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AC Driven Microplasma for Multielement Excitation and Determination by Optical Emission Spectrometry Yi Cai, Yong-Liang Yu, and Jian-Hua Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02904 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Analytical Chemistry

AC Driven Microplasma for Multielement Excitation and Determination by Optical Emission Spectrometry

Yi Cai, Yong-Liang Yu,* and Jian-Hua Wang Research Center for Analytical Sciences, Department of Chemistry, College of Sciences, Northeastern University, Box 332, Shenyang 110819, China

Corresponding Author *E-mail: [email protected] (Y.-L. Yu) Tel: +86 24 83688944; Fax: +86 24 83676698

ABSTRACT: Microplasma-optical emission spectrometry (OES) is a promising technique for developing portable analytical instrumentations for the real-time and on-site measurement of trace elemental species. However, its analytical performance is far from satisfactory for multielement analysis. Herein, a miniature OES system is developed for simultaneous multielement analysis with AC driven microplasma generated on the nozzle of a pneumatic micronebulizer as excitation source. Due to the strong excitation capability of the microplasma and its sufficient contact with solution, a series of elements, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Pb and Zn, are directly excited in spray with solution nebulization at a flow rate of 8 µL s-1. The characteristic optical emissions are measured by a charge-coupled device (CCD) spectrometer. In addition, hydride generation is compatible to the present system,

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which makes it feasible for simultaneous excitation of the hydrides of As, Ge, Hg, Sb and Sn by reaction with 0.8% (m/v) NaBH4. The microplasma OES system exhibits a powerful capability for multielement analysis with favorable limits of detection for the mentioned elements.

KEYWORDS: AC driven microplasma, optical emission spectrometry, multielement analysis.

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INTRODUCTION Microplasma is produced by confining the dimension of plasma to 1 mm or less.1 Currently, various configurations of microplasma have been applied for optical emission, including glow discharge (GD),2-4 microhollow-cathode discharge (MHCD),5,6 dielectric barrier discharge (DBD),7-9 point discharge (PD),10,11 capacitively coupled microplasma (µCCP),12 microfabricated inductively coupled plasma (mICP),13 and microwave microstrip plasma (MSP).14 Compared to the conventional inductively coupled plasma (ICP) as excitation source, microplasma possesses a series of excellent characteristics, including simplicity, small size, nonthermal source, low power and gas consumption.15 Therefore, the development of nonthermal optical emission spectrometric (OES) systems based on microplasma instead of conventional ICP as excitation source shows promising potential as a portable analytical instrumentation for real-time and on-site measurement of trace elemental species.16 Generally, the common nonthermal OES systems consist of a microplasma excitation source, a small charge-coupled device (CCD) spectrometer or other photodetector, and a well-matched sampling approach.17 Due to the use of low power supply for microplasma generation, the detection sensitivity of the nonthermal OES systems is usually inferior to that of the conventional ICP-OES instruments. In order to make up the deficiency of the limited excitation capability of microplasma itself, the employment of a well-matched sampling approach is essential for improving the detection sensitivity of the nonthermal OES systems. Initially, various gaseous species 3



can be directly introduced into the microplasma for the excitation and OES detection.18 Whereas various species in aqueous medium should be converted into volatile species as “pure” and “dry” species for their excitation and detection by the microplasma OES system,19 such as chemical vapor generation7,10,11,20-22 or electrothermal vaporization,23-27 in order to avoid the concomitant products and residual moisture. Although gaseous introduction avoids the decrease of the excitation capability or even extinguishing of microplasma resulting from the presence of a large amount of water molecules, multielement analysis capability of OES itself has to be highly dependent on gaseous species produced or electrothermal vaporization device. In order to make the microplasma OES system possess real multielement analysis capability like ICP-OES instrument, liquid introduction is also employed to facilitate the direct detection of more elements in solution. In this case, microplasma is usually formed between a flowing/nonflowing liquid surface and an electrode.28 Franzke et al. evaluated analytical performance for the direct determination of 23 elements in solution by a microplasma OES system at a low sampling flow rate of 20 µL min-1, giving rise to the LODs between 0.016 mg L-1 for Li and 41 mg L-1 for Bi.29 Zhu et al. applied microplasma formed between the nonflowing liquid surface and the tungsten electrode for the direct excitation of five elements including Na, K, Cu, Zn and Cd in solution, achieving the LODs between 7 µg L-1 for Na and 79 µg L-1 for Zn by OES measurements.30 However, the detection sensitivity of the microplasma OES system with liquid introduction is far from satisfactory for multielement analysis, especially for the concerned trace heavy metal elements.

