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Direct determination of Cu by liquid cathode glow discharge-atomic emission spectrometry
Spectrochimica Acta Part B 125 (2016) 136–139
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Technical note
Direct determination of Cu by liquid cathode glow discharge-atomic emission spectrometry Quanfang Lu a,b,⁎, Shuxiu Yang a, Duixiong Sun c, Jidong Zheng a, Yun Li a, Jie Yu a,⁎, Maogen Su c a b c
Key Lab of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China Editorial Department of the University Journal, Northwest Normal University, Lanzhou 730070, China Key Lab of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China
a r t i c l e
i n f o
Article history: Received 30 November 2015 29 September 2016 Accepted 29 September 2016 Available online 03 October 2016 Keywords: Liquid cathode glow discharge (LCGD) Atomic emission spectrometry (AES) Copper Ionic surfactant Water analysis
a b s t r a c t In this study, a novel liquid cathode glow discharge-atomic emission spectrometry was developed for the direct determination of Cu in aqueous solutions, in which the glow discharge plasma was produced in the solution between the needle-like Pt cathode and the electrolyte around it. The effects of discharge voltage, solution pH, and the ionic surfactant cetyltrimethylammonium chloride (CTAC) on emission intensities were investigated. The limit of detection (LOD) of Cu was compared with those measured by closed-type electrolyte cathode discharge-atomic emission spectrometry (ELCAD-AES). The results showed that the optimal operation conditions are voltage of 135 V, a pH of 1, and addition of 0.15% CTAC. CTAC can enhance the emission intensity and lower the LOD of Cu I. The net intensity of atomic emission lines of Cu I at 324.8 nm with 0.15% CTAC improved by 1.5 fold, and the LODs of the Cu at 135 V with 0.15% CTAC and without CTAC are 0.019 and 0.234 mg L−1, respectively. The analytical capability of Cu in this study is comparable to the closed-type ELCAD-AES, and it satisfied the recommended levels of Cu in the WHO standards for drinking-water quality. This technique can be effectively used for on-line monitoring of metal ions in aqueous samples. © 2016 Elsevier B.V. All rights reserved.
1. Introduction With the development of chemical industries, the environmental pollution caused by heavy metal elements has been dramatically increased. Accordingly, rapid and accurate monitoring of heavy metals in water samples is of great importance for timely assessment and effective prevention of heavy metal pollution. The well-known high-performance spectrometric instruments (e.g., ICP-AES and ICP-MS) are very stable and sensitive for the detection of heavy metals. However, they are mainly operated at laboratories, require high power consumption and special gases, and perform at high temperature even under vacuum [1,2]. Therefore, these methods are not suitable for achieving in situ, real time, and on-line analysis [3,4]. In recent years, electrolyte cathode discharge-atomic emission spectrometry (ELCAD-AES) has been successfully used for the on-line analysis of multiple elements [1]. Compared with ICP, ELCAD has more compact and portable instrument, lower power consumption (b70 W), and operates in atmospheric pressure air [4–6]. However, because of a higher limit of detection (LOD), this technique cannot achieve determination of tracemetals in environmental samples. To achieve more miniaturized analytical methods, the stability of discharge, the emission efficiency, and the analytical capability should ⁎ Corresponding authors. E-mail addresses: [email protected] (Q. Lu), [email protected] (J. Yu).
http://dx.doi.org/10.1016/j.sab.2016.09.019 0584-8547/© 2016 Elsevier B.V. All rights reserved.
be improved. Many modifications of discharge structure for ELCAD have been developed on the basis of the design originally reported by Cserfalvi and coworkers [7,8], including solution-cathode glow discharge (SCGD) [9], liquid sampling-atmospheric pressure glow discharge (LS-APGD) [10], direct current atmospheric pressure glow discharge (DC-APGD) [11], alternating current driven-liquid discharge (ACD-LD) [12], and liquid film-dielectric barrier discharge (LF-DBD) [13]. Moreover, the addition of some organic substances can also increase the emission intensity and sensitivity of the determination [11,12,14–16]. In this study, a novel liquid cathode glow discharge-atomic emission spectrometry (LCGD-AES) was established, in which the carbon rod anode (diameter: 5 mm) and needle-like Pt cathode (diameter: 0.5 mm, length: 0.5 mm) were introduced into the sample solution. When the voltage was sufficiently high, the glow discharge plasma was directly produced in the sample solution between the needle-like Pt cathode and the electrolyte around it. This device can offer several advantages over conventional ELCAD. For example, in the LCGD process, two electrodes are immersed in solution to produce discharge plasma and reduce the interference of nitrogen. In addition, the process of the driving the sample by a peristaltic pump is excluded. This discharge equipment provides a new approach for the detection of metals in liquid with advantages of low cost, portability, simplicity, and easy operation. To verify the analytical performance of the device, the determination of Cu by LCGD-AES was chosen as the subject of this study. The influence of
Q. Lu et al. / Spectrochimica Acta Part B 125 (2016) 136–139
137
Fig. 1. Experimental setup of LCGD-AES.
discharge voltage, solution pH, and cetyltrimethylammonium chloride (CTAC) concentrations on emission intensities of Cu was examined. The LOD of Cu was calculated and then compared with those of other techniques.
