Point Discharge Microplasma Optical Emission Spectrometer: Hollow


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Point Discharge Microplasma Optical Emission Spectrometer: Hollow Electrode for Efficient Volatile Hydride/Mercury Sample Introduction and 3D-Printing for Compact Instrumentation Mengtian Li, Kai Li, Lin He, Xiaoliang Zeng, Xi Wu, Xiandeng Hou, and Xiaoming Jiang Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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Point Discharge Microplasma Optical Emission Spectrometer: Hollow Electrode for Efficient Volatile Hydride/Mercury Sample Introduction and 3D-Printing for Compact Instrumentation Mengtian Li,a Kai Li,b Lin He,b Xiaoliang Zeng,b Xi Wu,b Xiandeng Hou,a,

b*

Xiaoming Jiang b*

a

Key Laboratory of Green Chemistry and Technology of MOE, and College of

Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. b

*

Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China.

Corresponding authors: [email protected] or [email protected]

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ABSTRACT A miniaturized optical emission spectrometer was constructed with an improved point discharge microplasma as an excitation source, to enhance the sample introduction efficiency and excitation efficiency. By using a hollow electrode as one of the discharge electrodes, analyte-containing chemical vapor yielded via hydride generation was transported and confined into the hollow electrode, and subsequently guided into the microplasma, with high sample introduction efficiency. Moreover, gaseous analyte species was directly diffused from inside the electrode into the center of the microplasma instead of traditional external diffusion into the microplasma, with sufficiently participating in interaction and excitation in the plasma, thus high excitation efficiency and stability can be achieved. 3D-printing technique was used to fabricate some components for compact integration of this spectrometer. Physical characteristics of the microplasma, 3D-printing and experimental parameters were all investigated to better understand the excitation capability and obtain optimal analytical performance. Under optimized conditions, As, Bi, Ge, Hg, Pb, Sb, Se and Sn were successfully detected, with detection limits of 2.5, 0.44, 1.6, 0.10, 2.8, 1.5, 31 and 0.24 μg L-1, respectively, and relative standard deviations all less than 4%. It was applied to analyze Certified Reference Materials (water, soil and biological samples) and real water samples with satisfactory results. Due to its advantages of compactness, robustness, easy fabrication and cost-effectiveness, it has a great prospect for a portable spectrometer for field analytical chemistry.

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INTRODUCTION Optical emission spectrometry (OES) is widely used in miniaturized atomic spectrometers for its simple instrumental structure without any extra radiation source and capability of simultaneous multi-element detection, in which various microplasmas have been used as excitation sources for compact instrumentation because of their unique advantages including small size, low gas and power consumption, easy operation and low manufacturing and running cost.1-3 These excellent characteristics also make them suitable for real-time and field analysis. Various types of microplasma-based excitation sources have been reported in recent years, such as micro-discharge plasma including glow discharge (GD),4-6 corona discharge (CD),7 dielectric barrier discharge (DBD),8-10 and point discharge (PD).11-12 They have been predominantly used for OES determination of various analytes, such as metal elements, inorganic anions, volatile carbon-containing compounds, and halohydrocarbons.13-16 Due to the reduction of power consumption and size, atomization/excitation capability of a microplasma is consequently lowered. Therefore, requirements for other instrumental components would be more rigorous to improve the final analytical performance to facilitate field applications, especially the sample introduction strategy and plasma design. A microplasma excitation source is usually interfered by sample moisture and sample matrix, which could consume most of the energy for evaporation, thus lower the atomization/excitation capability and deteriorate the stability, or even extinguish it. Therefore, gaseous species of analytes is preferred to introduce into the microplasma, which conveniently provides separation of analyte from complex sample matrix with high transportation efficiency and minimal moisture. The most frequently used technique is chemical vapor generation (CVG)17-18, including cold vapor generation (CV),19 hydride generation (HG)20-21 and photochemical vapor generation (Photo-CVG).22-23 Volatile species separated from sample matrix could be produced through the vapor generation process, although concomitants could also be inevitably yielded sometimes, such as hydrogen in HG process. However, recent

