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A Portable Dielectric Barrier Discharge-Atomic Emission Spectrometer Na Li, Zhongchen Wu, Yingying Wang, Jing Zhang, Xiangnan Zhang, Hengnan Zhang, Wenhai Wu, Jing Gao, and Jie Jiang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03523 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017
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Analytical Chemistry
A Portable Dielectric Barrier Discharge-Atomic Emission Spectrometer Na Lia, Zhongchen Wub, Yingying Wangc, Jing Zhanga, Xiangnan Zhanga, Hengnan Zhanga, Wenhai Wua, Jing Gaoa, Jie Jianga* a
School of Marine Science and Technology, Harbin Institute of Technology at Weihai, Weihai, 264209 P. R. China School of Space Science and Physics, Shandong University at Weihai, Weihai, 264209 P. R. China c Department of Optoelectronic Science, Harbin Institute of Technology at Weihai, Weihai, 264209 P. R. China b
*Corresponding Author. E-mail:
[email protected]. Fax: + (86)-631-5685-359.
ABSTRACT: This paper describes the first demonstration of a portable dielectric barrier discharge-atomic emission spectrometer (DBD-AES). The instrument primarily consists of a miniature electro-thermal vaporizer (ETV), DBD and optical signal acquisition units. It weighs only 4.5 kg and is powered by a 24 V DC battery with a maximum power consumption of 37 W. The accompanying software can be operated on a laptop computer. A specially designed quartz tube integrates the ETV unit with the DBD chamber. The effects of experimental parameters were investigated. The limit of detection (LOD) for mercury was 0.4 µg L-1 (1.2 pg) with a sampling volume of 3 µL. The instrument is applicable for multi-element analysis, and the LODs ranged from 0.16 to 11.65 µg L-1for Zn, Pb, Ag, Cd, Au, Cu, Mn, Fe, Cr and As. The instrument was also validated by in-field analysis of seawater samples. The experimental results demonstrated the sensitivity, reliability and practicality of the instrument.
The increasing contamination of the environment with heavy metals poses a growing threat to ecosystems and human health. Traditional methods for metal analysis based on laboratory instruments provide precise, accurate and sensitive analytical results and include atomic absorption spectrometry (AAS),1-3 atomic fluorescence spectrometry (AFS),4 inductively coupled plasma atomic emission spectrometry (ICP-AES)5 and inductively coupled plasma mass spectrometry (ICPMS).6,7 However, these techniques are labor-intensive, bulky, expensive and time-consuming, and require high power consumption. These shortcomings hinder the use of traditional laboratory-based analytical methods in remote regions and in cases in which analytical data are desired in real-time. The demand for fast, inexpensive analytical methods for field use has been motivated by the increasing interest in field analytical chemistry in recent years, particularly in the fields of environmental monitoring and industrial process control. Due to their small size and low power consumption, microplasmas are increasingly being used as radiation sources in the development of miniaturized and portable systems.8-11 Dielectric barrier discharge (DBD) microplasma sources possess advantages such as simplicity, compactness, stability, low operating temperature and high excitation ability, and thus display great potential for the development of portable field instruments.12 DBD microplasma was first used as an excitation source for AES by Yu et al. and Zhu et al. to determine mercury(Hg) in solution.13,14 The system was further applied to determine various other elements, and the results demonstrated the reliability and practicability of the system.15-18 In addition to the excitation source, sample introduction plays an important role in improving efficiency and system miniaturization. The presence of liquid can deteriorate the stability of the
DBD microplasma, even extinguishing the plasma, and decrease the excitation efficiency of the analyte.19 The preferred strategy is to transform the analyte to the gaseous form to separate the analyte from the sample solution and eliminate interference from moisture and the matrix. Chemical vapor generation (CVG), e.g., cold vapor generation (CV),13,14 hydride generation(HG),17 and photochemical vapor generation(PVG),18,20 is a widely used approach. However, the requirements of multiple agents and complicated components involving solution injection systems and gas-liquid separators limit further miniaturization of the entire system. In addition, this approach can be used only for certain elements.7 Compared with CVG, electro-thermal vaporization (ETV) is a competitive sample introduction approach with the advantages of high sample introduction efficiency, efficient separation of the sample matrix, low sample consumption and the capability for direct analysis of slurries and solids.21-23 Hou et al. first applied miniature ETV for sample introduction; the liquid sample was directly added to the tungsten (W) coil electro-thermal vaporizer, and the analyte-containing gasified species obtained by heating was introduced into the DBD microplasma.24 Zheng et al. successfully applied this system for the determination of lead (Pb) in water samples.25 Their results demonstrated the improved sensitivity and stability of ETV-DBD. The ETV device eliminates interference from the sample solvent and matrix by controlling temperature and can also simplify the entire DBD-AES system. Although the DBD-AES system has been greatly miniaturized and simplified, reports on the use of this portable DBD-AES instrument for field analysis are limited. In this work, a portable field instrument based on DBDAES was developed for the first time. The components were rationally designed. A miniaturized ETV device based on a