2. Experimental 2.1. Experimental set-up of LCGD-AES The experimental setup of LCGD-AES is shown in Fig. 1. It contains a reactor to produce glow discharge plasma, a high-voltage power supply, an oscilloscope, and a spectrometer. The reactor is a 250-mL quartz vessel equipped with a 0.5-mm-diameter needle (Pt) cathode sealed into a quartz tube to generate the glow discharge plasma in aqueous solution, and a 5-mm-diameter graphite rod anode. The thickness, diameter, and height of the quartz vessel are 2, 60, and 90 mm, respectively. The distance between the anode and cathode is about 10 mm. The 10-mm-diameter quartz window is opened on the side of vessel to place the fiber optic probe. This design can reduce the light absorption by solution. The reactor is coated with an outer water jacket to keep the solution at a constant temperature. A magnetic stirring bar is placed at the bottom of reactor to enable constant mixing of the solution. During the experiments, the gas produced in the liquid glow discharge is expelled from the outlet port. The high-voltage DC source is a DH 1722-6 power supply (Shanghai Liyou Radio Factory, Shanghai, China) that provides the voltage of 0–1000 V and the current of 0–0.5 A. The discharge powers are lower than 60 W in all experiments. The voltage and current signals are recorded on a Tektronix TDS 3052C digital oscilloscope, and the details of the study are shown in S1 (Appendix A). The optical emission spectra from 250 to 550 nm are measured by a multichannel optical fiber spectrometer (Avaspec-2048-8) with a resolution of 0.07 nm. When the applied voltage is 115–140 V, the stable glow discharge plasma is produced around the tip of the cathode, and the spectrum signal of plasma can be detected by the spectrometer.
2.2. Reagents and samples Cu(NO3)2·3H2O and Zn(NO3)2·6H2O (analytical reagent grade) were purchased from Shanghai Chemical Reagent Corporation. Nitric acid (superior reagent grade) was supplied by Sinopharm Chemical Reagent Co., Ltd. The HNO3 solution of pH 1 with 0.5 g L−1 Zn(NO3)2 was prepared as a blank solution. Zn(NO3)2 was added to the HNO3 solution to enhance the conductivity of solution and to further improve the stability of discharge. Stock solution (1000 mg L−1) of Cu was prepared with the blank solution. Working standards (1–6 mg L−1) of Cu were prepared by diluting the stock solution with the blank solution. The pH of the solutions was measured with a pH meter (PHS-3E, INESA). CTAC (analytical reagent grade) was purchased from J&K Scientific Ltd. and was added to the working standards of Cu solutions with mass percentages of 0%, 0.05%, 0.10%, 0.15%, 0.20%, and 0.25% for the LCGD-AES analysis.
Fig. 2. Identified atomic emission spectra lines of the LCGD-AES in the blank (a) and 50 mg L−1 Cu solution (b).
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Fig. 3. Effect of discharge voltage on emission intensity of Cu I by LCGD-AES (concentration of Cu: 5 mg L−1, pH: 1.0).
3. Results and discussion 3.1. Emission spectra of the LCGD-AES The emission spectra of the LCGD-AES in blank (a) and 50 mg L−1 Cu solution (b) are shown in Fig. 2. As shown in Fig. 2a, there are several lines and bands present in the spectra. The main lines and bands of excited species result from OH, H I, O II, and Ca I, which are marked in the graph. The bands in the wavelength range of 283.0–309.0 nm are attributed to the emission of OH (A2Σ+ → X2Π). Because a high amount of water is vaporized during discharge, electrons collide with the water molecules to produce OH+, which affects the electrons to engender a large number of OH [4]. The strongest emission lines at 486.1 and 435.8 nm are ascribed to Hβ and Hγ, which stem from the electrolyte around the cathode that is bombarded by the high-energy electrons [17]. A series of O II lines are distributed from 391.2 to 470.1 nm, which are produced from water vapor by electron impact [5,18]. A spectral line of Ca I also appears at 422.7 nm, which suggests that the blank sample still contains a small amount of impurities. Compared with Fig. 2a, the spectral lines of 324.8 and 327.4 nm, which can be assigned to the Cu I line, appear completely in Fig. 2b, indicating that LCGD can be used to detect the Cu in aqueous solution. As shown in Fig. 2b, emission intensity of Cu I 324.8 nm is much stronger than that of Cu I 327.4 nm; therefore, the line of 324.8 nm is selected as the analytical line of Cu I. The contribution of the intensity from the OH rotational band to Cu I 324.8 nm was around 1%, and was deducted by data processing. 3.2. Optimization of determination conditions 3.2.1. Effect of discharge voltage on emission intensity With other conditions remaining unchanged (5 mg L−1 of Cu), the effect of discharge voltage on emission intensity was studied. The applied discharge voltage b120 V was not beneficial to Cu excitation
Fig. 5. Effect of surfactant CTAC on the signal-to-noise ratio (S/N) of Cu by LCGD-AES (concentration of Cu: 5 mg L−1, discharge voltage: 135 V, pH: 1.0).