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works have demonstrated that hydrogen has almost no significant effect on the elemental determination,24 and could even compromise spectral background.11 Microplasma commonly has a small plasma region, which is a relatively closed area with slight expansion derived from abundant radicals and thermal effect in the plasma. In a point discharge, for example, the gaseous analyte species are usually transported to the microplasma externally and diffused across/bypass the plasma region; without entering into plasma for effective excitation, the retention time and the portion of the analyte for excitation are also greatly reduced. Pohl et al.25 and Wang et al.6, 26 utilized CVG sample introduction for glow discharge excitation with a metal tube and a solution electrode. Volatile analytes were delivered into the discharge through the metal tube anode, with well improved vapor transport efficiency. However, moisture was also introduced through the solution electrode which could deteriorate analytical performance, and additional device for pumping electrolyte was required which further complicated the system. Therefore, a point discharge microplasma structure can be further improved to enhance analytical performance. In addition, the final analytical performance of an instrument is largely limited by the fabrication technology, including the analytical stability, instrumental size and portability. Recently, three-dimensional (3D) printing technique has achieved revolutionary developments, as an additive manufacturing enabling cost-effective, facile and fast fabrication of objects, especially instrumental components and even a whole instrument with favorable capability of assemblage, compatibility and integration.27 However, there has been few studies on using 3D-printing in analytical atomic spectrometry for miniaturized apparatus. In this work, a hollow electrode was utilized to construct a point discharge microplasma as the excitation source for a miniaturized optical emission spectrometer, coupling with HG for sample introduction. Sample vapor was all transported and confined into the hollow electrode and guided into the microplasma, and diffused from inside out of the microplasma instead of traditional external diffusion, thus obtaining high sampling efficiency and sufficient interaction to improve excitation efficiency, sensitivity and stability. In addition, a 3D-printing technique was employed 4

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to fabricate the instrumental apparatus for conveniently and modularly assembling the miniaturized hydride generation-hollow electrode point discharge-optical emission spectrometer (HG-HEPD-OES). Physical characteristics of the new type microplasma, 3D-printing and experimental parameters were investigated in detail to better understand the excitation capability and obtain optimal analytical performance.

EXPERIMENTAL SECTION Instrumentation. Schematic diagram of the experimental setup is shown in Figure 1. It mainly included a hydride generation unit, a hollow electrode point discharge unit and a handheld commercial CCD spectrometric detector. The miniaturized HG-HEPD-OES was constructed by involving a 3D-printer (Hope 6000H, Hongpu Technology, Co., Ltd., Chengdu, China), with stereolithography (SLA) technique and photosensitive resin as printing material, for components of reaction coil, two-stage gas/liquid separator (GLS), discharge unit and its holder, and fixed mount for the CCD spectrometer. All the components were integrated on a 3D-printed baseplate (250 mm×115 mm×5 mm), by using matched concave slots and convex fixing rods. The helical reaction coil (4 mm i.d., 25 cm length, 3 mL volume) was printed in a square cylinder (30 mm×30 mm×50 mm), with two “Y” type three-way interfaces for mixing sample and HG reagents in the front end and supplementing carrier gas to flush the reaction mixture into the GLS in the rear end of the coil, respectively. The two-stage GLS (10 mL inner volume) was printed in the same size square cylinders, with the same size cavities (20 mm×20 mm×35 mm). The 1st GLS was printed with three ports for importing reaction products, releasing waste and exporting separated analyte-containing vapor, respectively. The 2nd one had an import and an export for mainly removing residual moisture, and a waste port for pouring waste conveniently, which was sealed during operation. The point discharge unit and its holder (30 mm×30 mm×45 mm) were printed separately and assembled together. The holder is hollow designed to match the point discharge unit, which could be inserted into the hollow holder with different depths,

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thus height adjustment capability obtained. A hole (8 mm diameter) was designed on the discharge unit (10 mm thickness) with two quartz pieces covered forming a discharge cavity. A tungsten needle electrode (1.6 mm diameter, 15 mm length) and a stainless steel hollow electrode (2 mm i.d., 2.5 mm o.d., 15 mm length) fixed on the base in opposite directions formed a gap (6 mm) at the center of cavity to generate the point discharge microplasma. The two electrodes were coaxial to form a diffuse and stable discharge when applying a bipolar alternating-current high voltage (NG.B408BL, Electronic Equipment Factory of Jinshi, Guangzhou, China). An additional export (1.6 mm i.d.) was made on the top of discharge unit for exhausting waste gas. The optical emission in the discharge region was focused with a lens mounted on the detector entrance into the slit of the CCD spectrometer (EMBED, 175-400 nm, 25 μm slit, 0.5 nm spectral resolution, Ocean Optics (Shanghai) Co., China), with 100 ms integration time and peak area recorded for quantitation. The total analytical time for one measurement was typically 30 s. Additional instrumental information and analytical procedure could be found in Sections 1 and 2 of the Supporting Information (SI).

