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Elemental Determination of Microsamples by Liquid Film Dielectric Barrier Discharge Atomic Emission Spectrometry Qian He,†,‡ Zhenli Zhu,*,† Shenghong Hu,†,‡ Hongtao Zheng,§ and Lanlan Jin† †

State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, China 430074 Faculty of Earth Sciences, China University of Geosciences, Wuhan, China 430074 § Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, China 430074 ‡

ABSTRACT: In this study, a new liquid-film dielectric barrier discharge (LFDBD) atomic emission source was developed for microsample elemental determination. It consists of a copper electrode, a tungsten wire electrode, and a piece of glass slide between them, which serves as the dielectric barrier as well as the sample plate. The sample solution with 1 mol L−1 nitric acid, when deposited onto the surface of the glass slide, forms a thin liquid film. The plasma is generated between the tip of the tungsten wire electrode and the liquid film surface when alternating-current (ac) high voltage (peak voltage ∼3.7 kV, frequency ∼30 kHz) is applied on the electrodes. Qualitative and quantitative determinations of metal ions in the sample solution were achieved by atomic emission measurements in the plasma and were demonstrated in this study with elements Na, K, Cu, Zn, and Cd. Detection limits were in the range from 0.6 ng (7 μg L−1) for Na to 6 ng (79 μg L−1) for Zn. Repeatability, expressed as relative standard deviation from seven repetitive analyses of samples with analyte concentrations at 1 mg L−1, varied from 2.1% to 4.4%. Compared with other liquid discharge systems that operate at atmospheric pressure, the current system offers several advantages: First, it eliminates the use of a sample flow system (e.g., syringe or peristaltic pump); instead, a small aliquot of sample is directly pipetted onto the glass slide for analysis. Second, it is a microanalysis system and requires sample volume ≤80 μL, a benefit when a limited amount of sample is available. Third, because the sample is applied in aliquot, there is no washout time, and the analysis can be easily extended to sample array for high-throughput analysis. The proposed LFDBD is promising for in-f ield elemental determination because of its simplicity, cost effectiveness, low power supply, and no inert gas requirement. flow rates (typically at 8−10 mL min−1) for sustainment of stable plasma. Webb et al.9,10 proposed a simplified design of ELCAD, termed as solution-cathode glow discharge (SCGD), exhibiting lower detection limits and lower flow rates (2.5−3.5 mL min−1). Shekhar et al.11 reported a new ELCAD system that yields stable plasma at even lower flow rates (0.96 mL min−1) by attaching a V-groove onto the liquid glass-capillary. Furthermore, Jenkins et al.12 integrated the ELCAD system to a microfluidic chip; the total electrical power to sustain the discharge was reported to be less than 70 mW. A variation of the ELCAD source, termed liquid sampling-atmospheric pressure glow discharge (LS-APGD),13−15 was developed by Marcus and Davis for direct analysis of electrolytic solutions. Different from ELCAD systems, which use a nonconductive capillary and reduced/no gas flow, LS-APGD systems generally employ a conductive capillary and often use a low gas flow. The liquid-filled capillary can act either as the cathode or the anode of the discharge in LS-APGD. It can be operated at low flow rates (0.3−1.0 mL min−1).

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he determinations of metal concentrations in water and wastewaters are very important issues in industrial or environmental fields. Well established analytical techniques, such as AAS, AFS, ICP-AES, and ICPMS, can perform fast, sensitive, and accurate elemental determinations for samples of virtually of all kinds. However, these instruments often require high power and inert or special (e.g., acetylene) gases and operate at high temperature and even under high vacuum, which are not practical for many on-site measurements. Therefore, it is of much significance to develop compact and low-cost instruments,1 for example, for in situ monitoring of metal ions. In recent years, liquid discharge microplasmas2−5 are especially promising for the development of portable instruments for elemental analysis because of their small size, low power consumption, cost effectiveness, and reduced/no inert gas requirement. The electrolyte as cathode glow discharge (ELCAD),6−8 which is an atmospheric-pressure glow discharge between a metal anode and the surface of a flowing electrolyte sample solution, has attracted much attention for determination of trace metals in water samples. Atomic emission from elements in the sample solution can be observed from the plasma. Early versions of ELCAD apparatus, however, required high sample © 2012 American Chemical Society

Received: February 21, 2012 Accepted: April 10, 2012 Published: April 10, 2012 4179

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These advantages make it attractive as a potential miniaturized AES system for in-f ield elemental determination.