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Figure 1. (a) Schematic of the entire instrument and (b) photograph of the prototype instrument. molybdenum (Mo) filament was used for sample introduction Mo filament (0.2 mm) through the sample inlet. Two highand was specially integrated into the DBD chamber to miniavoltage electrodes at a distance of 10 mm were coiled around turize the system. The optical signal acquisition (OSA) unit the narrow tubule (i.d. 2 mm), which formed the discharge was optimized. The effects of key experimental parameters region of the DBD. High-purity Ar gas was used as a working were investigated. The performance of the instrument was gas to convey the atomic analyte and to maintain stable disevaluated by analyzing metallic and non-metallic elements and charge in the DBD. A portable Ar cylinder of 0.2 L was used by in-field analysis of seawater samples. Compared with the as external gas source of DBD-AES. The cylinder can change common commercial instruments, this instrument is portable easily and a larger cylinder can be used when necessary. The and inexpensive. Moreover, it can realize the rapid and in-field gas flow rate was controlled by a rotameter. analysis of various elements. An HR2000+ spectrometer from Ocean Optics was used for synchronous data acquisition of the emission spectra rangEXPERIMENTAL SECTION ing from 200 to 420 nm. In the OSA unit, a miniaturized Chemicals and Reagents. Nitric acid of analytical grade was Ocean Optics 74-DA collimator was directly connected to the purchased from Laiyang Fine Chemical Factory (Laiyang, spectrometer, and a thin UV quartz window chip was placed in China). Stock solutions of Hg(Ⅱ), Zn (Ⅱ), Pb (Ⅱ), Ag (Ⅰ), the front of the collimator for protection (Figure 1a). (The Cd (Ⅱ), Au (III), Cu(Ⅱ), Mn(Ⅱ),Fe(III), Cr(III), As(Ⅱ) and detail information was given in the Supporting Information). -1 Be( Ⅱ )(1000 µg mL ) were purchased from the National In preliminary experiments, a circular lens and an optical fiber Analysis Center for Iron and Steel, China. The working soluwith a lens fixture26 were also investigated to focus light into tion of Hg was prepared by diluting the stock solution with 1% the entrance slit of the spectrometer. Performance was as(v/v) HNO3. The multi-element working solution was prepared sessed based on the molecular emission intensity of OH radiby mixing the single-element stock solutions and diluting with cals at 307.4 nm. As shown in Figure 3a, an intensity of 16383 deionized water. High-purity argon gas (99.99%, v/v) was was obtained by the miniaturized collimator, accompanied by purchased from LongkouHuadong GAS Co. (Longkou, Chia flat peak limited by the optical saturation of the CCD detecna). The Certified References Material (CRM) of water tor. By contrast, the intensities obtained by the circular lens (GBW08603) was purchased from the National Research Cenand the optical fiber with a lens fixture were 6400 and 16100, ter for Standard Materials (NRCSM) of China. respectively. An optical simulation of the miniaturized colliInstruments. The custom built instrument is schematically mator (Figure 3b) was used to provide an intuitive description illustrated in Figure 1. As shown in Figure 1a, the instrument of the optical process. The diameter and focal length of the primarily consisted of ETV, DBD and OSA units with correinternal lens of the collimator were 5 and 10 mm, respectively. sponding circuits. A coaxial circuit consisting of a battery, Parallel light was focused to a minimal spot to ensure accurate constant current sources and alternating current (AC) HV cirtransmission through the entrance slit of the spectrometer. The cuit was designed to support the working instrument. The inresults demonstrated the collimator was able to accurately strument weighs only 4.5kg and has the following dimensions: match the light path and effectively focus the light into the 30.5 cm (l) × 27 cm (w) × 14.5 cm (h). entrance slit of the spectrometer, thus significantly improving the sensitivity of the instrument. In addition, the miniaturized The designed quartz tube consisted of a wide tube used for collimator reduces the size of portable devices. ETV and a narrow tubule used for DBD. The quartz tube was more compact and miniaturized than that reported in previous A customized program was written using LabVIEW to constudies.24,25 A wrapped metal filament was placed inside the trol the spectrometer and for