Dielectric Barrier Discharge-Induced

and low cost of instrumentation and maintenance.1,2 Traditional. CL systems .... reported by Siemens in 1857.18 It is a typical nonequilibrium high- p...
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Anal. Chem. 2007, 79, 4674-4680

Dielectric Barrier Discharge-Induced Chemiluminescence: Potential Application as GC Detector Yihua He,† Yi Lv,† Yaming Li,† Huarong Tang,† Li Tang,† Xi Wu,‡ and Xiandeng Hou*†,‡

College of Chemistry and Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China

Atmospheric pressure dielectric barrier discharge (DBD) plasma can be used to split low molecular weight organic compounds, and the DBD-split/excited species can be swept into luminol solution to induce chemiluminescence (CL) emission. Based on this observation, a novel optical detector was proposed and preliminarily tested as a potential gas chromatographic (GC) detector in this work. The advantages of this new type of detector include the following: direct detection, fast response, high sensitivity, versatility (sensitive to a broad range of volatile organic compounds), simple and easy instrumentation, compactness (3.0 mm i.d. × 4.0 mm o.d. × 20 mm length of the DBD device), and low power (less than 5 W). Twelve volatile organic compounds, including methanol, ethanol, propanol, formaldehyde, acetaldehyde, benzene, dichloromethane, trichloromethane, tetrachloromethane, tetrahydrofuran, carbon bisulfide, and ethyl ether, were tested with this detector, and each of them produced a large signal. It was found that the CL signal was proportional to the analyte concentration and affected by the DBD parameters. Under the optimized experimental conditions, the limits of detection down to the tens of nanogram level were achieved for methanol, ethanol, propanol, formaldehyde, and acetaldehyde. It was then preliminarily tested as a GC detector for the separation of formaldehyde, ethanol, and propanol. This is the new application of DBD in analytical chemistry, and CL was for the first time generated in this way. The new detector can be a potential GC detector suitable for a wide range of volatile organic compounds. Chemiluminescence (CL) enjoys widespread applications in analytical chemistry, mainly owing to its unique advantages of low or even no background nature, high sensitivity, rapidity, simplicity, and low cost of instrumentation and maintenance.1,2 Traditional CL systems can be divided into two types: liquid-phase and gasphase CL systems. The former is usually based on the enhance* To whom correspondence should be addressed. E-mail: [email protected]. † College of Chemistry. ‡ Analytical & Testing Center. (1) Weeks, I. In Chemiluminescence Immunoassay Wilson and Wilson’s Comprehensive Analytical Chemistry; Svehla, G., Ed.; Elsevier: Amsterdam, 1992; Vol. 29. (2) Lin, J. M. In Chemiluminescence: Basic Principles and Applications; Chemical Industry Press: Beijing, 2004.

