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Simultaneously determination of multi metal elements in water samples by liquid cathode glow discharge-atomic emission spectrometry
Simultaneously determination of multi metal elements in water samples by liquid cathode glow discharge-atomic emission spectrometry Jie Yu, Shuxiu Yang, Duixiong Sun, Quanfang Lu, Jidong Zheng, Xiaomei Zhang, Xing Wang PII: DOI: Reference:
S0026-265X(16)30071-6 doi: 10.1016/j.microc.2016.05.019 MICROC 2490
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
13 April 2016 25 May 2016 25 May 2016
Please cite this article as: Jie Yu, Shuxiu Yang, Duixiong Sun, Quanfang Lu, Jidong Zheng, Xiaomei Zhang, Xing Wang, Simultaneously determination of multi metal elements in water samples by liquid cathode glow discharge-atomic emission spectrometry, Microchemical Journal (2016), doi: 10.1016/j.microc.2016.05.019
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ACCEPTED MANUSCRIPT Submitted to < Microchem. J. > on April 13, 2016; Manuscript No: MICROC-D-16-00275; revised on May 25, 2016
Simultaneously determination of multi metal elements in water
spectrometry
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samples by liquid cathode glow discharge-atomic emission
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Jie Yua,, Shuxiu Yanga, Duixiong Sunb,c, Quanfang Lua,c,, Jidong Zhenga,
a
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Xiaomei Zhanga, Xing Wanga
Key Lab of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of
b
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Chemistry and Chemical Engineering, 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; Editorial Department of the University Journal, Northwest Normal University, Lanzhou 730070,
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c
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China
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Graphical Abstract
E-mail address: [email protected] (J. Yu). Tel.: +86-931-7971533; [email protected] (Q.F. Lu). Tel.: +86-931-7971692.
ACCEPTED MANUSCRIPT Abstract: In this work, a liquid cathode glow discharge-atomic emission spectrometry (LCGD-AES) was constructed for simultaneously determination of K, Na, Ca, Mg and Zn in water samples (tap water, mineral water, Yellow River water
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and waste water), in which the glow discharge plasma was produced between the needle-like Pt anode and electrolyte around a quartz capillary cathode. The effects of
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the supporting electrolyte, discharge voltage and organic additives on emission intensity were investigated in detail. The limits of detection (LOD) of metals were compared
with
those
measured
by
the
closed-type
electrolyte-cathode
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discharge-atomic emission spectrometry (ELCAD-AES). In addition, the measured results of water samples using LCGD were verified by ICP. The results showed that
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the optimal operation conditions are pH=1 HNO3 as supporting electrolyte, addition of 0.15% formic acid and 650 V discharge voltage. The R2 and the RSD are ranged
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from 0.9887 to 0.9990 and 1.10% to 2.19%, respectively. LODs of K, Na, Ca, Mg and
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Zn are 0.02, 0.04, 0.19, 0.04 and 0.05 mg L-1, which are in agreement with the ELCAD-AES, and satisfied the recommended levels of the China standards for
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drinking water quality. The obtained results of Na, K, Ca, Mg and Zn in water samples by LCGD and ICP have certain difference. This result provides an alternative
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analytical method for the determination of metal elements in water samples.
Keywords: liquid cathode glow discharge (LCGD); atomic emission spectrometry (AES); determination; metal elements; water samples
ACCEPTED MANUSCRIPT 1. Introduction The determination of metal elements in real water samples is a very hot topic,
spectrometric
instruments,
such
as
inductively
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high-performance
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especially for environment monitoring and environmental protection. The well-known coupled
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plasma-mass spectrometry (ICP-MS), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and atomic absorption spectrometry (AAS), are widely used to accurately, sensitively and rapidly measure the micro and trace metal elements in
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all kinds of water samples. However, these methods are commonly performed at laboratories, requiring complicated equipment and operation [1]. These defects limit
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their applications mainly to laboratory settings, and hinder their use for real-time and on-line analyses under field conditions [1,2]. To meet the requirements of field arrangement and rapid determination, more compact low-cost micro-plasma emission
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element [3,4].
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source are highly desirable for in situ, real-time and on-line monitoring of metal
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Over the past two decades, electrolyte cathode discharge-atomic emission spectrometry (ELCAD-AES) has received a rapid development [1-5]. It is considered a very promising alternative miniaturized excitation source that possesses potential
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advantages over commercially and analytically successful plasma sources because of its more compact and portable instruments, lower power consumption (<75 W), no inert gas requirement and operation in atmospheric pressure air [6,7]. Also, ELCAD has been successfully employed for the real time and on-line simultaneously detection of multi-elements in aqueous solution [8,9]. For the ELCAD, the solution containing the metal elements is the cathode and above it (about 1-5 mm) a metal rod, i.e., W or Pt, is the anode [5]. When the applied voltage was sufficient high, a glow discharge is produced between the anode and the cathode at atmospheric air pressure. The atomic lines of metals dissolved in water samples appear instantaneously in the spectrum emitted by the ELCAD, in this way the content of metals in water samples can be detected [10,11]. Glow discharge devices are typically used as primary atomic spectrometry atomization/excitation
ACCEPTED MANUSCRIPT sources for direct solid analysis with favorable analytical features [12]. In 1887, the first glow discharge apparatus using solution as an electrode was described by Gubkin. Despite observations of atomic emissions from glow discharge electrolysis throughout
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the 1950s and 1960s, the electrolyte cathode glow discharge (ELCAD) as a glow discharge electrolysis-like system was successfully designed for metal analysis only
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in 1993 [13]. However, the devices and stability of discharge need to be further improved, and moreover, their higher limits of detections (LOD) do not meet the requirements for detection of trace metals in environmental and biological
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samples. In order to improve the analytical performance, enhance the emission efficiency, and increase the sensitivity of determination, many improvements for
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discharge structure of ELCAD have been established based on the original design by Cserfalvi group [13,14], including the solution-cathode glow discharge (SCGD) [15],
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liquid sampling-atmospheric pressure glow discharge (LS-APGD) [16], drop spark
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discharge (DSD) [17], direct current atmospheric pressure glow discharge (DC-APGD) [18], alternating-current electrolyte atmospheric liquid discharge (AC-EALD) [19],
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liquid electrode discharge [20], and so on. In addition, the excitation principle and fundamental feature of ELCAD were studied by many authors [21-23] and also reviewed in recent years [5,10,11].
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In the present work, we modified and designed a system termed as liquid cathode glow discharge-atomic emission spectrometry (LCGD-AES) based on the principle and characteristics of ELCAD, in which the glow discharge plasma was produced between the needle-like Pt anode and electrolyte around a quartz capillary cathode. This improvement of excitation source makes the solutions flow over the top of the capillary to be exhausted at any time by many grooves on the graphite carbon rod, result in conductive state of the circuit, and thereby increase the discharge plasma stability. Moreover, it can operate in air under atmospheric pressure. All these features make it attractive as a field instrument for metal elements analysis and determination. In order to demonstrate the method's feasibility, direct determination of K, Na, Ca, Mg and Zn in water samples (tap water, mineral water, Yellow River water and waste water) by using LCGD-AES was chosen as the subjects. The effects of the supporting
ACCEPTED MANUSCRIPT electrolyte, discharge voltage and addition of organic substances on emission signal were investigated in detail. The limits of detection (LOD) of metal elements were calculated and then compared with closed-type ECLAD-AES techniques. In addition,
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the measured results of water samples using LCGD were verified by ICP.
