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Combination of knotted reactor with portable tungsten coil electrothermal atomic absorption spectrometer for on-line determination of trace cadmium
Microchemical Journal 124 (2016) 60–64
Contents lists available at ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Combination of knotted reactor with portable tungsten coil electrothermal atomic absorption spectrometer for on-line determination of trace cadmium Xiaodong Wen ⁎, Shengchun Yang, Haizhu Zhang, Qingwen Deng College of Pharmacy and Chemistry, Dali University, Dali, Yunnan 671000, China
a r t i c l e
i n f o
Article history: Received 3 June 2015 Received in revised form 26 July 2015 Accepted 26 July 2015 Available online 1 August 2015 Keywords: Knotted reactor Tungsten coil electrothermal atomic absorption spectrometer Preconcentration On-line determination Cadmium
a b s t r a c t In this work, flow injection (FI) on-line precipitation–dissolution in a knotted reactor (KR) was established and combined with a portable tungsten coil electrothermal atomic absorption spectrometer (W-coil ET-AAS) firstly for preconcentration and determination of ultra-trace cadmium. As a new instrument, the application of the portable W-coil spectrometer was expanded to carry out on-line preconcentration and detection through the coupling with a KR system. A self-assembled FI system was employed to hyphenate the KR system with W-coil ET-AAS. The instrumental conditions and influencing factors relevant to KR efficiency, such as concentration of ammonia, pH, conditions of sampling and elution were studied systematically. The coupling improved the analytical performance of the portable spectrometer considerably. Under the optimal conditions, the limit of detection (LOD) for cadmium was 0.006 μg/L, with sensitivity enhancement factor (EF) of 41. The established method could be expanded and used for on-line preconcentration and detection of some other trace metal ions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Accurate analysis of ultra trace metal elements has been an academic topic for the application of analytical chemistry. Atomic absorption spectrometry (AAS) is an important and effective means of metal element analysis. Among the various instrumental techniques, electrothermal atomic absorption spectrometry (ET-AAS) is relatively costeffective with high sensitivity, and graphite furnace atomic absorption spectrometry (GF-AAS) occupies an important position. As another method of ET-AAS, tungsten coil electrothermal atomic absorption spectrometry (W-coil ET-AAS) has gained attention for the potential to provide portable instrument and field analysis [1]. The portable tungsten coil electrothermal atomic absorption spectrometer is a new developed electrothermal atomic absorption spectrometer based on a tungsten coil atomizer, charge coupled device (CCD) [2,3] and years of academic accumulation in the filed of W-coil ET-AAS [4–8]. This instrument is designed as portable spectrometer, which can be applied in field analysis and move traditional AAS analysis out of lab. When portable W-coil spectrometer is applied to field analysis, some rapid and simple preconcentration and separation methods are still necessary before instrumental analysis due to low concentrations
⁎ Corresponding author. E-mail address: [email protected] (X. Wen).
http://dx.doi.org/10.1016/j.microc.2015.07.019 0026-265X/© 2015 Elsevier B.V. All rights reserved.
and matrix effects of environmental and biological samples. In our previous works, many preconcentration methods have been coupled with this instrument to improve its analytical performance and expanded its applications [1,6–13]. But almost all the methods were coupled with W-coil ET-AAS through the off-line mode. In this work, a KR preconcentration method was firstly combined with the portable instrument by FI system for on-line preconcentration and determination. Knotted reactor (KR) was made from polytetrafluoroethylene (PTFE) tubing for the flow injection on-line separation/preconcentration of trace metals, which combined with different atomic spectrometric techniques [7]. The use of KRs for the online sorption preconcentration combined with FAAS determination was first reported by Fang et al. [14]. Thereafter, the use of KRs has been greatly extended to its applications through the combination with FAAS [7], atomic fluorescence spectrometry (AFS) [15], ET-AAS [16,17], inductively coupled plasma mass spectrometry (ICP-MS) [18], etc. On-line coprecipitation involving collection of precipitates on a PTFE or Microline KR was proposed by Fang et al. as an effective preconcentration method for ET-AAS [19,20]. This approach opens another important application area for KR tubing as an excellent filterless collector for inorganic precipitations [7]. To the best of our knowledge, the KR technique was first hyphenated with the portable W-coil ET-AAS instrument for on-line preconcentration and determination in this work. The developed combination expanded the applications of KR technique and the new portable AAS instrument, while improved the analytical performance of this instrument considerably. As one of the applications of the method established
X. Wen et al. / Microchemical Journal 124 (2016) 60–64
by the present work, trace cadmium was determined as a prevalent toxic element. Peng Wu et al. reviewed some recent works corresponding to the detection of cadmium in biological samples [21,22]. The analytical method established in this work has certain advantages in terms of sensitivity. The characteristics and performance parameters of the established on-line method KR–W-coil ET-AAS were described below.
2. Experimental
61
2.2. Reagents Cadmium standard solution (1000 mg/L) was purchased from National Center of Analysis and Testing for Nonferrous Metals and Electronic Materials (NCATN, Beijing, China). Working standard solution was obtained daily by stepwise dilution from standard stock solution in ultra pure water. Ammonia and nitric acid (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) were diluted to optimal concentrations (0.8 mol/L and 1 mol/L) by ultra pure water daily. Other chemical reagents were all of analytical grade.