Compared to those well established analytical techniques for elemental analysis, the aforementioned liquid discharge devices eliminate the use of a nebulizer and are operated at low power with reduced/no need for inert gas supply, and thus have promising potentials for on-site or in-f ield applications. However, they are operated with flowing solution as one electrode, which necessitates the use of a sample pump in the system. Approach with nonflowing solution electrode, which can further miniaturize the system, is desirable. Liquid electrode plasma atomic emission spectrometry (LEP-AES)16−18 with pulsed dc power based portable device (Micro Emission, Japan) is currently the only commercial liquid discharge unit for elemental determination with a nonflowing approach. In LEPAES, both the cathode and anode electrodes of the pulsed discharge are solutions. Recently, Kitano et al.19 developed a highly sensitive LEP-AES by combining quartz chip with sample flow system, whose detection limits for Cd and Pb were 0.52 μg L−1 and 19.0 μg L−1, respectively. Another simple nonflowing device is liquid electrode spectral emission chip (LEd-SpEC),20 which produces glow discharge in air or at moderate vacuum with liquid electrodes. This device can be used to measure sodium ion at concentrations of less than 10 mg L−1, lead at 5 mg L−1, and aluminum and chromium impurities at 10 mg L−1 in water; unfortunately, detection limits of the LEd-SpEC have not yet been reported. Dielectric barrier discharge (DBD) is a typical nonthermal plasma, which was first introduced to the analytical-chemistry community by Miclea and co-workers.21 Because of its several unique advantages such as simple construction, low power consumption, long lifetime, and operation at atmospheric pressure, new measurement methods based on DBD are of growing interest in the field of analytical spectrometry.22−24 For example, DBD has been used as a low-temperature atom reservoir for AAS25 and AFS,26 as an excitation source for AES,27,28 as a soft-ionization source for MS29−31 or GC detector,32,33 and as a sample introduction method for ICPMS34 or AFS.35 In most DBD systems, the analyte in the solution sample is often required to be transferred into gaseous form (e.g., through hydride or cold-vapor generation) to maintain plasma stability. Direct solution analysis with DBD was very challenging, partly because typical DBD cannot provide sufficient power for complete solvent evaporation and the subsequent analyte atomization. Tombrink et al.36 first demonstrated direct liquid analysis by using a specially designed capillary DBD. It consists of a fused-silica capillary where the plasma is formed between the flowing liquid and the metal electrode. The advantage of this system is the very low sample flow rate at just 1 μL min−1 and the low power consumption. The same research group37 further modified this system, used higher flow rates (20 μL min−1), and achieved detection limits of 0.02 mg L−1 for potassium and 6.9 mg L−1 for barium. In the present study, a nonflowing liquid-film DBD (LFDBD) source was developed for elemental determination of solution samples with AES. The nonflowing sample configuration eliminates the need of a pump and thus can significantly reduce the size of the whole device. The characteristics of this LFDBD and the effect of electrolyte identity were investigated. The analytical performance was evaluated with Na, Cd, Cu, Zn, and K as analytes. This LFDBD source consumes low power (≤18 W), operates at open atmosphere without the need for any inert gas, and is capable of analyzing volume-limited samples (≤80 μL). Moreover, multielements determination can be achieved within 1 min.