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2. Experimental 2.1. The setup of LCGD system
voltage power supply,
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The experiment setup of LCGD system is shown in Fig. 1. It contains DC high sample introduction,
glow
discharge system,
and
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spectral detection. The DC high voltage source was a DH 1722-6 power supply (Beijing Dahua radio factory, Beijing, China) providing the voltage of 0-1000 V and the current of 0-0.5 A.
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Water samples were introduced into the LCGD system by a quartz capillary
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(internal diameter 1 mm, external diameter 1.2 mm) with the aid of a peristaltic pump (Beijing Dongnan yicheng Laboratory Equipment Co., Ltd., YZ1515x) and its flow
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rate was controlled at 4.5 mL min-1. To decrease signal fluctuations of glow discharge, several knots were tied in rubber tubing of peristaltic-pump. The discharge system mainly consists of two parts: a pointed platinum anode with a diameter of 1 mm was
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sealed into a quartz tube, while the water samples was pumped through the quartz capillary and flowed over the top of the capillary which in turn was served as the cathode. The quartz capillary was passed through a hole on the graphite rod. The excess part of capillary with graphite carbon rod was 2.5 mm. The graphite rod was fixed on the plug of reservoir. The discharge gap between the tip of the platinum and the top of the quartz capillary was about 2 mm. The solution, which overflowed from the tip of quartz capillary, was flowed into the reservoir by means of many grooves on the graphite rod. When the voltage was sufficient high, the glow discharge plasma was produced between the electrolyte around quartz capillary cathode and needle-like Pt anode. The glow discharge system was mounted on a manual precision translation stage
ACCEPTED MANUSCRIPT with three orthogonal micrometer screw gauges, which could be controlled precisely in the x, y, and z directions to adjust position of the glow discharge plasma, obtain the maximum signal output and focus the discharge image into the entrance slit of
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monochromator (Zolix Instruments Co., Ltd., Omni-λ500) with a 1800 grooves/mm holographic grating. The discharge photographs were taken by ICCD camera of
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Andor technology (MN: DH334T-18U-03; SN: ICCD-05703). The emission spectrometry of discharge plasma was imaged with a quartz lens onto the vertical adjustable entrance slit of the monochromator. A PMTH-S1-CR131 photomultiplier
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tube (PMT) biased at –1000 V was used as the detector. Spectral resolution of the
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spectrometer is 0.05 nm.
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2.2. Reagents and samples
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Insert Figure 1 here
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HNO3, HCl and H2SO4 were of superior-reagent grade. KCl, NaCl, CaCl2, MgCl2·6H2O, ZnCl2·6H2O were all of analytical reagent grade. Deionized water (>18 MΩ cm resistivity) was obtained by a water purification system. Stock solutions
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(1000 mg L-1) of K, Na, Ca, Mg and Zn were prepared with pH=1 HNO3. Working standards were prepared through diluting stock solution and adjusted to pH 1.0 with HNO3. The pH of the solutions was determined with a pH meter. Formic acid, acetic acid and ethanol were of analytical reagent grade, and were added into the working standard solution at volume percentages of 0%, 0.25%, 0.5%, 0.75%, 1% and 1.5% to study the enhancement effect of organic substances. The tap water, mineral water, Yellow River water and waste water were taken directly from Lanzhou Water Works, Nongfu Spring, Yintan bridge of Lanzhou and Dongdagou Valley of Baiyin City, respectively. All data points represent the average values from the 10 successive measurements. The background subtraction was achieved by determination of a blank solution (pH=1 HNO3).
ACCEPTED MANUSCRIPT 3. Results and discussion 3.1. Emission spectra of the LCGD
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In order to demonstrate the capability of the method, the emission spectra of the LCGD for qualitative determination of K, Na, Ca, Mg and Zn in blank solution (pH=1
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HNO3) and waste water samples (adjusted to pH=1.0 with HNO3) are shown in Fig. 2. It is clearly observed from Fig. 2, several molecular emission lines at 283.0 and 308.9 nm ascribed to OH bands (A2Σ+→X2Π) are found [24]. It was reported that the OH
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band shown in this spectrum is attributed to a recombination process of the precursor species of OH+ [25]. Molecular band spectra of N2 ranged from 315-406 nm and
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assigned to the C3 Πu→B3 Πg systems are observed in LCGD because ambient air is used as the discharge gas [26]. In addition, the atomic lines of hydrogen Balmer (α
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and β) are at 656.3 and 486.1 nm. All these lines are consistent even if no metal ions
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are dissolved in the solution. The emission lines of K, Na, Ca, Mg and Zn in waste water samples are recorded at 766.5 or 769.9 nm, 589.0 or 589.6 nm, 422.7 nm, 285.2
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nm and 213.8 nm, respectively [10]. The results suggested that it is feasible to use the
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designed LCGD source to determine metal elements in aqueous solutions.
Insert Figure 2 here
3.2. Optimization of the experimental conditions 3.2.1. Effect of the supporting electrolyte The effect of the different supporting electrolyte on the emission intensity of Zn I (213.8 nm) was investigated. Three Zn standard solutions (1-10 mg L-1) was adjusted to pH=1.0 with HNO3, HCl and H2SO4, respectively, and then introduced into the LCGD system. It is observed that all these acidic mediums can sustain the stability of discharge. The anion illustrates a positive effect on the emission intensity of Zn I with the same H+ in the order: Cl- > NO3- > SO42- (shown in Fig. 3). The result is consistent with earlier reported by Mezei [5,27] and Webb [28].
ACCEPTED MANUSCRIPT When the size of the anion is increased, the mobility (conductivity) of the ions in electrolyte is decreased, and then, the current and power is led to lower [1,27]. Because the size of Cl- is close to NO3-, the change of emission intensity is not
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significant (ionic radius of Cl-, NO3-, and SO42- are 181, 165, and 244 pm,
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respectively). The size of SO42- is larger than that of Cl- and NO3-, accordingly, the
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lower emission intensity is observed. However, HCl and H2SO4 are easy to generate precipitation with several metal ions. Meanwhile, HNO3 is a promising reagent for digestion of water samples, and has good sensitivity and chemical compatibility [29].
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So we selected pH=1 HNO3 as supporting electrolyte mediums.
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Insert Figure 3 here
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3.2.2. Effect of discharge voltage
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There is a voltage threshold (600 V) below which the metal lines could not be
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determined. To optimize the experimental condition, the effects of discharge voltage on the emission intensity of Mg I (285.2 nm) were investigated from 600 to 700 V, as shown in Fig. 4. From 600 to 675 V, an approximately linear increase in analyte
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emission intensity of the Mg with the increasing voltage is found, but then, emission intensity begins to diverge from 675 to 700 V. This is because both the noise and signal increase simultaneously with applied voltage from 600 to 675 V, but the signal increases more rapidly than the noise [2]. However, when the voltages is reached 675 V, the increases both the signal and noise are leveled off [30,31].