2.1. Apparatus Portable tungsten coil electrothermal atomic absorption spectrometer model WFX-910 (Beijing Rayleigh Analytical Instrument Co., Ltd, Beijing, China) was introduced for determination and investigation as a new commercial instrument, which consisted of three main parts arranged horizontally, including a hollow cathode lamp (HCL), a W-coil atomizer enclosed in a quartz cell and a spectrometer-charge coupled device (CCD). This instrument is developed to accomplish field analysis and the portable design embodies in the following aspects. Overall dimension (length × width × height): 610 mm × 230 mm × 335 mm, weight: 18 kg. A customized box is equipped to be carried out conveniently. A rechargeable lithium battery is equipped in the portable instrument to provide enough power for field analysis without commercial power supply. A specially customized and portable cylinder of argon containing 20% H2 is equipped with this spectrometer for field analysis. A lab-assembled flow injection system was used to accomplish online KR preconcentration and sample elution/introduction, which was consisted of two peristaltic pumps (Model: HL-2D, Shanghai Huxi Instrument Factory, Shanghai, China) and a standard rotary injection valve (eight ports on the rotor and eight ports on the stator). The knotted reactor was a 0.30 mm i.d. PTFE tubing (ca. 150 cm) by tying interlaced 30–35 knots. The pH values were measured by a pH-meter Model pHS-25 (Shanghai Hongyi Instrument Co., Ltd, Shanghai, China). A laboratory pure water system Model DZG-303A (Chengdu Tangshi Kangning Science and Technology Development Co., Ltd, Chengdu, China) was used to prepare ultra pure water. A model ELAN DRC-e ICP-MS instrument (PerkinElmerSCIEX, USA) was used to detect the real water samples to compare with the developed method. The major instrumental parameters include: ICP RF power, 1150 W; plasma gas flow, 15 L/min; auxiliary gas flow, 1.20 L/min; nebulizer gas flow, 0.93 L/min; lens voltage, 6.50 V; dual detector mode, pulse counting; and isotope monitored, 111Cd.
2.3. Knotted reactor (KR) on-line preconcentration procedure The experimental procedure of flow injection on-line precipitation preconcentration included two steps for adsorption and elution respectively, which was illustrated in Fig. 1. In the first step, both the pumps were activated and the injection valve was set in the fill position. The sample or standard solution and ammonia solution were mixed in a three way valve just before entering the KR. The resultant precipitate of cadmium hydroxide was adsorbed by the KR. In the next step, the collected precipitate should be eluted and transferred to instrumental analysis. Pump 2 was paused and pump 1 was still activated at the flow rate of 60 rpm, whereas the valve was turned to the inject position to pump the elution of 1 mol/L HNO3 through the KR. The precipitate adsorbed on the inner wall of the KR was eluted and then introduced into W-coil ET-AAS at the flow rate of 10 rpm for 4 s to ensure the volume of sample was 20 μL. After determination, the KR should be rinsed with some dilute nitric acid and ultra pure water to make ready for next measurement. 2.4. Operation procedure of KR–W-coil ET-AAS The KR preconcentration system was firstly combined with W-coil ET-AAS in this work. Via the flow injection analysis system, the precipitation preconcentration process was accomplished and then elution and sampling were respectively accomplished as mentioned above and illustrated in Fig. 1. The adsorbed cadmium hydroxide was efficiently eluted by pump 1 at a flow rate of 60 rpm. When the eluted solution approached the W-coil atomizer through the sampling tube, the flow rate of pump 1 was set at 10 rpm to introduce the resultant sample onto the atomizer. A total sampling time was 4 s, which was counted from the sample solution dripping out from the end of the sampling tube. Thus the sampling volume was controlled at 20 μL. After determination, the KR was rinsed to make ready for next measurement.
Fig. 1. Instrumental arrangement of the KR–W-coil ET-AAS system.
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Table 1 Heating program of W-coil (sampling amount: 20 μL). Steps Current (A)/duration (s)
Without KR After KR
Desolvation
Pyrolysis
Cooling
Atomization
Cleaning
2.9/70 2.9/70
2.9/20 2.9/30
0/4 0/4
8.5/4 8.5/4
9.0/2 9.0/2
2.5. Sample collection and preparation
3.2. Investigation of KR parameters
Tap water was collected in our laboratory after flowing for about 5 min, river and lake water were gotten from Xier River and Erhai Lake (Dali, China). Bottled mineral water was purchased from a local supermarket. All the real water samples above were filtered through a 0.45 μm micropore membrane prior to use.
3.2.1. Effect of pH on the KR preconcentration In this work, the preconcentration was accomplished on the basis of the formation of precipitation and the subsequent adsorption on the inner wall of the KR. Similar to other preconcentration methods, the acidity of sample solution always plays a key role during the pretreatment process. In order to obtain the desired preconcentration efficiency, the pH values were studied in the range of 1.0–7.0 adjusted by diluted HNO3 and NH3·H2O. The results shown in Fig. 2 indicated that the absorbance of cadmium increased in the pH range of 1.0–4.0 and then maintained steady in the pH range of 4.0–7.0. In the subsequent experiments the pH of 4.0 was selected and the pH range of 4.0–7.0 was feasible for the determination of real samples.
3. Results and discussion 3.1. Optimization of instrumental parameters For a portable W-coil ET-AAS spectrometer, routine optimization should be carried out for detection of target element, including desolvation current, pyrolysis current, atomization current, duration for each step and carrier gas flow rate of Ar/H2. As listed in Table 1, the optimum W-coil heating procedures (including applied current and duration for each step) were found for the on-line determination of cadmium combined with KR. Compared to direct determination of cadmium without KR preconcentration, the duration for pyrolysis step after KR preconcentration was 10 s longer due to the matrix of elution. Considering the individual differences of tungsten coils as well as the production of circuit boards and other factors, the heating procedure is not immutable and frozen. Specific optimization of current and lasting time should be carried out when changing tungsten coils or using different instruments. After optimization, the optimal carrier gas flow rate of Ar/H2 (mL/min) was 800/200 to prevent the coil from oxidizing and to produce a reducing environment during atomization. The position of hollow cathode lamp (HCL) and charge coupled device (CCD) was recommended by instrument manufacturer. Peak-height absorbance at 228.9 nm was used for quantification and the emission line of 226.6 nm was used for near-line background correction. The sampling amount used in this developed method was 20 μL of the eluted rich phase via controlling the peristaltic pump at the flow rate of 10 rpm for 4 s, as described above.
