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Rapid determination of indium in water samples using a portable solution cathode glow discharge-atomic emission spectrometer
Microchemical Journal 137 (2018) 266–271
Contents lists available at ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Rapid determination of indium in water samples using a portable solution cathode glow discharge-atomic emission spectrometer Wenchuan Zu, Yin Yang, Yu Wang ⁎, Xiaotao Yang, Cong Liu, Min Ren Beijing Center for Physical & Chemical Analysis, Beijing 100089, China Beijing Academy of Science and Technology, Key Laboratory of Analysis and Testing Technology, Beijing 100089, China
a r t i c l e
i n f o
Article history: Received 1 September 2017 Received in revised form 2 November 2017 Accepted 2 November 2017 Available online 03 November 2017 Keywords: Solution cathode glow discharge Indium Water Portable atomic emission spectrometer
a b s t r a c t A novel method for fast determination of indium in water was established using a homemade solution cathode glow discharger coupled with a portable fiber optical spectrometer. The instrumental parameters and operation conditions which may remarkably influence the analytical performance including the analytical emission line, the solution acidity, etc. were optimized. The resolution of the instrument based on the peak width at half height at 451 nm for 2.0 mg/L indium solution was about 1.1 nm. Under the optimized parameters and conditions, the limit of detection (LOD) was 0.032 mg/L calculated by 3SDBLANK/k (n = 11). Moreover, the analytical stability was inspected by 7 parallel tests of 0.5 mg/L indium standard solution, and the relative standard deviation (RSD) was below 5%. The interferences of potential co-existing metal ions on indium determination were inspected and no significant interferences were observed. This method was applied to the analysis of real water samples available in Beijing and the spiked recoveries were in the range of 94.0%–103.6%, which proved the feasibility of indium determination of real samples. Besides, this method can satisfy the demands of field tests due to the simple and small-sized instrument and less consumption of reagent. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Indium, which is known as a heavy-metal element, is now widely used in different fields, including alloy, semi-conductor, biomedicine, etc. [1–5]. Meanwhile, the risk of indium pollution tends to be enhanced accordingly as the release of indium into the environment is remarkably increasing due to the continuously growing indium consumption [6]. Moreover, indium compounds are proved to be with certain toxicity and damages to health will be caused if the income amount exceeds a certain degree, especially for tooth decay, pain in joints and the germinal and inherit system [7]. Thus, it makes significant sense to monitor the indium content level of environmental water and industrial effluents to prevent from indium pollution. For the time being, the methods for indium determination have already been reported in literature, including atomic spectrometry [8– 10], inorganic mass spectrometry [11] and electrochemical technique [12–14]. Generally speaking, flame atomic absorption spectrometry (F-AAS) is widely used in indium determination. However, the limit of determination is somewhat unsatisfactory, and the use of fuel gas is unavoidable. As a high sensitivity technique, inductively coupled plasma-mass spectrometry (ICP-MS) is suitable for ultra-trace indium ⁎ Corresponding author at: Beijing Center for Physical & Chemical Analysis, Beijing 100089, China. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.microc.2017.11.001 0026-265X/© 2017 Elsevier B.V. All rights reserved.
determination, yet the instrumental and analytical cost is high and the apparatus is cumbersome. Furthermore, all the atomic spectrometry and ICP-MS techniques above can't work without gas cylinders so that instrumental portability tends to be difficult. As for electrochemical methods for indium detection, field test can be realized while the daily treatment or replacement of the electrodes is tedious and the memory effect is remarkable. As a novel technique for metal elemental analysis, solution cathode glow discharge atomic emission spectrometry (SCGD-AES) has been successfully applied in metal elemental quantitative analysis, such as Pb, Cu, Li, Na, etc. and showed favorable analytic performance [15–22]. As for the solution cathode discharge system, the solution is the cathode while a metal electrode is employed as the anode. The plasma will come into being due to the gas ionization when a high voltage is applied between the two electrodes. The solution is volatilized and the elements dissolved in the solution will enter the plasma, followed by the elemental atomization and exciting process. Thus, the characteristic emission spectrum is ultimately formed. In our previous work, the method for thallium determination by SCGD-AES was established and favorably verified in real samples [23]. However, the study on indium analysis has never been involved in the above literatures. In this paper, a novel method for indium determination using a portable SCGD equipment coupled with a fiber optical spectrometer is established. This method is feasible for field test as the dependence on gas cylinders for traditional atomic spectrometry
W. Zu et al. / Microchemical Journal 137 (2018) 266–271
267
in the 10% HNO3 for 24 h, washed clean with ultra pure water and finally dried for standby application. 2.2. Sample preparation The clearness of the water samples should be pre-judged before the follow-up treatment. If the samples are clear enough, direct determination is feasible after only an acidification operation. To say concretely, 1 mL HNO3 is added into each 100 mL water sample to ensure the acidity consistent with the standard solutions. As for the muddy samples, the acidification is implemented only after filtration with 0.45 μm filter membranes-aquo system. The blanks and spiked samples were prepared in parallel. 2.3. Apparatus Fig. 1. Diagram of SCGD cell with discharge on.
