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Dielectric Barrier Discharge Molecular Emission Spectrometer as Multichannel GC Detector for Halohydrocarbons Wei Li,† Chengbin Zheng,† Guangyu Fan,† Li Tang,† Kailai Xu,† Yi Lv,† and Xiandeng Hou*,†,‡ † ‡
College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China
bS Supporting Information ABSTRACT: A novel microplasma molecular emission spectrometer based on an atmospheric pressure dielectric barrier discharge (DBD) is described and further used as a promising multichannel GC detector for halohydrocarbons. The plasma is generated in a DBD device consisting of an outer electrode (1.2 mm in diameter) and an inner electrode (1.7 mm in diameter) within a small quartz tube (3.0 mm i.d. 5.0 mm o.d. 50 mm), wherein analyte molecules are excited by the microplasma to generate molecular emission. Therefore, the analytes are selectively and simultaneously detected with a portable chargecoupled device (CCD) via multichannel detection of their specific emission lines. The performance of this method was evaluated by separation and detection of a model mixture of chlorinated hydrocarbons (CHCl3 and CCl4), brominated hydrocarbons (CH2Br2 and CH2BrCH2Br), and iodinated hydrocarbons (CH3I and (CH3)2CHI) undergoing GC with the new detector. The completely resolved identification of the tested compounds was achieved by taking advantages of both chromatographic and spectral resolution. Under the optimized conditions with the CCD spectrometer set at 258, 292, and 342 nm channels for determination of chlorinated hydrocarbons, brominated hydrocarbons, and iodinated hydrocarbons, respectively, this detector with direct injection provided detection limits of 0.07, 0.06, 0.3, 0.04, 0.05, and 0.02 μg mL1 for CCl4, CHCl3, CH2Cl2, CH3I, CH3CH2I, and (CH3)2CHI, respectively.
D
ielectric barrier discharge (DBD) is a typical nonequilibrium ac gas discharge generated from the collision between high-energy electrons and ambient gas molecules. A frequency of a few Hz to MHz and an ac voltage with an amplitude of 1 100 kV is required to produce the discharge.1 Because of its several unique advantages including simple construction, low power consumption, high molecular dissociation capability, and operation at atmospheric pressure, DBD has been widely used in industrial fields such as ozone generation, pollution control, synthesis gas production, surface modification, excimer lamps, and large-area flat plasma display panels.1 However, applications of DBD to analytical chemistry still remain limited. Because various radicals, ions, atoms, and molecules are favorably produced in its plasma, the DBD has been used as a chemiluminescence (CL) based detector for gas chromatography (GC),2 as an alternative to traditional radioactive source (63Ni foil) for ion mobility spectrometry (IMS),3 as a source of soft ionization for mass spectrometry (MS),4,5 or as sampling mode for inductively coupled plasma mass spectrometry (ICPMS).6,7 Nevertheless, the most common application of DBD in analytical chemistry is as a miniaturized atomizer to atomic absorption spectrometry r 2011 American Chemical Society
(AAS)/atomic fluorescence spectrometry (AFS). Since Miclea et al. first used DBD as an atomizer for the determination of halogenated hydrocarbons by diode laser atomic absorption spectrometry (DLAAS),8,9 the application has been extended to the determination of traditional hydride-forming elements.1012 It should be noted that hydrogen plays an important role in this atomization process such as serving as the source of the hydrogen radicals. Most recently, Yu13 and Zhu14 have reported that DBD was used as a radiation source in a miniaturized atomic optical emission spectrometer (OES) to determine trace mercury. In contrast to conventional radiation source of OES such as inductively coupled plasma (ICP), microwave inductively plasma (MIP), and capacitively coupled plasma (CCP), DBD offers several important advantages including compactness, lower power and gas consumption, field-portability and convenient operation. Although DBD retains excellent excitation capability, its operation temperature is only several hundred Kelvin, much Received: March 22, 2011 Accepted: May 22, 2011 Published: May 23, 2011 5050
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Figure 1. (a) A schematic of the DBD-molecular emission spectrometer as a GC detector and (b) the plasma in the DBD chamber.