because of the low energy (excitation temperature) [19]. The emission intensity increased significantly with the increase in the discharge voltage from 120 to 135 V. However, as the discharge voltage increased to 140 V, the emission intensity decreased (Fig. 3). Above 140 V, the Pt cathode became red-hot, and the water samples around the Pt cathode started to boil, which caused the plasma to become unstable [4]. In addition, the Pt cathode would be destroyed under the strong glow discharge. Therefore, the voltage of 135 V was chosen as the optimal discharge voltage in this study. 3.2.2. Effect of solution pH on emission intensity It is known that the emission intensity strongly depends on the solution pH in ELCAD [7] and ACD-LD [12,20]. It was found that the Pt cathode would melt when the pH was below 0.6 because of the higher conductivity; furthermore, the emission intensity of Cu I could not be determined when the pH was above 1.4 because of the lower conductivity. Therefore, the effect of solution pH on emission intensity of Cu I was studied in the pH range of 0.6–1.4. As shown in Fig. 4, the emission intensities of Cu I increased from pH 0.6 to 0.8, and then decreased from pH 0.8 to 1.4. However, the Pt cathode became red-hot, and the solutions around the Pt cathode started to boil quickly below pH 1.0, which led to fluctuation in plasma. In addition, the discharge plasma remained relatively stable and the RSD (Table S1 in Appendix A) was lower at pH 1.0 (7.0%) than at pH 0.8 (11.9%). Therefore, the pH of 1.0 was selected for the optimum pH. 3.2.3. Effect of CTAC on emission intensity The effect of CTAC concentration on the enhancement of the signalto-noise ratio was also investigated. It was found that the signal-tonoise ratio could be significantly enhanced with the addition of even a small amount (0.05%) of CTAC (Fig. 5). In addition, the maximum enhancement of Cu signal-to-noise ratio with 0.15% CTAC was improved 1.5-fold compared with that in solutions without CTAC. Comparison of emission intensity of Cu I at 324.8 nm with 0.15% CTAC and without CTAC is shown in S2 (Appendix A). The findings indicate that the presence of CTAC could enhance the emission intensity of LCGD in the case of Cu. This is because surfactant CTAC could decrease the surface tension and increase the viscosity of solution samples, which could allow the metal ions to stay in their excitation states longer in LCGD, improve vaporization rates, and increase excitation temperature and ion density of the plasma [3,11,15,16]. Table 1 Analytical performance of LCGD-AES in Cu solution with and without CTAC.
Fig. 4. Effect of solution pH on emission intensity of Cu I by LCGD-AES (concentration of Cu: 5 mg L−1, discharge voltage: 135 V).
Cu
Linear equation
R2
S
LOD (mg L−1)
Without CTAC With CTAC
I = 218.7 + 864.9C I = 468.2 + 1096.9C
0.9840 0.9455
864.9 1096.8
0.234 0.019
Q. Lu et al. / Spectrochimica Acta Part B 125 (2016) 136–139
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Table 2 Comparison of LODs obtained by the LCGD-AES system with other ELCAD-AES systems and ICP for the detection of Cu, and Cu standards for drinking water quality. Methods
LOD (mg L−1)
Methods
LOD (mg L−1)
This study Earlier SCGD [5] Modified ELCAD [6] ELCAD [8] SCGD [9]
0.234 (without CTAC) 0.019 (with CTAC) 0.031 0.011 0.060 0.004
LS-APGD [10] DC-APGD [11] LE-DBD [13] ICP-AES [21] Cu standards for drinking water quality [22]
0.650 0.020 0.074 0.002 2.000
3.3. Analytical performance The analytical performance of the proposed technique was evaluated under optimal operating conditions (applied voltage: 135 V, pH: 1). A series of concentrations of Cu standard solutions with its optimal concentration of CTAC and the blank solutions were used to determine the LOD and sensitivity (S). The results are shown in Fig. S4 (Appendix A). Table 1 lists the LOD values of Cu with and without CTAC. The results showed that the LOD value of Cu is lower with CTAC than that without CTAC. A comparison of the LOD obtained by LCGD-AES with other closedtype ECLAD-AES techniques [5,6,8–11,13], ICP-AES [21], and Cu standards for drinking water quality of WHO [22] is shown in Table 2. The table shows that the LOD for LCGD-AES is found to be comparable to those of similar systems and mostly in the range of tens of parts per billion. In addition, the LOD value of Cu in this study is much lower than that of the standard for drinking water quality; thus, this technique can be effectively used to determine the safety of Cu in drinking water quality. 4. Conclusions The LCGD-AES was successfully applied for the determination of Cu in aqueous solution. The optimization analytical conditions of LCGDAES for the detection of Cu were 135 V, pH 1, and addition of 0.15% CTAC. The results showed that at the applied voltage of 135 V, the net intensity of atomic emission lines of Cu with 0.15% CTAC showed a 1.5-fold increase. The LODs of Cu at 135 V with 0.15% CTAC and without CTAC were 0.019 and 0.234 mg L−1, respectively. The analytical capability of Cu in this study is comparable to the closed-type ELCAD-AES, and satisfied the recommended levels of the WHO standards for drinking water quality. Compared with other ELCAD-AES systems, LCGD-AES has advantages such as lower setup cost, portable equipment, simple operation, and easy design. In addition, it is very easy to achieve in situ, real-time, and on-line analysis of samples. Therefore, LCGD-AES is a very promising technique for high-efficiency detection of metal ions in aqueous solutions. Acknowledgments This work was supported in part by the National Natural Science Foundation of China (Nos. 21367023, 21567025, and 11564037), the Natural Science Foundation of Gansu Province (Nos. 1308RJZA144 and 1208RJZA161), the Scientific Research Project in Higher Education Institutions of Gansu Province (No. 2013-019), and Key Project of Young Teachers' Scientific Research Promotion of Northwest Normal University (No. NWNU-LKQN-12-9), China. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.sab.2016.09.019.
