mm, 99.9%) were used as electrodes.
evaporated continuously until it was nearly dried. After being cooled to room temperature, 2 ml (1 + 1) H2SO4 and 10 ml (2 + 98) HCl was added and heated to eliminate Pb. Concentrated HBr (2.5 ml) and concentrated HCl (2.5 ml) were added to the beaker and heated to remove tin in the form of tin chloride or tin bromide. Finally, the sample solution was transferred to a 50 ml volumetric flask and brought up to volume with 1.5 M H2SO4 following the addition of 5 ml of 50 g l− 1 thiourea and ascorbic acid. The same reagents were chosen as the blank samples (controls), which were subjected to the same procedure as the sample. 2.5. Analytical procedures At first, the catholyte was pumped into a cross-tube at a flow rate of 3.4 ml min− 1. After being mixed with sweeping gas, the mixture of catholyte solution and gases was delivered to the cathode chamber. At the same time, the anolyte was pumped into the anode chamber at the same flow rate. After the anode and cathode chambers were filled, the flow rate was reduced to 1.8 ml min− 1 and electrolysis took place at 1.5 A. The supply time and sample volume consumed for a signal measurement were about 30 s and 8.4 ml, respectively. Oxygen produced in the anode chamber was driven out along with the anolyte and then diffused to the surroundings at the outlet of the transferring tube. The anolyte was reclaimed and recycled. Mixture of gases and solution from the cathode chamber were delivered to the two sequential GLS. The hydride-containing gas was then directed to the atomizer and finally detected by AFS. Peak area measurement with an integration time of 10 s was used for quantification. CHG-AFS was adopted for the validation of the proposed EC-HGAFS method. The sample solution was swept by 5% (v/v) HCl carrier
2.4. Sample preparation Samples were prepared according to GB/T 10574.13-2003 [17]. Sample aliquots of 200 mg were transferred to 50 ml beakers in which a mixture of 12 ml of concentrated HNO3 and 3 ml of concentrated HCl was added under gentle heating. The sample solution was then
Fig. 2. The effect of cathode materials on the atomic fluorescence intensity of 200 µg l− 1 Te (IV).
712
X. Jiang et al. / Spectrochimica Acta Part B 63 (2008) 710–713
at a flow rate of 3.0 ml min− 1, and was merged with stream of 3.0 ml min− 1 2% (m/v) NaBH4 in 0.5% (m/v) NaOH. Then, the generated TeH2 was introduced to the quartz atomizer by argon carrier gas for AFS determination.
Table 1 Analytical results of Te in soldering tin samples Sample
3. Results and discussion 3.1. Cathode materials and configuration
1 2 3 a
Platinum was used as the anode because of its stability and inertness. As for the cathode, conventional materials including Pt foil, Pb foil and W foil were first considered. The results in Fig. 2 show that the atomic fluorescence intensities of Te under the aforementioned cathode materials are relative to the cathode material overpotential (Pt b W b Pb) as previously reported [18,19] . However, the sensitivity of the method is not satisfactory when using Pt foil, Pb foil or W foil as a cathode. For improving the sensitivity of the method, a reticular W filament was used as the cathode. The results (also in Fig. 2) showed that the atomic fluorescence intensity clearly increased by using a reticular W filament cathode. This increase was a result of the larger analyte access of the reticular W filament cathode. Furthermore, the bubbles formed on the reticular cathode surface were very small, which caused the generated hydride to be effectively driven off the surface of the cathode by the sweeping gas, and also caused a reduction of the decomposition of TeH2. As a result, the reticular W filament cathode was selected for this study. 3.2. Effect of sweeping gas According to our previous work [15], the gas stream (400 ml min− 1 Ar and 200 ml min− 1 H2) was employed for effectively transferring hydride to the AFS and for maintaining the hydrogen–argon–air entrained flame. In the present work, the gas stream also acts as sweeping gas to drive hydride out of cathode chamber. In order to evaluate the influence of the sweeping gas on atomic fluorescence intensity using W foil and a reticular W filament cathode, the gas stream (400 ml min− 1 Ar and 200 ml min− 1 H2) was divided into two portions. One portion, the sweeping gas, was directed into the inlet of the cathode chamber, and the other portion was directed into the cross-tube at the outlet of cathode chamber. In Fig. 3, it was found that the atomic fluorescence intensity greatly increased as the sweeping gas flow rate increased when the reticular W filament cathode was used. However, the atomic fluorescence intensity increased slightly when W foil was used as the cathode. The results indicated that the multilayer reticulated configuration of the W
b
CHG-AFS
GFAAS
Determined
ECHG-AFS Recoveryb
Determined
Determined
(μg g− 1)a
(%)
(μg g− 1)a
(μg g− 1)a
8.81 ± 0.31 12.57 ± 0.28 16.52 ± 0.54
99.6 103.8 104.4
8.88 ± 0.25 11.75 ± 0.74 15.80 ± 0.56
8.75 ± 0.24 11.78 ± 0.32 16.32 ± 0.21
Mean value ± standard deviation (n = 3). 10 μg ml− 1 Te standard solution spiked in the sample.