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EXPERIMENTAL SECTION Instrumentation. Schematic diagram for the experimental setup is shown in Figure 1. The LFDBD device was composed

Figure 1. (a) Schematic diagram of the LFDBD atomic emission spectrometry setup. (b) An inset shows the cross-sectional shape of the concave well of the glass slide.

of a tungsten wire electrode (0.4 mm diameter × 30 mm length), a copper sheet electrode (15 mm × 15 mm), and a commercially available glass slide (25.4 mm × 76.2 mm × 1.2 mm) with a single concave well of diameter 15 mm and depth 0.5 mm (SAIL BRAND, CAT.NO.7103, Jiangsu, China). The glass slide was placed directly on the top surface of the copper electrode. The glass slide not only acts as the discharge barrier but also serves as a platform for the samples. Sample solution of precisely determined volume was pipetted into the concave well before each measurement. The sample forms a thin liquid film in the concave well. The tungsten electrode is aligned close to the center of the concave well. The distance between the liquid surface and the tip of the tungsten electrode was about 2 mm. Plasma power was provided from an ac power supply (Ozone generator power, Beijing Guoke Ozone Application Technology Co. Ltd., Beijing, China) with about 30 kHz frequency, 3.7 kV peak voltage, and 18 W maximum output power. When ac high voltage was applied, discharge plasma was formed between the tip of the tungsten electrode and the surface of the liquid film. The plasma resembled the shape of an inverted cone with diameter about 1.0 mm on the solution surface. The discharge was imaged 1:1 with a fused-silica lens (focal length = 100 mm, diameter = 25.4 mm) to the entrance slit of a monochromator (Princeton Instruments, Acton SP 2500, focal length = 0.5 m), which was equipped with a 1200 grooves/mm holographic grating. The monochromator has motorized slits whose width can be adjusted from 5 μm to 3 mm. Measured spectral resolution was 0.04 nm (at 435.8 nm) with slit width at 10 μm. The slit width was set at 80 μm for all experiments presented in this paper, unless otherwise indicated. The slit height was set at about 5 mm. A Hamamatsu R928 photomultiplier tube (PMT), biased at −700 V, was used as the detector. Integration time for the photocurrent was set at 300 ms. Procedure. An aliquot of 80 μL solution was pipetted directly into the concave well of the glass slide. When the highvoltage ac power supply was turned on, a plasma spontaneously ignited between the tip of the tungsten electrode and the surface of the liquid film. The plasma was spatially stationary during the discharge process. Due to thermal effect of the plasma, the sample solution slowly evaporated; the plasma would eventually extinguish when the local solution where the 4180

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tungsten electrode aligned to starts to dry up. During the discharge process, the analyte in the sample solution was continuously volatilized, atomized, and excited in the plasma and resulted in atomic emission. In this work, emission signals were collected during the entire lifespan of the plasma from its ignition to its extinguishment, which took less than 60 s. Afterward, the glass slide was cleaned by simple rinsing with deionized water and dried up with some lint-free wipers for next sample analysis. No memory effect was observed. Reagents and Samples. All chemicals used in this work were of at least analytical-reagent grade including nitric acid, sulfuric acid, hydrochloric acid, sodium nitrate, potassium nitrate, copper nitrate, zinc nitrate, and cadmium nitrate. Stock solutions (1000 mg L−1) of Na+, K+, Cu2+, Zn2+, and Cd2+ were prepared by dissolution of appropriate masses of solid reagent in high purity deionized water (90005-02, Labconco water pro ps, Canada). Working standards were prepared from diluting stocking solution with nitric acid (pH = 0). The pH of the solution was adjusted by adding concentrated HNO3 and measured with a pH meter (HACH, HQ40d, America). The reference materials, simulated natural water samples GBW(E)080397(Cu), GBW(E)080400 (Zn), and GBW(E)080402 (Cd) (National Research Center for Standard Materials, Nanjing, China), were used to validate the accuracy of our method.