References [1] Q. He, Z. Zhu, S. Hu, Flowing and nonflowing liquid electrode discharge microplasma for metal ion detection by optical emission spectrometry, Appl. Spectrosc. Rev. 49 (2014) 249–269. [2] P. Mezei, T. Cserfalvi, Electrolyte cathode atmospheric glow discharges for direct solution analysis, Appl. Spectrosc. Rev. 42 (2007) 573–604. [3] Z. Zhang, Z. Wang, Q. Li, H. Zou, Y. Shi, Determination of trace heavy metals in environmental and biological samples by solution cathode glow discharge-atomice mission spectrometry and addition of ionic surfactants for improved sensitivity, Talanta 119 (2014) 613–619. [4] J. Yu, S.X. Yang, D.X. Sun, Q.F. Lu, J.D. Zheng, X.M. Zhang, X. Wang, Simultaneously determination of multimetal elements in water samples by liquid cathode glow discharge-atomic emission spectrometry, Microchem. J. 128 (2016) 325–330. [5] M.R. Webb, F.J. Andrade, G. Gamez, R. McCrindle, G.M. Hieftje, Spectroscopic and electrical studies of a solution-cathode glow discharge, J. Anal. At. Spectrom. 20 (2005) 1218–1225. [6] R. Shekhar, D. Karunasagar, M. Ranjit, J. Arunachalam, Determination of elemental constituents in different matrix materials and flow injection studies by the electrolyte cathode glow discharge technique with a new design, Anal. Chem. 81 (2009) 8157–8166. [7] T. Cserfalvi, P. Mezei, P. Apai, Emission studies on a glow discharge in atmospheric pressure air using water as a cathode, J. Phys. D. Appl. Phys. 26 (1993) 2184–2188. [8] T. Cserfalvi, P. Mezei, Direct solution analysis by glow discharge: electrolyte-cathode discharge spectrometry, J. Anal. At. Spectrom. 9 (1994) 345–349. [9] M.R. Webb, F.J. Andrade, G.M. Hieftje, Compact glow discharge for the elemental analysis of aqueous samples, Anal. Chem. 79 (2007) 7899–7905. [10] A. Shaltout, Micro plasma generation using liquid sampling-atmospheric pressure glow discharge, Microchim. Acta 155 (2006) 447–452. [11] K. Gręda, P. Jamróz, P. Pohl, The improvement of the analytical performance of direct current atmospheric pressure glow discharge generated in contact with the smallsized liquid cathode after the addition of non-ionic surfactants to electrolyte solutions, Talanta 108 (2013) 74–82. [12] Q. Xiao, Z. Zhu, H. Zheng, H. He, C. Huang, S. Hu, Significant sensitivity improvement of alternating current driven-liquid discharge by using formic acid medium for optical determination of elements, Talanta 106 (2013) 144–149. [13] Q. He, Z. Zhu, S. Hu, H. Zheng, L. Jin, Elemental determination of microsamples by liquid film dielectric barrier discharge atomic emission spectrometry, Anal. Chem. 84 (2012) 4179–4184. [14] T.A. Doroski, M.R. Webb, Signal enhancement in solution-cathode glow dischargeoptical emission spectrometry via low molecular weight organic compounds, Spectrochim. Acta B 88 (2013) 40–45. [15] K. Gręda, P. Jamróz, P. Pohl, Effect of the addition of non-ionic surfactants on the emission characteristic of direct current atmospheric pressure glow discharge generated in contact with a flowing liquid cathode, J. Anal. At. Spectrom. 28 (2013) 134–141. [16] A. Kitano, A. Iiduka, T. Yamamoto, Y. Ukita, E. Tamiya, Y. Takamura, Highly sensitive elemental analysis for Cd and Pb by liquid electrode plasma atomic emission spectrometry with quartz glass chip and sample flow, Anal. Chem. 83 (2011) 9424–9430. [17] P. Jamroz, K. Greda, P. Pohl, Development of direct-current, atmospheric-pressure, glow discharges generated in contact with flowing electrolyte solutions for elemental analysis by optical emission spectrometry, Trends Anal. Chem. 41 (2012) 105–121. [18] Y.S. Park, S.H. Ku, S.H. Hong, H.J. Kim, E.H. Piepmeier, Fundamental studies of electrolyte-as-cathode glow discharge–atomic emission spectrometry for the determination of trace metals in flowing water, Spectrochim. Acta B 53 (1998) 1167–1179. [19] T. Cserfalvi, P. Mezei, Investigations on the element dependency of sputtering process in the electrolyte cathode atmospheric discharge, J. Anal. At. Spectrom. 20 (2005) 939–944. [20] R. Huang, Z. Zhu, H. Zheng, Z. Liu, S. Zhang, S. Hu, Alternating current driven atmospheric-pressure liquid discharge for the determination of elements with optical emission spectrometry, J. Anal. At. Spectrom. 26 (2011) 1178–1182. [21] S.J. Hill, A. Fisher, M. Foulkes, In Inductively Coupled Plasma Spectrometry and Its Application, 2nd ed. Blackwell Publishing Ltd., Oxford, 2007 61–97. [22] World Health Organisation, Guidelines for Drinking-water Quality, 4th ed., Geneva, 2011 240–342.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Technical note
Direct determination of Cu by liquid cathode glow discharge-atomic emission spectrometry Quanfang Lu a,b,⁎, Shuxiu Yang a, Duixiong Sun c, Jidong Zheng a, Yun Li a, Jie Yu a,⁎, Maogen Su c a b c
Key Lab of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China Editorial Department of the University Journal, Northwest Normal University, Lanzhou 730070, China Key Lab of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China
a r t i c l e
i n f o
