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A portable solution cathode glow discharge-atomic emission spectrometer for the rapid determination of thallium in water samples
Author’s Accepted Manuscript A portable solution cathode glow discharge-atomic emission spectrometer for the rapid determination of thallium in water samples Wenchuan Zu, Yu Wang, Xiaotao Yang, Cong Liu www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(17)30599-4 http://dx.doi.org/10.1016/j.talanta.2017.05.073 TAL17607
To appear in: Talanta Received date: 13 March 2017 Revised date: 17 May 2017 Accepted date: 25 May 2017 Cite this article as: Wenchuan Zu, Yu Wang, Xiaotao Yang and Cong Liu, A portable solution cathode glow discharge-atomic emission spectrometer for the rapid determination of thallium in water samples, Talanta, http://dx.doi.org/10.1016/j.talanta.2017.05.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A portable solution cathode glow discharge-atomic emission spectrometer for the rapid determination of thallium in water samples Wenchuan Zu1,2*ˈ Yu Wang1,2*ˈ Xiaotao Yang1,2ˈ Cong Liu1,2 1 2
Beijing center for physical & chemical analysis, Beijing, China, 100089
Beijing Academy of Science and Technology key laboratory of Analysis and Testing Technology, Beijing, China,
100089
[email protected]˄W. Zu˅ [email protected]˄Y. Wang˅ *Corresponding author.
Abstract A novel and high performance method for rapid determination of thallium in water samples was established by using a portable solution cathode glow discharger with a fiber-optical spectrometer. The operating conditions including the solution acidity, the electrolyte cathode, etc were optimized with 1.0 mg.L-1 thallium standard solution. The resolution of the fiber-optical spectrometer investigated with the peak width at half height of thallium was tested to be about 1.8 nmˈand thallium was determined at the emission wavelength of 535.0 nm. The caliberation curve was favorably linear when the concentrations of thallium standard solutions were in the range of 0.1 mg.L-1 ~ 5.0 mg.L-1. Under the optimized conditions, the limit of detection (LOD) for thallium was 11.8 ng.mL-1ˈ and the precision evaluated by relative standard deviation was 3.2 % for six times 1.0 mg.L-1 standard solution replicates. This method was used for detection of thallium in water samples. The results were satisfying, and the average recoveries for thallium spiked samples were found to be in the range of 91.3 %~ 107.5%ˈwhich showed this method was applicable for real samples analysis.
Besidesˈthis method is suitable for field tests due to the portable instrumental size and weight as well as the less consumption of time and reagent. Graphical abstract
1
Keywords: Solution cathode glow discharge; Thallium; Water; Portable atomic emission spectrometer 1 introduction Thalliumˈwhich is well-known as an extremely toxic heavy-metal elementˈis in a low-level content within the earth's crust[1]. Howeverˈwith the development of mining and metal smeltingˈthe release of thallium into the atmosphere is increasing and regional water thallium pollutions tend to be serious[2-4]. Besidesˈthallium can be remarkably accumulated in human tissues and organs by the food chainˈand when the income of thallium exceeds a certain allowable contentˈgreat harms could be caused[5]. Therefore, it makes great sense to field and fast determine thallium in environmental water samples as the monitoring of the thallium pollution level is realized, and the thallium-poisoning risk can be decreased effectively as well. At the present time, the determination of Tl (elemental symbol for thallium) is usually carried out by atomic spectrometry methods, such as atomic absorption spectrometry˄AAS˅[6-8]ˈinductively coupled plasma-atomic emission spectrometry (ICP-AES)
[9]
, etc. Although the sensitivity is ideal for the above meansˈmatrix
interference is serious and the analytic process is time-consuming for the graphite furnace˄GF˅-AAS method and spectral interference is prominent for the ICP-AES method due to the complicate emission spectral lines. In some works, the applications of inductively coupled plasma-mass spectrometry (ICP-MS) was used to improve the analytic performance[10], yet the instrumental operation is complicated and high-cost. Moreover, all the techniques above can’t be portable so that the demand of field test can’t be satisfied. Moreover, certain simple-instrumental electrochemical methods, such as anodic-stripping voltammetry, polarographic analysis etc, were reported in former study to detect Tl and showed good detection limits and precision[11,
12]
.