techniques is cast off and the instrument is with excellent portability due to the ideally small size and weight. Moreover, the home-made SCGD works under the normal pressure and this method is friendly to environment as only dilute acid is demanded in the test process, by which pollution brought from large amount of reagents can be effectively reduced. 2. Experimental 2.1. Reagents The nitric acid was of guarantee grade (G.R., from Beijing Fine Chemicals Ltd.), Indium standard stock solution (1000 mg·L−1) was acquired from national standard materials research center. The calibration solutions (0.2 mg/L–5.0 mg/L) were prepared by stepwise dilution of the standard stock solution with 1% (V/V) HNO3. Ultra pure water was produced by a Millipore ultra pure water system and the electrical resistivity is above 18.2 MΩ·cm. Besides, all the glassy wares were immersed
A home-made solution cathode glow discharge cell similarly as reported in our former study [23] was further employed except for certain improvement. The SCGD cell with discharge on was as showed in Fig. 1 while the detailed SCGD-AES configuration was as showed in Fig. 2. Generally speaking, the anode is a tungsten pin (20 mm long and 1 mm diameter) and the solution plays the role of cathode. When the gap between the two electrodes is applied a high voltage (500– 1000 V) with a high voltage supply module (DW-P102 - 100C5D, Beijing Yuan Bo Sheng Electronic Technology Co., Ltd.), the gas in the gap is ionized and the plasma is generated. The solution is progressively gasified and enters the plasma so that indium ions dissolved in the solution together enter the plasma and are atomized. Then the ground-state indium atoms are excited and give out the characteristic spectrum determined by a fiber-optical spectrometer (HR4000, Ocean Optics, Inc.). The whole apparatus is portable due to the small size and weight. In this work, a 6-way valve is adopted for the sampling module as showed in Fig. 1. In the loading process, the sample solution is pumped into the storage coil from 1, and fills in the storage coil between 3 and 6 while the surplus solution flows out as waste liquid through 2.
Fig. 2. The configuration of the SCGD-atomic emission spectrometry device.
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Table 1 Working parameters of SCGD-AES. SCGD-AES parameters
Adopted
The cathode glow discharge voltage/V The inner size of the glass capillary/mm The discharge distance/mm The detecting wavelength of indium/nm The integral time for the fiber-optical spectrometer/ms The carrier flow Measurement mode of the fiber-optical spectrometer
700 0.3 × 0.3 3 451 100 1%(V/V) HNO3 Peak height
Meanwhile, the plunger pump is pulled down so that 5 mL 1% HNO3 as carrier flow is pumped into the pump cylinder. In the injection process, one end of the storage coil is switched to 4 and the other end is switched to 5. When the plunger pump is pushed up and restored to the initial state, the sample solution in the storage coil can be pushed into the SCGD cell through 5 by the carrier flow. The excessive carrier flow can also plays the role of cleaning the pipeline to eliminate memory effect. In our work, the optimal working parameters adopted for the SCGDAES were as follows in Table 1. 3. Results and discussion 3.1. Optimization of operating conditions 3.1.1. The analytical emission line Generally speaking, the emission lines for each element are numerous. The selection of analytical emission line should consider both low spectrum interference and appropriate sensitivity. As for our SCGDAES system, characteristic emission spectrum for indium was observed by the inside CCD of a fiber optical spectrometer. As showed in Fig. 3, the sensitive emission line for indium under 451 nm was observed while a sub-sensitive emission peak under 410 nm was presented as well. The sensitivity of indium determination under 451 nm and 410 nm were compared, and the results were showed in Fig. 4. Under the same instrumental parameters and experimental conditions, the sensitivity for indium determination under 410 nm was 52.9% of that under 451 nm (calculated as the ratio of slope of the linear equation of working curve). The 410 nm wavelength is useful for certain samples with high indium content or serious interference at 451 nm. 3.1.2. The solution acidity and discharge gap Acidity is a crucial factor significantly influencing the analytical sensitivity and stability. Therefore, the influence of acidity on indium analytical sensitivity was investigated. A group of 1.0 mg/L indium standard solutions with different acidity were used to observe the emission intensity, and the acidity range covered 0.5%–1.5%(V/V) HNO3. The results showed in Fig. 5 presented the similar regulation as our former
Fig. 4. Sensitivity for In detection under 451 nm compared with that under 410 nm.