Article history: Received 30 November 2015 29 September 2016 Accepted 29 September 2016 Available online 03 October 2016 Keywords: Liquid cathode glow discharge (LCGD) Atomic emission spectrometry (AES) Copper Ionic surfactant Water analysis
a b s t r a c t In this study, a novel liquid cathode glow discharge-atomic emission spectrometry was developed for the direct determination of Cu in aqueous solutions, in which the glow discharge plasma was produced in the solution between the needle-like Pt cathode and the electrolyte around it. The effects of discharge voltage, solution pH, and the ionic surfactant cetyltrimethylammonium chloride (CTAC) on emission intensities were investigated. The limit of detection (LOD) of Cu was compared with those measured by closed-type electrolyte cathode discharge-atomic emission spectrometry (ELCAD-AES). The results showed that the optimal operation conditions are voltage of 135 V, a pH of 1, and addition of 0.15% CTAC. CTAC can enhance the emission intensity and lower the LOD of Cu I. The net intensity of atomic emission lines of Cu I at 324.8 nm with 0.15% CTAC improved by 1.5 fold, and the LODs of the Cu at 135 V with 0.15% CTAC and without CTAC are 0.019 and 0.234 mg L−1, respectively. The analytical capability of Cu in this study is comparable to the closed-type ELCAD-AES, and it satisfied the recommended levels of Cu in the WHO standards for drinking-water quality. This technique can be effectively used for on-line monitoring of metal ions in aqueous samples. © 2016 Elsevier B.V. All rights reserved.
1. Introduction With the development of chemical industries, the environmental pollution caused by heavy metal elements has been dramatically increased. Accordingly, rapid and accurate monitoring of heavy metals in water samples is of great importance for timely assessment and effective prevention of heavy metal pollution. The well-known high-performance spectrometric instruments (e.g., ICP-AES and ICP-MS) are very stable and sensitive for the detection of heavy metals. However, they are mainly operated at laboratories, require high power consumption and special gases, and perform at high temperature even under vacuum [1,2]. Therefore, these methods are not suitable for achieving in situ, real time, and on-line analysis [3,4]. In recent years, electrolyte cathode discharge-atomic emission spectrometry (ELCAD-AES) has been successfully used for the on-line analysis of multiple elements [1]. Compared with ICP, ELCAD has more compact and portable instrument, lower power consumption (b70 W), and operates in atmospheric pressure air [4–6]. However, because of a higher limit of detection (LOD), this technique cannot achieve determination of tracemetals in environmental samples. To achieve more miniaturized analytical methods, the stability of discharge, the emission efficiency, and the analytical capability should ⁎ Corresponding authors. E-mail addresses: [email protected] (Q. Lu), [email protected] (J. Yu).
http://dx.doi.org/10.1016/j.sab.2016.09.019 0584-8547/© 2016 Elsevier B.V. All rights reserved.
be improved. Many modifications of discharge structure for ELCAD have been developed on the basis of the design originally reported by Cserfalvi and coworkers [7,8], including solution-cathode glow discharge (SCGD) [9], liquid sampling-atmospheric pressure glow discharge (LS-APGD) [10], direct current atmospheric pressure glow discharge (DC-APGD) [11], alternating current driven-liquid discharge (ACD-LD) [12], and liquid film-dielectric barrier discharge (LF-DBD) [13]. Moreover, the addition of some organic substances can also increase the emission intensity and sensitivity of the determination [11,12,14–16]. In this study, a novel liquid cathode glow discharge-atomic emission spectrometry (LCGD-AES) was established, in which the carbon rod anode (diameter: 5 mm) and needle-like Pt cathode (diameter: 0.5 mm, length: 0.5 mm) were introduced into the sample solution. When the voltage was sufficiently high, the glow discharge plasma was directly produced in the sample solution between the needle-like Pt cathode and the electrolyte around it. This device can offer several advantages over conventional ELCAD. For example, in the LCGD process, two electrodes are immersed in solution to produce discharge plasma and reduce the interference of nitrogen. In addition, the process of the driving the sample by a peristaltic pump is excluded. This discharge equipment provides a new approach for the detection of metals in liquid with advantages of low cost, portability, simplicity, and easy operation. To verify the analytical performance of the device, the determination of Cu by LCGD-AES was chosen as the subject of this study. The influence of
Q. Lu et al. / Spectrochimica Acta Part B 125 (2016) 136–139
137
Fig. 1. Experimental setup of LCGD-AES.
discharge voltage, solution pH, and cetyltrimethylammonium chloride (CTAC) concentrations on emission intensities of Cu was examined. The LOD of Cu was calculated and then compared with those of other techniques.