lower than that of an ICP (6 00010 000 K) or even of an airacetylene flame (20003000 K). Therefore, the use of DBD to excite analytes to generate characteristic spectrum also results in a serious drawback, namely, low excitation energy, which limits the scope of its analytical application. Moreover, condense liquid droplets are simultaneously transported to a DBD cell even when using chemical vapor generation (CVG) for sample introduction. These liquid droplets may further consume the energy for DBD. Consequently, apart from determination of mercury, DBDmicroplasma OES for sensitive determination of other elements has not been realized yet. In contrast to atomic emission, the energy required for generation of molecular emission is generally much lower. With power less than 0.2 W, an atmospheric pressure, portable, and pulsed plasma source-based molecular emission detector has been constructed with a home-built high-voltage pulsed power supply and used for determination of a variety of organic compounds by Duan et al.15 Other plasma-based molecular emission detectors with low power, small size, and simple construction including microwave,16 direct current,17 alternating current,18 and high-voltage pulsed19 plasma have also been reported. GC is an effective analytical approach for volatile and semivolatile organic compounds. However, for the analysis of complex mixtures, such as fuel, the GC peak capacities20,21 often do not suffice to achieve complete separation of analyte compounds. In these situations, a powerful multichannel detector can provide additional information that aids the resolution of unseparated analytes. However, the conventional GC detectors such as thermal conductivity detector (TCD), flame ionization detector (FID), electron capture detector (ECD), photoionization detector (PID), or helium ionization detector (HID) are single channel detectors. Although recent successes have been achieved with the application of ICP-OES/mass spectrometry (MS),2225 glow discharge (GD) AES/MS,26,27 MIP-AES/MS,2830 and molecular MS31 as GC detectors, they are too expensive to be accepted in every laboratory. Moreover, it is impossible to use these detectors for portable instrumentation for field analytical chemistry. As a digital optical recorder, a charge-coupled device (CCD) not only has multichannel advantage but a miniaturized size. Therefore, multichannel simultaneous analysis and improved resolution of overlapped peaks of complex mixtures will
be achieved without multidimensional separation but with a CCD-based molecular emission spectrometer (MES) as a multichannel GC detector. To our knowledge, there have been no reports on using DBD as a miniaturized radiation source for MES of organic compounds. We herein report a miniaturized DBD molecular emission spectrometer with advantages of compactness, rapidness, simplicity and low power consumption. This spectrometer was further used to couple with a 1D GC to achieve high resolution through multichannel simultaneous detection, thus realizing high-throughput analysis.
’ EXPERIMENTAL SECTION Reagents. All chemicals used in this work were of at least analytical-reagent grade. CH2Cl2, CHCl3, CCl4, CH2Br2, CH2BrCH2Br, CH3I, and (CH3)2CHI were purchased from Kelong Reagent Co. (Chengdu, China). Argon (99.99%, Taiyu Gas Co. Ltd., Chengdu, China) was used as both carrier gas and discharge gas. He (99.999%) and N2 (99.999%) were also tested as the discharge gas. Instrumentation. Figure 1a shows the whole instrumental setup, which mainly consisted of a GC and a DBD-MES detector using a miniaturized CCD spectrometer. The DBD-MES detector was a cylindrical laboratory-built DBD device consisting of a quartz tube (50 mm 3.0 mm i.d. 5.0 mm o.d.) and two copper wire electrodes. One of the electrodes (1.2 mm in diameter) was tightly wrapped around the outer side of the quartz tube evenly; the other (1.7 mm in diameter) was inserted into the tube. A compact ac ozone generation power supply (YG. BP105P, Electronic Equipment Factory of Guangzhou Salvage, Guangzhou, China; 6 cm long 4 cm wide 3 cm high, with a rated output of 4 kV, 20 kHz, and 12 W at 220 V, 50 Hz input) was connected to the electrodes to provide high voltage for generation of the DBD plasma. The ozone generation power supply was connected to a transformer (TPGC2J-1, Shanghai Pafe Electronic Equipment Ltd. Co., Shanghai, China) for convenient adjustment of the discharge power. The optical radiation from the DBD plasma was collected onto the entrance slit of a commercial hand-held CCD spectrometer (USB 2000; Ocean Optics Inc., Dunedin, FL). As a GC detector, this CCD based DBD-MES was connected to a Techcomp GC7890F GC 5051
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Figure 2. Effect of discharge gas type on the background emission spectra of the DBD plasma. Experimental conditions: input voltage, 190 V; discharge gas flow rate, 400 mL min1; and integration time, 200 ms.