work [23]. Generally, when the concentration of HNO3 was within the above range, the emission intensity turned larger with the increasing of the acidity. Meanwhile, in our work, the discharge got more difficult and the glow stability turned worse when the acidity was either too large or small. When the acidity was below 0.5% or beyond 1.5%(V/V) HNO3, the discharge can't even be generated. The regularity can be well-explained by the work T. Cserfalvi, et al. [24] ever reported. Considering both the sensitivity and stability, we chose 1% HNO3 for our further work. Therefore, for the real sample analysis, we should keep the consistent acidity with the standard solutions. The discharge gap is another crucial parameter of the SCGD system closely associated with the emission intensity. The discharge gap, which means the distance between the tip of the tungsten anode and the surface of the test solution, should neither too high nor too low to achieve the ideal analytical performance. As Z Wang, et al. [25] ever reported, the narrower discharge gap does bad to the improvement of the sensitivity. And in the range of 1–3 mm discharge gap, the emission intensity increased with the broadening of the gap due to the longer residence time of the analytic target in the plasma and when the gap exceeds 3 mm, the corresponding opposite trend was showed due to the loss of the energy or number of electrons available for excitation. Moreover, the optimizing process indicated that the discharge gap beyond 3 mm went against the stability of the plasma. Therefore, we chose 3 mm as the discharge gap. 3.1.3. Integral time for the fiber-optical spectrometer Integral time is a parameter of the fiber-optical spectrometer affecting the exposure time of the inside CCD. The influence of integral time on the emission intensity of indium was investigated. As Fig. 6 showed, the emission intensity with integral time of 200 ms was twice as large as that of 100 ms because the time for protons acceptance doubled.
Fig. 3. The spectrum of In observed by SCGD-AES.
W. Zu et al. / Microchemical Journal 137 (2018) 266–271
Fig. 5. The effect of solution acidity on indium detection sensitivity.
However, the noise of instrument increased to the similar degree as well, which showed integral time is not a crucial parameter to improve the limit of detection. In our work, we chose the integral time of 100 ms. 3.2. Analytic characteristic 3.2.1. The resolution capacity To ensure the portability of the instrument, a small-sized fiber optical spectrometer with internal optical dispersion system and CCD was employed for indium determination at 451 nm. The analytical accuracy and interference were directly affected by the resolution performance. Thus, the instrumental resolution capacity was inspected by the peak width at half height for 2.0 mg/L indium solution. As showed in Fig. 7, the peak width at half height at 451 nm was about 1.1 nm, which was favorable. 3.2.2. Interference The potential co-existing elements in water samples with complicated compositions may bring in matrix, spectral or even chemical interference for indium determination. Therefore, in our work, interference of potential co-existing mental ions for indium determination was studied. Generally, 2.0 mg/L indium solution was chosen as reference and a series of solutions containing different co-existing metal ions with certain concentrations were spiked into 2 mg/L indium, respectively. Subsequently, the emission intensity acquired was compared with the reference. As showed in Fig. 8, the recoveries were within 93.0%–105.0%, which showed no remarkable interference was discovered at the wavelength of 451 nm.