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In view of the limited excitation capability of the microplasma, it is essential to increase the effective contact between the liquid surface and microplasma for improving the detection sensitivity of the microplasma OES system with liquid introduction. For this reason, pneumatic nebulization approach provides an alternative for liquid introduction into the microplasma. It has been demonstrated that microplasma generated on the nozzle of a pneumatic micronebulizer can focus the energy on a confined space and directly excite the metal elements in spray, achieving higher detection sensitivity to Zn, Cd and Hg by OES measurements.31,32 Therefore, the microplasma OES system with pneumatic nebulization introduction is promising to achieve real multielement analysis. In this work, AC driven microplasma as a nonthermal excitation source is integrated on a pneumatic micronebulizer for developing a miniature OES system. Due to the strong excitation capability of the proposed microplasma and its sufficient contact with the aqueous solution, the present OES system possesses a powerful capability for multielement excitation and provides comparable limits of detection with respect to conventional bulky ICP-OES instrumentation. 14 elements, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Pb and Zn, are directly excited in spray with solution nebulization, while As, Ge, Hg, Sb and Sn are excited in the form of hydride. The characteristic optical emissions produced are recorded by a CCD spectrometer for quantitative analysis. The detection limits for these elements are between 0.9 µg L-1 for Cd and 880 µg L-1 for Cr. The reliability and practicability of the present OES system are validated by multielement determination in water samples.

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EXPERIMENTAL SECTION Experimental Setup and Operating Procedure. Figure 1 illustrates the configuration of the AC driven microplasma OES system. The core part is a homemade pneumatic micronebulizer integrating a microplasma excitation source. The micronebulizer shows a basic structure of the double concentric tube type. The inner tube of the micronebulizer is a 10-mm quartz capillary (0.20 mm i.d. and 0.32 mm o.d.) connected to a 40-mm quartz tube (1 mm i.d. and 2 mm o.d.) for sample introduction. A tungsten filament with a diameter of 0.06 mm as an electrode is used to surround the outside of the quartz capillary. The outer tube of the micronebulizer is a 40-mm quartz tube (4 mm i.d. and 6 mm o.d.) with a 0.4-mm i.d. nozzle for argon introduction. The distance between two outlets of inner and outer nozzles is set at 2 mm. A tungsten rod with a diameter of 1.0 mm as counter electrode is placed at a distance of 2 mm from the outer nozzle of micronebulizer. The discharging gap between two tungsten electrodes is adjusted by a manual positioner (TSM25-1, Zolix, China). The microplasma is directly generated between two tungsten electrodes, when applying a high-frequency and high-voltage electric field from a neon power supply (ENT-106B, Guangzhou Xinxing Neon Light Supply, China) at a sinusoidal discharging voltage with a frequency of ca. 40 kHz. Power consumption is measured by plasma generator (CTP-2000K, Nanjing Suman Plasma Technology Co., Ltd, China). Sample introduction is achieved with solution nebulization or chemical vapor generation approach by a sequential injection system (FIA lab Instruments, USA). A sensitive CCD spectrometer (AvaSpec-ULS2048-4-USB2, Avantes, Netherlands) is 6


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combined with a fiber-optic probe (600 µm core i.d. and 50 cm length) for recording the emission spectra,31,32 offering spectral resolutions of ca. 0.2 nm and 0.4-0.5 nm within wavelength ranges of 200-620 nm and 620-1000 nm, respectively. A halogen tungsten lamp (LS-1, Ocean Optics, USA) is employed as a calibration lamp to calibrate the CCD detector.