Fig. 2. Effect of pH on the absorbance. KR on-line preconcentration conditions: Cd, 0.2 µg/L; ammonia concentration/flow rate, 0.8 mol/L/20 rpm; sample volume/flow rate, 5 mL/40 rpm; elution solvent, 1 mol/L HNO3. The error bars standard for “± one standard deviation of three trials”.
3.2.2. Optimization of ammonia concentration and flow rate In the work, ammonia was used to react with cadmium to form the precipitation of cadmium hydroxide. The optimization of ammonia concentration was carried out in the concentration range of 0.1–1.2 mol/L. As shown in Fig. 3, the absorbance signals increased sharply with the ammonia concentrations increasing from 0.1 to 0.8 mol/L. When the concentrations of ammonia kept increasing, the signals dramatically decreased, probably due to the dissolution of analyte precipitate in excessive ammonia as ammonia complexes. After investigation, the ammonia concentration of 0.8 mol/L was selected for further experiments. For flow injection analysis, another important factor influencing preconcentration efficiency and analytical performance was flow rate of peristaltic pumps. The effect of ammonia flow rate was carefully investigated in the range of 5–40 rpm. The results indicated that 10 rpm was the optimal flow rate and higher flow rate decreased analytical signals significantly. During FIA process, the precipitation and adsorption need a period of time and higher flow rates may result in less reaction and adsorption time and thus less cadmium hydroxide preconcentrated on the inner wall of the KR.
Fig. 3. Optimization of ammonia concentration. KR on-line preconcentration conditions: Cd, 0.2 µg/L; pH, 4; sample volume/flow rate, 5 mL/40 rpm; elution solvent, 1 mol/L HNO3. The error bars standard for “± one standard deviation of three trials”.
X. Wen et al. / Microchemical Journal 124 (2016) 60–64
63
Table 3 Analytical characteristics.
Calibration equation (μg/L) Upper linear range (μg/L) LODa (μg/L) Absolute LODb (pg) R2 (correlation coefficient) RSD (n = 7, after KR) Sensitivity enhancement factorc
With KR
Without KR
A = 1.2627 C − 0.0156 0.75 0.006 0.12 0.9982 4.3% (0.20 μg/L) 41
A = 0.031 C + 0.0206 30 0.25 5 0.9971
a
LOD, limit of detection, was based on 3σ criterion for 11 blank measurements. Absolute LOD, with quality as the unit to represent the detection limit, LOD was multiplied by the sampling volume. c Sensitivity enhancement factor, was calculated by the slope ratio of the calibration curves for cadmium determination with and without KR. b
Fig. 4. Optimization of sample flow rate. KR on-line preconcentration conditions: Cd, 0.2 µg/L; ammonia concentration/flow rate, 0.8 mol/L/10 rpm; pH, 4; sample volume, 5 mL; elution solvent, 1 mol/L HNO3. The error bars standard for “± one standard deviation of three trials”.
3.2.3. Optimization of sample volume and flow rate Considering the preconcentration capacity of the KR and the speed of whole analytical procedure, the sample volume was fixed at 5 mL. As mentioned above, the flow rate of sample solution was a key factor and should be investigated to find out the optimal condition. As revealed in Fig. 4, the absorbance signals increased dramatically with the increasing of flow rate from 10 rpm to 40 rpm (corresponding to real sampling rate from 5 μL/s to 20 μL/s) and then obviously decreased as the flow rate further increased. The higher sample flow rate shortened the reaction time of cadmium and ammonia thus decreased the formation of the precipitation of cadmium hydroxide. The adsorption efficiency of the precipitation on the inner wall of the KR was also decreased in the high flow rate. Therefore, the sample flow rate of 40 rpm (corresponding to 20 μL/s of real sampling rate) was used in this work. 3.2.4. Effect of elution solvent In this work, the kind of elution solvent and the flow rate of elution affected the preconcentration efficiency and analytical performance. After optimization, the solution of 1 mol/L HNO3 was selected as optimal elution solvent due to its high efficiency for the on-line dissolution of the precipitation and its optimal flow rate was 60 rpm. After dissolution of the cadmium hydroxide precipitation, the pump rate was adjusted to 10 rpm to facilitate the sampling to W-coil atomizer for the measurement, as described in the operation procedure of KR–W-coil ET-AAS. Table 2 Tolerant limits of coexisting ions for the determination of cadmium (0.5 μg/L). Potential interfering ions
Interference/metal ratio
Recovery (%)
K+ Na+ Ca2+ Mg2+ ClSO24 Pb2+ Cu2+ Fe3+ Co2+ Mn2+ Zn2+ Ni2+ Cr3+
600 12,000 8000 4000 10,000 7000 20 50 50 20 30 50 50 20
107 106 93.7 94.6 104 103 92.6 96.4 93.8 96.7 105 93.5 106 91.8