real-time monitoring of the emiscylindrical quartz tube (i.d. 10 mm) as a miniaturized ETV sion spectra. The following spectra were displayed synchrodevice. (The detail information was given in the Supporting nously: the real-time spectrum of emission intensity versus Information). The photograph of DBD-ETV at working state wavelength (200-420 nm), the spectrum of emission intensity was shown in Figure 2. In a preliminary experiment, W and versus time at a selected wavelength, and the whole spectrum Mo filaments were tested, and the Mo filament provided a of emission intensity versus wavelength (200-420nm) when cleaner emission background. Thus, the Mo filament was used the emission intensity of the selected peak reached the maxiin subsequent experiments. The sample was injected onto the mum value. In addition, this program was used to set the inte-
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Analytical Chemistry
gration time and monitoring wavelengths. The program operated well on laptop computers during field tests. The circuit consisted of a power supply, constant current sources and an AC HV circuit. Power for the entire device was provided by an internal battery with an output voltage of 24 V, which maintained the instrument in a working state for 3 hours at a maximum power consumption of 37 W or 4.8 hours at normal consumption. Two constant-current source modules were employed in the instrument to generate adjustable constant currents (0–5A and 0–8A) for the ETV. The AC HV circuit included a 36-kHz signal generator, power amplifier, and transformer. The transformer consisted of an air-coil transformer and a HV capacitor. The transformer was driven by power amplifier and generated HV which applied to DBD region by two high-voltage electrodes; the circuit could achieve an adjustable AC voltage output of 0–8000 Vp-p. AC HV circuit was used to drive the DBD discharge and generate microplasma. The instrument is with portability in terms of small dimensions and weight, low power consumption, battery-powered, laptop-operated and with miniature gas source.
Figure 2. Photograph of ETV-DBD at working state.
Figure 3. (a) Effect of the optical path on the background emission intensity. (b) Schematic diagram of the optical simulation of the miniaturized collimator. The quartz chip is not shown because of its high transparency to UV radiation.
Experimental Procedure. A 3-µL aliquot of analyte solution was pipetted onto the Mo filament by a micropipette. Then, the heating program for the Mo filament ETV started. At first, a low current of 1.4 A was applied and maintained for 160 s to remove moisture. Then, the plasma was ignited. After the DBD process is stable, a high current of 4.0 A was used for atomization/vaporization of the sample and maintained for 5 s. Finally, a high current of 6.0 A was maintained for 5 s to eliminate residues. The flow rate of Ar was 0.5 L min-1. RESULTS AND DISCUSSION. Optimization of Experimental Conditions. The effects of some key experimental parameters on excitation capability of Hg, Cd and Pb were investigated. The typical emission lines of Hg, Cd and Pb were selected at 253.65 nm13,14,24, 228.8 nm and 368.35 nm, respectively, in subsequent experiments because of their high intensity and clear isolation from background spectra. Emission spectra of Hg was shown in Figure 4.
Figure 4. Optical emission spectra of blank and mercury standard solutions (50 µg L-1). Effect of drying current and drying time. The ETV process plays an important role in elemental analysis. By multi-step heating of ETV, the analyte was separated from the solvent and the matrix, and gasified for DBD excitation. A low drying current was first used to remove the solvent. The effect of drying current was investigated (Figure S3, Supporting Information). A high current is beneficial for the removal of solvent and the excitation/emission of the analytes, whereas an excessively high current can result in loss of analyte during the drying step. Therefore, drying currents were selected at 1.4 A for Hg, and 1.8A for Cd and Pb in the following experiments. The drying time was optimized (Figure S4, Supporting Information). An increase in drying time is beneficial for the effective removal of moisture. However, an excessively long drying time can result in a higher temperature of the Mo filament and the loss of analyte. Finally, a drying time of 160 s was selected for subsequent experiments. Effect of vaporization current. After the drying step, a high current was directly applied to the atomization/vaporization of the analytes. The vaporization current affects the temperature of the Mo filament and, consequently, the sample introduction efficiency and analyte atomization/vaporization efficiency. The effect of vaporization current was investigated (Figure S5, Supporting Information).The release of the analytes is slow at low vaporization current, the emission peaks of the analytes significantly broadened and the precision decreased. A high current would lead to rapid release of the analytes from the