filament cathode enabled the hydride to be easily driven out from the cathode chamber. Consequently, the gas stream (400 ml min− 1 Ar and 200 ml min− 1 H2) was totally introduced into the inlet of cathode chamber. 3.3. Operating parameters The entire electrochemical cell employed in our work was immersed into a constant temperature trough for increased sensitivity. The results demonstrated that the atomic fluorescence intensity increased along with decreasing temperature from 40 °C to 10 °C, and then leveled off. Thus, the constant temperature was set at 10 °C for the study. In this work, 0.5 M H2SO4 was selected as the anolyte. The effects of different concentrations of catholytes consisting of HCl, H2SO4, H3PO4 and HNO3 on the atomic fluorescence intensity of 200 µg l− 1 Te (IV) were studied. The results indicated that 1.5 M H2SO4 was the optimum catholyte in the present work. The atomic fluorescence intensity of Te (IV) increased along with a sample flow rate that varied from 1 ml min− 1 up to 1.8 ml min− 1. The intensity then decreased with a further increase in sample flow rate. A constant sample flow rate of 1.8 ml min− 1 was chosen. An electrolysis current of 1.5 A was used throughout so that maximum atomic fluorescence could be obtained. The corresponding current density was about 68.24 mA cm− 2. The influence of the transferring tube length (from the first gas liquid separator to the second) on atomic fluorescence intensity was studied. The atomic fluorescence intensity decreased slowly as the tube length varied from 100 to 400 mm due to TeH2 decomposition in the transferring process. As a result, a 100 mm length transferring tube was used. No discernible signal was observed when the Te (VI) solution was used in this experiment. In this experiment, 5 g l− 1 ascorbic acid and 5 g l− 1 thiourea was found to be suitable for the reduction of Te (VI) to Te (IV) and selected as reducing agent. The ascorbic acid helped to avoid the influence of matrix with oxidation as well as to reduce Te (VI) to Te (IV). Because excessive ascorbic acid can reduce Te to elemental form, the maximum concentration of the ascorbic acid should not exceed 6 g l− 1. 3.4. Effect of coexisting ions
Fig. 3. The effect of flow rate of sweeping gas on 200 µg l− 1 Te (IV).