2. Experimental 2.1. Experimental set-up of LCGD-AES The experimental setup of LCGD-AES is shown in Fig. 1. It contains a reactor to produce glow discharge plasma, a high-voltage power supply, an oscilloscope, and a spectrometer. The reactor is a 250-mL quartz vessel equipped with a 0.5-mm-diameter needle (Pt) cathode sealed into a quartz tube to generate the glow discharge plasma in aqueous solution, and a 5-mm-diameter graphite rod anode. The thickness, diameter, and height of the quartz vessel are 2, 60, and 90 mm, respectively. The distance between the anode and cathode is about 10 mm. The 10-mm-diameter quartz window is opened on the side of vessel to place the fiber optic probe. This design can reduce the light absorption by solution. The reactor is coated with an outer water jacket to keep the solution at a constant temperature. A magnetic stirring bar is placed at the bottom of reactor to enable constant mixing of the solution. During the experiments, the gas produced in the liquid glow discharge is expelled from the outlet port. The high-voltage DC source is a DH 1722-6 power supply (Shanghai Liyou Radio Factory, Shanghai, China) that provides the voltage of 0–1000 V and the current of 0–0.5 A. The discharge powers are lower than 60 W in all experiments. The voltage and current signals are recorded on a Tektronix TDS 3052C digital oscilloscope, and the details of the study are shown in S1 (Appendix A). The optical emission spectra from 250 to 550 nm are measured by a multichannel optical fiber spectrometer (Avaspec-2048-8) with a resolution of 0.07 nm. When the applied voltage is 115–140 V, the stable glow discharge plasma is produced around the tip of the cathode, and the spectrum signal of plasma can be detected by the spectrometer.
2.2. Reagents and samples Cu(NO3)2·3H2O and Zn(NO3)2·6H2O (analytical reagent grade) were purchased from Shanghai Chemical Reagent Corporation. Nitric acid (superior reagent grade) was supplied by Sinopharm Chemical Reagent Co., Ltd. The HNO3 solution of pH 1 with 0.5 g L−1 Zn(NO3)2 was prepared as a blank solution. Zn(NO3)2 was added to the HNO3 solution to enhance the conductivity of solution and to further improve the stability of discharge. Stock solution (1000 mg L−1) of Cu was prepared with the blank solution. Working standards (1–6 mg L−1) of Cu were prepared by diluting the stock solution with the blank solution. The pH of the solutions was measured with a pH meter (PHS-3E, INESA). CTAC (analytical reagent grade) was purchased from J&K Scientific Ltd. and was added to the working standards of Cu solutions with mass percentages of 0%, 0.05%, 0.10%, 0.15%, 0.20%, and 0.25% for the LCGD-AES analysis.
Fig. 2. Identified atomic emission spectra lines of the LCGD-AES in the blank (a) and 50 mg L−1 Cu solution (b).
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Q. Lu et al. / Spectrochimica Acta Part B 125 (2016) 136–139
Fig. 3. Effect of discharge voltage on emission intensity of Cu I by LCGD-AES (concentration of Cu: 5 mg L−1, pH: 1.0).
3. Results and discussion 3.1. Emission spectra of the LCGD-AES The emission spectra of the LCGD-AES in blank (a) and 50 mg L−1 Cu solution (b) are shown in Fig. 2. As shown in Fig. 2a, there are several lines and bands present in the spectra. The main lines and bands of excited species result from OH, H I, O II, and Ca I, which are marked in the graph. The bands in the wavelength range of 283.0–309.0 nm are attributed to the emission of OH (A2Σ+ → X2Π). Because a high amount of water is vaporized during discharge, electrons collide with the water molecules to produce OH+, which affects the electrons to engender a large number of OH [4]. The strongest emission lines at 486.1 and 435.8 nm are ascribed to Hβ and Hγ, which stem from the electrolyte around the cathode that is bombarded by the high-energy electrons [17]. A series of O II lines are distributed from 391.2 to 470.1 nm, which are produced from water vapor by electron impact [5,18]. A spectral line of Ca I also appears at 422.7 nm, which suggests that the blank sample still contains a small amount of impurities. Compared with Fig. 2a, the spectral lines of 324.8 and 327.4 nm, which can be assigned to the Cu I line, appear completely in Fig. 2b, indicating that LCGD can be used to detect the Cu in aqueous solution. As shown in Fig. 2b, emission intensity of Cu I 324.8 nm is much stronger than that of Cu I 327.4 nm; therefore, the line of 324.8 nm is selected as the analytical line of Cu I. The contribution of the intensity from the OH rotational band to Cu I 324.8 nm was around 1%, and was deducted by data processing. 3.2. Optimization of determination conditions 3.2.1. Effect of discharge voltage on emission intensity With other conditions remaining unchanged (5 mg L−1 of Cu), the effect of discharge voltage on emission intensity was studied. The applied discharge voltage b120 V was not beneficial to Cu excitation
Fig. 5. Effect of surfactant CTAC on the signal-to-noise ratio (S/N) of Cu by LCGD-AES (concentration of Cu: 5 mg L−1, discharge voltage: 135 V, pH: 1.0).