3.2.5. Interferences Interferences may occur due to the formation of precipitation of other metal ions under alkaline conditions and the subsequent coprecipitation and adsorption, which would decrease the adsorption efficiency of target analytes on the inner wall of the KR. The eluted sample including some other precipitation possibly affected atomic absorption measurement during pyrolysis and atomization process. To evaluate the selectivity of the developed method, the effect of typically potential interfering ions was investigated and the results were shown in Table 2 (tolerable limit was taken as a relative error ≤10%). According to the results, some major matrix ions in natural water samples have no obvious interferences, while some other heavy metal ions have less tolerable limits due to the competition effect mentioned above. 3.2.6. Analytical figures of merit Analytical figures of merit of the established on-line KR–W-coil ETAAS method were obtained under optimal conditions and summarized in Table 3. At least 0.9971 of R2 (correlation coefficient) showed good linearity of the calibration curves. It can be seen from Table 3 that superior sensitivity of cadmium detection was obtained after the on-line KR preconcentration. The developed combination showed relatively high extraction efficiency, ultra-high sensitivity, simplicity and rapidity. 3.2.7. Determination of cadmium in real water samples Considering the biological and environmental significance, several water samples including Erhai Lake, Xier River, tap water in our lab and bottled mineral water were analyzed to validate the applicability and accuracy of the developed method. The analytical results were summarized in Table 4. The recoveries for the spiked samples were in the acceptable range of 93.7–104% and the results were compared with ICPMS detection, which showed good coincidence. 4. Conclusion In this work, a knotted reactor (KR) was firstly combined with a portable tungsten coil electrothermal atomic absorption spectrometer for the determination of ultra-trace cadmium via on-line precipitation– dissolution pattern. This combination expanded the application of this Table 4 Analytical results for cadmium in real water samples (avg. ± SD of three trials). Real sample
Found in natural sample (μg/L)
Determined by ICP-MS (μg/L)
Spiked (μg/L)
Recovery (%)
Tap water Erhai Lake Xier River Bottled mineral water
a
a
0.5 2.0 2.0 0.5
97.6 104 93.7 102
a
ND, not detected.
ND 1.80 ± 0.10 2.90 ± 0.20 a ND
ND 1.76 ± 0.10 2.82 ± 0.18 ND
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new portable W-coil ET-AAS instrument, as well as the application of KR method. Through this combination, the limit of detection (LOD) for cadmium was 0.006 μg/L, with a sensitivity enhancement factor (EF) of 41. The analytical performance of W-coil ET-AAS for the detection of cadmium was greatly improved. The established method was applied to the detection of some real water samples with satisfactory analytical results. Acknowledgments Xiaodong Wen gratefully acknowledges the financial support for this project from the National Natural Science Foundation of China [No. 21165001] and the financial and instrumental support from Beijing Rayleigh Analytical Instrument Co., Ltd. References [1] X.D. Wen, H.Z. Zhang, S.C. Yang, X. Zhao, J. Guo, Int. J. Environ. Anal. Chem. 94 (2014) 1243. [2] X.D. Hou, B.T. Jones, Microchem. J. 66 (2000) 115.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
X.D. Hou, Z. Yang, B.T. Jones, Spectrochim. Acta Part B 56 (2001) 203. A.C. Davis, C.P. Calloway Jr., B.T. Jones, Talanta 71 (2007) 1144. P. Wu, X.D. Wen, L. He, Y.H. He, M.Z. Chen, X.D. Hou, Talanta 74 (2008) 505. P.P. Chen, Y.J. Deng, K.C. Guo, X.M. Jiang, C.B. Zheng, X.D. Hou, Microchem. J. 112 (2014) 7–12. X.D. Wen, P. Wu, L. Chen, X.D. Hou, Anal. Chim. Acta 650 (2009) 33. C.J. Zeng, X.D. Wen, Z.Q. Tan, P.Y. Cai, X.D. Hou, Microchem. J. 96 (2010) 238. X.D. Wen, S.C. Yang, H.Z. Zhang, J.W. Wang, Anal. Methods 6 (2014) 8773. X.D. Wen, H.Z. Zhang, Q.W. Deng, S.C. Yang, Anal. Methods 6 (2014) 5047. X.D. Wen, Q.W. Deng, J.W. Wang, X. Zhao, Anal. Methods 5 (2013) 2978. A. Salido, C.L. Sanford, B.T. Jones, Spectrochim. Acta Part B 54 (1999) 1167. J.Y. Neira, J. Mendoza, C.G. Bruhn, Quim. Nova 28 (2005) 217. Z.L. Fang, S.K. Xu, L.P. Dong, W.Q. Li, Talanta 41 (1994) 2165. H.j. Zi, W.E. Gan, S.P. Han, X.J. Jiang, L.Z. Wan, Chin. J. Anal. Chem. 37 (2009) 1029. R.A. Gil, J.A. Gásquez, R. Olsina, L.D. Martinez, S. Cerutti, Talanta 76 (2008) 669. A.C. Grijalba, E.M. Martinis, G.E. Lascalea, R.G. Wuilloud, Spectrochim. Acta B 103–104 (2015) 49. Y. Li, Y.F. Huang, Y. Jiang, B.L. Tian, F. Han, X.P. Yan, Anal. Chim. Acta 692 (2011) 42. Z.L. Fang, M. Sperling, B. Welz, J. Anal. At. Spectrom. 6 (1991) 301. Z.L. Fang, L.P. Dong, J. Anal. At. Spectrom. 7 (1992) 439. A.C. Davis, P. Wu, X.F. Zhang, X.D. Hou, B.T. Jones, Appl. Spectrosc. Rev. 41 (2006) 35. P. Wu, C.H. Li, J.B. Chen, C.B. Zheng, X.D. Hou, Appl. Spectrosc. Rev. 47 (2012) 327.