269
3.2.3. Analytic performance Under the optimized parameters and working conditions, the standard series were tested from low concentration to high. The calibration curve was acquired by linear fitting, where the emission intensities were as the vertical coordinates while indium concentrations were as horizontal coordinates. In the range of 0.2–5.0 mg/L, the linearity is favorable and the equation of linear regression was as follows. I = 3.4 + 294.7ρIn; R = 0.9993,where I meant the emission intensity while ρIn stood by the indium concentrations, and R was the linear correlation coefficient. The limit of detection (LOD) based on 3 SDBLANK/k was 0.032 mg/L, where SDBLANK was the standard deviation of emission intensity for 11 parallel blank tests and k meant the slope of the linear regression equation for the calibration curve. And the limit of quantification (LOQ) calculated by 10 SDBLANK/k was 0.11 mg/L. Besides, the precision was investigated by 7 parallel determination of the 0.5 mg/L indium standard solution. The emission intensity obtained were 146, 143, 131, 141, 147, 141 and 131. As a result, the relative standard deviation (RSD) was 4.7%. 3.3. Sample determination by SCGD-AES Three kinds of water samples with different matrix in Beijing, i.e. river water, waste water and tap water were chosen for validation of real sample determination. All the samples were prepared as the proposed method and indium content was determined by this SCGD-AES technique. Besides, 0.4 mg/L–2.0 mg/L indium standard solutions were spiked into the samples to validate the availability of the experimental results. The average recoveries were acquired in the range of 94.0%–103.6%, and the relative standard deviations for the parallel recovery results didn't exceed 7.5% (n = 5) (Table 2). It was concluded that the results were credible and the SCGD-AES method implemented with portable instrument was applicable for real sample analysis. 4. Conclusion A novel method for determination of indium in water samples based on homemade portable instrument was established. It offers the determination of indium in water, especially for the field test an ideal alternative due to the favorable analytical performance. As CCD in the fiber optical spectrometer was applied as final detector, the information of elemental emission spectrum was direct and concrete so that the spectral interference can be effectively judged. This proposed technique for indium determination is with high performance as it takes only 1.5 min to complete a whole detection process. Besides, only dilute acid (1% HNO3) is needed for this technique so that it's less reagent consumption, which is more environment-friendly and brings the decrease of reagent pollution risk and test background. This method is more suitable for
Fig. 6. Spectrum of 4.0 mg/L In at 451 nm with 100 ms and 200 ms integral time.
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W. Zu et al. / Microchemical Journal 137 (2018) 266–271
Fig. 7. The resolution test based on the 2.0 mg/L In standard solution.
Fig. 8. The interference of co-existing metal elements for In determination. Conditions: Indium concentration of the reference was 2 mg/L. Ni, Co, Cd, Pb, Mn, Cu concentrations were 2 mg/L, and the concentrations of other ions were 10 mg/L. All the tests were under the same conditions.
field test compared with the traditional atomic spectrometric method as the glow discharge was generated in normal atmosphere and it needs no assisted gas while the instrument is simple and portable. It makes significant sense for this technique to be applied in the indium pollution monitor of water samples to decrease the risk of indium damage.
Acknowledgments The authors are grateful to the supports of (Beijing Municipal Science and Technology Project: Z161100003016010 and Beijing Financial Project: PXM2016-178305-000017).
Table 2 The determination and recovery results for water samples. Sample
Content/mg·L−1 a
River water
–
Waste water
1.92
Tap water
–a
a b
Add/mg·L−1
Found/mg·L−1
Recovery/%
Average recovery/%
RSDb/%
1.00 1.00 1.00 1.00 1.00 2.00 2.00 2.00 2.00 2.00 0.400 0.400 0.400 0.400 0.400
0.911 0.921 1.064 0.921 0.884 2.04 2.09 1.98 2.05 2.20 0.420 0.383 0.383 0.400 0.424
91.1 92.1 106.4 92.1 88.4 102.0 104.5 99.0 102.5 110.0 105.0 95.6 95.6 100.0 106.0
94.0
7.5
103.6
3.9
100.4
4.9
The value is below LOQ. RSD values are based on 5 times parallel spiked recovery results for each sample.
W. Zu et al. / Microchemical Journal 137 (2018) 266–271