However, matrix effects were serious, and the electrodes were easily to be polluted and a tedious polishing treatment was demanded. Solution cathode glow discharge (SCGD) atomic emission spectrometry has already reported as a novel technique to determine heavy-metal elements, including Pb, Cd, CuˈHgˈetc[13-18]. and some methods were reported to improve the analytical
2
sensitivity as well[19-21] .However, the determination of Tl has not been involved. In the SCGD system, metal electrodes are used as the anode while sample solution is employed as cathode. When a high voltage is applied, gas between the two electrodes will be ionized and the plasma will be generated. During the glow discharge process, the cathode solution is continuous gasified. As a result, the metal ions dissolved in the solution cathode also enters the plasma and was excited so that the emission spectrum comes into being[22]. In our work, the method for rapid detecting thallium by a home-made solution cathode glow discharger coupled with a fiber-optical spectrometer was established. The working conditions and parameters influencing the analytic performance were studied ˈ after which the analysis capabilities were validated. The detailed spectra information of the interference could be acquired by an inside CCD detector of a fiber-optical spectrometer. This method is feasible for the rapid thallium determination in water samples. Besides, the SCGD apparatus we used is portable so that the field analysis is realized. What’s moreˈonly less dilute acid is needed for the test so that more chemical reagents could be saved, for which it’s more environmental-friendly as well. 2 Experimental 2.1 Reagents All the acids were of guarantee grade (G.R., from Beijing Fine Chemicals Ltd.), Ultra pure water (>18.2 MΩ) was obtained from a Millipore ultra pure water system. Thallium standard stock solution (ICP-60N-1, 1000 mg.L-1) was acquired from J&K Scientific Ltd. Calibration solutions ( 0.1 μg.mL-1~ 5.0 μg.mL-1) were prepared by appropriate dilution of the standard stock solution with 1%(v/v) hydrochloric acid. Besides, all the glassy wares were immersed in the 10% nitrous acid for 24 hours and washed clean with ultra pure water. 2.2 Sample preparation The water samples were directly determined only after an acidification treatment if the samples are clean. To say exactlyˈ1mL HNO3 is added into each 100 mL water sample to keep the same acidity with the standard solutions. As for the muddy samplesˈthe acidification treatment is accomplished after an filtration process using 0.45 m filter membranes-aquo system. And the blanks and spiked samples were
3
prepared in parallel. 2.3 Apparutus In the SCGD-atomic emission spectrometry system ( as Fig .1), a home-made solution cathode glow discharge cell was employed in our study. Simply, the anode is a tungsten pin (20 mm long and 1 mm diameter) housed in the upper partˈand a solution cathode is placed in the lower partˈwhich consists of a quartz glass capillary inserted into a graphite cylinder which in turn was fixed to a PTFE reservoir for the overflowing solution. The gap of the two parts were applied a voltage at the level of 700 V., which was transferred and amplified from a 12V output DC power source ˄HF150W-S-12, Hengfu CorporationˈChina˅.The sample solution was pumped into the glass capillary by a peristaltic pumpˈand the liquid waste was discharged out of the electrolyte reservoir through a specified pipeline. The glow discharge emission spectrometry is observed by a high resolution fiber-optical spectrometer˄HR400, Ocean Optics, Inc˅. The whole set of the instrument is portable.
Fig.1 The configuration of the SCGD-atomic emission spectrometry device
In our work, the optimal working parameters adopted for the SCGD-AES were as follows in Tab.1. 4
Tab. 1 Working parameters of SCGD-AES SCGD-AES parameters
Setting
The cathode glow discharge current /mA
35
The inner size of the glass capillary /mm
0.3h0.3
The flow velocity of the electrolyte cathode /mL.min-1 The detecting wavelength of Tl emission line /nm
535.0
The integral time for the fiber-optical spectrometer /ms 3
4
100
Results and discussion
3.1 Optimization of operating conditions 3.1.1 The solution acidity It has ever been reported that the discharge of SCGD-AES strongly depends on the pH of the solution due to the association between pH and cathode fall [23]. T. Cserfalvi etal [24] observed the cathode fall as a function with pH and found that it was almost constant when the pH range was between 4-8, but significantly decreased when the pH was dropped to be below 4. The emission intensity is increased as the cathode fall decreases. In our work, the influence of electrolyte cathode acidity on Tl analytical sensitivity was examined using HNO3 medium. Generally, a series of 1.0 mg/L Tl standard solutions with different acidity were used to observe the emission intensity, including 0.5%-2 %(v/v) HNO3. The results were showed in Fig.2. The sensitivity was improved with the increase of the acidity, which could be explained by the formulas[14] reported by T. Cserfalvi below.
Uef
=
1 1 · ln(1 + ) h g
g = 0.526 aH + 5
( pH<2.5 )
Uef stands for the cathode fall and η is the count of ion pairs caused by cathode fall. αH+ is the concentration of H+. Therefore, in our work( pH<2.5 ), when the concentration of H+ increased, the value of γ got larger and the cathode fall was decreased. Thusˈthe emission intensity was increased. Although the sensitivity was better when the acidity was increased, the generation of the discharging became difficult and the glow tended to be unstable when the acidity was above 1%. Therefore, taking the factors of electrolyte cathode glow stability and facility of the glow discharging generation except for the sensitivity into consideration, 1% HNO3 (pH=0.85) was used in our following work.