Contents lists available at ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Combination of knotted reactor with portable tungsten coil electrothermal atomic absorption spectrometer for on-line determination of trace cadmium Xiaodong Wen ⁎, Shengchun Yang, Haizhu Zhang, Qingwen Deng College of Pharmacy and Chemistry, Dali University, Dali, Yunnan 671000, China
a r t i c l e
i n f o
Article history: Received 3 June 2015 Received in revised form 26 July 2015 Accepted 26 July 2015 Available online 1 August 2015 Keywords: Knotted reactor Tungsten coil electrothermal atomic absorption spectrometer Preconcentration On-line determination Cadmium
a b s t r a c t In this work, flow injection (FI) on-line precipitation–dissolution in a knotted reactor (KR) was established and combined with a portable tungsten coil electrothermal atomic absorption spectrometer (W-coil ET-AAS) firstly for preconcentration and determination of ultra-trace cadmium. As a new instrument, the application of the portable W-coil spectrometer was expanded to carry out on-line preconcentration and detection through the coupling with a KR system. A self-assembled FI system was employed to hyphenate the KR system with W-coil ET-AAS. The instrumental conditions and influencing factors relevant to KR efficiency, such as concentration of ammonia, pH, conditions of sampling and elution were studied systematically. The coupling improved the analytical performance of the portable spectrometer considerably. Under the optimal conditions, the limit of detection (LOD) for cadmium was 0.006 μg/L, with sensitivity enhancement factor (EF) of 41. The established method could be expanded and used for on-line preconcentration and detection of some other trace metal ions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Accurate analysis of ultra trace metal elements has been an academic topic for the application of analytical chemistry. Atomic absorption spectrometry (AAS) is an important and effective means of metal element analysis. Among the various instrumental techniques, electrothermal atomic absorption spectrometry (ET-AAS) is relatively costeffective with high sensitivity, and graphite furnace atomic absorption spectrometry (GF-AAS) occupies an important position. As another method of ET-AAS, tungsten coil electrothermal atomic absorption spectrometry (W-coil ET-AAS) has gained attention for the potential to provide portable instrument and field analysis [1]. The portable tungsten coil electrothermal atomic absorption spectrometer is a new developed electrothermal atomic absorption spectrometer based on a tungsten coil atomizer, charge coupled device (CCD) [2,3] and years of academic accumulation in the filed of W-coil ET-AAS [4–8]. This instrument is designed as portable spectrometer, which can be applied in field analysis and move traditional AAS analysis out of lab. When portable W-coil spectrometer is applied to field analysis, some rapid and simple preconcentration and separation methods are still necessary before instrumental analysis due to low concentrations
⁎ Corresponding author. E-mail address: [email protected] (X. Wen).
http://dx.doi.org/10.1016/j.microc.2015.07.019 0026-265X/© 2015 Elsevier B.V. All rights reserved.
and matrix effects of environmental and biological samples. In our previous works, many preconcentration methods have been coupled with this instrument to improve its analytical performance and expanded its applications [1,6–13]. But almost all the methods were coupled with W-coil ET-AAS through the off-line mode. In this work, a KR preconcentration method was firstly combined with the portable instrument by FI system for on-line preconcentration and determination. Knotted reactor (KR) was made from polytetrafluoroethylene (PTFE) tubing for the flow injection on-line separation/preconcentration of trace metals, which combined with different atomic spectrometric techniques [7]. The use of KRs for the online sorption preconcentration combined with FAAS determination was first reported by Fang et al. [14]. Thereafter, the use of KRs has been greatly extended to its applications through the combination with FAAS [7], atomic fluorescence spectrometry (AFS) [15], ET-AAS [16,17], inductively coupled plasma mass spectrometry (ICP-MS) [18], etc. On-line coprecipitation involving collection of precipitates on a PTFE or Microline KR was proposed by Fang et al. as an effective preconcentration method for ET-AAS [19,20]. This approach opens another important application area for KR tubing as an excellent filterless collector for inorganic precipitations [7]. To the best of our knowledge, the KR technique was first hyphenated with the portable W-coil ET-AAS instrument for on-line preconcentration and determination in this work. The developed combination expanded the applications of KR technique and the new portable AAS instrument, while improved the analytical performance of this instrument considerably. As one of the applications of the method established
X. Wen et al. / Microchemical Journal 124 (2016) 60–64
by the present work, trace cadmium was determined as a prevalent toxic element. Peng Wu et al. reviewed some recent works corresponding to the detection of cadmium in biological samples [21,22]. The analytical method established in this work has certain advantages in terms of sensitivity. The characteristics and performance parameters of the established on-line method KR–W-coil ET-AAS were described below.
2. Experimental
61
2.2. Reagents Cadmium standard solution (1000 mg/L) was purchased from National Center of Analysis and Testing for Nonferrous Metals and Electronic Materials (NCATN, Beijing, China). Working standard solution was obtained daily by stepwise dilution from standard stock solution in ultra pure water. Ammonia and nitric acid (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) were diluted to optimal concentrations (0.8 mol/L and 1 mol/L) by ultra pure water daily. Other chemical reagents were all of analytical grade.