References [1] V.P. Vassiliev, A. Benaissa, A.F. Taldrik, Thermodynamics analysis of the rare earth metals and their alloys with indium in solid state, J. Alloys Compd. 572 (2013) 118–123. [2] H. Hasegawa, I.M.M. Rahman, Y. Egawa, et al., Recovery of indium from end-of-life liquid-crystal display panels using aminopolycarboxylate chelants with the aid of mechanochemical treatment, Microchem. J. 106 (2013) 289–294. [3] A. Urbaniak, M. Pawłowski, M. Marzantowicz, Opto-electrical characterisation of Indoped SnS thin films for photovoltaic applications, Thin Solid Films 636 (2017) 158–163. [4] A.K. Singh, A comparative study on optical properties of Se–Zn–In and Se–Zn–Te–In chalcogenide glasses, Int. J. Light Electron. Optics 124 (15) (2013) 2187–2190. [5] S. Srivastava, E.A. Fergason-Cantrell, R.I. Nahas, et al., Synthesis and opioid receptor binding of indium (III) and [111In]-labeled macrocyclic conjugates of diprenorphine: novel ligands designed for imaging studies of peripheral opioid receptors, Tetrahedron 72 (40) (2016) 6127–6135. [6] J. Połedniok, A sensitive spectrophotometric method for determination of trace quantities of indium in soil, Water Air Soil Pollut. 186 (1–4) (2007) 242–349. [7] M.M. Hassanien, I.M. Kenawy, M.R. Mostafa, et al., Extraction of gallium, indium and thallium from aquatic media using amino silica gel modified by gallic acid, Microchim. Acta 172 (1–2) (2011) 137–145. [8] J. Uhrovčík, J. Lesný, Determination of indium in liquid crystal displays by flame atomic absorption spectrometry, J. Ind. Eng. Chem. 21 (2015) 163–165. [9] Y. Arslan, E. Kendüzler, O.Y. Ataman, Indium determination using slotted quartz tube-atom trap-flame atomic absorption spectrometry and interference studies, Talanta 85 (4) (2011) 1786–1791. [10] M. Tuzen, M. Soylak, A solid phase extraction procedure for indium prior to its graphite furnace atomic absorption spectrometric determination, J. Hazard. Mater. 129 (1–3) (2006) 179–185. [11] K.J. Orians, E.A. Boyle, Determination of picomolar concentrations of titanium, gallium and indium in sea water by inductively coupled plasma mass spectrometry following an 8-hydroxyquinoline chelating resin preconcentration, Anal. Chim. Acta 282 (1) (1993) 63–74. [12] M.A. Taher, Differential pulse polarography determination of indium after column preconcentration with [1-(2-pyridylazo)-2-naphthol]-naphthalene adsorbent or its complex on microcrystalline naphthalene, Talanta 52 (2) (2000) 301–309.
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Contents lists available at ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Rapid determination of indium in water samples using a portable solution cathode glow discharge-atomic emission spectrometer Wenchuan Zu, Yin Yang, Yu Wang ⁎, Xiaotao Yang, Cong Liu, Min Ren Beijing Center for Physical & Chemical Analysis, Beijing 100089, China Beijing Academy of Science and Technology, Key Laboratory of Analysis and Testing Technology, Beijing 100089, China
a r t i c l e
i n f o
Article history: Received 1 September 2017 Received in revised form 2 November 2017 Accepted 2 November 2017 Available online 03 November 2017 Keywords: Solution cathode glow discharge Indium Water Portable atomic emission spectrometer
a b s t r a c t A novel method for fast determination of indium in water was established using a homemade solution cathode glow discharger coupled with a portable fiber optical spectrometer. The instrumental parameters and operation conditions which may remarkably influence the analytical performance including the analytical emission line, the solution acidity, etc. were optimized. The resolution of the instrument based on the peak width at half height at 451 nm for 2.0 mg/L indium solution was about 1.1 nm. Under the optimized parameters and conditions, the limit of detection (LOD) was 0.032 mg/L calculated by 3SDBLANK/k (n = 11). Moreover, the analytical stability was inspected by 7 parallel tests of 0.5 mg/L indium standard solution, and the relative standard deviation (RSD) was below 5%. The interferences of potential co-existing metal ions on indium determination were inspected and no significant interferences were observed. This method was applied to the analysis of real water samples available in Beijing and the spiked recoveries were in the range of 94.0%–103.6%, which proved the feasibility of indium determination of real samples. Besides, this method can satisfy the demands of field tests due to the simple and small-sized instrument and less consumption of reagent. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Indium, which is known as a heavy-metal element, is now widely used in different fields, including alloy, semi-conductor, biomedicine, etc. [1–5]. Meanwhile, the risk of indium pollution tends to be enhanced accordingly as the release of indium into the environment is remarkably increasing due to the continuously growing indium consumption [6]. Moreover, indium compounds are proved to be with certain toxicity and damages to health will be caused if the income amount exceeds a certain degree, especially for tooth decay, pain in joints and the germinal and inherit system [7]. Thus, it makes significant sense to monitor the indium content level of environmental water and industrial effluents to prevent from indium pollution. For the time being, the methods for indium determination have already been reported in literature, including atomic spectrometry [8– 10], inorganic mass spectrometry [11] and electrochemical technique [12–14]. Generally speaking, flame atomic absorption spectrometry (F-AAS) is widely used in indium determination. However, the limit of determination is somewhat unsatisfactory, and the use of fuel gas is unavoidable. As a high sensitivity technique, inductively coupled plasma-mass spectrometry (ICP-MS) is suitable for ultra-trace indium ⁎ Corresponding author at: Beijing Center for Physical & Chemical Analysis, Beijing 100089, China. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.microc.2017.11.001 0026-265X/© 2017 Elsevier B.V. All rights reserved.