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ment or depression of CL signal from a CL reagent such as luminol or lucigenin, and the latter includes the CL of nitrogen-, phosphorus-, or sulfur-containing compounds in flame, which has been successfully used as a GC detector. Recent development in CL analysis has been reviewed by many authors.3-5 It is known that conventional CL analysis can only detect a limited number of substances. For instance, the popular luminol CL system is based on the oxidation [usually by H2O2, MnO4-, Cr2O7-, Ce(IV), Fe(CN)63-] or catalysis (by transition metal ions) of luminol to give intense CL emission;6 thus, only those substances can be detected. Recent studies in cataluminescence7,8 extended the detectable range of analytes and may provide a new trend in CL research.9 The cataluminescence-based detector has been successfully used in liquid chromatography10 and capillary electrophoresis.11 Although the cataluminescence can cover more substances directly, different catalysts and diverse catalysis conditions have to be used to detect different compounds. Therefore, more universal CL systems are desired. Radicals have high reactivity and oxidation power, and they have been widely studied in CL, especially the reactive oxygen species such as hydroxyl radical (HO•),12,13 superoxide (•O2-),14 and singlet oxygen (1O2).15 These reactive oxygen species generated from chemical reactions16,17 can oxidize CL reagent to emit intense CL. Other ways to generate radicals may include flame, (3) Su, Y. Y.; Chen, H.; Wang, Z. M.; Lv, Y. Appl. Spectrosc. Rev. 2007, 42, 139-176. (4) Marquette, C. A.; Blum, L. J. Anal. Bioanal. Chem. 2006, 385, 546-554. (5) Zhang, Z. Y.; Zhang, S. C.; Zhang, X. R. Anal. Chim. Acta 2005, 541, 3747. (6) Garcı´a-Campan ˜a, A. M., Baeyens, W. R. G., Eds. Chemiluminescence in Analytical Chemistry; Marcel Dekker Inc.: New York, 2001. (7) Breysse, M.; Claudel, B.; Faure, L.; Guenin, M.; Williams, R. J. J. J. Catal. 1976, 45, 137-144. (8) McCord, P.; Yan, S. L.; Bard, A. J. Science 1992, 257, 68-69. (9) Nakagawa, M. Sens. Actuators, B: Chem. 1995, 29, 94-100. (10) Lv, Y.; Zhang, S. C.; Liu, G. H.; Huang, M. W.; Zhang, X. R. Anal. Chem. 2005, 77, 1518-1525. (11) Huang, G. M.; Lv, Y.; Zhang, S. C.; Yang, C. D.; Zhang, X. R. Anal. Chem. 2005, 77, 7356-7365. (12) Zheng, J.; Springston, S. R.; Weinstein-Lloyd, J. Anal. Chem. 2003, 75, 46964700. (13) He, C.; Zhang, Z. J.; He, D. Y.; Xiong, Y. Anal. Bioanal. Chem. 2006, 385, 128-133. (14) Mujahid, A.; Yoshiki, Y.; Akiba, Y.; Toyomizu, M. Poultry Sci. 2005, 84, 307-314. (15) Lau, C. W.; Lu, J. Z.; Kai, M. Anal. Chim. Acta 2004, 503, 235-239. (16) Yu, W. L.; Zhao, Y. P.; Shu, B. Food Chem. 2004, 86, 525-529. (17) Chen, T. Z.; Chiou, J. F.; Tsai, C. H.; Shu, C. W.; Lin, M. H.; Liu, T. Z.; Tsai, L. Y. J. Agric. Food Chem. 2006, 54, 9297-9302. 10.1021/ac070321u CCC: $37.00

© 2007 American Chemical Society Published on Web 05/10/2007

Figure 1. Schematic diagram of the instrumental setup of the DBD-CL system.

plasma, and the like. A simple and effective way for radical generation may be the low-power plasma technique. With this strategy, a more universal CL system can be realized with a simple device, such as dielectric barrier discharge (DBD). In fact, DBD itself is an old plasma technique that was first reported by Siemens in 1857.18 It is a typical nonequilibrium highpressure ac gas discharge that can work at atmospheric pressure.19 The instrumentation of a DBD device is simple, two electrodes and at least one dielectric barrier; yet the design of such device is diverse.20,21 By applying a few hundred to several thousand volts with a frequency ranging from tens of hertz to several megahertz ac power to the electrodes, discharge will be ignited in the DBD device, and the high energetic electrons (1-10 eV) can collide with the ambient gas molecules to produce plasmas containing various radicals, ions, atoms, molecular fragments, and the like.22 Compared to other plasma generation techniques, DBD possesses some attractive characteristics: excellent dissociation capability for molecular species, simplicity, capability of working with various gases, low gas temperature (∼600 K), low cost for instrumentation and maintenance, and capability of working at atmospheric pressure. Therefore, DBD is widely used in industry, including ozone generation,23-25 synthesis gas production,26,27 pollution control,28-30 surface treatment (modification, coating, deposition, or cleaning),31,32 excimer-based lamps/laser,33 and flat large-area plasma displays.34 In contrast with its industrial application, the analytical application of a DBD device is very limited. To the best of our (18) Siemens, W.; Poggendorff’s Ann. Phys. Chem. 1857, 102, 66-122. (19) Xu, X. J. Thin Solid Films 2001, 390, 237-242. (20) Pietsch, G. J. Contrib. Plasma Phys. 2001, 41, 620-628. (21) Kogelschatz, U. Plasma Chem. Plasma Process 2003, 23, 1-46. (22) Snyder, H. R.; Anderson, S. K. IEEE Trans. Plasma Sci. 1998, 26, 16951699. (23) Sung, Y.-M.; Sakoda, T. Surf. Coat. Technol. 2005, 197, 148-153. (24) Takayama, M.; Ebihara, K.; Strgczewska, H.; Ikegami, T.; Gyoutoku, Y.; Kubo, K.; Tachibana, M. Thin Solid Films 2006, 506-507, 396-399. (25) Park, S.-L.; Moon, J.-D.; Lee, S.-H.; Shin, S.-Y. J. Electrostat. 2006, 64, 275282. (26) Song, H.-K.; Choi, J.-W.; Yue, S. H.; Lee, H.; Na, B.-K. Catal. Today 2004, 89, 27-33. (27) Indarto, A.; Choi, J.-W.; Lee, H.; Song, H. K. J. Nat. Gas Chem. 2006, 15, 87-92. (28) Lu, B.; Zhang, X.; Yu, X.; Feng, T.; Yao, S. J. Hazard. Mater. 2006, 137, 633-637. (29) Ban, J.-Y.; Son, Y.-H.; Kang, M.; Choung, S.-J. Appl. Surf. Sci. 2006, 253, 535-542. (30) Subrahmanyam, Ch.; Magureanu, M.; Reuken, A.; Kiwi-Minsker, L. Appl. Catal. B: Environ. 2006, 65, 150-156. (31) Goossens, O.; Dekempeneer, E.; Vangeneugden, D.; Van de Leest, R.; Leys, C. Surf. Coat. Technol. 2001, 142-144, 474-481. (32) Liu, C. Z.; Cui, N. Y.; Brown, N. M. D.; Meenan, B. J. Surf. Coat. Technol. 2004, 185, 311-320. (33) Boyd, I. W.; Zhang, J.-Y. Solid State Electron. 2001, 45, 1413-1431. (34) Boeuf, J. P. J. Phys. D: Appl. Phys. 2003, 36, R53-R79.