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RESULTS AND DISCUSSION At atmospheric pressure, DBD can be sustained either in inert gas or in air. In our LFDBD system, when liquid film is absent, the DBD plasma is difficult to ignite and sustain in air. On the other hand, when electroconductive liquid film is present, selfignited and stable plasma is readily formed between the surface of the liquid film and the tungsten wire tip. The liquid film is subject to both Joule heating by the electrical current and radiation heating by the plasma. The characteristic yellow color of Na atomic emission was easily observable with naked eyes at a Na concentration of 10 mg L−1. When the sample solution evaporates, the plasma automatically extinguishes and thus protects the glass slide from plasma sputtering. The lifetime of the device is very long, and the same glass slide can be used in hundreds of measurements with simple rinsing as pretreatment before each measurement. For demanding situations, which even simple rinsing is not possible to be performed on-site (e.g., remote area where deionized water is precious or even unavailable), the glass slide can be developed to a disposable device. Moreover, the nonflowing LFDBD avoids the use of pumping apparatus, thus miniaturizes the total system. In addition, the LFDBD can be operated with sample volume as low as 30 μL and is thus well suited for volume-limited samples. Furthermore, analysis can be finished within 1 min, which is preferred in high-throughput analysis. All these advantages suggest that LFDBD has the potential to become a promising technique for on-site applications. Spectral Characteristics. Emission spectra of the plasma were studied from a nitric acid solution (pH = 0) in the absence or presence of 20 mg L−1 Na, K, and Cu. Figure 2 shows typical emission spectra, measured by a portable spectrometer (Ocean Optics; USB 4000) with an optical fiber (AVANTES FCUV400-1-ME-SR 0905108), over the wavelength region between 200 and 900 nm. Background emission is weak below 280 nm. Above this wavelength, several molecular emission features attributed to OH bands (bandheads at 281 and 306 nm), and the N2 second positive system (bandheads at

Figure 2. Typical LFDBD emission spectrum of (a) an acid blank solution (HNO3, pH = 0) and (b) that spiked with Na, K, Cu at 20 mg L−1 each.

337, 357, 380, and 405 nm) were found. The atomic spectral lines of Na I (589 nm), K I (766 nm, 770 nm) are very strong, and those of Cu I (324 and 327 nm) are also evident. Clearly, the feasibility that the LFDBD system is an effective excitation source for atomic emission measurements of metal ions is demonstrated. However, the resolution of the USB 4000 spectrometer is not high enough to distinguish the background molecular structure and emission lines of some elements. Several elements like Cu I (324.7 and 327.4 nm), Cr I (357.8 nm), Pb I (405.8 nm) would be interfered by the N2 second positive system nearby. Meanwhile, the portable CCD spectrometer detection system might not be so sensitive. Therefore, a higher resolution spectrometer with a PMT detector was used for the elemental determination described in later sections of this paper. Optimization of Experimental Parameters. Due to their relatively large spans in excitation potentials, sodium, copper, and zinc were chosen as model analyte to further investigate the characteristics of the LFDBD system. In this study, the effect of experimental parameters such as the effects of the identity of background electrolyte, solution pH, input voltage, and sample volume on the emission intensity of sodium, copper, and zinc were evaluated. As the measured temporal emission intensity was not constant (cf. Figure 6) during the whole discharge duration, the emission signal was collected continuously during the whole discharge process. Background correction was achieved by subtraction of the blank emission signal measured at the same analyte wavelength. For the optimization studies, the temporal-integrated net-emission signal was used as the target to be maximized, and five replicates were performed at each set of studied parameters (e.g., solution pH, input voltage). 4181

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The effect of background electrolyte on the net-intensity of metal atomic emission was investigated in our work (Figure 3).

Figure 4. Effect of pH on the normalized integrated net-emission intensities of Na (589.0 nm), Cu (324.7 nm), and Zn (213.8 nm). Error bars in the figure represent standard deviations of five replicates. Discharge conditions: input voltage for discharge power supply, 220 V; electrolyte, HNO3; sample volume, 80 μL; concentration of Na, Cu, or Zn, 10 mg L−1.

Figure 3. Effect of acid electrolyte on the normalized integrated netemission intensities of Na (589.0 nm), Cu (324.7 nm) and Zn (213.8 nm). Error bars in the figure represent standard deviations of five replicates. Discharge conditions: input voltage for discharge power supply, 220 V; pH, 0; sample volume, 80 μL; concentration of Na, Cu, or Zn, 10 mg L−1.