(PLVVLRQLQWHQVLW\
1LWULFDFLGFRQFHQWUDWLRQ
Fig.2 The effect of electrolyte cathode acidity on Tl detection sensitivity
3.1.2 The validation of electrolyte cathode Four kinds of common inorganic acids, i.e. HCl, HNO3, H2SO4 and H3PO4with the equal pH as 1% HNO3 (pH=0.85) were used as the acid medium for 1.0 mg/L Tl standard solution. The emission intensity determined were as showed in Fig.3. The emission intensity was not ideal when HCl and H3PO4 were used, and the glow generation is better when HNO3 is used compared with H2SO4. Considering both the sensitivity and simplicity of the glow discharge syntheticallyˈwe chose HNO3 as the acidic medium.
6
Fig.3 The effect of electrolyte cathode on Tl detection sensitivity 3.1.3 The glow discharge gap The glow discharge gap is separated between the tip of the tungsten anode and the electrolyte solution surface. An ideal glow discharge gap should not only supply a strong emission intensity, but ensure the stability of the glow. As Z.Wang, etal [25] ever reported, when the discharge gap exceeded 3 mm, the emission intensity would become weaker with the increasing of the gap due to the lower discharge potential which decreased the energy available for excitation. Besides, the glow discharge turned to be instable because of the greater susceptibility to air currents. On the other hand, if the discharge gap fell below 3 mm, the emission intensity increased as the analytic target could remain in the plasma for longer time. Therefore, we preferred 3mm glow discharge gap in our further study.
3.2 Analytic characteristic 3.2.1 The resolution capacity and the emission line In our workˈthe atomic spectrometry emission signals were collected and processed by the fiber-optical spectrometerˈwhich consists of the optical splitting system and the CCD detecting system. The resolution capacity for the fiber-optical spectrometer greatly effect the interference performance. Thus ˈ the resolution capacity was inspected using 10 mg /L Tl standard solutions and the spectrum near 535.0 nm was as Fig.4ˈand the resolution calculated in peak width at half height was about 1.8 nm. 7
Fig.4 The peak width at half height test result of 10mg.L-1 Tl at 535.0 nm
Besides, it was interesting that a sub-sensitive emission line at 377.6 nm
was
discovered except for the sensitive line at 535.0 nm for Tl as Fig.5 showed. The emission intensity at 377.6 nm we acquired was about half (47.5% exactly) as that at the 535.0 nm sensitive line, which could be greatly useful for the high concentration Tl determination.
Fig.5 Spectrum of Tl observed by the fiber-optical spectrometer of SCGD-AES
3.2.2 Interference The potential interferences of other ions for thallium determination were examined. 8
All the ions were added into 500 ng.mL-1 thallium standard solution, and the concrete emission spectrum acquired by the high resolution fiber-optical spectrometer was as showed in Fig.6 , no obvious emission peaks appeared close neighbor the thallium emission peak (535.0nm). The recovery results showed that no significant interference was observed.
Fig.6 The interference of co-existing metal elements for Tl determination . The red line was the spectrum for 1.0 mg.L-1 Tl while the black line was 1.0 mg.L-1 Tl with equal concentration of co-existing metal elements ions of Pb, Mn, Ni, Cu, Mg, Co, Cd, Cr, Li and Zn.
3.2.3 Analytic performance 1.0 mL each standard solution was pumped into the cathode glow discharge cell and determined by the high resolution fiber-optical spectrometer continuously from low concentration to high. The emission intensity read by peak height presented a favorable linearity when the concentrations of Tl working solutions were in the range of 0.1 mg.L-1~ 5.0 mg.L-1 (as Tl). The equation of linear regression acquired in this work was following as Fig.7 showed : Ie = 101.9+1404.7²Tl ; R=0.9996, where Ie is
9
the peak height of the emission intensity within the reading time while²Tl means the concentration of Tl solutions. The value of linear correlation coefficient R convincingly confirmed the good linear-ship between the emission intensity and Tl concentrations in the selected range. The limit of detection (LOD) for Tl (calculated as 3SDBLANK/slope; n=11) was 11.8 ng.mL-1and the limit of quantification (LOQ) for Tl (calculated as 10SDBLANK/slope; n=11) was 39.3 ng.mL-1, and the precision evaluated by relative standard deviation (RSD) was 3.2 % for six times replicates of 1.0 mg.L-1 Tl solution as can be seen in Fig.8. . \ [
5
,H
&PJ/
Fig.7 The working curve of Tl
,H
7HVWWLPH
Fig.8 Test results for 6 times replicates of 1.0 mg.L-1 Tl solution 10