Figure 1. (a) Schematic diagram of the instrumental arrangement (HEPD unit rotated 45ºfor convenient observation); and (b) photo of the prototype spectrometer.

Reagents and Samples. All reagents used in this work were at least of analytical 6

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grade. Working solutions were prepared by serial dilution of the stock solutions. Certified Reference Materials from Beijing North Carolina Souren Biotechnology Research Institute (Beijing, China), National Research Center of China (NRCC) and the National Research Council Canada (NRC); as well as real water samples were used to validate the accuracy of the proposed technique. Detailed reagents and samples could be found in Section 3 of the SI.

RESULTS AND DISCUSSION Spectral Characteristics. To demonstrate the utility of the proposed instrumentation, the optical emission spectra in the microplasma were firstly investigated. Due to HG was used for sample introduction, working gas He was mixed with H2 yielded in the HG process, and a He-H2 microplasma was formed for the atomization/excitation of the hydrides. The addition of H2 could minimize most of the background spectral emission of a pure He microplasma, thus obtaining more analytical spectral lines available for analyte elements with less background interference (Section 4 of the SI). Owing to the clean and smooth optical emission background, numerous atomic emission lines could be obviously identified by comparing with the blank background and reference database28, as shown in Figure 2. Considering high sensitivity and minimal spectral interference, As 234.98 nm, Bi 306.77 nm, Ge 265.12 nm, Hg 253.65 nm, Pb 368.35 nm, Sb 259.80 nm, Se 203.98 nm and Sn 317.50 nm were chosen as analytical lines in this work.

Hollow Discharge Electrode for Sample Introduction. The hollow electrode additionally plays another important role of introducing sample vapor into the discharge region efficiently, with which a cone-shape mciroplasma was obtained, and its outlet was at the tapered bottom of the microplasma, thus with a larger plasma region. Besides, gaseous analyte-containing species was confined into the hollow electrode and guided to its outlet where was full of plasma atmosphere, and subsequently entirely entered the inside of the microplasma. Therefore, analyte

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species directly diffused from inside to outside of the plasma, with sufficient interaction and excitation in the plasma for high excitation efficiency. Compared with traditional external diffusion introduction (under the same experimental conditions

Figure 2. Emission spectra of As, Bi, Ge, Hg, Pb, Sb, Se and Sn (units for all wavelengths are nm). Experimental details: 20, 1.0, 10, 0.20, 5.0, 5.0, 50 and 0.50 mg L-1 for As, Bi, Ge, Hg, Pb, Sb, Se and Sn, respectively; and CCD integration time, 100 ms.

with similar discharge unit described in Section 1 of the SI), the excited optical emission intensities with HEPD were totally improved for these elements, as shown in Figure 3. The achieved higher sensitivity had a maximum of 10-fold enhancement compared with our previous work11. Besides, more elements could be further detected owing to the improvement in detection sensitivity and stability.

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Figure 3. Comparison of sample vapor introduction between PD and HEPD: (a) introduction illustration; and (b) relative intensity. Experimental details: 5.0, 1.0, 1.0, 0.10, 1.0, 5.0, 20 and 1.0 mg L-1 for As, Bi, Ge, Hg, Pb, Sb, Se and Sn, respectively.

Plasma Characteristic Calculation. The excitation capability of the microplasma is correlated with extensive physical conditions, such as working gas type and flow rate, electrode structure and distance, as well as discharge voltage and frequency. To further understand the excitation efficiency of the HEPD and the possible excitation mechanism, the physical characteristics of the microplasma were further studied, including three classic parameters: rotational temperature (Trot), excitation temperature (Texc) and electron density (ne). Considering the microplasma accorded with local thermodynamic equilibrium (LTE), the parameters would agree with the Boltzmann distribution. Therefore, Trot and Texc were calculated by Boltzmann slope method and ne was according to the Stark broadening. The spectral information was spatially obtained from 7 observation positions, with 6 mm electrode distance and 1 mm gap step between electrodes. The results demonstrated that the HEPD had good excitation capability for optical emission spectra (Section 5 of the SI).