Insert Figure 4 here
Fig. 5 shows the discharge images under different applied voltage. As can be seen from Fig. 5, with the increase of voltage, the energy of plasma is raised, and accordingly, the density of excited atoms is increased. As a result, the larger the volume of glow is generated, the stronger the intensity of glow plasma is obtained,
ACCEPTED MANUSCRIPT thereby, resulting in the emission intensity of Mg I (285.2 nm) is increased gradually.
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Insert Figure 5 here
The stability of plasma was detected as a function of time at fixed wavelengths
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[31]. Fig. 6 shows the temporal tracing of the emission intensity of 10 mg L-1 Mg solution in different voltage over 30 mins. Over 675 V, the further increase of voltage can cause increasing fluctuation of the emission intensities because of unstable
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discharge plasma [8]. In addition, if the voltage is too high (>700 V), the quartz capillary of cathode will be molten and destroyed due to the strong glow discharge
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and high energy. What is more, when the discharge voltage is over 675 V, the tip of the Pt wire becomes red-hot and water samples start to boil. When the discharge
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voltage is further increased to 700 V, the anode is completely red-hot and the plasma
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unstability is shown. A similar result was reported by Mottaleb et al [8] and Xiao et al [19]. The results suggested that the stability of the emission signals is realized at 650
Insert Figure 6 here
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V. Thus, 650 V was chosed as the best discharge voltage.
3.2.3. Enhancement effect of organic substances It was reported that the emission intensity of metals in ELCAD could be improved by some low molecular weight organic substances and surfactants [19,32-34]. Accordingly, the effects of ethanol, formic acid and acetic acid on the emission intensity of K I at 766.5 nm (Signal intensity of K at 766.5 nm was heighter than that at 769.9 nm) were also investigated in this work. K solutions of 10 mg L-1 containing different volume ratio (0%, 0.25%, 0.5%, 0.75%, 1% and 1.5%) of organic substances as well as their corresponding solutions without organic substances were adjusted to pH 1.0 with HNO3. It is observed from Fig. 7 that three organic substances can enhance the emission intensity of K and the enhancement effect is in the order:
ACCEPTED MANUSCRIPT HCOOH > CH3COOH > CH3CH2OH. This result was in agreement with the study of Ag, Cd and Pb by AC-EALD [19,33]. In addition, the enhancement effects of these organic substances at various volume ratios were also investiaged, and the results
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indicated that 0.5% HCOOH is shown to present the maximum enhancement for K. Because HCOOH produced large singal enhancement factors for several metals and
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did not suppress emission for any metals [33], therefore, 0.5% HCOOH was chosen for further experiments. It was reported that enhancement mechanism of addition of HCOOH can decrease the surface tension of water samples, which can promote the
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generation of small droplets and favor the evaporation rate of analytes into the plasma
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zone for atomization and excitation [7,35].
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Insert Figure 7 here
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As shown in Fig. 8, linear fitting are obtained with 0.5% HCOOH and without HCOOH for K solution in the range of 0-25 mg L-1 and the square regression
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coefficients (R2) are all close to 1. The limits of detection (LOD), using the definition 3σ/s (σ is the standard deviation corresponding to 10 blank measurements and s is the slope of the calibration graph) [36,37], are listed in Table 1. Obviously, the LOD
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value of K is lower with 0.5% HCOOH (0.02 mg L-1) than that without HCOOH (0.07 mg L-1). The values of relative standard deviation (RSD) with 0.5% HCOOH and without HCOOH are 1.10% and 0.94 %, respectively, suggesting this method has a high precision.
Insert Figure 8 here Insert Table 1 here
3.3. Analytical performance The analytical performance of LCGD was evaluated under optimal operating parameters (supporting electrolytes: pH=1.0 HNO3, added with 0.5% HCOOH,
ACCEPTED MANUSCRIPT discharge voltage: 650 V, flow rate: 4.5 mL min-1, inter-electrode gap: 2 mm). Standard solutions of K, Na, Ca, Mg ranged from 5 to 30 mg L-1 and Zn ranged from 1 to 10 mg L-1 were prepared and established calibration curves. The results showed
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that all calibration curves have a good linear relationship. The linear equation, LOD,
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sensitivity, R2 and RSD are listed in Table 2. It is obvious that the R2 and the RSD are
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ranged from 0.9887 to 0.9990 and 1.10% to 2.19%, respectively. The results suggested that LCGD can be employed for the quantitative determination of metal
0.04 and 0.05 mg L-1, respectively.
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elements in water samples. The LODs of K, Na, Ca, Mg and Zn are 0.02, 0.04, 0.19,
comparison
of
the
LODs
obtained
by
other
electrolyte-cathode
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A
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Insert Table 2 here
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discharge-atomic emission spectrometry (ECLAD-AES) techniques is listed in Table 3. Obviously, the LODs for LCGD are found to be comparable to those of similar
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systems [14,18,20,23,38]. In addition, the LOD value of Zn in this work is lower than that standard for drinking water quality [39], so this method can be used to detect the
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safety of Zn in drinking water quality.
Insert Table 3 here
3.4. Analytical application of water samples and validation To demonstrate the analytical performance on real samples of the developed method, the LCGD-AES was used to determine the K, Na, Ca, Mg and Zn in water samples (tap water, mineral water, yellow river water and waste water). In addition, LCGD results were also verified by using ICP analysis of the samples. Except for the dilution, adjustment of pH=1 using nitric acid, and addition of 0.5% formic acid, no other sample pretreatment was performed. The determination results of LCGD and ICP are listed in Table 4. It is observed from Table 4, the measured values of Na, K,
ACCEPTED MANUSCRIPT Ca, Mg and Zn by LCGD and ICP have some differences. As a new method, this is an acceptable result. It suggested that LCGD can be employed for simultaneously
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Insert Table 4 here
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determination of multi metal elements in complex water samples.
4. Conclusions
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The liquid cathode glow discharge-atomic emission spectrometry (LCGD-AES) was established and successfully employed to measurement of K, Na, Ca, Mg and Zn
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in water samples, such as tap water, mineral water, Yellow River water and waste water. The optimization analytical conditions of LCGD-AES for detection of water
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samples were pH=1 HNO3 as electrolyte, addition of 0.15% formic acid and 650 V
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voltage. The LODs of K, Na, Ca, Mg and Zn were 0.02, 0.04, 0.19, 0.04 and 0.05 mg L-1, respectively. The LODs of metals in this work agreed well with the closed-type
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ECLAD-AES, and satisfied the recommended levels of the China standards for drinking water quality. Compared with ICP-MS, ICP-AES and AAS, LCGD-AES has some features, such as lower cost in set-up, portable equipment, convenient operation
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and easy design. In addition, it is very easy in achieving real-time and on-line analysis for samples. All the results suggested that LCGD-AES is a very promising technique for highly efficient determination of metal elements in aqueous solution.
Acknowledgments This work was supported in part by National Natural Science Foundation of China (No. 21567025, 21367023 and 11564037), and Natural Science Foundation of Gansu Province (Nos. 1308RJZA144 and 1208RJZA161), China.
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atomic emission spectrometry, J. Anal. At. Spectrom. 28 (2013): 234-240. [31] R. Shekhar, D. Karunasagar, M. Ranjit, J. Arunachalam, Determination of
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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.
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[32] R. Shekhar, Improvement of sensitivity of electrolyte cathode discharge atomic emission spectrometry (ELCAD-AES) for mercury using acetic acid medium,
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Talanta 93 (2012) 32-36.