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Mo filament and fast thermal expansions and would thus improve the efficiency of sample introduction. Additionally, the extra energy of the gasified analytes is beneficial to the DBD excitation/emission process. 24 However, an excessively high current could shorten the time of the analytes in the excitation chamber due to the rapid release and transportation of the analyte. The vaporization currents were selected at 4.0A for Hg, and at 4.3A for Cd and Pb. Effect of input voltage of DBD. The plasma potential can affect the state of microplasma and, consequently, the emission intensity of the analyte. The effect of discharge voltage of DBD was investigated (Figure S6, Supporting Information). A high discharge voltage can enhance the excitation capability of the DBD microplasma and thus improve the emission intensity of analyte. However, the stability and homogeneity of the DBD microplasma are diminished at an excessively high voltage. Breakdown of the air outside the DBD chamber was observed at an applied voltage of 7730Vp-p. Consequently, the discharge voltages were set at 7400 Vp-p, 7070 Vp-p and 7400 Vp-p for Hg, Cd and Pb, respectively. Effect of Ar flow rate. In this work, Ar gas was used as the working gas to transport the vaporized analyte from the ETV to the DBD region and to maintain a stable discharge of the DBD. The Ar gas also protected the Mo filament. The flow rate of the working gas can affect both the transportation efficiency and the concentration of the analyte. The effect of Ar flowrate was investigated in the range of 0.2 to 1.0 L min-1 (the detail information was given in the Supporting Information). With the increase of gas flowrate, the shape of emission peaks sharpened but the emission intensities of the analytes decreased clearly because of the dilution of the analytes by working gas and the reduction of the residence time of the analyte in the DBD chamber. Consequently, the flow rate of Ar was set at 0.5 L min-1. Interference. The interference of common metal ions was investigated. No interference was found within a 5% error range for 50 mg L-1 Na+, K+, Ca2+ and Mg2+ in determination of 50 µg L-1 Hg. In real water samples, the contents of these species are generally well within the tolerant concentrations. The results demonstrated the robustness of the instrument. Analytical Figures of Merit. The standard curves were constructed by plotting the emission intensities recorded at the selected emission lines versus the analyte concentrations of working solutions. The regression equations of Hg, Cd and Pb were with linear coefficient (R2) better than 0.99 (Table S1, Supporting Information). The limit of detection (LOD) was derived as three times the standard deviation of the blank determination. The LODs for Hg, Cd and Pb were 0.4, 0.65 and 8.95 µg L-1, respectively, which correspond to 1.2, 2.0 and 26.8 pg of absolute mass. Table 1 summarizes the analytical figures of merit characterizing this method and compares its performance to other similar methods. The LOD of Hg obtained by this method is comparable to those obtained by other
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methods. In addition, the sampling volume in this method is much lower, and the LOD might be further decreased by simply increasing the sample volume. To evaluate the stability of the instrument, the solution with 50 µg L-1 Hg, 100 µg L-1 Cd and 500 µg L-1 Pb was analyzed per 0.5 h and continuously analyzed for 10 times. The average recoveries of Hg, Cd and Pb ranged from 99.9% to 102.4% with the RSDs lower than 6.7% and the results demonstrated the stability of the instrument. The detail information was given in Supporting Information. The analytical performance of the instrument was further evaluated by simultaneous multi-element analysis. At high temperatures, the background emission from the filament significantly affected the analysis. To further reduce the emission background interference, a carbonized Mo filament prepared by the addition of lubrication oil to the heated Mo filament was used. To improve the stability of determination, background correction was conducted in simultaneous multielement analysis. The final emission spectrum was obtained by subtracting the background emission intensity of the blank solution from that of the analyte solution at the same acquisition time. The simultaneous emission spectra for Zn, Hg, Be, Cd, Ag and Pb are shown in Figure 5 with typical emission peaks. The applicability of the instrument was validated by determining various metals. The LODs for Zn (213.86 nm), Pb (368.35 nm), Ag (328.07 nm), Cd (228.80 nm), Au (242.80 nm), Cu (324.75 nm), Mn (260.57 nm), Fe (302.06 nm) and Cr (360.53 nm) were 1.89, 8.95, 0.16, 0.65, 7.94, 7.94, 7.0, 26 and 6.5 µg L-1 for a sample volume of 3 µL and a vaporization current ranging from 4.0 A to 4.6A. The specific atomic emission of As at 228.81 nm was also obtained, and the LOD for As was 11.65 µg L-1(34.95 pg). Table 1. Analytical Figures of Merit Compared with Other Methods in Hg analysis Method
LOD (µ µg L-1) 0.2
CV-DBDAES13
LOD (pg)
RSD (%)
-
2.1 a
Sample volume (µ µL) 500
CV-DBDAES14
0.024
7.2