determination, yet the instrumental and analytical cost is high and the apparatus is cumbersome. Furthermore, all the atomic spectrometry and ICP-MS techniques above can't work without gas cylinders so that instrumental portability tends to be difficult. As for electrochemical methods for indium detection, field test can be realized while the daily treatment or replacement of the electrodes is tedious and the memory effect is remarkable. As a novel technique for metal elemental analysis, solution cathode glow discharge atomic emission spectrometry (SCGD-AES) has been successfully applied in metal elemental quantitative analysis, such as Pb, Cu, Li, Na, etc. and showed favorable analytic performance [15–22]. As for the solution cathode discharge system, the solution is the cathode while a metal electrode is employed as the anode. The plasma will come into being due to the gas ionization when a high voltage is applied between the two electrodes. The solution is volatilized and the elements dissolved in the solution will enter the plasma, followed by the elemental atomization and exciting process. Thus, the characteristic emission spectrum is ultimately formed. In our previous work, the method for thallium determination by SCGD-AES was established and favorably verified in real samples [23]. However, the study on indium analysis has never been involved in the above literatures. In this paper, a novel method for indium determination using a portable SCGD equipment coupled with a fiber optical spectrometer is established. This method is feasible for field test as the dependence on gas cylinders for traditional atomic spectrometry
W. Zu et al. / Microchemical Journal 137 (2018) 266–271
267
in the 10% HNO3 for 24 h, washed clean with ultra pure water and finally dried for standby application. 2.2. Sample preparation The clearness of the water samples should be pre-judged before the follow-up treatment. If the samples are clear enough, direct determination is feasible after only an acidification operation. To say concretely, 1 mL HNO3 is added into each 100 mL water sample to ensure the acidity consistent with the standard solutions. As for the muddy samples, the acidification is implemented only after filtration with 0.45 μm filter membranes-aquo system. The blanks and spiked samples were prepared in parallel. 2.3. Apparatus Fig. 1. Diagram of SCGD cell with discharge on.
techniques is cast off and the instrument is with excellent portability due to the ideally small size and weight. Moreover, the home-made SCGD works under the normal pressure and this method is friendly to environment as only dilute acid is demanded in the test process, by which pollution brought from large amount of reagents can be effectively reduced. 2. Experimental 2.1. Reagents The nitric acid was of guarantee grade (G.R., from Beijing Fine Chemicals Ltd.), Indium standard stock solution (1000 mg·L−1) was acquired from national standard materials research center. The calibration solutions (0.2 mg/L–5.0 mg/L) were prepared by stepwise dilution of the standard stock solution with 1% (V/V) HNO3. Ultra pure water was produced by a Millipore ultra pure water system and the electrical resistivity is above 18.2 MΩ·cm. Besides, all the glassy wares were immersed
A home-made solution cathode glow discharge cell similarly as reported in our former study [23] was further employed except for certain improvement. The SCGD cell with discharge on was as showed in Fig. 1 while the detailed SCGD-AES configuration was as showed in Fig. 2. Generally speaking, the anode is a tungsten pin (20 mm long and 1 mm diameter) and the solution plays the role of cathode. When the gap between the two electrodes is applied a high voltage (500– 1000 V) with a high voltage supply module (DW-P102 - 100C5D, Beijing Yuan Bo Sheng Electronic Technology Co., Ltd.), the gas in the gap is ionized and the plasma is generated. The solution is progressively gasified and enters the plasma so that indium ions dissolved in the solution together enter the plasma and are atomized. Then the ground-state indium atoms are excited and give out the characteristic spectrum determined by a fiber-optical spectrometer (HR4000, Ocean Optics, Inc.). The whole apparatus is portable due to the small size and weight. In this work, a 6-way valve is adopted for the sampling module as showed in Fig. 1. In the loading process, the sample solution is pumped into the storage coil from 1, and fills in the storage coil between 3 and 6 while the surplus solution flows out as waste liquid through 2.
Fig. 2. The configuration of the SCGD-atomic emission spectrometry device.