instrument (Techcomp Ltd., Shanghai, China) equipped with a stainless steel polarity column (3 m long, 2 mm i.d. and filled with 10% polyethylene glycol 20 M (PEG-20M) on 101 support, Fuji Instrument, Chengdu, China). Procedure. In the direct determination of halohydrocarbons, the analyte standards were injected into a 50 mL heated vessel to generate the vapor of halohydrocarbons. The vapor was then flushed to the DBD plasma by Ar flow. Finally, the optical emission of halogen molecules was directly measured by the CCD. A blank was measured by using Ar carrier gas only. All experimental conditions were optimized for maximum response. The analyte mixture was obtained by mixing 410 μL of CHCl3, 800 μL of CCl4, 500 μL of CH2Br2, 1000 μL of CH2BrCH2Br, 140 μL of CH3I, and 80 μL of (CH3)2CHI in a 10 mL quartz vessel. Then 2 μL of the mixture was injected into the GC for separation and detected by the proposed detector. The operation parameters of GC are as follows: column temperature, 60 °C; injection temperature, 150 °C; and Ar carrier gas, 26.57 mL min1. The working parameters of the DBD plasma: input voltage, 200 V; power, 12 W; Ar gas flow rate, 560 mL min1; and integration time, 500 ms. The peak area measurement was used for quantification throughout the work.
’ RESULTS AND DISCUSSION It has been reported that the type and concentration of discharge gas are strongly related with the analytical application and even the design of a DBD device. No plasma was observed by using N2 as the discharge gas, but the maximum atomic absorbance response of arsenic could be achieved with He containing 4% H2.4 On the other hand, He et al.2 found that the DBD plasma could be generated with all the tested gases including N2 in their DBD design. In this work, therefore, initial experiments were performed to explore the effect of discharge gases (Ar, He, N2, and air) on the plasma generation. Under the atmospheric
pressure, the DBD plasma could be maintained easily with all the tested gases (Figure 1b and Figure 2). The spectra obtained from the plasmas with air, Ar, He, and N2 were shown in Figure 2. Typical molecular emission bands of OH (283, 309 nm), NH (337 nm), and N2 (316, 358, and 380 nm) species can be found in all the plasmas, which agree with the earlier results by using a different type of plasma.15 For Ar and He, these molecular emission bands may arise from air entrainment or from impurities. Despite the DBD plasma can be maintained stable in all the tested gases, the optical emission signal from the plasma was too weak in the case of air as the discharge gas. Many strong emission bands from the N2 DBD plasma were found in the wavelength range of 200 nm∼300 nm where specific spectral peaks of molecular halogen are located. These emission bands elevated the blank and resulted in serious interferences to subsequent quantification. As for the cost, He is more expensive than Ar. Therefore, Ar was the choice as the discharge gas for the subsequent experiments in this work. For investigation of the stability of the Ar DBD plasma, the background emission intensity (from the impurities) in a period of 60 min was monitored at most found emission bands (OH 309 nm (A2Σþ f X2Π), NH 337 nm (A3Π f X3Σ) and N2 358 nm (C3Π f B3Π)).32 As evident from Figure S1 in the Supporting Information, there is only a slight drift for the background, yielding the relative standard deviation (RSD) of 0.9%, 1.0%, and 1.1% for 309, 337, and 358 nm, respectively. To demonstrate the feasibility of the generation of specific halogen molecular emission in the DBD and the practicability as an element-specific detector for determination of halohydrocarbons, CCl4, CH2Br2, and CH3I were chosen as model analytes for chlorinated hydrocarbons, brominated hydrocarbons, and iodinated hydrocarbons, respectively, with results shown in Figure 3. Compared to the emission spectrum of the pure Ar DBD plasma, obvious specific molecular emission bands were found when these halohydrocarbons were introduced into the 5052
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Figure 3. The molecular emission spectra from pure argon and those from argon containing 0.5 μL different halohydrocarbons. Experimental conditions: input voltage, 190 V; Ar flow rate, 400 mL min1; and integration time, 200 ms. Note the atomic emission of iodine at 206.20 nm.