because of the low energy (excitation temperature) [19]. The emission intensity increased significantly with the increase in the discharge voltage from 120 to 135 V. However, as the discharge voltage increased to 140 V, the emission intensity decreased (Fig. 3). Above 140 V, the Pt cathode became red-hot, and the water samples around the Pt cathode started to boil, which caused the plasma to become unstable [4]. In addition, the Pt cathode would be destroyed under the strong glow discharge. Therefore, the voltage of 135 V was chosen as the optimal discharge voltage in this study. 3.2.2. Effect of solution pH on emission intensity It is known that the emission intensity strongly depends on the solution pH in ELCAD [7] and ACD-LD [12,20]. It was found that the Pt cathode would melt when the pH was below 0.6 because of the higher conductivity; furthermore, the emission intensity of Cu I could not be determined when the pH was above 1.4 because of the lower conductivity. Therefore, the effect of solution pH on emission intensity of Cu I was studied in the pH range of 0.6–1.4. As shown in Fig. 4, the emission intensities of Cu I increased from pH 0.6 to 0.8, and then decreased from pH 0.8 to 1.4. However, the Pt cathode became red-hot, and the solutions around the Pt cathode started to boil quickly below pH 1.0, which led to fluctuation in plasma. In addition, the discharge plasma remained relatively stable and the RSD (Table S1 in Appendix A) was lower at pH 1.0 (7.0%) than at pH 0.8 (11.9%). Therefore, the pH of 1.0 was selected for the optimum pH. 3.2.3. Effect of CTAC on emission intensity The effect of CTAC concentration on the enhancement of the signalto-noise ratio was also investigated. It was found that the signal-tonoise ratio could be significantly enhanced with the addition of even a small amount (0.05%) of CTAC (Fig. 5). In addition, the maximum enhancement of Cu signal-to-noise ratio with 0.15% CTAC was improved 1.5-fold compared with that in solutions without CTAC. Comparison of emission intensity of Cu I at 324.8 nm with 0.15% CTAC and without CTAC is shown in S2 (Appendix A). The findings indicate that the presence of CTAC could enhance the emission intensity of LCGD in the case of Cu. This is because surfactant CTAC could decrease the surface tension and increase the viscosity of solution samples, which could allow the metal ions to stay in their excitation states longer in LCGD, improve vaporization rates, and increase excitation temperature and ion density of the plasma [3,11,15,16]. Table 1 Analytical performance of LCGD-AES in Cu solution with and without CTAC.
Fig. 4. Effect of solution pH on emission intensity of Cu I by LCGD-AES (concentration of Cu: 5 mg L−1, discharge voltage: 135 V).
Cu
Linear equation
R2
S
LOD (mg L−1)
Without CTAC With CTAC
I = 218.7 + 864.9C I = 468.2 + 1096.9C
0.9840 0.9455
864.9 1096.8
0.234 0.019
Q. Lu et al. / Spectrochimica Acta Part B 125 (2016) 136–139
139
Table 2 Comparison of LODs obtained by the LCGD-AES system with other ELCAD-AES systems and ICP for the detection of Cu, and Cu standards for drinking water quality. Methods
LOD (mg L−1)
Methods
LOD (mg L−1)
This study Earlier SCGD [5] Modified ELCAD [6] ELCAD [8] SCGD [9]
0.234 (without CTAC) 0.019 (with CTAC) 0.031 0.011 0.060 0.004
LS-APGD [10] DC-APGD [11] LE-DBD [13] ICP-AES [21] Cu standards for drinking water quality [22]
0.650 0.020 0.074 0.002 2.000
3.3. Analytical performance The analytical performance of the proposed technique was evaluated under optimal operating conditions (applied voltage: 135 V, pH: 1). A series of concentrations of Cu standard solutions with its optimal concentration of CTAC and the blank solutions were used to determine the LOD and sensitivity (S). The results are shown in Fig. S4 (Appendix A). Table 1 lists the LOD values of Cu with and without CTAC. The results showed that the LOD value of Cu is lower with CTAC than that without CTAC. A comparison of the LOD obtained by LCGD-AES with other closedtype ECLAD-AES techniques [5,6,8–11,13], ICP-AES [21], and Cu standards for drinking water quality of WHO [22] is shown in Table 2. The table shows that the LOD for LCGD-AES is found to be comparable to those of similar systems and mostly in the range of tens of parts per billion. In addition, the LOD value of Cu in this study is much lower than that of the standard for drinking water quality; thus, this technique can be effectively used to determine the safety of Cu in drinking water quality. 4. Conclusions The LCGD-AES was successfully applied for the determination of Cu in aqueous solution. The optimization analytical conditions of LCGDAES for the detection of Cu were 135 V, pH 1, and addition of 0.15% CTAC. The results showed that at the applied voltage of 135 V, the net intensity of atomic emission lines of Cu with 0.15% CTAC showed a 1.5-fold increase. The LODs of Cu at 135 V with 0.15% CTAC and without CTAC were 0.019 and 0.234 mg L−1, respectively. The analytical capability of Cu in this study is comparable to the closed-type ELCAD-AES, and satisfied the recommended levels of the WHO standards for drinking water quality. Compared with other ELCAD-AES systems, LCGD-AES has advantages such as lower setup cost, portable equipment, simple operation, and easy design. In addition, it is very easy to achieve in situ, real-time, and on-line analysis of samples. Therefore, LCGD-AES is a very promising technique for high-efficiency detection of metal ions in aqueous solutions. Acknowledgments This work was supported in part by the National Natural Science Foundation of China (Nos. 21367023, 21567025, and 11564037), the Natural Science Foundation of Gansu Province (Nos. 1308RJZA144 and 1208RJZA161), the Scientific Research Project in Higher Education Institutions of Gansu Province (No. 2013-019), and Key Project of Young Teachers' Scientific Research Promotion of Northwest Normal University (No. NWNU-LKQN-12-9), China. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.sab.2016.09.019.