Figure 1. Schematic diagram of the AC driven microplasma OES system incorporating the photographs of the microplasma before and after liquid sampling. Route (1) and (2) represent two different sample introduction approaches, i.e., solution nebulization and chemical vapor generation. GLS: gas-liquid separator (The overall volume (O.V.) and dead volume (D.V.) are 14 mL and 13 mL, respectively). With solution nebulization at the flow rates of liquid 8 µL s-1 and argon 600 mL min-1, 14 elements including Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Pb and Zn in solution are able to be directly excited by the microplasma and measured by OES. Alternatively, with chemical vapor generation by reaction of sample solution

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with 0.8% (m/v) NaBH4, As, Ge, Hg, Sb and Sn in aqueous solution are converted into volatile species and dried by 6 mL of concentrated H2SO4 for the excitation and measurement by the microplasma OES system at an argon flow rate of 200 mL min-1. Reagents and Sample Pretreatment. The details of the reagents and sample pretreatment are shown in the Supporting Information.

RESULTS AND DISCUSSION Emission Spectral Characteristics. Due to the presence of argon atmosphere around the nozzle of micronebulizer, stable microplasma is formed between two tungsten electrodes at a certain discharging voltage under the open circumstance. When sample solution passes through the micronebulizer at a flow rate of 8 µL s-1, it can be nebulized at the nozzle of micronebulizer by an argon stream and keep a sufficient contact with the microplasma. In this case, analytes in spray would be simultaneously atomized and excited by the microplasma to produce the characteristic optical emissions, due to the presence of high-energy electrons and active species including ·H, ·OH, and H2O2 in microplasma.33 As illustrated in Figure 2, the characteristic optical emission lines of 14 common elements including Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Pb and Zn appear clearly within the range of 200-800 nm. Compared to the conventional ICP-OES (Table S1), only sensitive atomic emission lines of elements can be observed by the microplasma OES system without their ionic emission lines. Although the number of characteristic optical emission lines of elements for quantitative analysis is less by the proposed microplasma OES, multielement analysis can still be achieved. 8


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3000

Mg 285.2

A 2500

Intensity

2000 1500 1000

Sample Blank Cu 324.8

Ni Mn 231.1 279.5 Cd 232.0 Fe 248.3 228.8 Co Zn 240.7 213.9

Cr 359.4 360.5

Pb 405.8

Ca 422.7

500 0 200

3000

250

300

350

400

450

500

Wavelength / nm

B

K 766.5 769.9

2500 Li 670.8

2000

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1500

Na 589.0 589.6

1000 500 0 500

550

600

650

700

750

800

Wavelength / nm

Figure 2. Optical emission spectra of argon microplasma within the wavelength ranges of (A) 200-500 nm and (B) 500-800 nm by introducing a sample solution (red line) or blank solution (black line) via nebulization. Sample solution in 5% (v/v) alcohol contains 10 µg L-1 Cd2+, 50 µg L-1 Li+ and Zn2+, 80 µg L-1 Mg2+, 100 µg L-1 Cu2+ and Mn2+, 200 µg L-1 Na+ and Pb2+, 300 µg L-1 Co2+ and Ni2+, 500 µg L-1 Ca2+ and Fe3+, 1.5 mg L-1 K+ and 8 mg L-1 Cr3+. Blank solution: 5% (v/v) alcohol. Full details of the experimental parameters are given in Table S2. Besides solution nebulization approach, chemical vapor generation can also be directly applied for sample introduction in the present system. Some elements that are capable of vapor generation are converted into volatile species by reaction with

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NaBH4, and afterwards directly introduced into the microplasma by an argon stream for the excitation and OES detection. As illustrated in Figure 3, the characteristic optical emission lines of As, Ge, Hg, Sb and Sn appear clearly within the range of 200-320 nm. These five elements are more sensitively detected by the present system with chemical vapor generation rather than solution nebulization approach. The microplasma is able to tolerate the presence of a large amount of concomitant hydrogen and achieve the excitation of elements, without the occurrence of microplasma quenching. Chemical vapor generation approach as a supplement further extends the scope of application for the microplasma OES system. There is a broad continuum extending between 200 nm and 450 nm due to the dissociation of hydrogen in microplasma.7,34 Considering the fluctuation of the microplasma, background correction is performed by deducting the background signal at correction wavelength from the raw emission signal at detection wavelength (Figure S1). 5000 Sample Blank