knowledge, there are only a few publications dealing with the application of DBD in analytical chemistry, and most of them used it as an atomizer. In 2001, Niemax et al.35 first developed diode laser atomic absorption spectrometry (DL AAS) with a DBD device as an atomizer. CCl2F2, CClF3, and CHClF2 were dissociated in the DBD, and Cl and F atoms were detected using DL AAS. They later successfully applied the DBD-DL AAS as a GC detector to analyze C6H5F, C3H6Cl2, C4H9Br, C5H11Cl, C4H9I, and C6H8S by detection of S, I, Br, Cl, and F atomic absorption.36 Later in 2006, Zhang et al.37 developed a high-performance liquid chromatography (HPLC)-hydride generation (HG)-DBD-AAS system for arsenic speciation analysis. After the HPLC separation, As(III), As(V), monomethylarsonic acid, and dimethylarsinic acid were reduced by KBH4-HCl to form hydrides, which were then dissociated/atomized in the DBD for AAS detection. Using the HG-DBD-AAS system, they also successfully measured Se, Sb, and Sn down to the nanogram per milliliter level.38 The beauties of the DBD atomizer are its simplicity, compactness, low surface temperature, and cost-effectiveness. Luong et al.39 used a simple DBD device as a GC detector: two electrodes were used to collect the ionized substances in the DBD plasma and connected to an electrometer for voltage measurement. Besides atoms, the dissociation products of compounds in a DBD plasma are diverse, and this can be explored for various analytical purposes. In this work, a small cylindrical DBD was constructed to dissociate low molecular weight volatile organic compounds, and the resulting products (radicals, excited species, and the like) were directed to react with luminol for the generation of CL emission in an attempt to establish a more universal CL detector for the detection of gaseous organic compounds. Performance of the new detector was evaluated using alcohol (methanol, ethanol, propanol) and aldehyde (formaldehyde, acetaldehyde) as examples. The potential application of the proposed system as a GC detector was also preliminarily examined. EXPERIMENTAL SECTION Instrumentation. As shown in Figure 1, the DBD-CL instrumental setup consists of two major parts, a laboratory-built DBD device and a commercial BPCL ultraweak luminescence analyzer (35) Miclea, M.; Kunze, K.; Musa, G.; Franzke, J.; Niemax, K. Spectrochim. Acta, Part B 2001, 56, 37-43. (36) Kunze, K.; Miclea, M.; Franzke, J.; Niemax, K. Spectrochim. Acta, Part B 2003, 58, 1435-1443. (37) Zhu, Z. L.; Zhang, S. C.; Lv, Y.; Zhang, X. R. Anal. Chem. 2006, 78, 865872. (38) Zhu, Z. L.; Zhang, S. C.; Xue, J. H.; Zhang, X. R. Spectrochim. Acta, Part B 2006, 61, 926-921. (39) Gras, R.; Luong, J.; Monagle, M.; Winniford, B. J. Chromatogr. Sci. 2006, 44, 101-107.