3.3 Sample determination by SCGD-AES Three different kinds of real samples were completely disposed beforehand as we did in the sample preparation step above. And then the Tl contents of the standard samples including the blank were determined by the optimized SCGD-AES technique to assess the feasibility of this method for real sample analysis. The results were elaborately recapitulated in Tab.2. Meanwhile, 0.4 mg/L~2.0 mg/L Tl standard solutions were spiked into the samples to validate the availability of the experimental results. The average recoveries were acquired in the range of 91.3 %~ 107.5%ˈand the relative standard deviations for the parallel recovery results were not beyond 5.22% (n=5). It was concluded that the results were credible and the method was applicable for the real sample analysis. Tab.2 The water samples determination and recovery results Sample
Content/ mg.L-1
--a
River water
Tl-containing waste water
0.682
Water leaching liquor of stream sediment
a
--
Add/ mg.L-1
Found/ mg.L-1
Recovery/ %
0.400
0.388
97.0
0.400
0.360
90.0
0.400
0.350
87.5
0.400
0.389
97.3
0.400
0.352
88.0
2.00
2.17
108.5
2.00
2.19
109.5
2.00
2.21
110.5
2.00
2.18
109.0
2.00
2.00
100.0
0.400
0.380
95.0
0.400
0.377
94.3
0.400
0.361
90.3
0.400
0.363
90.8
0.400
0.345
86.3
The value is below LOQ 11
Average
Recovery/ %
RSD b / %
92.0
5.22
107.5
3.96
91.3
3.83
b
RSD values are based on 5 times parallel spiked recovery results for each sample
4
Conclusion
The portable solution cathode glow discharge-atomic emission spectrometer is a novel technique to quantitatively and rapidly determine the trace amount of Tl. It offers the field detection of Tl at trace amount level an ideal alternative due to the high analytic performance and low spectrometric or matrix interference. It just takes about 1.5 min to complete the total analysis process of a sample, and no memory effect is observed due to the adequate washing process. Besides, SCGD-AES instrument is with small size and weight and needs no additional gas cylinders which is essential for other atomic spectrometry techniques so that it’s with approving portability. Furthermore, this method is convenient and practical in virtue of its simple instrument, low cost, low power and small sample consumption. The method can be successfully applied to the fast Tl analysis for the real water samples. The field experiments or test for environmental Tl pollution risk monitoring is realized due to the instrumental portability so that the damage caused by Tl pollution is expected to be greatly decreased.
Acknowledgments The authors are grateful to the support of ( Beijing Municipal Science and Technology Project: Z161100003016008, Z161100003016010). References [1] T. F. Xiao, F. Yang, S. H. Li, etal. Thallium pollution in China: A geo-environmental perspective[J], Science of The Total Environment,2012,421-422(1):51-58. [2] E. Álvarez-Ayuso, V. Otones, A. Murciego,etal. Zinc, cadmium and thallium distribution in soils
and
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12
[5] C. L. Wang, Y. H.Chen, J. Liu,etal. Health risks of thallium in contaminated arable soils and food crops irrigated with wastewater from a sulfuric acid plant in western Guangdong province, China [J], Ecotoxicology and Environmental Safety,2013,90(1):76-81. [6] Z. D. Firouzabadia, A. M. H. Shabania, S. Dadfarnia,etal. Preconcentration and speciation of thallium by ferrofluid based dispersive solid phase extraction and flame atomic absorption spectrometry[J], Microchemical Journal,2017,130:428-435. [7]
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[8] N. N. Meeravali, K. Madhavi, S. J. Kumar. Microwave assisted aqua regia extraction of thallium from sediment and coal fly ash samples and interference free determination by continuum source ETAAS after cloud point extraction[J], Talanta, 2013, 104(30):180-186. [9] N. Velitchkova, E.N. Pentcheva, N. Daskalova.Determination of arsenic, mercury, selenium, thallium, tin and bismuth in environmental materials by inductively coupled plasma emission spectrometry[J], Spectrochimica Acta Part B: Atomic Spectroscopy,2004,59(6):871-882. [10] B. Krasnodębska-Ostręga, M. Sadowska, K. Piotrowska,etal. Thallium (III) determination in the Baltic seawater samples by ICP MS after preconcentration on SGX C18 modified with DDTC[J], Talanta,2013,112(15):73-79. [11] R. Chester, M. Sohail, M. I. Ogden,etal. A calixarene-based ion-selective electrode for thallium(I) detection[J], Analytica Chimica Acta,2014,851(3):78-86. [12] J.K. Bhadra, P. Bandyopadhyay, B.K. Sen. Polarographic determination of thallium[J], Analytica Chimica Acta,1974,68˄2˅˖475-479. [13] M. A. Mottaleb, Y. A. Woo, H. J. Kim. Evaluation of open-air type electrolyte-as-cathode glow discharge-atomic emission spectrometry for determination of trace heavy metals in liquid samples[J], Microchemical Journalˈ2001,69 (3):219-230. [14] T. Cserfalvi ˈ P. Mezei. Subnanogram sensitive multimetal detector with atmospheric electrolyte cathode glow discharge[J], Journal of Analytical Atomic Spectrometryˈ2003, 18 (6)˖596–602 [15] M. R. Webb, F. J. Andrade, G. M. Hieftje. Compact Glow Discharge for the Elemental Analysis of Aqueous Samples[J], Analytical Chemistryˈ2007, 79 (20)˖7899-7905. [16] Y. Kohara, Y. Terui, M. Ichikawaˈetal. Characteristics of liquid electrode plasma for atomic emission spectrometry[J], Journal of Analytical Atomic Spectrometryˈ 2012, 27 (27)˖1457– 1464. [17] J. Wu, J. Yu, J. Liˈetal. Detection of metal ions by atomic emission spectroscopy from