268
W. Zu et al. / Microchemical Journal 137 (2018) 266–271
Table 1 Working parameters of SCGD-AES. SCGD-AES parameters
Adopted
The cathode glow discharge voltage/V The inner size of the glass capillary/mm The discharge distance/mm The detecting wavelength of indium/nm The integral time for the fiber-optical spectrometer/ms The carrier flow Measurement mode of the fiber-optical spectrometer
700 0.3 × 0.3 3 451 100 1%(V/V) HNO3 Peak height
Meanwhile, the plunger pump is pulled down so that 5 mL 1% HNO3 as carrier flow is pumped into the pump cylinder. In the injection process, one end of the storage coil is switched to 4 and the other end is switched to 5. When the plunger pump is pushed up and restored to the initial state, the sample solution in the storage coil can be pushed into the SCGD cell through 5 by the carrier flow. The excessive carrier flow can also plays the role of cleaning the pipeline to eliminate memory effect. In our work, the optimal working parameters adopted for the SCGDAES were as follows in Table 1. 3. Results and discussion 3.1. Optimization of operating conditions 3.1.1. The analytical emission line Generally speaking, the emission lines for each element are numerous. The selection of analytical emission line should consider both low spectrum interference and appropriate sensitivity. As for our SCGDAES system, characteristic emission spectrum for indium was observed by the inside CCD of a fiber optical spectrometer. As showed in Fig. 3, the sensitive emission line for indium under 451 nm was observed while a sub-sensitive emission peak under 410 nm was presented as well. The sensitivity of indium determination under 451 nm and 410 nm were compared, and the results were showed in Fig. 4. Under the same instrumental parameters and experimental conditions, the sensitivity for indium determination under 410 nm was 52.9% of that under 451 nm (calculated as the ratio of slope of the linear equation of working curve). The 410 nm wavelength is useful for certain samples with high indium content or serious interference at 451 nm. 3.1.2. The solution acidity and discharge gap Acidity is a crucial factor significantly influencing the analytical sensitivity and stability. Therefore, the influence of acidity on indium analytical sensitivity was investigated. A group of 1.0 mg/L indium standard solutions with different acidity were used to observe the emission intensity, and the acidity range covered 0.5%–1.5%(V/V) HNO3. The results showed in Fig. 5 presented the similar regulation as our former
Fig. 4. Sensitivity for In detection under 451 nm compared with that under 410 nm.
work [23]. Generally, when the concentration of HNO3 was within the above range, the emission intensity turned larger with the increasing of the acidity. Meanwhile, in our work, the discharge got more difficult and the glow stability turned worse when the acidity was either too large or small. When the acidity was below 0.5% or beyond 1.5%(V/V) HNO3, the discharge can't even be generated. The regularity can be well-explained by the work T. Cserfalvi, et al. [24] ever reported. Considering both the sensitivity and stability, we chose 1% HNO3 for our further work. Therefore, for the real sample analysis, we should keep the consistent acidity with the standard solutions. The discharge gap is another crucial parameter of the SCGD system closely associated with the emission intensity. The discharge gap, which means the distance between the tip of the tungsten anode and the surface of the test solution, should neither too high nor too low to achieve the ideal analytical performance. As Z Wang, et al. [25] ever reported, the narrower discharge gap does bad to the improvement of the sensitivity. And in the range of 1–3 mm discharge gap, the emission intensity increased with the broadening of the gap due to the longer residence time of the analytic target in the plasma and when the gap exceeds 3 mm, the corresponding opposite trend was showed due to the loss of the energy or number of electrons available for excitation. Moreover, the optimizing process indicated that the discharge gap beyond 3 mm went against the stability of the plasma. Therefore, we chose 3 mm as the discharge gap. 3.1.3. Integral time for the fiber-optical spectrometer Integral time is a parameter of the fiber-optical spectrometer affecting the exposure time of the inside CCD. The influence of integral time on the emission intensity of indium was investigated. As Fig. 6 showed, the emission intensity with integral time of 200 ms was twice as large as that of 100 ms because the time for protons acceptance doubled.
Fig. 3. The spectrum of In observed by SCGD-AES.
W. Zu et al. / Microchemical Journal 137 (2018) 266–271
Fig. 5. The effect of solution acidity on indium detection sensitivity.
However, the noise of instrument increased to the similar degree as well, which showed integral time is not a crucial parameter to improve the limit of detection. In our work, we chose the integral time of 100 ms. 3.2. Analytic characteristic 3.2.1. The resolution capacity To ensure the portability of the instrument, a small-sized fiber optical spectrometer with internal optical dispersion system and CCD was employed for indium determination at 451 nm. The analytical accuracy and interference were directly affected by the resolution performance. Thus, the instrumental resolution capacity was inspected by the peak width at half height for 2.0 mg/L indium solution. As showed in Fig. 7, the peak width at half height at 451 nm was about 1.1 nm, which was favorable. 3.2.2. Interference The potential co-existing elements in water samples with complicated compositions may bring in matrix, spectral or even chemical interference for indium determination. Therefore, in our work, interference of potential co-existing mental ions for indium determination was studied. Generally, 2.0 mg/L indium solution was chosen as reference and a series of solutions containing different co-existing metal ions with certain concentrations were spiked into 2 mg/L indium, respectively. Subsequently, the emission intensity acquired was compared with the reference. As showed in Fig. 8, the recoveries were within 93.0%–105.0%, which showed no remarkable interference was discovered at the wavelength of 451 nm.