In this work, 200 μg l− 1 Te (IV) solution possibly containing coexisting ions was analyzed. Four microgrammes per milliliter Bi (III), 2 μg ml− 1 Cu (II), 5 μg ml− 1 Fe (II), 2 μg ml− 1 Zn (II), 2 μg ml− 1 Cd (II) and 1 μg ml− 1 Ni (II) have no significant influence on the atomic fluorescence intensity of Te (IV). Hydride-forming elements such as As (III) (1 μg ml− 1) and Sb (III) (1 μg ml− 1) also do not interfere with the detection of Te. The matrix interference was also taken into account, as the soldering tin sample is a type of Pb–Sn alloy. It was found that 0.2 μg ml− 1 Pb (II) and 1 μg ml− 1 Sn (IV) would not interfere with the determination of the Te (IV). The concentrations of Pb (II) and Sn (IV) in sample solutions were determined by GF-AAS and were less than
X. Jiang et al. / Spectrochimica Acta Part B 63 (2008) 710–713
0.2 and 1 μg ml− 1, respectively. As a result, the interference of matrix elements was neglected in this work. 3.5. Analytical figures of merit A regression line between fluorescence signal response (If) and the concentration (C) are expressed by If = −72.66 + 3.14C with the regression coefficients R of 0.9990. The Te limit of detection for the sample blank solution is 2.2 ng ml− 1 (3σ). The relative standard deviation (RSD) was 3.9% for nine consecutive measurements of 200 μg l− 1 Te (IV) standard solution. The calibration curve was linear up to 800 µg l− 1. 3.6. Method validation and analytical results of samples Three types of soldering tin material produced according to GB/ T3131-2001 were analyzed using the present method. These results are listed in Table 1. The sample blank experiment was conducted for the same time with blank values less than 0.67 μg g− 1. In order to validate the developed method, standard solutions were spiked into the actual sample as shown by Table 1. Recovery values varied from 99.9% to 104.4%. Further validation of this method was carried out by comparison of the data found by ECHG-AFS with those obtained by GFAAS and HGAFS. The results are also listed in Table 1. The statistical method used was the paired t-test (95% confidence level), which shows that the results of EC-HG-AFS are statistically equivalent to that of CHG-AFS and GF-AAS. 4. Conclusions In the present work, a reticular W filament cathode EC-HG system operating at low temperatures was fabricated for the determination of trace amounts of Te. The developed EC-HG system could effectively drive out the generated TeH2 by sweeping gas from the cathode chamber and reduce TeH2 decomposition in the transferring procedure. Coupled with AFS, the proposed method was successfully used for the determination of Te in soldering tin material. The precision and accuracy of the method were satisfactory. This method offers the advantages of avoiding the use of sodium tetrahydroborate and NaOH and only requires the use of acids, which can easily be obtained at ultra pure levels, thereby greatly decreasing the potential risk of reagent contamination as well as material costs. It should be noted that detection limit obtained by the EC-HG method is not as low that of CHG as mentioned in the literature [20,21] . Acknowledgement This work is supported by the National Nature Science Foundation of PR China (No. 20675074). References [1] F. Laborda, E. Bolea, J.R. Castillo, Tubular electrolytic hydride generator for continuous and flow injection sample introduction in atomic absorption spectrometry, J. Anal. At. Spectrom. 15 (2000) 103–107.