4000

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ge Sb 265.1, 269.1 271.0, 275.5 Hg 259.8 Sb 253.7 252.9

3000 Ge Sb 206.9 2000 206.8

1000 0 200

Sb As 231.1 228.8

Sn 300.9 303.4

Sb 217.6

220

240

260

280

300

320

Wavelength / nm

Figure 3. Optical emission spectra of argon microplasma within the wavelength range of 200-320 nm by introducing a sample solution (red line) or blank solution (black line) via vapor generation. Sample solution: 30 µg L-1 Hg2+, 100 µg L-1 As(III), Ge4+,

10


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Sb3+ and Sn4+ in 1.5% (v/v) HCl. Blank solution: 1.5% (v/v) HCl. Full details of the experimental parameters are given in Table S3. Micronebulizer Design. A homemade pneumatic micronebulizer integrating a microplasma excitation source is applied for sample introduction and subsequent excitation. There are two key structure parameters for the present double concentric tube type of micronebulizer, i.e., the distance between two outlets of inner and outer nozzles, and the outlet size of the outer nozzle. As illustrated in Figure 4A, position a, b, c, and d of inner nozzle of micronebulizer correspond to the different distances between two outlets of inner and outer nozzles, i.e., 0.5, 1, 2, and 4 mm. This distance would directly influence the effect of solution nebulization, and then affect the optical emissions (Figure 4B). Due to the poor nebulization effect, the optical emissions of five representative elements are barely obtained at a short distance of 0.5 mm. A favorable nebulization effect would highly improve the detection sensitivity by virtue of the sufficient contact between the solution and microplasma, so this distance is better to be set at 2 mm. At a long distance of 4 mm, fine water mist after nebulization tends to condense into the droplets at the outer nozzle of the micronebulizer, making nebulization effect worse and optical emission intensities decrease. In the present micronebulizer, the outlet size of the outer nozzle should match with the channel size of the microplasma (Figure 4C). The smaller outlet size of the outer nozzle would decrease the cross section of microplasma, resulting in a decrease of contact area between the solution and microplasma. On the other hand, the larger outlet size of the outer nozzle would lead to an insufficient contact between the

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solution and microplasma. In both cases, the optical emission intensities would decline sharply with solution nebulization approach. The optimal optical emissions of most elements are obtained with a 0.4 mm i.d. outlet of the outer nozzle, due to well agreement between cross sections of solution and microplasma.

A Nozzle

0.5

0.50 a b 1.0 1 2 c d 4

Normalized intensity

1.2 1.0

1

2

4 Unit: mm

0.8 0.6 0.4 0.2 Cd

1.2

Unit: mm

0.5

Distance

B

0.0

Normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0

C

Co

Li

Outlet inner diameter

Mn 0.2

0.3

Pb 0.4

0.5 Unit: mm

0.8 0.6 0.4 0.2 0.0

Cd

Co

Li

Mn

Pb

Figure 4. (A) Schematic diagram of a homemade pneumatic micronebulizer, and dependence of optical emission intensities on (B) the distance between two outlets of inner and outer nozzles, and (C) the outlet size of the outer nozzle. Sample solution: 0.2 mg L-1 Cd2+, 2 mg L-1 Co2+, 1 mg L-1 Li+, 1 mg L-1 Mn2+, and 2 mg L-1 Pb2+ in 5% (v/v) alcohol. Full details of the experimental parameters are given in Table S2. Microplasma Feature. A stronger excitation capability of the present microplasma has been presented by directly exciting 14 elements in spray and 5 elements in gas phase to produce their characteristic optical emissions. First, it is