Mezei et al.38 found in the ELCAD system that using acids as the supporting electrolyte results in stronger emission than using salts and that the acid anions also affect the emission intensity. Therefore, 10 mg L−1 Na, Cu, and Zn standard solutions were prepared in various acids such as nitric, sulfuric, and hydrochloric acids, with pH adjusted to 0. Stable discharges were produced under all these three studied acids. As shown in Figure 3, the net-intensities of Na, Cu, and Zn are all affected by the acid anions. The emission follows the order NO3− > Cl− > SO42‑, which is different from those reported in ELCAD systems38 and might suggest that the characteristics or mechanisms of LFDBD and ELCAD are somewhat different. On the other hand, emission intensities of the three studied elements were affected to similar extents by the three acids; the net-emission ratios of the three analytes in nitric, hydrochloric, and sulfuric acids were approximately 6:1.5:1. Therefore, nitric acid was selected as the electrolyte in the following studies. Another known behavior about ELCAD is that the element emission intensity strongly depends on the pH of the solution. The intensity is significantly reduced as the pH increases, and no visual emission is observed when the pH of the solution is above 3.0.6,39 In this study, the effect of solution pH, acidified with nitric acid, from 0 to 1.0 on the emission intensities of 10 mg L−1 Na, Cu, and Zn were investigated (Figure 4). The netemission intensities of Na, Cu, and Zn all declined, but to different extents, when pH was increased. For all three tested elements, no determined net-signal was observed at pH above 1.0. Thus, pH = 0 was selected in subsequent studies. When compared to the ELCAD system, the range of solution pH that can generate atomic emission of the analyte is narrower. A possible reason is that the dielectric layer increases the resistance and hence lower solution pH (higher conductivity) is needed to support the discharge. The influence of the input voltage for the discharge power supply on the net-emission intensity of 10 mg L−1 Na, Cu, and Zn was also investigated in this work. Input voltage from 180 to 230 V ac was examined, and the results are presented in Figure 5. When the input voltage increased from 180 to 230 V, the

Figure 5. Effect of input voltage for discharge power supply on the normalized integrated net-emission intensity of Na (589.0 nm), Cu (324.7 nm), and Zn (213.8 nm). Error bars in the figure represent standard deviations of five replicates. Discharge conditions: electrolyte, HNO3; solution pH, 0; sample volume, 80 μL; concentration of Na, Cu, and Zn, 10 mg L−1.

net-emission intensities of Na, Cu, and Zn all rose approximately linearly but with different proportionality. When the input voltage was set lower than 180 V, no plasma was formed. When the discharge voltage was increased beyond 230 V, it was found that the glass slide would break and the tungsten electrode could melt due to the elevated temperature. Therefore, an input voltage of 230 V was adopted in our study. The effect of sample volume on the net-emission intensity of the analyte and discharge duration was also evaluated. Three sample volumes of 50 μL, 80 μL, and 100 μL were pipetted onto the concave well with 10 mg L−1 Cu as analyte. The larger the volume was, the longer the discharge lasted. Both the discharge duration and the net-emission intensity increased linearly when the sample volume grew, and the maximum discharge duration for different sample volumes was less than 1 min. Taking the analysis time and the sensitivity into account, a 4182

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Table 1. Analytical Characteristics of Elemental Determinations with LFDBD Atomic Emission Spectrometry System and Other Liquid Discharge Sources element

wavelength (nm)

present work RSDa (%)

present work LOD (ng)

present work LOD (μg L−1)

ELCAD7 LOD (μg L−1)

SCGD10 LOD (μg L−1)

LEP-AES (MH-5000)40 LOD (μg L−1)

Na K Cu Zn Cd

589.0 766.5 324.7 213.8 228.8

3.9 2.2 4.4 2.1 2.7

0.6 2 6 6 3

7 25 74 79 38

1 1 10  30

0.1  4  2

1−10 10−100 100−1000 10−100 10−100

a

Standard concentration, 1 mg L−1, n = 7.