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(BP-II, Institute of Biophysics, Academia Sinica, Beijing, China). The DBD device was simply constructed with a 3.0 mm i.d. × 4.0 mm o.d. × 20 mm long glass/quartz tube as a dielectric barrier (which means the thickness of the dielectric barrier was the thickness of the tube wall, 1 mm) and two metal wire electrodes. The two electrodes were of 1 mm in diameter, and one of them was inserted into the tube; thus, the discharge gap was also 1 mm. The other electrode was tightly wrapped around the outer side of the dielectric barrier (i.e., the tube) evenly. The electrodes were connected to a compact ac ozone generation power supply (YG.BP101P, 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 5 W at 220 V, 50 Hz input. In order to change the discharge power, the ozone generation power supply was connected via a transformer (TPGC2J1, Shanghai Pafe Electronic Equipment Ltd. Co., Shanghai, China) to adjust the input voltage. The discharge part of the DBD device was placed inside the detection chamber of the BPCL analyzer to shorten the distance between the DBD outlet and the detection coil. During the discharge process, the DBD will also emit light radiation, so the DBD part was sealed in a black box to eliminate its emission influence on the CL detection. With a peristaltic pump (HL-2D, Shanghai Qingpu Huxi Instrument Factory, Shanghai, China), the CL reagent (luminol aqueous solution) and the substances from the DBD were mixed through a Y-shape connector and then flushed immediately into the glass detection coil for CL detection by the BPCL analyzer. The data acquisition time (i.e., the data integration time) of the BPCL analyzer was set to 0.1 s/spectrum, and a working voltage of -780 V was set for the analyzer’s photomultiplier tube. The CL intensity was recorded by the computer and further processed with Microsoft Excel or OriginPro. When used as a GC detector, the DBD-CL system was connected to the outlet of a Dikma SS4M-M (4 m long × 2 mm i.d. and filled with 10% PEG-20M on 101 support) stainless steel column (Dikma Technologies, Beijing, China), and the column was accommodated in a Techcomp GC7890F GC instrument (Techcomp Ltd., Shanghai, China). Reagents. All reagents used in this work were of at least analytical grade. Sub-boiled doubly distilled water (DDW) was used to prepare solutions throughout the whole work. Luminol was used as the CL reagent, and the stock solution of 2 × 10-2 mol L-1 was prepared by dissolving 0.44 g of luminol powder in 250 mL of 0.1 mol L-1 NaOH. NaHCO3-NaOH buffer was used to adjust the pH value of the luminol solution. Methanol, ethanol, propanol, formaldehyde (40%), acetaldehyde (37%), n-hexane, dichloromethane, trichloromethane, tetrachloromethane, carbon bisulfide, propanone, cyclohexanone, ethyl ether, tetrahydrofuran, ethyl acetate, and benzene were used as analytes, and the alcohols and aldehydes were used to evaluate the performance of the proposed method. The alcohols and aldehydes were diluted to the desired concentration with DDW as specified in the relevant figure captions later. Ar (99.99%), He (99.999%), or N2 (99.999%) was tested as the discharge gas. RESULTS AND DISCUSSION Optimization for Best CL Signal-to-Noise (S/N) Ratio. As partially shown in Figure 2, when 0.1-0.2 µL of methanol, ethanol, propanol, formaldehyde, acetaldehyde, benzene, n-hexane, dichlo4676 Analytical Chemistry, Vol. 79, No. 12, June 15, 2007

Figure 2. CL emissions of (1) methanol, (2) ethanol, (3) propanol, (4) formaldehyde, (5) acetaldehyde, (6) benzene, (7) dichloromethane, (8) trichloromethane, (9) tetrachloromethane, (10) tetrahydrofuran, (11) carbon bisulfide, and (12) ethyl ether. Experimental conditions: injection volume, 0.1-0.2 µL of the chemicals (without any dilution); discharge voltage, 1.7 kV; N2 discharge gas flow rate, 300 mL min-1; and luminol concentration and flow rate, 8 × 10-5 mol L-1 and 1.6 mL min-1, respectively.