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3.2.3. Analytic performance Under the optimized parameters and working conditions, the standard series were tested from low concentration to high. The calibration curve was acquired by linear fitting, where the emission intensities were as the vertical coordinates while indium concentrations were as horizontal coordinates. In the range of 0.2–5.0 mg/L, the linearity is favorable and the equation of linear regression was as follows. I = 3.4 + 294.7ρIn; R = 0.9993,where I meant the emission intensity while ρIn stood by the indium concentrations, and R was the linear correlation coefficient. The limit of detection (LOD) based on 3 SDBLANK/k was 0.032 mg/L, where SDBLANK was the standard deviation of emission intensity for 11 parallel blank tests and k meant the slope of the linear regression equation for the calibration curve. And the limit of quantification (LOQ) calculated by 10 SDBLANK/k was 0.11 mg/L. Besides, the precision was investigated by 7 parallel determination of the 0.5 mg/L indium standard solution. The emission intensity obtained were 146, 143, 131, 141, 147, 141 and 131. As a result, the relative standard deviation (RSD) was 4.7%. 3.3. Sample determination by SCGD-AES Three kinds of water samples with different matrix in Beijing, i.e. river water, waste water and tap water were chosen for validation of real sample determination. All the samples were prepared as the proposed method and indium content was determined by this SCGD-AES technique. Besides, 0.4 mg/L–2.0 mg/L indium standard solutions were spiked into the samples to validate the availability of the experimental results. The average recoveries were acquired in the range of 94.0%–103.6%, and the relative standard deviations for the parallel recovery results didn't exceed 7.5% (n = 5) (Table 2). It was concluded that the results were credible and the SCGD-AES method implemented with portable instrument was applicable for real sample analysis. 4. Conclusion A novel method for determination of indium in water samples based on homemade portable instrument was established. It offers the determination of indium in water, especially for the field test an ideal alternative due to the favorable analytical performance. As CCD in the fiber optical spectrometer was applied as final detector, the information of elemental emission spectrum was direct and concrete so that the spectral interference can be effectively judged. This proposed technique for indium determination is with high performance as it takes only 1.5 min to complete a whole detection process. Besides, only dilute acid (1% HNO3) is needed for this technique so that it's less reagent consumption, which is more environment-friendly and brings the decrease of reagent pollution risk and test background. This method is more suitable for
Fig. 6. Spectrum of 4.0 mg/L In at 451 nm with 100 ms and 200 ms integral time.
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Fig. 7. The resolution test based on the 2.0 mg/L In standard solution.
Fig. 8. The interference of co-existing metal elements for In determination. Conditions: Indium concentration of the reference was 2 mg/L. Ni, Co, Cd, Pb, Mn, Cu concentrations were 2 mg/L, and the concentrations of other ions were 10 mg/L. All the tests were under the same conditions.
field test compared with the traditional atomic spectrometric method as the glow discharge was generated in normal atmosphere and it needs no assisted gas while the instrument is simple and portable. It makes significant sense for this technique to be applied in the indium pollution monitor of water samples to decrease the risk of indium damage.
Acknowledgments The authors are grateful to the supports of (Beijing Municipal Science and Technology Project: Z161100003016010 and Beijing Financial Project: PXM2016-178305-000017).
Table 2 The determination and recovery results for water samples. Sample
Content/mg·L−1 a
River water
–
Waste water
1.92
Tap water
–a
a b
Add/mg·L−1
Found/mg·L−1
Recovery/%
Average recovery/%
RSDb/%
1.00 1.00 1.00 1.00 1.00 2.00 2.00 2.00 2.00 2.00 0.400 0.400 0.400 0.400 0.400
0.911 0.921 1.064 0.921 0.884 2.04 2.09 1.98 2.05 2.20 0.420 0.383 0.383 0.400 0.424
91.1 92.1 106.4 92.1 88.4 102.0 104.5 99.0 102.5 110.0 105.0 95.6 95.6 100.0 106.0
94.0
7.5
103.6
3.9
100.4
4.9
The value is below LOQ. RSD values are based on 5 times parallel spiked recovery results for each sample.
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