References [1] Q. He, Z. Zhu, S. Hu, Flowing and nonflowing liquid electrode discharge microplasma for metal ion detection by optical emission spectrometry, Appl. Spectrosc. Rev. 49 (2014) 249–269. [2] P. Mezei, T. Cserfalvi, Electrolyte cathode atmospheric glow discharges for direct solution analysis, Appl. Spectrosc. Rev. 42 (2007) 573–604. [3] Z. Zhang, Z. Wang, Q. Li, H. Zou, Y. Shi, Determination of trace heavy metals in environmental and biological samples by solution cathode glow discharge-atomice mission spectrometry and addition of ionic surfactants for improved sensitivity, Talanta 119 (2014) 613–619. [4] J. Yu, S.X. Yang, D.X. Sun, Q.F. Lu, J.D. Zheng, X.M. Zhang, X. Wang, Simultaneously determination of multimetal elements in water samples by liquid cathode glow discharge-atomic emission spectrometry, Microchem. J. 128 (2016) 325–330. [5] M.R. Webb, F.J. Andrade, G. Gamez, R. McCrindle, G.M. Hieftje, Spectroscopic and electrical studies of a solution-cathode glow discharge, J. Anal. At. Spectrom. 20 (2005) 1218–1225. [6] R. Shekhar, D. Karunasagar, M. Ranjit, J. Arunachalam, Determination of elemental constituents in different matrix materials and flow injection studies by the electrolyte cathode glow discharge technique with a new design, Anal. Chem. 81 (2009) 8157–8166. [7] T. Cserfalvi, P. Mezei, P. Apai, Emission studies on a glow discharge in atmospheric pressure air using water as a cathode, J. Phys. D. Appl. Phys. 26 (1993) 2184–2188. [8] T. Cserfalvi, P. Mezei, Direct solution analysis by glow discharge: electrolyte-cathode discharge spectrometry, J. Anal. At. Spectrom. 9 (1994) 345–349. [9] M.R. Webb, F.J. Andrade, G.M. Hieftje, Compact glow discharge for the elemental analysis of aqueous samples, Anal. Chem. 79 (2007) 7899–7905. [10] A. Shaltout, Micro plasma generation using liquid sampling-atmospheric pressure glow discharge, Microchim. Acta 155 (2006) 447–452. [11] K. Gręda, P. Jamróz, P. Pohl, The improvement of the analytical performance of direct current atmospheric pressure glow discharge generated in contact with the smallsized liquid cathode after the addition of non-ionic surfactants to electrolyte solutions, Talanta 108 (2013) 74–82. [12] Q. Xiao, Z. Zhu, H. Zheng, H. He, C. Huang, S. Hu, Significant sensitivity improvement of alternating current driven-liquid discharge by using formic acid medium for optical determination of elements, Talanta 106 (2013) 144–149. [13] Q. He, Z. Zhu, S. Hu, H. Zheng, L. Jin, Elemental determination of microsamples by liquid film dielectric barrier discharge atomic emission spectrometry, Anal. Chem. 84 (2012) 4179–4184. [14] T.A. Doroski, M.R. Webb, Signal enhancement in solution-cathode glow dischargeoptical emission spectrometry via low molecular weight organic compounds, Spectrochim. Acta B 88 (2013) 40–45. [15] K. Gręda, P. Jamróz, P. Pohl, Effect of the addition of non-ionic surfactants on the emission characteristic of direct current atmospheric pressure glow discharge generated in contact with a flowing liquid cathode, J. Anal. At. Spectrom. 28 (2013) 134–141. [16] A. Kitano, A. Iiduka, T. Yamamoto, Y. Ukita, E. Tamiya, Y. Takamura, Highly sensitive elemental analysis for Cd and Pb by liquid electrode plasma atomic emission spectrometry with quartz glass chip and sample flow, Anal. Chem. 83 (2011) 9424–9430. [17] P. Jamroz, K. Greda, P. Pohl, Development of direct-current, atmospheric-pressure, glow discharges generated in contact with flowing electrolyte solutions for elemental analysis by optical emission spectrometry, Trends Anal. Chem. 41 (2012) 105–121. [18] Y.S. Park, S.H. Ku, S.H. Hong, H.J. Kim, E.H. Piepmeier, Fundamental studies of electrolyte-as-cathode glow discharge–atomic emission spectrometry for the determination of trace metals in flowing water, Spectrochim. Acta B 53 (1998) 1167–1179. [19] T. Cserfalvi, P. Mezei, Investigations on the element dependency of sputtering process in the electrolyte cathode atmospheric discharge, J. Anal. At. Spectrom. 20 (2005) 939–944. [20] R. Huang, Z. Zhu, H. Zheng, Z. Liu, S. Zhang, S. Hu, Alternating current driven atmospheric-pressure liquid discharge for the determination of elements with optical emission spectrometry, J. Anal. At. Spectrom. 26 (2011) 1178–1182. [21] S.J. Hill, A. Fisher, M. Foulkes, In Inductively Coupled Plasma Spectrometry and Its Application, 2nd ed. Blackwell Publishing Ltd., Oxford, 2007 61–97. [22] World Health Organisation, Guidelines for Drinking-water Quality, 4th ed., Geneva, 2011 240–342.