13
liquid-electrode discharge plasma[J], Spectrochimica Acta Part Bˈ2007ˈ62 (11)˖1269-1272. [18] R. Manjusha, M. A. Reddy, R. ShekharˈSunil Jaikumar. Determination of major to trace level elements in Zircaloys by electrolyte cathode discharge atomic emission spectrometry using formic acid[J], Journal of Analytical Atomic Spectrometryˈ2013, 28(12)˖1932–1939. [19] T. A. Doroski, M. R. Webb. Signal enhancement in solution-cathode glow discharge-optical emission spectrometry via low molecular weight organic compounds[J], Spectrochimica Acta Part B,2013, 88 : 40̢45. [20] R. Shekhar, Improvement of sensitivity of electrolyte cathode discharge atomic emission spectrometry
(ELCAD-AES)
for
mercury
using
acetic
acidmedium[J],
Talanta,
2012,93:32-36. [21] Q. Xiao, Z. Zhu, H. Zheng, etc. Significant sensitivity improvement of alternating current driven-liquid discharge by using formic acid medium for optical determination of elements[J], Talanta, 2013, 106 :144-149. [22] W Zhang, T S FisherˈS V J Garimella. Simulation of ion generation and breakdown in atmospheric air[J] Journal of Applied Physicsˈ2004ˈ96˄11˅:6066-6072 . [23] Y. S. Park, S. H. Ku, S. H. Hongˈetal. Fundamental studies of electrolyte-as-cathode glow discharge-atomic emission spectrometry for the determination of trace metals in flowing water[J], Spectrochimica Acta Part Bˈ1998,53 (6)˖1167–1179. [24] T. Cserfalvi ˈP. Mezei. Direct solution analysis by glow discharge: electrolyte-cathode discharge spectrometry[J], Journal of Analytical Atomic Spectrometryˈ1994, 9(3):345-349. [25] Z. Wang, A. J. Schwartz, S. J. Rayˈetal. Determination of trace sodium, lithium, magnesium, and
potassium
impurities
in
colloidal
silica
by
slurry
introduction
into
an
atmospheric-pressure solution-cathode glow discharge[J], Journal of Analytical Atomic Spectrometryˈ2012, 28(2):234-240. Highlights l
Field monitoring of thallium using a portable homemade solution cathode glow discharger is proposed.
l
Thallium is sensitively determined with no other reagents but only dilute acidˈ which is environmental-friendly.
l
The proposed method is rapid as only 1.5 min is needed for the single determination.
l
The reproducibility and precision are favorable due to the less interference.
14
PII: DOI: Reference:
S0039-9140(17)30599-4 http://dx.doi.org/10.1016/j.talanta.2017.05.073 TAL17607
To appear in: Talanta Received date: 13 March 2017 Revised date: 17 May 2017 Accepted date: 25 May 2017 Cite this article as: Wenchuan Zu, Yu Wang, Xiaotao Yang and Cong Liu, A portable solution cathode glow discharge-atomic emission spectrometer for the rapid determination of thallium in water samples, Talanta, http://dx.doi.org/10.1016/j.talanta.2017.05.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A portable solution cathode glow discharge-atomic emission spectrometer for the rapid determination of thallium in water samples Wenchuan Zu1,2*ˈ Yu Wang1,2*ˈ Xiaotao Yang1,2ˈ Cong Liu1,2 1 2
Beijing center for physical & chemical analysis, Beijing, China, 100089
Beijing Academy of Science and Technology key laboratory of Analysis and Testing Technology, Beijing, China,
100089
[email protected]˄W. Zu˅ [email protected]˄Y. Wang˅ *Corresponding author.
Abstract A novel and high performance method for rapid determination of thallium in water samples was established by using a portable solution cathode glow discharger with a fiber-optical spectrometer. The operating conditions including the solution acidity, the electrolyte cathode, etc were optimized with 1.0 mg.L-1 thallium standard solution. The resolution of the fiber-optical spectrometer investigated with the peak width at half height of thallium was tested to be about 1.8 nmˈand thallium was determined at the emission wavelength of 535.0 nm. The caliberation curve was favorably linear when the concentrations of thallium standard solutions were in the range of 0.1 mg.L-1 ~ 5.0 mg.L-1. Under the optimized conditions, the limit of detection (LOD) for thallium was 11.8 ng.mL-1ˈ and the precision evaluated by relative standard deviation was 3.2 % for six times 1.0 mg.L-1 standard solution replicates. This method was used for detection of thallium in water samples. The results were satisfying, and the average recoveries for thallium spiked samples were found to be in the range of 91.3 %~ 107.5%ˈwhich showed this method was applicable for real samples analysis.