[33] T.A. Doroski, M.R. Webb, Signal enhancement in solution-cathode glow
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discharge-optical emission spectrometry via low molecular weight organic
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compounds, Spectrochim. Acta B 88 (2013) 40-45. [34] Z. Zhang, Z. Wang, Q. Li, H. Zou, Y. Shi, Determination of trace heavy metals and
biological
samples
by
solution
cathode
glow
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inenvironmental
discharge-atomic emission spectrometry and addition of ionic surfactants for improved sensitivity, Talanta 119 (2014) 613-619.
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[35] R.Shekhar, K. Madhavi, N. N. Meeravali, S.J. Kumar, Determination of thallium at trace levels by electrolyte cathode discharge atomic emission spectrometry with improved sensitivity, Anal. Methods 6 (2014)732-740. [36] Z. Liu, Z. Zhu, Q. Wu, S. Hu, H. Zheng, Dielectric barrier discharge-plasma induced vaporization and its application to the determination of mercury by atomic fluorescence spectrometry, Analyst 136 (2011)4539-4544. [37] 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. [38] A. Shaltout, Micro plasma generation using liquid sampling-atmospheric pressure glow discharge, Microchim. Acta 155 (2006) 447-452. [39] National Standards of the People's Republic of China, Standards for Drinking
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Water Quality (GB5749-2006), Standards Press of China, Beijing, 2007.
ACCEPTED MANUSCRIPT Figure captions
Fig. 1. Schematic diagram of the LCGD.
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Fig. 2. Emission spectra of the LCGD in blank solution (a) and waste water samples (b) (blank solution: adjusted to pH 1.0 with HNO3, waste water samples: adjusted
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to pH 1.0 with HNO3, discharge voltage: 650 V, inter-electrode gap: 2 mm, flow rate: 4.5 mL min-1).
Fig. 3. Comparison of the Zn I (213.8 nm) emission intensity in different supporting
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electrolyte mediums (zinc concentration: 0-10 mg L-1 adjusted to pH 1.0 with HNO3, discharge voltage: 650 V, inter-electrode gap: 2 mm, flow rate: 4.5 mL
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min-1).
Fig. 4. Optimization of discharge voltage (magnesium concentration: 10 mg L-1
).
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-1
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adjusted to pH 1.0 with HNO3, inter-electrode gap: 2 mm, flow rate: 4.5 mL min
Fig. 5. Images of the LCGD at different discharge voltage (magnesium concentration:
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10 mg L-1 adjusted to pH 1.0 with HNO3, inter-electrode gap: 2 mm, flow rate: 4.5 mL min -1).
Fig. 6. Stability of emission intensity as a function of time of LCGD (magnesium
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concentration: 10 mg L-1 adjusted to pH 1.0 with HNO3, inter-electrode gap: 2 mm, flow rate: 4.5 mL min -1). Fig. 7. Enhancement effect of different organic substances (formic acid, acetic acid and ethanol) on the emission intensity of K I (766.5 nm) (potassium concentration: 10 mg L-1 adjusted to pH 1.0 with HNO3, discharge voltage: 650 V, inter-electrode gap: 2 mm, flow rate: 4.5 mL min -1). Fig. 8. Calibration curve of the emission intensity versus concentration of potassium solution without HCOOH and with 0.5% HCOOH (potassium concentration: 10 mg L-1 adjusted to pH 1.0 with HNO3, discharge voltage: 650 V, inter-electrode gap: 2 mm, flow rate: 4.5 mL min -1).
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Figures
Fig. 1
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Fig. 2
HNO3 HCl H2SO4
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2
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1
0 2
4 6 8 -1 Concentration (mg L )
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0
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3
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4
Emission intensity (x10 Counts)
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Fig. 3
10
1.8
V V V V V
1.6
284.8
285.0
285.2
285.4
285.6
W avelength (nm )
-1
10 mg L Mg
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1.4
600
620
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1.2 1.0
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600 625 650 675 700
6
Emission intensity (x10 Counts)
2.0
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640
660
680
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Discharge voltage (V)
Fig. 4
700
625 V
650 V
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600 V
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Fig. 5
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675 V
700 V
2.0
700 V
1.8
1.4
625 V
1.2
600 V
200 400 600 800 1000 1200 1400 1600 1800 Time (s)
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0
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1.0
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Fig. 6
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650 V
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1.6
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675 V
6
Emission intensity (x10 Counts)
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HCOOH CH3COOH
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7.8
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CH3CH2OH
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7.6 7.4
7.0 6.8
0.0
0.2
0.4
0.6
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7.2
0.8
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4
Emission intensity (x10 Counts)
8.0
1.0
1.2
Organic content (%)
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Fig. 7
1.4
1.6
Without HCOOH With 0.5% HCOOH
4
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2 1
5
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5
Emission intensity (x10 Counts)
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10
15
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Concentration (mg L )
Fig. 8
25
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Tables
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Table 1. Analytical performance of LCGD-AES in K solution with 0.5% HCOOH and without HCOOH.
LOD (mg L-1)
R2
RSD (%)*
y=2400+4964x
4964
0.07
0.9997
0.94
y=41122+17645x
17645
0.02
0.9932
1.10
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S (Counts mg-1 L)
Linear equation
Without HCOOH With 0.5% HCOOH
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K
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* Standard concentration: 10 mg L-1, n = 10.
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K
766.5
Na
589.0
Mg Zn
S(Counts mg-1 L)
LOD (mg L-1)
R2
RSD* (%)
y=41122+17645x
17645
0.02
0.9932
1.10
y=102395+30923x
30923
0.04
0.9887
1.65
422.7
y=2311+684x
684
0.19
0.9971
2.19
285.2
y=62611+20619x
20619
0.04
0.9937
2.06
213.8
y=783+1587x
1587
0.05
0.9990
1.35
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Ca
Linear equation
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Wavelength (nm)
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Analytical element
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solution with 0.5% HCOOH
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Table 2 Analytical performance of LCGD-AES in K, Na, Ca, Mg and Zn standard
* Standard concentration: 10 mg L-1, n = 10.
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Table 3 Comparison of the LODs obtained by LCGD-AES system with other ELCAD-AES systems for the detection of Na, K, Ca, Mg, Zn and standards for
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drinking water quality.
LOD (mg L-1)
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Methods LCGD-AES
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ELCAD
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DC-APGD
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CD SCGD LS-APGD Standards for Drinking Water Quality
Reference
K
Na
Ca
Mg
Zn
0.02
0.04
0.19
0.04
0.05
This work
0.2
0.06
0.4
0.8
0.1
[14]
0.001
0.001
0.04
0.008
0.04
[18]
0.013 -
0.001 -
1 0.023 0.3 -
1 0.019 -
0.042 0.7 1
[20] [23] [38] [39]
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Table 4. Measurement results of K, Na, Ca, Mg and Zn in real water samples by LCGD-AES and ICP
144.2
Na
1001.0
K
12.5
Mg
58.3
Zn
1.6
Ca Na
ICP (mg L-1)
Element
This work (mg L-1)
ICP (mg L-1)
Ca
6.7
30.8
Na
0.3
2.2
K
0.7
1.1
13.0
Mg
0.6
0.2
0.2
Zn
--
0.006
Ca
37.5
70.9
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Ca
533.2
1045.3
Water Sample
Mineral water
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25.9
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Yellow river water
This work (mg L-1)
57.5
76.7
19.5
28.3
Na
19.7
29.2
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Waste water
Element
Tap water
K
1.6
2.2
K
1.7
2.1
Mg
16.1
1.9
Mg
16.8
1.9
Zn
--
0.003
Zn
--
0.005
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Water Sample
ACCEPTED MANUSCRIPT Highlights ►
The
liquid
cathode
glow
discharge-atomic
emission
spectrometry
(LCGD-AES) was constructed.
additives on emission intensity were investigated.