2.1. Apparatus Portable tungsten coil electrothermal atomic absorption spectrometer model WFX-910 (Beijing Rayleigh Analytical Instrument Co., Ltd, Beijing, China) was introduced for determination and investigation as a new commercial instrument, which consisted of three main parts arranged horizontally, including a hollow cathode lamp (HCL), a W-coil atomizer enclosed in a quartz cell and a spectrometer-charge coupled device (CCD). This instrument is developed to accomplish field analysis and the portable design embodies in the following aspects. Overall dimension (length × width × height): 610 mm × 230 mm × 335 mm, weight: 18 kg. A customized box is equipped to be carried out conveniently. A rechargeable lithium battery is equipped in the portable instrument to provide enough power for field analysis without commercial power supply. A specially customized and portable cylinder of argon containing 20% H2 is equipped with this spectrometer for field analysis. A lab-assembled flow injection system was used to accomplish online KR preconcentration and sample elution/introduction, which was consisted of two peristaltic pumps (Model: HL-2D, Shanghai Huxi Instrument Factory, Shanghai, China) and a standard rotary injection valve (eight ports on the rotor and eight ports on the stator). The knotted reactor was a 0.30 mm i.d. PTFE tubing (ca. 150 cm) by tying interlaced 30–35 knots. The pH values were measured by a pH-meter Model pHS-25 (Shanghai Hongyi Instrument Co., Ltd, Shanghai, China). A laboratory pure water system Model DZG-303A (Chengdu Tangshi Kangning Science and Technology Development Co., Ltd, Chengdu, China) was used to prepare ultra pure water. A model ELAN DRC-e ICP-MS instrument (PerkinElmerSCIEX, USA) was used to detect the real water samples to compare with the developed method. The major instrumental parameters include: ICP RF power, 1150 W; plasma gas flow, 15 L/min; auxiliary gas flow, 1.20 L/min; nebulizer gas flow, 0.93 L/min; lens voltage, 6.50 V; dual detector mode, pulse counting; and isotope monitored, 111Cd.
2.3. Knotted reactor (KR) on-line preconcentration procedure The experimental procedure of flow injection on-line precipitation preconcentration included two steps for adsorption and elution respectively, which was illustrated in Fig. 1. In the first step, both the pumps were activated and the injection valve was set in the fill position. The sample or standard solution and ammonia solution were mixed in a three way valve just before entering the KR. The resultant precipitate of cadmium hydroxide was adsorbed by the KR. In the next step, the collected precipitate should be eluted and transferred to instrumental analysis. Pump 2 was paused and pump 1 was still activated at the flow rate of 60 rpm, whereas the valve was turned to the inject position to pump the elution of 1 mol/L HNO3 through the KR. The precipitate adsorbed on the inner wall of the KR was eluted and then introduced into W-coil ET-AAS at the flow rate of 10 rpm for 4 s to ensure the volume of sample was 20 μL. After determination, the KR should be rinsed with some dilute nitric acid and ultra pure water to make ready for next measurement. 2.4. Operation procedure of KR–W-coil ET-AAS The KR preconcentration system was firstly combined with W-coil ET-AAS in this work. Via the flow injection analysis system, the precipitation preconcentration process was accomplished and then elution and sampling were respectively accomplished as mentioned above and illustrated in Fig. 1. The adsorbed cadmium hydroxide was efficiently eluted by pump 1 at a flow rate of 60 rpm. When the eluted solution approached the W-coil atomizer through the sampling tube, the flow rate of pump 1 was set at 10 rpm to introduce the resultant sample onto the atomizer. A total sampling time was 4 s, which was counted from the sample solution dripping out from the end of the sampling tube. Thus the sampling volume was controlled at 20 μL. After determination, the KR was rinsed to make ready for next measurement.
Fig. 1. Instrumental arrangement of the KR–W-coil ET-AAS system.
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X. Wen et al. / Microchemical Journal 124 (2016) 60–64
Table 1 Heating program of W-coil (sampling amount: 20 μL). Steps Current (A)/duration (s)
Without KR After KR
Desolvation
Pyrolysis
Cooling
Atomization
Cleaning
2.9/70 2.9/70
2.9/20 2.9/30
0/4 0/4
8.5/4 8.5/4
9.0/2 9.0/2
2.5. Sample collection and preparation
3.2. Investigation of KR parameters
Tap water was collected in our laboratory after flowing for about 5 min, river and lake water were gotten from Xier River and Erhai Lake (Dali, China). Bottled mineral water was purchased from a local supermarket. All the real water samples above were filtered through a 0.45 μm micropore membrane prior to use.
3.2.1. Effect of pH on the KR preconcentration In this work, the preconcentration was accomplished on the basis of the formation of precipitation and the subsequent adsorption on the inner wall of the KR. Similar to other preconcentration methods, the acidity of sample solution always plays a key role during the pretreatment process. In order to obtain the desired preconcentration efficiency, the pH values were studied in the range of 1.0–7.0 adjusted by diluted HNO3 and NH3·H2O. The results shown in Fig. 2 indicated that the absorbance of cadmium increased in the pH range of 1.0–4.0 and then maintained steady in the pH range of 4.0–7.0. In the subsequent experiments the pH of 4.0 was selected and the pH range of 4.0–7.0 was feasible for the determination of real samples.
3. Results and discussion 3.1. Optimization of instrumental parameters For a portable W-coil ET-AAS spectrometer, routine optimization should be carried out for detection of target element, including desolvation current, pyrolysis current, atomization current, duration for each step and carrier gas flow rate of Ar/H2. As listed in Table 1, the optimum W-coil heating procedures (including applied current and duration for each step) were found for the on-line determination of cadmium combined with KR. Compared to direct determination of cadmium without KR preconcentration, the duration for pyrolysis step after KR preconcentration was 10 s longer due to the matrix of elution. Considering the individual differences of tungsten coils as well as the production of circuit boards and other factors, the heating procedure is not immutable and frozen. Specific optimization of current and lasting time should be carried out when changing tungsten coils or using different instruments. After optimization, the optimal carrier gas flow rate of Ar/H2 (mL/min) was 800/200 to prevent the coil from oxidizing and to produce a reducing environment during atomization. The position of hollow cathode lamp (HCL) and charge coupled device (CCD) was recommended by instrument manufacturer. Peak-height absorbance at 228.9 nm was used for quantification and the emission line of 226.6 nm was used for near-line background correction. The sampling amount used in this developed method was 20 μL of the eluted rich phase via controlling the peristaltic pump at the flow rate of 10 rpm for 4 s, as described above.