romethane, trichloromethane, tetrachloromethane, carbon bisulfide, propanone, cyclohexanone, ethyl ether, tetrahydrofuran, or ethyl acetate was respectively injected into the DBD as a sample, CL emission was observed, although with different intensities and peak shapes. For instance, chlorinated hydrocarbons (dichloromethane, trichloromethane, tetrachloromethane), especially tetrachloromethane, produced very strong CL emission, while the CL emission of carbon bisulfide was very weak (indicated by the arrow); some of them gave a very sharp peak shape (e.g., benzene) while others gave a broad peak shape (e.g., tetrahydrofuran). This varied signal magnitude or peak profile is probably controlled by the amount of produced radicals and its production rate from the target analyte molecules. Alcohol (methanol, ethanol, propanol) and aldehyde (formaldehyde, acetaldehyde) were used as the typical targets for further optimization. The electrode and dielectric barrier materials, the discharge gas species and their flow rate, and the discharge voltage can all affect the discharge power. As far as the CL influencing factors are concerned, the distance of the DBD outlet to the luminol tubing and the luminol concentration and its flow rate will affect the detection sensitivity. Peak area measurement of three replicates for each data point was used in the following optimization except for those specially mentioned, and the criteria for the optimal conditions were based on the best S/N ratio. (a) Effect of Electrode Material and Dielectric Barrier Material. Five metals were tested as the inside electrode in this work, including Al, Cu, Pt, Sn, and stainless steel. Figure 3 shows the signals and S/N ratios of ethanol on different electrode materials. Although Pt electrode could result in the highest CL intensity, the Cu electrode gave slightly the best S/N ratio. In addition, Cu wire is much less expensive and easier to obtain than Pt; thus, the Cu electrode was chosen for use in this work. The Cu electrode can still perform well after more than 60 working hours and 1300 injections. From Figure 3, it is also known that the noise level is significant, which may largely originate from the mixing of the gas phase (the carrier gas with the active species from the DBD) with the liquid phase (luminol solution). Hence,

Figure 3. CL signals and S/N ratios of ethanol using different electrode materials. The error bars stand for ( standard deviation (SD) of three replicates. Injection volume, 0.03 µL of ethanol; discharge voltage, 1.7 kV; N2 discharge gas flow rate, 300 mL min-1; and luminol concentration and flow rate, 8 × 10-5 mol L-1 and 1.6 mL min-1, respectively.

optimal design for better mixing should be further explored to reduce the noise level. Glass or quartz was tested as dielectric barrier material. It was found that the CL signal with quartz or glass was similar, but a better S/N ratio was obtained with glass. In addition, glass material is less costly and easily available, so glass was the choice for this work. The reactivity of DBD plasma is expected to be improved by increasing the permittivity/dielectric constant of the dielectric barrier.40 The relative permittivity of glass (4.9-7.5) is larger than that of quartz (3.8);41 as a consequence, the discharge should be stronger in a glass dielectric barrier. Nevertheless, the CL intensity using glass as dielectric barrier was a bit smaller than that using quartz in this work, probably because stronger discharge leads to stronger dissociation ability, which results in the decline of radical transport to the detection coil to oxidize luminol, thus leading to the decline of CL intensity. Except for glass or quartz, ceramic materials as well as thin enamel or polymer layers can also be used as dielectric barrier material, and sometimes additional protective or functional coatings can be applied.20 For instance, Li et al.40 used Li2Si2O5 as a sintering additive into the Ca0.7Sr0.3TiO3 ceramic as a dielectric barrier of DBD and greatly enhanced the conversion efficiency of CO2 to CO and O2. (b) Effect of Discharge Gas and Its Flow Rate. The discharge status of Ar and N2 in a DBD device is extremely different.42,43 Zhang et al.37 observed no discharge using N2 as the discharge gas, but He containing 4.1% H2 as the discharge gas led to the maximum absorbance for As under the same conditions. However, discharges were all observed using Ar, He, and N2 as the discharge gas in this work, probably due to a different DBD design in this work. Once the DBD switched on, all the carrier gases used in this work would induce CL signals, i.e., ∼160-200 counts for He and N2 and ∼1600 counts for Ar, compared to ∼50 counts without discharge. As shown in Figure 4a, in terms of CL (40) Li, R. X.; Yamaguchi, Y.; Yin, S.; Tang, Q.; Sato, T. Solid State Ionics 2004, 172, 235-238. (41) Kuphaldt, T. R. In Lessons on Electric Circuits; an electronic book on http:// www.allaboutcircuits.com/vol_1/chpt_13/3.html. (42) Massines, F.; Se´gur, P.; Gherardi, N.; Khamphan, C.; Ricard, A. Surf. Coat. Technol. 2003, 174-175, 8-14. (43) Zhang, J. L.; Sun, J.; Wang, D. Z.; Wang, X. G. Thin Solid Films 2006, 506-507, 404-408.