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attributed to a higher power consumption applied between two electrodes. As shown in Figure S2A, C, the intense optical emissions of most elements are obtained at a power consumption of more than 9.6 W with solution nebulization and 5.1 W with chemical vapor generation. Furthermore, the different kinds of work gas for the microplasma generation may lead to the different discharging modes. The ionization mechanisms for helium and argon microplasma are different due to the existence of N2 (impurity) with an ionization potential of 15.5 eV. Helium metastables (HeM, 19.8 eV and 20.6 eV) with longer lifetimes are formed by transitions from excited helium atoms (He*). N2+ is generated by Penning ionization of N2 and HeM.35,36 The N2+ ions have an optical transition under an emission band at 391 nm.37 However, with the same discharge geometry, a much higher voltage is required to ignite argon microplasma, which may result in filamentary microplasma and dramatically decrease the soft ionization efficiency. In contrast to helium discharge, argon discharge is based on another ionization pathway: N2+ is not involved, because N2 molecules fail to be Penning-ionized by argon metastables (ArM, 11.5 eV and 11.7 eV).35,36 The N2+ emission band at 391 nm can be observed in helium spectrum, while barely appears in argon spectrum (Figure S4). It illustrates that the discharge in argon is filamentary microplasma. In this case, the electron and current densities are far higher than those obtained in a homogeneous discharging mode.38 Due to the different discharging modes, argon and helium microplasmas as excitation source present the difference of excitation capability in this system. It is found that the characteristic emission lines of elements start to be observed clearly in argon microplasma at a power consumption of

13



5.1 W, and their intensities are enhanced with the increase of the power consumption from 5.1 to 11.4 W. By contrast, the characteristic emission lines of only Zn and Cd could be observed in helium microplasma even at a power consumption of 11.4 W with solution nebulization. A halogen tungsten lamp was employed as a calibration lamp to calibrate the CCD detector (Figure S3). The correction coefficients (Rλ) for the detector response at different wavelengths are shown in Figure S4. Based on the spectral parameters listed in Table S4, S5,39 and atomic emission lines shown in Figure S4, electronic excitation temperature (Tex) obtained from Boltzmann’s plot (Figure S5) is 7587 ± 370 K for argon microplasma and 3610 ± 238 K for helium microplasma at the power consumption of 9.6 W.40 Molecular rotational temperature (Trot) is obtained by utilizing LIFBASE Software Spectroscopy Tool (v2.1.1) to match the simulated spectrum of OH (0, 0) band to its experimental spectrum within the range of 305-315 nm.41 As shown in Figure S6, well agreement between the simulated spectrum of OH (0, 0) band and its experimental spectrum is achieved at the Trot values of 2213 ± 137 K for argon microplasma and 1095 ± 269 K for helium microplasma at the power consumption of 9.6 W. On one hand, through the measurement of Tex, the excitation capability of argon microplasma is higher than helium microplasma. It means that more elements can be excited in argon microplasma. On the other hand, the number density of ground state atom (N0) is another significant factor to emission intensity. The previous study had proved that liquid spray discharge induced plasma can be used as a kind of novel vapor generation technique.42 Some volatile species (probably hydrides) are generated through the

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microplasma. With the same introduction approach, Cd and Zn in aqueous sample can be converted to volatile species through the microplasma, leading to the increase of N0. Therefore, Zn and Cd emission lines could be observed in helium microplasma. It is noteworthy that the gas temperatures of both argon microplasma and helium microplasma are at a relatively low level, i.e., 335 K and 310 K measured roughly by a thermometer before and after liquid sampling, respectively.43 Obviously, both argon microplasma and helium microplasma generated belong to a kind of nonthermal excitation source. In addition, the emission intensities of elements are also highly dependent on the discharging gap between two electrodes. It is found in Figure S2B that the sensitive optical emissions of five elements including Cd, Co, Li, Mn and Pb are obtained at a short discharging gap of 4 mm. Sample Introduction Conditions. With solution nebulization, the conditions of sample solution highly influence the detection sensitivity of the present microplasma OES system. As shown in Figure S7A, the addition of a certain amount of alcohol in sample solution would play the role of sensitization for the determination of most elements by the present system. Alcohol can be partly decomposed into H2,44 and highly promotes the production of ·H radicals.45 The enhancement in the presence of alcohol is probably a consequence of more efficient hydride generation between cations and reducing radicals (·H radicals). In Figure S4A, B, the Hα line at 656 nm could be obviously observed in the spectra of argon and helium microplasma. There are ca. 13% and 23% enhancement of Hα (656 nm) for argon and helium microplasma in the presence of 5% (v/v) alcohol. When a certain amount of alcohol is added into