Fig. 2. Effect of pH on the absorbance. KR on-line preconcentration conditions: Cd, 0.2 µg/L; ammonia concentration/flow rate, 0.8 mol/L/20 rpm; sample volume/flow rate, 5 mL/40 rpm; elution solvent, 1 mol/L HNO3. The error bars standard for “± one standard deviation of three trials”.
3.2.2. Optimization of ammonia concentration and flow rate In the work, ammonia was used to react with cadmium to form the precipitation of cadmium hydroxide. The optimization of ammonia concentration was carried out in the concentration range of 0.1–1.2 mol/L. As shown in Fig. 3, the absorbance signals increased sharply with the ammonia concentrations increasing from 0.1 to 0.8 mol/L. When the concentrations of ammonia kept increasing, the signals dramatically decreased, probably due to the dissolution of analyte precipitate in excessive ammonia as ammonia complexes. After investigation, the ammonia concentration of 0.8 mol/L was selected for further experiments. For flow injection analysis, another important factor influencing preconcentration efficiency and analytical performance was flow rate of peristaltic pumps. The effect of ammonia flow rate was carefully investigated in the range of 5–40 rpm. The results indicated that 10 rpm was the optimal flow rate and higher flow rate decreased analytical signals significantly. During FIA process, the precipitation and adsorption need a period of time and higher flow rates may result in less reaction and adsorption time and thus less cadmium hydroxide preconcentrated on the inner wall of the KR.
Fig. 3. Optimization of ammonia concentration. KR on-line preconcentration conditions: Cd, 0.2 µg/L; pH, 4; sample volume/flow rate, 5 mL/40 rpm; elution solvent, 1 mol/L HNO3. The error bars standard for “± one standard deviation of three trials”.
X. Wen et al. / Microchemical Journal 124 (2016) 60–64
63
Table 3 Analytical characteristics.
Calibration equation (μg/L) Upper linear range (μg/L) LODa (μg/L) Absolute LODb (pg) R2 (correlation coefficient) RSD (n = 7, after KR) Sensitivity enhancement factorc
With KR
Without KR
A = 1.2627 C − 0.0156 0.75 0.006 0.12 0.9982 4.3% (0.20 μg/L) 41
A = 0.031 C + 0.0206 30 0.25 5 0.9971
a
LOD, limit of detection, was based on 3σ criterion for 11 blank measurements. Absolute LOD, with quality as the unit to represent the detection limit, LOD was multiplied by the sampling volume. c Sensitivity enhancement factor, was calculated by the slope ratio of the calibration curves for cadmium determination with and without KR. b
Fig. 4. Optimization of sample flow rate. KR on-line preconcentration conditions: Cd, 0.2 µg/L; ammonia concentration/flow rate, 0.8 mol/L/10 rpm; pH, 4; sample volume, 5 mL; elution solvent, 1 mol/L HNO3. The error bars standard for “± one standard deviation of three trials”.
3.2.3. Optimization of sample volume and flow rate Considering the preconcentration capacity of the KR and the speed of whole analytical procedure, the sample volume was fixed at 5 mL. As mentioned above, the flow rate of sample solution was a key factor and should be investigated to find out the optimal condition. As revealed in Fig. 4, the absorbance signals increased dramatically with the increasing of flow rate from 10 rpm to 40 rpm (corresponding to real sampling rate from 5 μL/s to 20 μL/s) and then obviously decreased as the flow rate further increased. The higher sample flow rate shortened the reaction time of cadmium and ammonia thus decreased the formation of the precipitation of cadmium hydroxide. The adsorption efficiency of the precipitation on the inner wall of the KR was also decreased in the high flow rate. Therefore, the sample flow rate of 40 rpm (corresponding to 20 μL/s of real sampling rate) was used in this work. 3.2.4. Effect of elution solvent In this work, the kind of elution solvent and the flow rate of elution affected the preconcentration efficiency and analytical performance. After optimization, the solution of 1 mol/L HNO3 was selected as optimal elution solvent due to its high efficiency for the on-line dissolution of the precipitation and its optimal flow rate was 60 rpm. After dissolution of the cadmium hydroxide precipitation, the pump rate was adjusted to 10 rpm to facilitate the sampling to W-coil atomizer for the measurement, as described in the operation procedure of KR–W-coil ET-AAS. Table 2 Tolerant limits of coexisting ions for the determination of cadmium (0.5 μg/L). Potential interfering ions
Interference/metal ratio
Recovery (%)
K+ Na+ Ca2+ Mg2+ ClSO24 Pb2+ Cu2+ Fe3+ Co2+ Mn2+ Zn2+ Ni2+ Cr3+
600 12,000 8000 4000 10,000 7000 20 50 50 20 30 50 50 20
107 106 93.7 94.6 104 103 92.6 96.4 93.8 96.7 105 93.5 106 91.8