Figure 4. (a) CL signals and S/N ratios of ethanol using Ar, He, and N2 as discharge gas. The error bars stand for ( SD of three replicates; (b) CL signal and S/N ratio of ethanol at different N2 flow rates. The error bars stand for ( SD of three replicates. Injection volume, 0.03 µL of ethanol; discharge voltage, 1.7 kV; and luminol concentration and flow rate, 8 × 10-5 mol L-1 and 1.6 mL min-1, respectively.

signal and S/N ratio, Ar was the last choice; He gave the best S/N ratio, while N2 was the best choice for the best signal with just slightly lower S/N ratio compared to He. Further observation revealed that the surface temperature (∼90 °C) of the DBD device with Ar as the discharge gas was much higher than that in the case of He or N2 (∼30 °C). The high surface temperature of the DBD device can accelerate the aging of the outlet latex tubing of the DBD device, and this can post a potential leaking problem. Moreover, He and Ar are both much more expensive than N2. Therefore, N2 was the best choice as the discharge gas in this work. The discharge gas in a DBD device plays two roles: one is to maintain the discharge, and the other is to act as the carrier gas. The effects of N2 flow rates ranging from 100 to 600 mL min-1 were examined in this work. Figure 4b shows the dependency of the S/N ratio of ethanol on N2 flow rate. Higher gas flow rate may cause severe disturbance for the mixing of radicals and luminol, which may increase the noise and decrease the S/N ratio. A lower gas flow rate less dilutes the active species, thus a larger signal; but it also prolongs the time for the radical to reach luminol, and this increases the uncertainty. For these considerations and the sake of minimizing gas consumption, 300 mL min-1 N2 flow rate for ethanol was chosen. (c) Optimization of Discharge Voltage. As shown in Figure 5, the CL signal and S/N ratio were dependent on the discharge power. They both increased but then decreased sharply as the Analytical Chemistry, Vol. 79, No. 12, June 15, 2007

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Table 1. Optimal Analytical Conditions of Methanol, Ethanol, Propanol, Formaldehyde, and Acetaldehyde for the DBD-CL System

substance

discharge voltage (kV)

N2 flow rate (mL min-1)

luminol pH value

Cluminol (mol L-1)

luminol flow rate (mL min-1)

methanol ethanol propanol formaldehyde acetaldehyde

1.7 1.7 1.7 2.0 2.0

300 300 300 400 200

12.0 12.0 12.0 12.0 12.0

8 × 10-5 8 × 10-5 8 × 10-5 8 × 10-5 8 × 10-5

1.6 1.6 1.6 1.6 1.6

Figure 5. CL signals and S/N ratios of ethanol at different DBD discharge voltages. The error bars stand for ( SD of three replicates. Injection volume, 0.03 µL of ethanol; N2 discharge gas flow rate, 300 mL min-1; and luminol concentration and flow rate, 8 × 10-5 mol L-1 and 1.6 mL min-1, respectively.

Figure 7. Signal intensities versus the amount of ethanol injected. Good reproducibility for three parallel injections was observed; the inset shows the calibration curves of ethanol in both peak height and peak area measurements. DBD-CL conditions: discharge voltage, 1.7 kV; N2 discharge gas flow rate, 300 mL min-1; and luminol concentration and flow rate, 8 × 10-5 mol L-1 and 1.6 mL min-1, respectively.