Besidesˈthis method is suitable for field tests due to the portable instrumental size and weight as well as the less consumption of time and reagent. Graphical abstract
1
Keywords: Solution cathode glow discharge; Thallium; Water; Portable atomic emission spectrometer 1 introduction Thalliumˈwhich is well-known as an extremely toxic heavy-metal elementˈis in a low-level content within the earth's crust[1]. Howeverˈwith the development of mining and metal smeltingˈthe release of thallium into the atmosphere is increasing and regional water thallium pollutions tend to be serious[2-4]. Besidesˈthallium can be remarkably accumulated in human tissues and organs by the food chainˈand when the income of thallium exceeds a certain allowable contentˈgreat harms could be caused[5]. Therefore, it makes great sense to field and fast determine thallium in environmental water samples as the monitoring of the thallium pollution level is realized, and the thallium-poisoning risk can be decreased effectively as well. At the present time, the determination of Tl (elemental symbol for thallium) is usually carried out by atomic spectrometry methods, such as atomic absorption spectrometry˄AAS˅[6-8]ˈinductively coupled plasma-atomic emission spectrometry (ICP-AES)
[9]
, etc. Although the sensitivity is ideal for the above meansˈmatrix
interference is serious and the analytic process is time-consuming for the graphite furnace˄GF˅-AAS method and spectral interference is prominent for the ICP-AES method due to the complicate emission spectral lines. In some works, the applications of inductively coupled plasma-mass spectrometry (ICP-MS) was used to improve the analytic performance[10], yet the instrumental operation is complicated and high-cost. Moreover, all the techniques above can’t be portable so that the demand of field test can’t be satisfied. Moreover, certain simple-instrumental electrochemical methods, such as anodic-stripping voltammetry, polarographic analysis etc, were reported in former study to detect Tl and showed good detection limits and precision[11,
12]
.
However, matrix effects were serious, and the electrodes were easily to be polluted and a tedious polishing treatment was demanded. Solution cathode glow discharge (SCGD) atomic emission spectrometry has already reported as a novel technique to determine heavy-metal elements, including Pb, Cd, CuˈHgˈetc[13-18]. and some methods were reported to improve the analytical
2
sensitivity as well[19-21] .However, the determination of Tl has not been involved. In the SCGD system, metal electrodes are used as the anode while sample solution is employed as cathode. When a high voltage is applied, gas between the two electrodes will be ionized and the plasma will be generated. During the glow discharge process, the cathode solution is continuous gasified. As a result, the metal ions dissolved in the solution cathode also enters the plasma and was excited so that the emission spectrum comes into being[22]. In our work, the method for rapid detecting thallium by a home-made solution cathode glow discharger coupled with a fiber-optical spectrometer was established. The working conditions and parameters influencing the analytic performance were studied ˈ after which the analysis capabilities were validated. The detailed spectra information of the interference could be acquired by an inside CCD detector of a fiber-optical spectrometer. This method is feasible for the rapid thallium determination in water samples. Besides, the SCGD apparatus we used is portable so that the field analysis is realized. What’s moreˈonly less dilute acid is needed for the test so that more chemical reagents could be saved, for which it’s more environmental-friendly as well. 2 Experimental 2.1 Reagents All the acids were of guarantee grade (G.R., from Beijing Fine Chemicals Ltd.), Ultra pure water (>18.2 MΩ) was obtained from a Millipore ultra pure water system. Thallium standard stock solution (ICP-60N-1, 1000 mg.L-1) was acquired from J&K Scientific Ltd. Calibration solutions ( 0.1 μg.mL-1~ 5.0 μg.mL-1) were prepared by appropriate dilution of the standard stock solution with 1%(v/v) hydrochloric acid. Besides, all the glassy wares were immersed in the 10% nitrous acid for 24 hours and washed clean with ultra pure water. 2.2 Sample preparation The water samples were directly determined only after an acidification treatment if the samples are clean. To say exactlyˈ1mL HNO3 is added into each 100 mL water sample to keep the same acidity with the standard solutions. As for the muddy samplesˈthe acidification treatment is accomplished after an filtration process using 0.45 m filter membranes-aquo system. And the blanks and spiked samples were
3
prepared in parallel. 2.3 Apparutus In the SCGD-atomic emission spectrometry system ( as Fig .1), a home-made solution cathode glow discharge cell was employed in our study. Simply, the anode is a tungsten pin (20 mm long and 1 mm diameter) housed in the upper partˈand a solution cathode is placed in the lower partˈwhich consists of a quartz glass capillary inserted into a graphite cylinder which in turn was fixed to a PTFE reservoir for the overflowing solution. The gap of the two parts were applied a voltage at the level of 700 V., which was transferred and amplified from a 12V output DC power source ˄HF150W-S-12, Hengfu CorporationˈChina˅.The sample solution was pumped into the glass capillary by a peristaltic pumpˈand the liquid waste was discharged out of the electrolyte reservoir through a specified pipeline. The glow discharge emission spectrometry is observed by a high resolution fiber-optical spectrometer˄HR400, Ocean Optics, Inc˅. The whole set of the instrument is portable.