713
[2] A. Matsumoto, T. Shiozaki, T. Nakahara, Simultaneous determination of bismuth and tellurium in steels by high power nitrogen microwave induced plasma atomic emission spectrometry coupled with the hydride generation technique, Anal. Bioanal. Chem. 379 (2004) 90–95. [3] P. Cava. Montesinos, M.L. Cervera, A. Pastor, M.D.L. Guardia, Hydride generation atomic fluorescence spectrometric determination of ultratraces of selenium and tellurium in cow milk, Anal. Chim. Acta 481 (2003) 291–300. [4] Y.L. Chen, S.J. Jiang, Determination of tellurium in a nickel-based alloy by flow injection vapor generation inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 15 (2000) 1578–1582. [5] M. Grotti, C. Lagomarsino, R. Frache, Multivariate study in chemical vapor generation for simultaneous determination of arsenic, antimony, bismuth, germanium, tin, selenium, tellurium and mercury by inductively coupled plasma optical emission spectrometry, J. Anal. At. Spectrom. 20 (2005) 1365–1373. [6] X. Li, J. Jia, Z.H. Wang, Speciation of inorganic arsenic by electrochemical hydride generation atomic absorption spectrometry, Anal. Chim. Acta 560 (2006) 153–158. [7] J. Liang, Q.Q. Wang, B.L. Huang, Electrochemical vapor generation of selenium species after online photolysis and reduction by UV-irradiation under nano TiO2 photocatalysis and its application to selenium speciation by HPLC coupled with atomic fluorescence spectrometry, Anal. Bioanal. Chem. 381 (2005) 366–372. [8] B. Özmen, F.M. Matysik, N.H. Bings, J.A.C. Broekaert, Optimization and evaluation of different chemical and electrochemical hydride generation systems for the determination of arsenic by microwave plasma torch atomic emission spectrometry, Spectrochim. Acta Part B 59 (2004) 941–950. [9] E. Denkhaus, A. Golloch, X.M. Guo, B. Huang, Electrolytic hydride generation (EC-HG) — a sample introduction system with some special features, J. Anal. At. Spectrom. 16 (2001) 870–878. [10] F. Laborda, E. Bolea, J.R. Castillo, Electrochemical hydride generation as a sampleintroduction technique in atomic spectrometry: fundamentals, interferences, and applications, Anal. Bioanal. Chem. 388 (2007) 743–751. [11] M.H. Arbab-Zavar, M. Chamsaz, A. Youssefi, M. Aliakbari, Electrochemical hydride generation atomic absorption spectrometry for determination of cadmium, Anal. Chim. Acta 546 (2005) 126–132. [12] X. Li, Z.H. Wang, Determination of mercury by intermittent flow electrochemical cold vapor generation coupled to atomic fluorescence spectrometry, Anal. Chim. Acta 588 (2007) 179–183. [13] M.H. Arbab-Zavar, G.H. Rounaghi, M. Chamsaz, M. Masrournia, Determination of Mecurry(II) ion by electrochemical cold vapor generation atomic absorption spectrometry, Anal. Sci. 19 (2003) 743–746. [14] V. Červený, P. Rychlovský, J. Netolická, J. Šíma, Electrochemical generation of mercury cold vapor and its in-situ trapping in gold-covered graphite tube atomizers, Spectrochim. Acta Part B 62 (2007) 317–323. [15] W.B. Zhang, W.E. Gan, X.Q. Lin, Electrochemical hydride generation atomic fluorescence spectrometry for the simultaneous determination of arsenic and antimony in Chinese medicine samples, Anal. Chim. Acta 539 (2005) 335–340. [16] W.B. Zhang, W.E. Gan, X.Q. Lin, Development of a new electrochemical hydride generator with W wire cathode for the determination of As and Sb by atomic fluorescence spectrometry, Talanta 68 (2006) 1316–1321. [17] Methods for chemical analysis of tin–lead solders—Determination of copper, iron, cadmium, silver, gold, arsenic, zinc, aluminum, bismuth, phosphorous content. GB/T 10574.13-2003. [18] Y.H. Lin, X.R. Wang, D.X. Yuan, P.Y. Yang, B.L. Huang, Z.X. Zhuang, Flow injectionelectrochemical hydride generation technique for atomic absorption spectrometry, J. Anal. Atom. Spectrom. 7 (1992) 287–291. [19] E. Denkhaus, F. Beck, P. Bueschler, R. Gerhard, A. Golloch, Electrolytic hydride generation atomic absorption spectrometry for the determination of antimony, arsenic, selenium, and tin – mechanistic aspects and figures of merit, Fresenius' J. Anal. Chem. 370 (2001) 735–743. [20] İ Menemenlioğlu, D. Korkmaz, O.Y. Ataman, Determination of antimony by using a quartz atom trap and electrochemical hydride generation atomic absorption spectrometry, Spectrochim. Acta Part B 62 (2007) 40–47. [21] J. Šíma, P. Rychlovský, Electrochemical selenium hydride generation with in situ trapping in graphite tube atomizers, Spectrochim. Acta Part B 58 (2003) 919–930.