DBD plasma. Meanwhile, the peak width of these molecular emission bands arising from these halogen-containing analytes is several nanometers, much broader than the peak width of normal atomic emission lines. This together with the report from the literature demonstrated these emission bands at 258.0 nm (D0 f A0 ), 292.0 nm (D0 f A0 ), and 342.0 nm (D0 f A0 ) belong to excited molecules of Cl2, Br2, and I2, respectively.1,33 To further confirm these emission bands are related to these halogen molecules, HCl, HBr, and I2 as alternatives for CCl4, CH2Br2, and CH3I were injected into the Ar DBD plasma, respectively. The same specific molecular emission bands were also clearly observed. In fact, in the studies of DBD-based vacuum ultraviolet (VUV) excimer lamps,1 many researchers have observed emission at 258 nm when Cl2* excimer molecules release its excitation energy and relax to the ground state. It should be noted that a narrow atomic emission line of iodine was also observed at 206.2 nm (Figure 3), and this is in agreement with an earlier report.33 Although the mechanism of generation of DBD has been discussed independently by many researchers, none of them focused on the application of DBD for molecular emission spectrometry. On the basis of a summarization of earlier works,1,3 we speculate that the mechanism of the proposed work may involve several steps including dissociation of halohydrocarbons to halogen radicals (X•) by the high energetic electrons and gas temperature from the DBD plasma, formation of X2 (Cl2, Br2, or I2) by recombination of X•, generation of X2* excimer molecules via excitation of X2 with the high energy electrons, and generation of the optical emission through release of energy of the X2* excimer to its respective ground state. As reported earlier,1 Ar DBD can emit UV irradiation with a peak wavelength at 126 nm; and it is well-known that the UV irradiation is widely used to
decompose organic pollutants and to induce molecular fluorescence. Therefore, this UV may also contribute to the dissociation of volatile chlorinated hydrocarbons and/or generation of X2* excimer molecules. The analytical figures of merit measured by directly injecting samples into the DBD plasma were evaluated by using chlorohydrocarbon and iodohydrocarbon as model analytes under optimal conditions (see the Supporting Information for Figure S2 and Figure S3). The peak area measurement was used for quantification throughout this work since it provided a wider linear dynamic range. Linear correlation coefficients for calibration curves of these analytes were better than 0.99. The precision, expressed as RSDs of six replicate measurements, were better than 4% (Figure S4 in the Supporting Information). The limits of detection (LODs) were LOD = 3N/S, where N and S are the standard deviation of 11 measurements of a blank solution and the slope of the calibration curve, respectively. The upper linear dynamic ranges are from tens of μg mL1 to 200 μg mL1, and the LODs are at the tens of ng mL1 level except for CH2Cl2 (Table 1). The performance of the GC-DBD MES setup was initially tested with chlorinated hydrocarbons that contained tetrachloromethane (CCl4), chloroform (CHCl3), and dichloromethane (CH2Cl2). It is worthwhile to note that the DBD cell should be coupled to GC as close as possible to reduce the dead volume. Therefore, the end of the GC column was perpendicularly connected to the DBD device to ensure that analytes mix with the plasma gas quickly. The full chromatogram of the tested mixture indicates that the proposed DBD-MES can be used as a GC detector with satisfactory resolution (also see the Supporting Information for Figure S5). 5053