Fig.1 The configuration of the SCGD-atomic emission spectrometry device
In our work, the optimal working parameters adopted for the SCGD-AES were as follows in Tab.1. 4
Tab. 1 Working parameters of SCGD-AES SCGD-AES parameters
Setting
The cathode glow discharge current /mA
35
The inner size of the glass capillary /mm
0.3h0.3
The flow velocity of the electrolyte cathode /mL.min-1 The detecting wavelength of Tl emission line /nm
535.0
The integral time for the fiber-optical spectrometer /ms 3
4
100
Results and discussion
3.1 Optimization of operating conditions 3.1.1 The solution acidity It has ever been reported that the discharge of SCGD-AES strongly depends on the pH of the solution due to the association between pH and cathode fall [23]. T. Cserfalvi etal [24] observed the cathode fall as a function with pH and found that it was almost constant when the pH range was between 4-8, but significantly decreased when the pH was dropped to be below 4. The emission intensity is increased as the cathode fall decreases. In our work, the influence of electrolyte cathode acidity on Tl analytical sensitivity was examined using HNO3 medium. Generally, a series of 1.0 mg/L Tl standard solutions with different acidity were used to observe the emission intensity, including 0.5%-2 %(v/v) HNO3. The results were showed in Fig.2. The sensitivity was improved with the increase of the acidity, which could be explained by the formulas[14] reported by T. Cserfalvi below.
Uef
=
1 1 · ln(1 + ) h g
g = 0.526 aH + 5
( pH<2.5 )
Uef stands for the cathode fall and η is the count of ion pairs caused by cathode fall. αH+ is the concentration of H+. Therefore, in our work( pH<2.5 ), when the concentration of H+ increased, the value of γ got larger and the cathode fall was decreased. Thusˈthe emission intensity was increased. Although the sensitivity was better when the acidity was increased, the generation of the discharging became difficult and the glow tended to be unstable when the acidity was above 1%. Therefore, taking the factors of electrolyte cathode glow stability and facility of the glow discharging generation except for the sensitivity into consideration, 1% HNO3 (pH=0.85) was used in our following work.
(PLVVLRQLQWHQVLW\
1LWULFDFLGFRQFHQWUDWLRQ
Fig.2 The effect of electrolyte cathode acidity on Tl detection sensitivity
3.1.2 The validation of electrolyte cathode Four kinds of common inorganic acids, i.e. HCl, HNO3, H2SO4 and H3PO4with the equal pH as 1% HNO3 (pH=0.85) were used as the acid medium for 1.0 mg/L Tl standard solution. The emission intensity determined were as showed in Fig.3. The emission intensity was not ideal when HCl and H3PO4 were used, and the glow generation is better when HNO3 is used compared with H2SO4. Considering both the sensitivity and simplicity of the glow discharge syntheticallyˈwe chose HNO3 as the acidic medium.
6
Fig.3 The effect of electrolyte cathode on Tl detection sensitivity 3.1.3 The glow discharge gap The glow discharge gap is separated between the tip of the tungsten anode and the electrolyte solution surface. An ideal glow discharge gap should not only supply a strong emission intensity, but ensure the stability of the glow. As Z.Wang, etal [25] ever reported, when the discharge gap exceeded 3 mm, the emission intensity would become weaker with the increasing of the gap due to the lower discharge potential which decreased the energy available for excitation. Besides, the glow discharge turned to be instable because of the greater susceptibility to air currents. On the other hand, if the discharge gap fell below 3 mm, the emission intensity increased as the analytic target could remain in the plasma for longer time. Therefore, we preferred 3mm glow discharge gap in our further study.
3.2 Analytic characteristic 3.2.1 The resolution capacity and the emission line In our workˈthe atomic spectrometry emission signals were collected and processed by the fiber-optical spectrometerˈwhich consists of the optical splitting system and the CCD detecting system. The resolution capacity for the fiber-optical spectrometer greatly effect the interference performance. Thus ˈ the resolution capacity was inspected using 10 mg /L Tl standard solutions and the spectrum near 535.0 nm was as Fig.4ˈand the resolution calculated in peak width at half height was about 1.8 nm. 7
Fig.4 The peak width at half height test result of 10mg.L-1 Tl at 535.0 nm
Besides, it was interesting that a sub-sensitive emission line at 377.6 nm
was
discovered except for the sensitive line at 535.0 nm for Tl as Fig.5 showed. The emission intensity at 377.6 nm we acquired was about half (47.5% exactly) as that at the 535.0 nm sensitive line, which could be greatly useful for the high concentration Tl determination.
Fig.5 Spectrum of Tl observed by the fiber-optical spectrometer of SCGD-AES
3.2.2 Interference The potential interferences of other ions for thallium determination were examined. 8
All the ions were added into 500 ng.mL-1 thallium standard solution, and the concrete emission spectrum acquired by the high resolution fiber-optical spectrometer was as showed in Fig.6 , no obvious emission peaks appeared close neighbor the thallium emission peak (535.0nm). The recovery results showed that no significant interference was observed.