Figure 6. CL signals and S/N ratios of ethanol using different DBD outlet tubing lengths. The error bars stand for ( SD of three replicates. Injection volume, 0.03 µL of ethanol; discharge voltage, 1.7 kV; N2 discharge gas flow rate, 300 mL min-1; and luminol concentration and flow rate, 8 × 10-5 mol L-1 and 1.6 mL min-1, respectively.

discharge voltage increased, which implies that the DBD-CL system is highly related to the dissociation power of the DBD. High discharge power results in high dissociation ability of the DBD to generate more activated substances that can be transported to react with luminol and induce CL emission, but once it exceeds a threshold, it may lead to a negative effect as mentioned earlier. The S/N ratio of ethanol reached a maximum at around 1.7-2.0 kV, and 1.7 kV was selected for use. (d) Optimization of the DBD Outlet Tubing Length. The DBD outlet tubing length is critical for CL intensity, because radicals will be lost via recombination during the transport. The DBD outlet tubing length here refers to the distance between the DBD (glass tube) outlet and the Y-shape connector. Figure 6 shows that shorter outlet length can lead to higher CL intensity, corresponding to the fact that longer tubing results in more recombination. However, the closer to the Y-shape connector, the more disturbance from the discharge gas to the detection system. The best DBD outlet tubing length for this work was 5.5 cm, as seen from Figure 6. 4678 Analytical Chemistry, Vol. 79, No. 12, June 15, 2007

(e) Optimization of the Luminol Solution and Flow Rate. Since the reaction with luminol is in an alkaline condition, the pH of the luminol solution was first optimized. According to Xiao et al.,44 carbonate could enhance the CL intensity; thus, NaHCO3NaOH buffer was selected as the reaction buffer. It was found that the CL intensity increased as the pH of the luminol solution increased and reached a plateau when pH was >12.0. Luminol solution of pH 12.0 was chosen for latter studies for it resulted in better S/N ratio. Then, the influences of luminol concentration ranging from 1 × 10-5 to 1 × 10-3 mol L-1 at pH 12.0 and its flow rate varying from 0.5 to 2.3 mL min-1 (corresponding to the peristaltic pump rate of 10 to 60 rpm) were investigated. A concentration of 8 × 10-5 mol L-1 with its flow rate at 1.6 mL min-1 resulted in the best S/N ratio, and such parameters were used for further experiments in this work. All the optimal analytical conditions for methanol, ethanol, propanol, formaldehyde, and acetaldehyde are summarized in Table 1. Analytical Characteristics. The analytical figures of merit for these five compounds were studied under optimal conditions. Figure 7 shows the signal intensities and reproducibility from a mass series of ethanol. By processing the data shown in Figure 7 in peak height or peak area, the calibration curves for ethanol were obtained and shown in the inset of Figure 7. It is found that peak area measurement provides a wider linear dynamic range (44) Xiao, C. B.; King, D. W.; Palmer, D. A.; Wesolowski, D. J. Anal. Chim. Acta 2000, 415, 209-219.

Table 2. Analytical Figures of Merit for Methanol, Ethanol, Propanol, Formaldehyde, and Acetaldehyde with the New Detector analyte

calibration equation (µg)

LDR (µg)

R

LOD (µg)

methanol ethanol propanol formaldehyde acetaldehyde

I ) 1149m - 3154 I ) 2609m - 4484 I ) 58262m - 34935 I ) 9977m + 3486 I ) 5431m - 6644