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Further, a comparison between the proposed method and GC-FID was made. Since there are no suitable samples available at this time, a model mixture containing chlorinated hydrocarbons (CHCl3 and CCl4), brominated hyrocarbons (CH2Br2 and CH2BrCH2Br), and iodinated hydrocarbons (CH3I and (CH3)2CHI) was use for this purpose. Both methods were performed on the same GC instrument with identical GC instrumental parameters. In the case of the FID detector, the chromatographic peaks of CCl4 and (CH3)2CHI almost completely overlapped and could not be resolved (Figure 4a). Meanwhile, a baseline separation of CH3I and CCl4 was also not achieved. In the case of the DBD-MES detector, the CCD spectrometer was set at 258, 292, and 342 nm channels to determine the molecular emission signals arising from chlorinated hydrocarbons, brominated hydrocarbons, and iodinated hydrocarbon, respectively. In contrast to the case of FID, Figure 4b shows these halohydrocarbon peaks appear completely resolved in the element specific mode, partially by taking advantage of the spectral resolution of the DBD-MES detector. Therefore, multidimensional GC separation or careful selection of instrumental parameters to achieve complete chromatographic separation of these compounds is no longer a must. It is worthwhile to note that the peaks in the case of the DBD-MES detector appear narrower than those with the FID. This is Table 1. Analytical Characteristics of the Proposed Method with Direct Injection
because a discharge gas was added in the former case, thus the analyte was more quickly flushed through the detector, leading to narrower chromatographic peaks. The width of the peak could be further reduced if the inner diameter of the discharge chamber is further reduced (can be down to 100 μm as reported by Zhang’s group6). Finally, a comparison of calibration curves with error bars obtained for determination of CCl4, CH2Br2, and CH3I with GC-FID and GC-DBD-MES was taken, as shown in Figure S6 in the Supporting Information. Table 2 summarizes the analytical figures of merit achieved by using GC-DBD-MES in comparison with those by GC-FID. Linear correlation coefficients for calibration curves obtained by using both methods are better than 0.99, but the sensitivities and the LODs of the proposed method are worse than those by FID and ECD for sure. The utility of the proposed technique was preliminarily tested by the determination of three representative halohydrocarbons, CCl4, CH2Br2, and CH3I in 93# gasoline. It was necessary to spike the sample with these analytes because their endogenous concentrations were far below the detection limits. The preliminary recoveries for CCl4, CH2Br2, and CH3I were from 80% to 150%, depending on the concentration level and the analyte species. Table 2. Comparison of the Performance by the Proposed Method with Those by GC-FID linear correlation coefficient,
upper linear range, DBD
RSD,a
LOD,b DBD
DBD (FID)
(FID), mg
DBD (FID)
(FID), mg
CCl4
0.997 (0.999)
0.64 (0.25)
0.53 (1.2)
CH2Br2
0.999 (0.999)
0.40 (0.40)
0.45 (0.80)
0.013 (0.00045)
CH3I
0.994 (0.988)
0.46 (0.27)
1.2 (4.6)
0.0048 (0.00039)
upper linear dynamic range (μg mL1)
R
LOD (μg mL1)
CH2Cl2
20
0.9988
0.3
CHCl3 CCl4
40 200
0.9989 0.9987
0.06 0.07
halohdrocarbons
CH3I
20
0.9915
0.04
CH3CH2I
30
0.9918
0.05
(CH3)2CHI
50
0.9983
0.02
analyte
0.0073 (0.00052)
a
RSD of 6 measurements of 0.15 mg of analyte. b Calculated by using 3-fold standard deviation of 11 measurements of a blank solution, divided by the slope of the calibration curve.