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► The effects of the supporting electrolyte, discharge voltage and organic
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► A multi metals simultaneously analyzer of water samples was achieved by
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LCGD-AES.
S0026-265X(16)30071-6 doi: 10.1016/j.microc.2016.05.019 MICROC 2490
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
13 April 2016 25 May 2016 25 May 2016
Please cite this article as: Jie Yu, Shuxiu Yang, Duixiong Sun, Quanfang Lu, Jidong Zheng, Xiaomei Zhang, Xing Wang, Simultaneously determination of multi metal elements in water samples by liquid cathode glow discharge-atomic emission spectrometry, Microchemical Journal (2016), doi: 10.1016/j.microc.2016.05.019
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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT Submitted to < Microchem. J. > on April 13, 2016; Manuscript No: MICROC-D-16-00275; revised on May 25, 2016
Simultaneously determination of multi metal elements in water
spectrometry
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samples by liquid cathode glow discharge-atomic emission
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Jie Yua,, Shuxiu Yanga, Duixiong Sunb,c, Quanfang Lua,c,, Jidong Zhenga,
a
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Xiaomei Zhanga, Xing Wanga
Key Lab of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of
b
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Chemistry and Chemical Engineering, 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; Editorial Department of the University Journal, Northwest Normal University, Lanzhou 730070,
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c
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China
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Graphical Abstract
E-mail address: [email protected] (J. Yu). Tel.: +86-931-7971533; [email protected] (Q.F. Lu). Tel.: +86-931-7971692.
ACCEPTED MANUSCRIPT Abstract: In this work, a liquid cathode glow discharge-atomic emission spectrometry (LCGD-AES) was constructed for simultaneously determination of K, Na, Ca, Mg and Zn in water samples (tap water, mineral water, Yellow River water
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and waste water), in which the glow discharge plasma was produced between the needle-like Pt anode and electrolyte around a quartz capillary cathode. The effects of
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the supporting electrolyte, discharge voltage and organic additives on emission intensity were investigated in detail. The limits of detection (LOD) of metals were compared
with
those
measured
by
the
closed-type
electrolyte-cathode
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discharge-atomic emission spectrometry (ELCAD-AES). In addition, the measured results of water samples using LCGD were verified by ICP. The results showed that
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the optimal operation conditions are pH=1 HNO3 as supporting electrolyte, addition of 0.15% formic acid and 650 V discharge voltage. The R2 and the RSD are ranged
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from 0.9887 to 0.9990 and 1.10% to 2.19%, respectively. LODs of K, Na, Ca, Mg and
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Zn are 0.02, 0.04, 0.19, 0.04 and 0.05 mg L-1, which are in agreement with the ELCAD-AES, and satisfied the recommended levels of the China standards for
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drinking water quality. The obtained results of Na, K, Ca, Mg and Zn in water samples by LCGD and ICP have certain difference. This result provides an alternative
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analytical method for the determination of metal elements in water samples.
Keywords: liquid cathode glow discharge (LCGD); atomic emission spectrometry (AES); determination; metal elements; water samples
ACCEPTED MANUSCRIPT 1. Introduction The determination of metal elements in real water samples is a very hot topic,
spectrometric
instruments,
such
as
inductively
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high-performance
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especially for environment monitoring and environmental protection. The well-known coupled
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plasma-mass spectrometry (ICP-MS), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and atomic absorption spectrometry (AAS), are widely used to accurately, sensitively and rapidly measure the micro and trace metal elements in
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all kinds of water samples. However, these methods are commonly performed at laboratories, requiring complicated equipment and operation [1]. These defects limit
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their applications mainly to laboratory settings, and hinder their use for real-time and on-line analyses under field conditions [1,2]. To meet the requirements of field arrangement and rapid determination, more compact low-cost micro-plasma emission
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element [3,4].
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source are highly desirable for in situ, real-time and on-line monitoring of metal
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Over the past two decades, electrolyte cathode discharge-atomic emission spectrometry (ELCAD-AES) has received a rapid development [1-5]. It is considered a very promising alternative miniaturized excitation source that possesses potential
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advantages over commercially and analytically successful plasma sources because of its more compact and portable instruments, lower power consumption (<75 W), no inert gas requirement and operation in atmospheric pressure air [6,7]. Also, ELCAD has been successfully employed for the real time and on-line simultaneously detection of multi-elements in aqueous solution [8,9]. For the ELCAD, the solution containing the metal elements is the cathode and above it (about 1-5 mm) a metal rod, i.e., W or Pt, is the anode [5]. When the applied voltage was sufficient high, a glow discharge is produced between the anode and the cathode at atmospheric air pressure. The atomic lines of metals dissolved in water samples appear instantaneously in the spectrum emitted by the ELCAD, in this way the content of metals in water samples can be detected [10,11]. Glow discharge devices are typically used as primary atomic spectrometry atomization/excitation
ACCEPTED MANUSCRIPT sources for direct solid analysis with favorable analytical features [12]. In 1887, the first glow discharge apparatus using solution as an electrode was described by Gubkin. Despite observations of atomic emissions from glow discharge electrolysis throughout
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the 1950s and 1960s, the electrolyte cathode glow discharge (ELCAD) as a glow discharge electrolysis-like system was successfully designed for metal analysis only
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in 1993 [13]. However, the devices and stability of discharge need to be further improved, and moreover, their higher limits of detections (LOD) do not meet the requirements for detection of trace metals in environmental and biological
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samples. In order to improve the analytical performance, enhance the emission efficiency, and increase the sensitivity of determination, many improvements for
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discharge structure of ELCAD have been established based on the original design by Cserfalvi group [13,14], including the solution-cathode glow discharge (SCGD) [15],
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liquid sampling-atmospheric pressure glow discharge (LS-APGD) [16], drop spark
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discharge (DSD) [17], direct current atmospheric pressure glow discharge (DC-APGD) [18], alternating-current electrolyte atmospheric liquid discharge (AC-EALD) [19],
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liquid electrode discharge [20], and so on. In addition, the excitation principle and fundamental feature of ELCAD were studied by many authors [21-23] and also reviewed in recent years [5,10,11].