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the sample, the intensities of Cd, Co and Pb are enhanced, but the intensity of Li remains unchanged. The presence of alcohol in sample solution facilitates the production of ·H in microplasma and further promotes the atomization of most elements, particularly for Zn and Cd, giving rises to the more sensitive optical emissions.46,47 In addition, solution pH is better under a weak acidic condition to avoid the suppression of ·H production at a lower pH value and hydrolyzation of metal ions at a higher pH value, as shown in Figure S7B. Therefore, sample solutions are prepared in 5% (v/v) alcohol at pH 3 for solution nebulization introduction. The flow rates of liquid and argon influence the sampling quantity and nebulization efficiency, and thus influence the detection sensitivity of the present microplasma OES system, as shown in Figure S8. After optimization, the flow rates of liquid and argon are fixed at 8 µL s-1 and 600 mL min-1, respectively. As a supplement, some elements are more suitable to be introduced into the microplasma by a chemical vapor generation approach. In such case, the concentrations of HCl in sample solution and NaBH4 have to be optimized for the simultaneous and efficient vapor generation of these elements. As shown in Figure S9, the optimal detection sensitivity for simultaneous determination of As, Ge, Hg, Sb and Sn is obtained by using 1.5% (v/v) HCl in sample solution and 0.8% (m/v) NaBH4 for vapor generation at an argon flow rate of 200 mL min-1. To avoid the occurrence of microplasma quenching due to a large amount of hydrogen produced, HCl in sample solution and NaBH4 are both at a relatively low concentration level. Analytical Performance Evaluation and System Validation. After a thorough

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optimization of the experimental conditions, the characteristic analytical performances of the microplasma OES system for the simultaneous determination of Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Pb and Zn with solution nebulization and As, Ge, Hg, Sb and Sn with chemical vapor generation are summarized in Table 1. The achieved detection limits (LODs) of 19 elements are between 0.9 µg L-1 for Cd and 880 µg L-1 for Cr, and RSD values are less than 6.2%. Compared to the conventional ICP-OES instrument, the consumptions of sample solution and argon gas are both in very small scale for each analysis by the present system. 120 µL of sample solution is introduced for solution nebulization at an argon flow rate of 600 mL min-1, while 500 µL of sample solution is used for chemical vapor generation at an argon flow rate of 200 mL min-1. Compared with other microplasma OES systems listed in Table S6, the present system balances both the scope of simultaneous detectable elements and detection sensitivity. Table 1. Analytical performances of the present microplasma OES system for multielement analysis Detection Element wavelength (nm)

Correction wavelength (nm)

LOD (µg L-1)

Linear range (µg L-1)

RSDa (%)

As

228.8

229.2

16

50-5000

1.9

4.1

Ca

422.7

423.0

65

200-20000

4.6

5.7

Cd

228.8

229.2

0.9

3-500

1.5

4.4

Co

240.7

240.3

32

100-2000

3.5

5.1

Cr

359.4

359.6

880

3000-100000

3.8

4.1

Cu

324.8

325.1

15

50-2000

4.4

4.7

Fe

248.3

248.6

52

200-2000

3.2

5.3

Ge

265.1

264.7

10

30-4000

0.8

5.6

17


RSDb (%)