sample volume of 80 μL was chosen in following studies. In addition, the sample volume could be further reduced with smaller well size. In our LFDBD, a 30 μL sample volume can be used in elemental determination with a protein fence chip containing a 6 × 2 array conglutinated on a glass slide, but, unfortunately, this biochip could burn easily for its rubber material which could not be heated. Analytical Performance. The analytical characteristics of our LFDBD technique were evaluated under optimal conditions (electrolyte, HNO3; Solution pH, 0; input voltage for discharge power supply, 230 V; sample volume, 80 μL), as shown in Table 1. A calibration series of 0, 0.5, 1, 5, and 10 mg L−1 for the five elements was used for the detection limit determinations; the calibration standard for Na was further extended to 0.1 mg L−1 (i.e., with a total of six standards for Na). Five replicates were performed at each standard concentration. Calibration curves from integrating the area of emission signals were linear for all studied elements − Na, K, Cu, Zn, and Cd with linear correlation coefficients (R) between 0.9980 and 0.9997. Limits of detection (LOD) were calculated using the equation, LOD = (ksbk)/m, where k = 3, s is the standard deviation corresponding to 11 blank measurements, and m is the slope of the calibration curve. After optimization, the slit width was fixed at 120 μm for all elements for LOD experiment. The LOD values ranged from 7 to 79 μg L−1. However, it should be noted that the sodium detection limit is believed to be currently constrained by Na contamination in the glass blanks because Na emission was observed even in the high-purity nitric acid blank. The detection limits of the proposed LFDBD system have also been compared with other liquid discharge emission sources (cf. Table 1). It can be observed that the detection limits for this new source are comparable to or one order higher than other sources. In addition, repeatability, expressed as relative standard deviation from four replicates, ranged from 2.1% to 4.4% for analyte concentration at 1 mg L−1. Figure 6 shows the temporal profile of emission signal from 7 consecutive determinations of a 1 mg L−1 Cd standard. Recoveries, calculated from spiking 5 mg L−1 standard into tap water, were satisfactory: 104.5%, 98.0%, and 94.8% for Cu, Zn, and Cd, respectively. To validate the proposed method, the LFDBD system was employed to determine Cu, Zn, and Cd in standard reference materials of simulated natural water samples (GBW(E)080397, GBW(E)080400, and GBW(E)080402, National Research Center for Standard Materials, Nanjing, China). No sample pretreatment other than pH adjustment was performed. Our measured results agree well with the reference values (Table 2). In addition to Na, K, Cu, Zn, and Cd, several other elements, such as Mg (285.2 nm), Ag (328.1 nm), and Au (242.8 nm), could also be determined by this system with LODs of 0.06, 0.08, and 0.6 mg L−1, respectively, estimated with a standard concen-

Figure 6. Temporal emission profile of 7 consecutive determinations of a 1 mg L−1 Cd (228.8 nm) standard. Discharge conditions: input voltage for discharge power supply, 230 V; electrolyte, HNO3; solution pH, 0; sample volume: 80 μL.

Table 2. Concentration of Different Elements in Simulated Natural Water Samples (GBW(E)080397, GBW(E)080400, and GBW(E)080402) Determined by LFDBD Atomic Emission Spectrometry System sample

element

certified value (mg L−1)

measured value (mg L−1)

GBW(E)080397 GBW(E)080400 GBW(E)080402

Cu Zn Cd

10.00 ± 0.20 5.0 ± 0.1 1.00 ± 0.02

10.50 ± 0.60 4.84 ± 0.40 0.98 ± 0.08

tration of 20 mg L−1. However, no determined emission line of Fe and Cr was found even with a 100 mg L−1 standard solution. A possible reason for the lack of determined emission line of Fe and Cr is the comparatively high atomization temperatures for these refractory elements. Ongoing research will focus on understanding the atomization and excitation mechanisms of this new emission source to further improve its analytical capability and analyte coverage.

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CONCLUSIONS The proposed LFDBD shows its potential as an efficient emission source for determination of metallic elements in solution. This device does not need a sample introduction (flow) system, consumes only low sample volume (≤80 μL) and low discharge power (≤18 W), and thus offers a low cost (both hardware and running costs) microanalysis system. Moreover, analysis can be achieved within 1 min. Additional advantages of the proposed device are its small size, simple configuration, and ease of operation (in air at atmospheric pressure). With all these merits, integration of the LFDBD with 4183

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a portable optical emission spectrometer should make it a potential promising device for in-f ield metal-ion detection.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-27-6788-3455. Fax: +86-27-6788-3456. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Nature Science Foundation of China (No. 20905066, No. 41173018) and the Chenguang Project of Science and Technology of Wuhan (201150431073).

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dx.doi.org/10.1021/ac300518y | Anal. Chem. 2012, 84, 4179−4184