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In the present work, we modified and designed a system termed as liquid cathode glow discharge-atomic emission spectrometry (LCGD-AES) based on the principle and characteristics of ELCAD, in which the glow discharge plasma was produced between the needle-like Pt anode and electrolyte around a quartz capillary cathode. This improvement of excitation source makes the solutions flow over the top of the capillary to be exhausted at any time by many grooves on the graphite carbon rod, result in conductive state of the circuit, and thereby increase the discharge plasma stability. Moreover, it can operate in air under atmospheric pressure. All these features make it attractive as a field instrument for metal elements analysis and determination. In order to demonstrate the method's feasibility, direct determination of K, Na, Ca, Mg and Zn in water samples (tap water, mineral water, Yellow River water and waste water) by using LCGD-AES was chosen as the subjects. The effects of the supporting
ACCEPTED MANUSCRIPT electrolyte, discharge voltage and addition of organic substances on emission signal were investigated in detail. The limits of detection (LOD) of metal elements were calculated and then compared with closed-type ECLAD-AES techniques. In addition,
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the measured results of water samples using LCGD were verified by ICP.
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2. Experimental 2.1. The setup of LCGD system
voltage power supply,
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The experiment setup of LCGD system is shown in Fig. 1. It contains DC high sample introduction,
glow
discharge system,
and
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spectral detection. The DC high voltage source was a DH 1722-6 power supply (Beijing Dahua radio factory, Beijing, China) providing the voltage of 0-1000 V and the current of 0-0.5 A.
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Water samples were introduced into the LCGD system by a quartz capillary
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(internal diameter 1 mm, external diameter 1.2 mm) with the aid of a peristaltic pump (Beijing Dongnan yicheng Laboratory Equipment Co., Ltd., YZ1515x) and its flow
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rate was controlled at 4.5 mL min-1. To decrease signal fluctuations of glow discharge, several knots were tied in rubber tubing of peristaltic-pump. The discharge system mainly consists of two parts: a pointed platinum anode with a diameter of 1 mm was
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sealed into a quartz tube, while the water samples was pumped through the quartz capillary and flowed over the top of the capillary which in turn was served as the cathode. The quartz capillary was passed through a hole on the graphite rod. The excess part of capillary with graphite carbon rod was 2.5 mm. The graphite rod was fixed on the plug of reservoir. The discharge gap between the tip of the platinum and the top of the quartz capillary was about 2 mm. The solution, which overflowed from the tip of quartz capillary, was flowed into the reservoir by means of many grooves on the graphite rod. When the voltage was sufficient high, the glow discharge plasma was produced between the electrolyte around quartz capillary cathode and needle-like Pt anode. The glow discharge system was mounted on a manual precision translation stage
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monochromator (Zolix Instruments Co., Ltd., Omni-λ500) with a 1800 grooves/mm holographic grating. The discharge photographs were taken by ICCD camera of
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Andor technology (MN: DH334T-18U-03; SN: ICCD-05703). The emission spectrometry of discharge plasma was imaged with a quartz lens onto the vertical adjustable entrance slit of the monochromator. A PMTH-S1-CR131 photomultiplier
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tube (PMT) biased at –1000 V was used as the detector. Spectral resolution of the
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spectrometer is 0.05 nm.
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2.2. Reagents and samples
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Insert Figure 1 here
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HNO3, HCl and H2SO4 were of superior-reagent grade. KCl, NaCl, CaCl2, MgCl2·6H2O, ZnCl2·6H2O were all of analytical reagent grade. Deionized water (>18 MΩ cm resistivity) was obtained by a water purification system. Stock solutions
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(1000 mg L-1) of K, Na, Ca, Mg and Zn were prepared with pH=1 HNO3. Working standards were prepared through diluting stock solution and adjusted to pH 1.0 with HNO3. The pH of the solutions was determined with a pH meter. Formic acid, acetic acid and ethanol were of analytical reagent grade, and were added into the working standard solution at volume percentages of 0%, 0.25%, 0.5%, 0.75%, 1% and 1.5% to study the enhancement effect of organic substances. The tap water, mineral water, Yellow River water and waste water were taken directly from Lanzhou Water Works, Nongfu Spring, Yintan bridge of Lanzhou and Dongdagou Valley of Baiyin City, respectively. All data points represent the average values from the 10 successive measurements. The background subtraction was achieved by determination of a blank solution (pH=1 HNO3).
ACCEPTED MANUSCRIPT 3. Results and discussion 3.1. Emission spectra of the LCGD
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In order to demonstrate the capability of the method, the emission spectra of the LCGD for qualitative determination of K, Na, Ca, Mg and Zn in blank solution (pH=1
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HNO3) and waste water samples (adjusted to pH=1.0 with HNO3) are shown in Fig. 2. It is clearly observed from Fig. 2, several molecular emission lines at 283.0 and 308.9 nm ascribed to OH bands (A2Σ+→X2Π) are found [24]. It was reported that the OH
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band shown in this spectrum is attributed to a recombination process of the precursor species of OH+ [25]. Molecular band spectra of N2 ranged from 315-406 nm and
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assigned to the C3 Πu→B3 Πg systems are observed in LCGD because ambient air is used as the discharge gas [26]. In addition, the atomic lines of hydrogen Balmer (α
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and β) are at 656.3 and 486.1 nm. All these lines are consistent even if no metal ions
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are dissolved in the solution. The emission lines of K, Na, Ca, Mg and Zn in waste water samples are recorded at 766.5 or 769.9 nm, 589.0 or 589.6 nm, 422.7 nm, 285.2
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nm and 213.8 nm, respectively [10]. The results suggested that it is feasible to use the
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designed LCGD source to determine metal elements in aqueous solutions.
Insert Figure 2 here
3.2. Optimization of the experimental conditions 3.2.1. Effect of the supporting electrolyte The effect of the different supporting electrolyte on the emission intensity of Zn I (213.8 nm) was investigated. Three Zn standard solutions (1-10 mg L-1) was adjusted to pH=1.0 with HNO3, HCl and H2SO4, respectively, and then introduced into the LCGD system. It is observed that all these acidic mediums can sustain the stability of discharge. The anion illustrates a positive effect on the emission intensity of Zn I with the same H+ in the order: Cl- > NO3- > SO42- (shown in Fig. 3). The result is consistent with earlier reported by Mezei [5,27] and Webb [28].
ACCEPTED MANUSCRIPT When the size of the anion is increased, the mobility (conductivity) of the ions in electrolyte is decreased, and then, the current and power is led to lower [1,27]. Because the size of Cl- is close to NO3-, the change of emission intensity is not
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significant (ionic radius of Cl-, NO3-, and SO42- are 181, 165, and 244 pm,
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respectively). The size of SO42- is larger than that of Cl- and NO3-, accordingly, the
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lower emission intensity is observed. However, HCl and H2SO4 are easy to generate precipitation with several metal ions. Meanwhile, HNO3 is a promising reagent for digestion of water samples, and has good sensitivity and chemical compatibility [29].
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So we selected pH=1 HNO3 as supporting electrolyte mediums.
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Insert Figure 3 here
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3.2.2. Effect of discharge voltage
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There is a voltage threshold (600 V) below which the metal lines could not be
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determined. To optimize the experimental condition, the effects of discharge voltage on the emission intensity of Mg I (285.2 nm) were investigated from 600 to 700 V, as shown in Fig. 4. From 600 to 675 V, an approximately linear increase in analyte
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emission intensity of the Mg with the increasing voltage is found, but then, emission intensity begins to diverge from 675 to 700 V. This is because both the noise and signal increase simultaneously with applied voltage from 600 to 675 V, but the signal increases more rapidly than the noise [2]. However, when the voltages is reached 675 V, the increases both the signal and noise are leveled off [30,31].