Fig.6 The interference of co-existing metal elements for Tl determination . The red line was the spectrum for 1.0 mg.L-1 Tl while the black line was 1.0 mg.L-1 Tl with equal concentration of co-existing metal elements ions of Pb, Mn, Ni, Cu, Mg, Co, Cd, Cr, Li and Zn.
3.2.3 Analytic performance 1.0 mL each standard solution was pumped into the cathode glow discharge cell and determined by the high resolution fiber-optical spectrometer continuously from low concentration to high. The emission intensity read by peak height presented a favorable linearity when the concentrations of Tl working solutions were in the range of 0.1 mg.L-1~ 5.0 mg.L-1 (as Tl). The equation of linear regression acquired in this work was following as Fig.7 showed : Ie = 101.9+1404.7²Tl ; R=0.9996, where Ie is
9
the peak height of the emission intensity within the reading time while²Tl means the concentration of Tl solutions. The value of linear correlation coefficient R convincingly confirmed the good linear-ship between the emission intensity and Tl concentrations in the selected range. The limit of detection (LOD) for Tl (calculated as 3SDBLANK/slope; n=11) was 11.8 ng.mL-1and the limit of quantification (LOQ) for Tl (calculated as 10SDBLANK/slope; n=11) was 39.3 ng.mL-1, and the precision evaluated by relative standard deviation (RSD) was 3.2 % for six times replicates of 1.0 mg.L-1 Tl solution as can be seen in Fig.8. . \ [
5
,H
&PJ/
Fig.7 The working curve of Tl
,H
7HVWWLPH
Fig.8 Test results for 6 times replicates of 1.0 mg.L-1 Tl solution 10
3.3 Sample determination by SCGD-AES Three different kinds of real samples were completely disposed beforehand as we did in the sample preparation step above. And then the Tl contents of the standard samples including the blank were determined by the optimized SCGD-AES technique to assess the feasibility of this method for real sample analysis. The results were elaborately recapitulated in Tab.2. Meanwhile, 0.4 mg/L~2.0 mg/L Tl standard solutions were spiked into the samples to validate the availability of the experimental results. The average recoveries were acquired in the range of 91.3 %~ 107.5%ˈand the relative standard deviations for the parallel recovery results were not beyond 5.22% (n=5). It was concluded that the results were credible and the method was applicable for the real sample analysis. Tab.2 The water samples determination and recovery results Sample
Content/ mg.L-1
--a
River water
Tl-containing waste water
0.682
Water leaching liquor of stream sediment
a
--
Add/ mg.L-1
Found/ mg.L-1
Recovery/ %
0.400
0.388
97.0
0.400
0.360
90.0
0.400
0.350
87.5
0.400
0.389
97.3
0.400
0.352
88.0
2.00
2.17
108.5
2.00
2.19
109.5
2.00
2.21
110.5
2.00
2.18
109.0
2.00
2.00
100.0
0.400
0.380
95.0
0.400
0.377
94.3
0.400
0.361
90.3
0.400
0.363
90.8
0.400
0.345
86.3
The value is below LOQ 11
Average
Recovery/ %
RSD b / %
92.0
5.22
107.5
3.96
91.3
3.83
b
RSD values are based on 5 times parallel spiked recovery results for each sample
4
Conclusion
The portable solution cathode glow discharge-atomic emission spectrometer is a novel technique to quantitatively and rapidly determine the trace amount of Tl. It offers the field detection of Tl at trace amount level an ideal alternative due to the high analytic performance and low spectrometric or matrix interference. It just takes about 1.5 min to complete the total analysis process of a sample, and no memory effect is observed due to the adequate washing process. Besides, SCGD-AES instrument is with small size and weight and needs no additional gas cylinders which is essential for other atomic spectrometry techniques so that it’s with approving portability. Furthermore, this method is convenient and practical in virtue of its simple instrument, low cost, low power and small sample consumption. The method can be successfully applied to the fast Tl analysis for the real water samples. The field experiments or test for environmental Tl pollution risk monitoring is realized due to the instrumental portability so that the damage caused by Tl pollution is expected to be greatly decreased.
Acknowledgments The authors are grateful to the support of ( Beijing Municipal Science and Technology Project: Z161100003016008, Z161100003016010). References [1] T. F. Xiao, F. Yang, S. H. Li, etal. Thallium pollution in China: A geo-environmental perspective[J], Science of The Total Environment,2012,421-422(1):51-58. [2] E. Álvarez-Ayuso, V. Otones, A. Murciego,etal. Zinc, cadmium and thallium distribution in soils
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atmospheric-pressure solution-cathode glow discharge[J], Journal of Analytical Atomic Spectrometryˈ2012, 28(2):234-240. Highlights l
Field monitoring of thallium using a portable homemade solution cathode glow discharger is proposed.
l
Thallium is sensitively determined with no other reagents but only dilute acidˈ which is environmental-friendly.
l
The proposed method is rapid as only 1.5 min is needed for the single determination.
l
The reproducibility and precision are favorable due to the less interference.
14