Figure 4. Chromatograms of halohydrocarbons mixture (a) detected with the FID detector and (b) detected with the CCD based DBD-MES detector. The working parameters of GC: column temperature, 60 °C; injection temperature, 150 °C; and argon carrier gas, 26.57 mL min1. The working parameter of FID: detection temperature, 110 °C. The working parameters of the DBD plasma: input voltage, 200 V; power, 12 W; and argon discharger gas flow rate, 560 mL min1. 5054
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Analytical Chemistry In summary, the DBD plasma was successfully used as an excitation source for a molecular emission spectrometer, which was further coupled to GC for element specific detection to enhance the resolution spectrally. As a GC detector, it provides for spectral resolution (multichannel), simplicity, compactness, fast analysis, and cost-effectiveness, in comparison to the traditional element specific detectors such as MS, ICP-AES/MS. Further, nitrogen and nitrogen-containing compounds were also preliminarily found detectable by this method; and molecular emission of sulfur and phosphorus could be observed, since it was realized with excitation by other low power plasma techniques.34,35 Therefore, we believe that application of DBDMES will be further broadened in the near future, together with other analytical applications of DBDs.36
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT X. D. Hou thanks the National Natural Science Foundation of China Grant No. 20835003 for financial support. W. Li and C. B. Zheng contributed equally to this work. Technical assistance from Mr. Xi Wu of the Analytical & Testing Center is very much appreciated. The authors also acknowledge parts of this work were reported in the 15th Organic and Biological Analysis Conference of China, Chongqing, November, 2009 and the 4th AsiaPacific Winter Conference on Plasma Spectrochemistry, Chengdu, November, 2010. ’ REFERENCES (1) Kogelschatz, U. Plasma Chem. Plasma Process. 2003, 23, 1–46. (2) He, Y. H.; Lv, Y.; Li, Y. M.; Tang, H. R.; Tang, L.; Wu, X.; Hou, X. D. Anal. Chem. 2007, 79, 4674–4680. (3) Michels, A.; Tombrink, S.; Vautz, W.; Miclea, M.; Franzke, J. Spectrochim. Acta, Part B 2007, 62, 1208–1215. (4) Zoltan, T.; Wiseman, J. M.; Cooks, R. G. J. Mass Spectrom. 2005, 40, 1261–1275. (5) Na, N.; Zhao, M. X.; Zhang, S. C.; Yang, C. D.; Zhang, X. R. J. Am. Soc. Mass Spectrom. 2007, 18, 1859–1862. (6) Liu, Y. Y.; Ma, X. X.; Lin, Z. Q.; He, M. J.; Han, G. J.; Yang, C. D.; Xing, Z.; Zhang, S. C.; Zhang, X. R. Angew. Chem., Int. Ed. 2010, 49, 4435–4437. (7) Xing, Z.; Wang, J.; Han, G. J.; Kuermaiti, B.; Zhang, S. C.; Zhang, X. R. Anal. Chem. 2010, 82, 5872–5877. (8) Miclea, M.; Kunze, K.; Musa, G.; Franzke, J.; Niemax, K. Spectrochim. Acta, Part B 2001, 56, 37–43. (9) Kunze, K.; Miclea, M.; Musa, G.; Franzke, J.; Vadla, C.; Niemax, K. Spectrochim. Acta, Part B 2002, 57, 137–146. (10) Zhu, Z. L.; Zhang, S. C.; Lv, Y.; Zhang, X. R. Anal. Chem. 2006, 78, 865–872. (11) Zhu, Z. L.; Liu, J. X.; Zhang, S. C.; Na, X.; Zhang, X. R. Spectrochimi. Acta, Part B 2008, 63, 431–436. (12) Yu, Y. L.; Du, Z.; Chen, M. L.; Wang, J. H. J. Anal. At. Spectrom. 2008, 23, 493–499.
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