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Hg

253.7

253.2

3

10-2000

1.4

4.5

K

766.5

765.9

150

500-10000

3.8

5.0

Li

670.8

671.5

6

20-1000

2.3

5.0

Mg

285.2

285.8

8

30-3000

2.6

4.4

Mn

279.5

279.1

10

30-3000

1.7

6.2

Na

589.0

588.4

25

80-3000

3.2

5.4

Ni

232.0

232.3

31

100-3000

2.2

3.9

Pb

405.8

406.0

25

80-3000

3.2

4.5

Sb

217.6

217.2

10

30-5000

1.4

4.5

Sn

303.4

303.0

12

40-3000

1.2

4.1

Zn

213.9

214.1

5

20-3000

1.8

4.3

a

A standard solution containing 0.2 mg L-1 Cd2+, 0.5 mg L-1 Hg2+ and Zn2+, 0.8 mg

L-1 Mg2+, 1 mg L-1 Cu2+, Fe3+, Li+, Mn2+, Na+ and Sn4+, 2 mg L-1 Ca2+, Co2+, Ge4+, Ni2+, Pb2+ and Sb3+, 5 mg L-1 As(III) and K+, and 50 mg L-1 Cr is employed for RSD evaluation at 9 successive measurements. b

A standard solution containing 10 µg L-1 Cd2+, 30 µg L-1 Hg2+, 50 µg L-1 Li+ and

Zn2+, 80 µg L-1 Mg2+, 100 µg L-1 As(III), Cu2+, Ge4+, Mn2+, Sb3+ and Sn4+, 200 µg L-1 Na+ and Pb2+, 300 µg L-1 Co2+ and Ni2+, 500 µg L-1 Ca2+ and Fe3+, 1.5 mg L-1 K+ and 8 mg L-1 Cr3+ is employed for RSD evaluation at 9 successive measurements. To validate the reliability and applicability of the present system, it is applied for the simultaneous determination of multielements in real water samples, including GBW 08608 and GBW(E) 080039 (water, from National Center for Standard Materials, China). It is obvious that reasonable agreements are obtained between the certified and the found values for water samples, as listed in Table 2. Meanwhile, the spiking recoveries obtained are favorable within the range of 93-110%. The

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experimental results have demonstrated that the present system is robust and reliable for multielement analysis. Table 2. Simultaneous determination of multielements in water samples by employing the present microplasma OES system Sample

GBW08608

Element

Certified Found (µg L-1) (µg L-1)

Ca

39000

Cd

12.0±0.5 9.9±1.9

Cr

33±2

Cu

41100±3300b 2.2

Spiked Recovery (µg L-1) (%) 40000

102

3.8

10

100

not detected

-

5000

104

50±2

53±4

3.1

100

105

K

2200

2100±200

2.7

1000

110

Mg

11000

11150±920b

0.58

10000

93

Ni

61±2

66±7

3.0

100

94

Pb

50±2

50±6

0.23

80

98

Zn

90±3

90±6

0.37

100

99

Cd

10.8±0.8 11.0±0.4

1.5

10

100

Cr

30±2

not detected

-

5000

106

100±3

103±6

1.6

100

99

Pb

30±2

35±4

4.1

50

104

Zn

800±24

760±33

4.2

200

106

GBW(E)080039 Cu

a

T valuea

Paired t test between the certified and the found values; T (critical) at the 95%

confidence level and two degrees of freedom is 4.30; The number of measurements is 3. b

GBW08608 was 10-fold diluted for the determination of Ca and Mg.

CONCLUSIONS In this study, AC driven microplasma as excitation source is subtly integrated on

19



the nozzle of a pneumatic micronebulizer for developing a nonthermal OES system. The present microplasma OES system as a portable instrument exhibits a powerful capability for multielement analysis, as demonstrated by the simultaneous determination of 14 elements with solution nebulization and that of five elements with chemical vapor generation. Compared to the conventional ICP-OES, the proposed microplasma OES system presents low power and argon consumption for multielement analysis. It would play an important role in the development of portable analytical instrumentations for the real-time and on-site multielement analysis.

ASSOCIATED CONTENT Supporting Information Additional information as noted in text. This material is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.-L. Yu) Tel: +86 24 83688944; Fax: +86 24 83676698 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China (21475018 and 21727811), and the Fundamental Research Funds for the 20


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Central Universities (N160504010and N170507001).

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For TOC only

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AC Driven Microplasma for Multielement Excitation ... - ACS Publications

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