Insert Figure 4 here
Fig. 5 shows the discharge images under different applied voltage. As can be seen from Fig. 5, with the increase of voltage, the energy of plasma is raised, and accordingly, the density of excited atoms is increased. As a result, the larger the volume of glow is generated, the stronger the intensity of glow plasma is obtained,
ACCEPTED MANUSCRIPT thereby, resulting in the emission intensity of Mg I (285.2 nm) is increased gradually.
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Insert Figure 5 here
The stability of plasma was detected as a function of time at fixed wavelengths
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[31]. Fig. 6 shows the temporal tracing of the emission intensity of 10 mg L-1 Mg solution in different voltage over 30 mins. Over 675 V, the further increase of voltage can cause increasing fluctuation of the emission intensities because of unstable
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discharge plasma [8]. In addition, if the voltage is too high (>700 V), the quartz capillary of cathode will be molten and destroyed due to the strong glow discharge
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and high energy. What is more, when the discharge voltage is over 675 V, the tip of the Pt wire becomes red-hot and water samples start to boil. When the discharge
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voltage is further increased to 700 V, the anode is completely red-hot and the plasma
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unstability is shown. A similar result was reported by Mottaleb et al [8] and Xiao et al [19]. The results suggested that the stability of the emission signals is realized at 650
Insert Figure 6 here
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V. Thus, 650 V was chosed as the best discharge voltage.
3.2.3. Enhancement effect of organic substances It was reported that the emission intensity of metals in ELCAD could be improved by some low molecular weight organic substances and surfactants [19,32-34]. Accordingly, the effects of ethanol, formic acid and acetic acid on the emission intensity of K I at 766.5 nm (Signal intensity of K at 766.5 nm was heighter than that at 769.9 nm) were also investigated in this work. K solutions of 10 mg L-1 containing different volume ratio (0%, 0.25%, 0.5%, 0.75%, 1% and 1.5%) of organic substances as well as their corresponding solutions without organic substances were adjusted to pH 1.0 with HNO3. It is observed from Fig. 7 that three organic substances can enhance the emission intensity of K and the enhancement effect is in the order:
ACCEPTED MANUSCRIPT HCOOH > CH3COOH > CH3CH2OH. This result was in agreement with the study of Ag, Cd and Pb by AC-EALD [19,33]. In addition, the enhancement effects of these organic substances at various volume ratios were also investiaged, and the results
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indicated that 0.5% HCOOH is shown to present the maximum enhancement for K. Because HCOOH produced large singal enhancement factors for several metals and
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did not suppress emission for any metals [33], therefore, 0.5% HCOOH was chosen for further experiments. It was reported that enhancement mechanism of addition of HCOOH can decrease the surface tension of water samples, which can promote the
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generation of small droplets and favor the evaporation rate of analytes into the plasma
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zone for atomization and excitation [7,35].
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Insert Figure 7 here
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As shown in Fig. 8, linear fitting are obtained with 0.5% HCOOH and without HCOOH for K solution in the range of 0-25 mg L-1 and the square regression
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coefficients (R2) are all close to 1. The limits of detection (LOD), using the definition 3σ/s (σ is the standard deviation corresponding to 10 blank measurements and s is the slope of the calibration graph) [36,37], are listed in Table 1. Obviously, the LOD
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value of K is lower with 0.5% HCOOH (0.02 mg L-1) than that without HCOOH (0.07 mg L-1). The values of relative standard deviation (RSD) with 0.5% HCOOH and without HCOOH are 1.10% and 0.94 %, respectively, suggesting this method has a high precision.
Insert Figure 8 here Insert Table 1 here
3.3. Analytical performance The analytical performance of LCGD was evaluated under optimal operating parameters (supporting electrolytes: pH=1.0 HNO3, added with 0.5% HCOOH,
ACCEPTED MANUSCRIPT discharge voltage: 650 V, flow rate: 4.5 mL min-1, inter-electrode gap: 2 mm). Standard solutions of K, Na, Ca, Mg ranged from 5 to 30 mg L-1 and Zn ranged from 1 to 10 mg L-1 were prepared and established calibration curves. The results showed
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that all calibration curves have a good linear relationship. The linear equation, LOD,
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sensitivity, R2 and RSD are listed in Table 2. It is obvious that the R2 and the RSD are
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ranged from 0.9887 to 0.9990 and 1.10% to 2.19%, respectively. The results suggested that LCGD can be employed for the quantitative determination of metal
0.04 and 0.05 mg L-1, respectively.
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elements in water samples. The LODs of K, Na, Ca, Mg and Zn are 0.02, 0.04, 0.19,
comparison
of
the
LODs
obtained
by
other
electrolyte-cathode
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A
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Insert Table 2 here
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discharge-atomic emission spectrometry (ECLAD-AES) techniques is listed in Table 3. Obviously, the LODs for LCGD are found to be comparable to those of similar
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systems [14,18,20,23,38]. In addition, the LOD value of Zn in this work is lower than that standard for drinking water quality [39], so this method can be used to detect the
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safety of Zn in drinking water quality.
Insert Table 3 here
3.4. Analytical application of water samples and validation To demonstrate the analytical performance on real samples of the developed method, the LCGD-AES was used to determine the K, Na, Ca, Mg and Zn in water samples (tap water, mineral water, yellow river water and waste water). In addition, LCGD results were also verified by using ICP analysis of the samples. Except for the dilution, adjustment of pH=1 using nitric acid, and addition of 0.5% formic acid, no other sample pretreatment was performed. The determination results of LCGD and ICP are listed in Table 4. It is observed from Table 4, the measured values of Na, K,
ACCEPTED MANUSCRIPT Ca, Mg and Zn by LCGD and ICP have some differences. As a new method, this is an acceptable result. It suggested that LCGD can be employed for simultaneously
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Insert Table 4 here
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determination of multi metal elements in complex water samples.
4. Conclusions
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The liquid cathode glow discharge-atomic emission spectrometry (LCGD-AES) was established and successfully employed to measurement of K, Na, Ca, Mg and Zn
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in water samples, such as tap water, mineral water, Yellow River water and waste water. The optimization analytical conditions of LCGD-AES for detection of water
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samples were pH=1 HNO3 as electrolyte, addition of 0.15% formic acid and 650 V
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voltage. The LODs of K, Na, Ca, Mg and Zn were 0.02, 0.04, 0.19, 0.04 and 0.05 mg L-1, respectively. The LODs of metals in this work agreed well with the closed-type
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ECLAD-AES, and satisfied the recommended levels of the China standards for drinking water quality. Compared with ICP-MS, ICP-AES and AAS, LCGD-AES has some features, such as lower cost in set-up, portable equipment, convenient operation
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and easy design. In addition, it is very easy in achieving real-time and on-line analysis for samples. All the results suggested that LCGD-AES is a very promising technique for highly efficient determination of metal elements in aqueous solution.
Acknowledgments This work was supported in part by National Natural Science Foundation of China (No. 21567025, 21367023 and 11564037), and Natural Science Foundation of Gansu Province (Nos. 1308RJZA144 and 1208RJZA161), China.