3D-Printing and Materials. The main components for the HG unit (including the reaction coil and the GLS) requires chemically inertness to acid and base solutions, and photosensitive resin was finally chosen for constructing these parts, as well as the discharge unit holder, the CCD spectrometer fixing mount and baseplate, and for the 9

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consideration of the printing precision of SLA technique and material costs. The 3D-printed point discharge unit was designed to support the electrodes and form a microplasma cavity for atomic excitation. For the local thermal effect derived from the compact integration and the small size of the discharge cavity, the printing materials should have good thermal stability and mechanical strength. Four types of materials with corresponding printing techniques were tested to investigate the 3D-printing performance in this work, including polycarbonate (PC), nylon, rubber-like material and resin. The result of the temperature tolerance experiment was shown in Figure 4. By heating the printed discharge units with different materials through a hot air heat gun for 30 min at each temperature, it was found that the limiting temperatures before heat deflection of unit were 200, 225 and 250 ºC for PC, nylon and rubber-like material, respectively. Resin would be fragile above 300 ºC but reverted quickly when stopping heating. Considering the material and manufacture cost and printing time, resin was utilized for the discharge unit (Section 6 of the SI).

Figure 4. Temperature tolerance of 3D-printed discharge unit with different materials.

Optimization of Experimental Conditions. To obtain optimal performance of the spectrometer, experimental parameters were investigated in detail. The optimal experimental conditions were as follows: working gas (He) flow rate, 240 mL min-1; electrode distance, 6 mm; input discharge voltage, 125 V (ca. 2150 V output peak voltage, measured with plasma on); HCl concentrations (v/v), 10%, 5%, 1%, 2%, 1%, 10%, 15%, and 0.5%; and NaBH4 concentrations (m/v), 0.2%, 0.1%, 0.6%, 0.01%, 0.2%, 0.2%, 0.2% and 0.2% for As, Bi, Ge, Hg, Pb, Sb, Se and Sn, respectively; HCl 10

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and NaBH4 solution flow rate, 6.5 mL min-1; thiourea concentration (m/v) for As and Sb, 1.0%; K3[Fe(CN)6] concentration (m/v) for Pb, 2.0% (Section 7 of the SI).

Analytical Performance. The analytical performance of the proposed system was evaluated under optimal experimental conditions. The linear correlation coefficients (R2) of calibration curves, limits of detection (LODs) and comparison with those obtained by similar plasma-based OES techniques, as well as the interferences were summarized in Sections 8 and 9 of the SI. LODs of 2.5, 0.44, 1.6, 0.10, 2.8, 1.5, 31 and 0.24 μg L-1 were obtained for As, Bi, Ge, Hg, Pb, Sb, Se and Sn, respectively, with relative standard deviations (RSDs) all less than 4%. It is worthwhile to note that the LODs were greatly improved than those by previous HG-PD-OES because much more gaseous analyte species participated in the plasma processing through the hollow electrode directly instead of diffusing from the outside of the microplasma (Figure 3). Besides, the instrumental stability could be guaranteed owing to compact integration of the whole system by using the 3D-printing.

Sample Analysis. The utility and accuracy of the proposed technique were firstly validated by analysis of CRMs including water (GBW08605, BW01101, GBW08603, GBW(E)080398, GBW(E)080545, soil (GBW07405, GBW07406), selenium enriched yeast (SELM-1) and biological sample (TORT-3 and DORT-5), obtaining consistent results with certified values (t-test, 95% confidence level) for the eight elements. Its applicability was further demonstrated through analyzing four real samples including river water, spring water, ground water and tap water, with recoveries from 88% to 111% achieved for all elements. Detailed analytical results of samples can be found in Section 10 of the SI.

CONCLUSION A prototype of compact optical emission spectrometer was constructed using 3D-printing technique, with a hollow electrode point discharge microplasma as the

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excitation source and hydride generation for sample introduction. The hollow electrode directed the sample vapor into the microplasma to ensure gaseous analyte species sufficiently participating in the diffusion, interaction and excitation process inside the microplasma. Therefore, high excitation capability, sensitivity and stability were achieved for determination of trace As, Bi, Ge, Hg, Pb, Sb, Se and Sn. Its reliability and potential application were demonstrated with satisfactory results. By improving the microplasma structure for high sample introduction/excitation efficiency and using the 3D-printing for the instrumental miniaturization, the HG-HEPD-OES is very promising for field analytical chemistry in the future.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (No. 21427810 and 21775105), National Key Research and Development Program of China (No. 2017YFD0801203), and the Fundamental Research Funds for Central Universities (No. 2016SCU04A12).

SUPPORTING INFORMATION This material is available free of charge via the Internet at http://pubs.acs.org. Instrumentation, analytical procedure, reagents and samples, spectral characteristics, discharge plasma characteristic calculation, 3D-printing technique and material, optimization of experimental conditions, interferences, analytical performance, and sample analysis.

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