3.2.5. Interferences Interferences may occur due to the formation of precipitation of other metal ions under alkaline conditions and the subsequent coprecipitation and adsorption, which would decrease the adsorption efficiency of target analytes on the inner wall of the KR. The eluted sample including some other precipitation possibly affected atomic absorption measurement during pyrolysis and atomization process. To evaluate the selectivity of the developed method, the effect of typically potential interfering ions was investigated and the results were shown in Table 2 (tolerable limit was taken as a relative error ≤10%). According to the results, some major matrix ions in natural water samples have no obvious interferences, while some other heavy metal ions have less tolerable limits due to the competition effect mentioned above. 3.2.6. Analytical figures of merit Analytical figures of merit of the established on-line KR–W-coil ETAAS method were obtained under optimal conditions and summarized in Table 3. At least 0.9971 of R2 (correlation coefficient) showed good linearity of the calibration curves. It can be seen from Table 3 that superior sensitivity of cadmium detection was obtained after the on-line KR preconcentration. The developed combination showed relatively high extraction efficiency, ultra-high sensitivity, simplicity and rapidity. 3.2.7. Determination of cadmium in real water samples Considering the biological and environmental significance, several water samples including Erhai Lake, Xier River, tap water in our lab and bottled mineral water were analyzed to validate the applicability and accuracy of the developed method. The analytical results were summarized in Table 4. The recoveries for the spiked samples were in the acceptable range of 93.7–104% and the results were compared with ICPMS detection, which showed good coincidence. 4. Conclusion In this work, a knotted reactor (KR) was firstly combined with a portable tungsten coil electrothermal atomic absorption spectrometer for the determination of ultra-trace cadmium via on-line precipitation– dissolution pattern. This combination expanded the application of this Table 4 Analytical results for cadmium in real water samples (avg. ± SD of three trials). Real sample
Found in natural sample (μg/L)
Determined by ICP-MS (μg/L)
Spiked (μg/L)
Recovery (%)
Tap water Erhai Lake Xier River Bottled mineral water
a
a
0.5 2.0 2.0 0.5
97.6 104 93.7 102
a
ND, not detected.
ND 1.80 ± 0.10 2.90 ± 0.20 a ND
ND 1.76 ± 0.10 2.82 ± 0.18 ND
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new portable W-coil ET-AAS instrument, as well as the application of KR method. Through this combination, the limit of detection (LOD) for cadmium was 0.006 μg/L, with a sensitivity enhancement factor (EF) of 41. The analytical performance of W-coil ET-AAS for the detection of cadmium was greatly improved. The established method was applied to the detection of some real water samples with satisfactory analytical results. Acknowledgments Xiaodong Wen gratefully acknowledges the financial support for this project from the National Natural Science Foundation of China [No. 21165001] and the financial and instrumental support from Beijing Rayleigh Analytical Instrument Co., Ltd. References [1] X.D. Wen, H.Z. Zhang, S.C. Yang, X. Zhao, J. Guo, Int. J. Environ. Anal. Chem. 94 (2014) 1243. [2] X.D. Hou, B.T. Jones, Microchem. J. 66 (2000) 115.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
X.D. Hou, Z. Yang, B.T. Jones, Spectrochim. Acta Part B 56 (2001) 203. A.C. Davis, C.P. Calloway Jr., B.T. Jones, Talanta 71 (2007) 1144. P. Wu, X.D. Wen, L. He, Y.H. He, M.Z. Chen, X.D. Hou, Talanta 74 (2008) 505. P.P. Chen, Y.J. Deng, K.C. Guo, X.M. Jiang, C.B. Zheng, X.D. Hou, Microchem. J. 112 (2014) 7–12. X.D. Wen, P. Wu, L. Chen, X.D. Hou, Anal. Chim. Acta 650 (2009) 33. C.J. Zeng, X.D. Wen, Z.Q. Tan, P.Y. Cai, X.D. Hou, Microchem. J. 96 (2010) 238. X.D. Wen, S.C. Yang, H.Z. Zhang, J.W. Wang, Anal. Methods 6 (2014) 8773. X.D. Wen, H.Z. Zhang, Q.W. Deng, S.C. Yang, Anal. Methods 6 (2014) 5047. X.D. Wen, Q.W. Deng, J.W. Wang, X. Zhao, Anal. Methods 5 (2013) 2978. A. Salido, C.L. Sanford, B.T. Jones, Spectrochim. Acta Part B 54 (1999) 1167. J.Y. Neira, J. Mendoza, C.G. Bruhn, Quim. Nova 28 (2005) 217. Z.L. Fang, S.K. Xu, L.P. Dong, W.Q. Li, Talanta 41 (1994) 2165. H.j. Zi, W.E. Gan, S.P. Han, X.J. Jiang, L.Z. Wan, Chin. J. Anal. Chem. 37 (2009) 1029. R.A. Gil, J.A. Gásquez, R. Olsina, L.D. Martinez, S. Cerutti, Talanta 76 (2008) 669. A.C. Grijalba, E.M. Martinis, G.E. Lascalea, R.G. Wuilloud, Spectrochim. Acta B 103–104 (2015) 49. Y. Li, Y.F. Huang, Y. Jiang, B.L. Tian, F. Han, X.P. Yan, Anal. Chim. Acta 692 (2011) 42. Z.L. Fang, M. Sperling, B. Welz, J. Anal. At. Spectrom. 6 (1991) 301. Z.L. Fang, L.P. Dong, J. Anal. At. Spectrom. 7 (1992) 439. A.C. Davis, P. Wu, X.F. Zhang, X.D. Hou, B.T. Jones, Appl. Spectrosc. Rev. 41 (2006) 35. P. Wu, C.H. Li, J.B. Chen, C.B. Zheng, X.D. Hou, Appl. Spectrosc. Rev. 47 (2012) 327.