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[35] R.Shekhar, K. Madhavi, N. N. Meeravali, S.J. Kumar, Determination of thallium at trace levels by electrolyte cathode discharge atomic emission spectrometry with improved sensitivity, Anal. Methods 6 (2014)732-740. [36] Z. Liu, Z. Zhu, Q. Wu, S. Hu, H. Zheng, Dielectric barrier discharge-plasma induced vaporization and its application to the determination of mercury by atomic fluorescence spectrometry, Analyst 136 (2011)4539-4544. [37] 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. [38] A. Shaltout, Micro plasma generation using liquid sampling-atmospheric pressure glow discharge, Microchim. Acta 155 (2006) 447-452. [39] National Standards of the People's Republic of China, Standards for Drinking
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Water Quality (GB5749-2006), Standards Press of China, Beijing, 2007.
ACCEPTED MANUSCRIPT Figure captions
Fig. 1. Schematic diagram of the LCGD.
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Fig. 2. Emission spectra of the LCGD in blank solution (a) and waste water samples (b) (blank solution: adjusted to pH 1.0 with HNO3, waste water samples: adjusted
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to pH 1.0 with HNO3, discharge voltage: 650 V, inter-electrode gap: 2 mm, flow rate: 4.5 mL min-1).
Fig. 3. Comparison of the Zn I (213.8 nm) emission intensity in different supporting
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electrolyte mediums (zinc concentration: 0-10 mg L-1 adjusted to pH 1.0 with HNO3, discharge voltage: 650 V, inter-electrode gap: 2 mm, flow rate: 4.5 mL
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min-1).
Fig. 4. Optimization of discharge voltage (magnesium concentration: 10 mg L-1
).
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-1
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adjusted to pH 1.0 with HNO3, inter-electrode gap: 2 mm, flow rate: 4.5 mL min
Fig. 5. Images of the LCGD at different discharge voltage (magnesium concentration:
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10 mg L-1 adjusted to pH 1.0 with HNO3, inter-electrode gap: 2 mm, flow rate: 4.5 mL min -1).
Fig. 6. Stability of emission intensity as a function of time of LCGD (magnesium
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concentration: 10 mg L-1 adjusted to pH 1.0 with HNO3, inter-electrode gap: 2 mm, flow rate: 4.5 mL min -1). Fig. 7. Enhancement effect of different organic substances (formic acid, acetic acid and ethanol) on the emission intensity of K I (766.5 nm) (potassium concentration: 10 mg L-1 adjusted to pH 1.0 with HNO3, discharge voltage: 650 V, inter-electrode gap: 2 mm, flow rate: 4.5 mL min -1). Fig. 8. Calibration curve of the emission intensity versus concentration of potassium solution without HCOOH and with 0.5% HCOOH (potassium concentration: 10 mg L-1 adjusted to pH 1.0 with HNO3, discharge voltage: 650 V, inter-electrode gap: 2 mm, flow rate: 4.5 mL min -1).
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Figures
Fig. 1
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Fig. 2
HNO3 HCl H2SO4
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2
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1
0 2
4 6 8 -1 Concentration (mg L )
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0
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3
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4
Emission intensity (x10 Counts)
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Fig. 3
10
1.8
V V V V V
1.6
284.8
285.0
285.2
285.4
285.6
W avelength (nm )
-1
10 mg L Mg
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1.4
600
620
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1.2 1.0
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600 625 650 675 700
6
Emission intensity (x10 Counts)
2.0
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640
660
680
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Discharge voltage (V)
Fig. 4
700
625 V
650 V
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600 V
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Fig. 5
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675 V
700 V
2.0
700 V
1.8
1.4
625 V
1.2
600 V
200 400 600 800 1000 1200 1400 1600 1800 Time (s)
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0
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1.0
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Fig. 6
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650 V
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1.6
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675 V
6
Emission intensity (x10 Counts)
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HCOOH CH3COOH
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7.8
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CH3CH2OH
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7.6 7.4
7.0 6.8
0.0
0.2
0.4
0.6
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7.2
0.8
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4
Emission intensity (x10 Counts)
8.0
1.0
1.2
Organic content (%)
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Fig. 7
1.4
1.6
Without HCOOH With 0.5% HCOOH
4
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2 1
5
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5
5
Emission intensity (x10 Counts)
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10
15
20 -1
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Concentration (mg L )
Fig. 8
25
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Tables
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Table 1. Analytical performance of LCGD-AES in K solution with 0.5% HCOOH and without HCOOH.
LOD (mg L-1)
R2
RSD (%)*
y=2400+4964x
4964
0.07
0.9997
0.94
y=41122+17645x
17645
0.02
0.9932
1.10
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S (Counts mg-1 L)
Linear equation
Without HCOOH With 0.5% HCOOH
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K
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* Standard concentration: 10 mg L-1, n = 10.
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K
766.5
Na
589.0
Mg Zn
S(Counts mg-1 L)
LOD (mg L-1)
R2
RSD* (%)
y=41122+17645x
17645
0.02
0.9932
1.10
y=102395+30923x
30923
0.04
0.9887
1.65
422.7
y=2311+684x
684
0.19
0.9971
2.19
285.2
y=62611+20619x
20619
0.04
0.9937
2.06
213.8
y=783+1587x
1587
0.05
0.9990
1.35
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Ca
Linear equation
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Wavelength (nm)
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Analytical element
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solution with 0.5% HCOOH
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Table 2 Analytical performance of LCGD-AES in K, Na, Ca, Mg and Zn standard
* Standard concentration: 10 mg L-1, n = 10.
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Table 3 Comparison of the LODs obtained by LCGD-AES system with other ELCAD-AES systems for the detection of Na, K, Ca, Mg, Zn and standards for
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drinking water quality.
LOD (mg L-1)
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Methods LCGD-AES
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ELCAD
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DC-APGD
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CD SCGD LS-APGD Standards for Drinking Water Quality
Reference
K
Na
Ca
Mg
Zn
0.02
0.04
0.19
0.04
0.05
This work
0.2
0.06
0.4
0.8
0.1
[14]
0.001
0.001
0.04
0.008
0.04
[18]
0.013 -
0.001 -
1 0.023 0.3 -
1 0.019 -
0.042 0.7 1
[20] [23] [38] [39]
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Table 4. Measurement results of K, Na, Ca, Mg and Zn in real water samples by LCGD-AES and ICP
144.2
Na
1001.0
K
12.5
Mg
58.3
Zn
1.6
Ca Na
ICP (mg L-1)
Element
This work (mg L-1)
ICP (mg L-1)
Ca
6.7
30.8
Na
0.3
2.2
K
0.7
1.1
13.0
Mg
0.6
0.2
0.2
Zn
--
0.006
Ca
37.5
70.9
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Ca
533.2
1045.3
Water Sample
Mineral water
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25.9
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Yellow river water
This work (mg L-1)
57.5
76.7
19.5
28.3
Na
19.7
29.2
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Waste water
Element
Tap water
K
1.6
2.2
K
1.7
2.1
Mg
16.1
1.9
Mg
16.8
1.9
Zn
--
0.003
Zn
--
0.005
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Water Sample
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The
liquid
cathode
glow
discharge-atomic
emission
spectrometry
(LCGD-AES) was constructed.
additives on emission intensity were investigated.
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► The effects of the supporting electrolyte, discharge voltage and organic
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► A multi metals simultaneously analyzer of water samples was achieved by
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LCGD-AES.