2.0-79.2 2.0-157.9 2.0-40.2 0.8-60.3 1.4-14.4

0.9997 0.9996 0.9998 0.9966 0.9983

0.26 0.08 0.01 0.03 0.04

(LDR). Also, peak area measurement is widely accepted in chromatographic analysis; thus, peak area measurement was used for quantification throughout this work. Using peak area measurement, at least five data point calibration curves with linear regression coefficients (R) of >0.99 for methanol, ethanol, propanol, formaldehyde, and acetaldehyde can be routinely obtained. The instrumental limit of detection (LOD) was calculated as, LOD ) 3N/S, in which N refers to the noise and S refers to the slope of the calibration curve. Table 2 lists the analytical figures of merit for the five substances. Brief Discussion of DBD-CL Mechanism. The discharge in a DBD has been modeled45,46 and compared to other nonequilibrium discharge plasmas in detail.47 Diode laser absorption spectroscopy of high spatial resolution for the plasma diagnostics of a low-pressure Ar DBD by Niemax et al.48 gave clear evidence for a thin, short-lived plasma layer near the temporary cathode. In this layer, the gas temperature reaches ∼1000 K and an electron density of greater than 1015 cm-3. Such high-density, energetic electrons and high gas temperature provide the high dissociation ability to form radicals, ions, and atoms. As mentioned earlier, DBD is also used to develop Hg-free UV lamp and excimer-based lamps/laser, since highly excited substances and excimers are also generated in the DBD. UV irradiation is also known as an efficient way to dissociate compounds and to generate radicals, and it is widely used for pollutant degradation treatment.49 Therefore, we believe that the UV irradiation in the DBD also contributes to the dissociation ability of the DBD. Furthermore, the highly changing frequency ac field in the DBD seems to generate a magnetic field, and whether this magnetic field will also contribute to the dissociation ability of the DBD needs further studies. Before the reactive species reaches the luminol solution, they have to travel through the transport tubing wherein they may recombine to form new species or partially loss their reactivity. By simply comparing the CL intensity of methanol, ethanol, propanol, formaldehyde, and acetaldehyde at the same concentration of 3 mol L-1, some interesting information can be obtained, as shown in Figure 8. The CL intensity of alcohol increased as the carbon chain increased, while the CL intensity of aldehyde decreased as the carbon chain increased. Among these five compounds, formaldehyde gave a strongest CL emission, which (45) Eliasson, B.; Kogelschatz, U. IEEE Trans. Plasma Sci. 1991, 19, 309-323. (46) Sjo ˜berg, M.; Serdyuk, Yu. V.; Gubanski, S. M.; Leijon, M. Å. S. J. Electrostat. 2003, 59, 87-113. (47) Eliasson, B.; Kogelschatz, U. IEEE Trans. Plasma Sci. 1991, 19, 10631077. (48) Kunze, K.; Miclea, M.; Musa, G.; Franzke, J.; Vadla, C.; Niemax, K. Spectrochim. Acta, Part B 2002, 57, 137-146. (49) Modirshahla, N.; Behnajady, M. A. Dyes Pigm. 2006, 70, 54-59.

Figure 8. Comparison of CL signals of 3 mol L-1 methanol, ethanol, propanol, formaldehyde, and acetaldehyde by the DBD-CL system. The error bars stand for ( SD of three replicates. Main experimental conditions: DBD input voltage of 1.7 kV; N2 gas flow rate of 300 mL min-1; and luminol concentration and flow rate, 8 × 10-5 mol L-1 and 1.6 mL min-1, respectively.

corresponds to the fact that formaldehyde is the compound easiest to be dissociated to generate radicals, and the CL intensity is closely related to the amount and the kinds of radicals and their production rate. The most probable radical products of these five compounds are as follows:

CH3OH f CH3• + HO• CH3CH2OH f CH3CH2• + HO•,

(1)

CH3CH2• f CH3•

(2)

CH3CH2CH2OH f CH3CH2CH2• + HO•, CH3CH2CH2• f CH3CH2• + CH3• (3) CH2O f CHO• + CO2• CH3CHO f CH3CO•,

(4)

CH3CO• f CH3• + CO2• (5)

Among these five compounds, propanol can generate the most species of radicals, thus leading to the stronger CL emission and the widest CL peak among these five substances. Yet, formaldehyde has the chemical structure easy to be dissociated and forms a more reactive radical of CO2•, thus leading to even stronger CL emission than propanol. Yet, the CL peak of formaldehyde is the narrowest among these five substances, and this corresponds to the fact that the number of radical species generated by formaldehyde is the least. Although acetaldehyde can also generate the CO2• radical, it requires more energy to break the chemical bond to generate it. To summarize, the mechanism is complicated, awaiting further exploration. Preliminary Application of DBD-CL as GC Detector. GC is an efficient separation technology widely used in environmental analysis, pharmaceutical analysis, biomedical analysis, and food industries for the analysis of volatile compounds and those nonvolatile ones that can be turned into stable volatiles.50 Conventional GC detectors are diverse,51 but except for the (50) Bartle, K. D.; Myers, P. Trends Anal. Chem. 2002, 21, 547-557. (51) Eiceman, G. A.; Gardea-Torresdey, J.; Overton, E.; Carney, K.; Korman, F. Anal. Chem. 2002, 74, 2771-2780.

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connected to the outlet of the GC column. The separation and detection of formaldehyde, ethanol, and propanol was used as an example. The recorded chromatogram is shown in Figure 9. By comparing Figure 9 to Figure 2 or Figure 7, it is clear that the peaks in Figure 9 are a bit wider, and this may be caused by the retention of the GC column and the dead volume between the connection of the GC and the DBD-CL detector. Restricted by the size of the GC instrument and the BPCL analyzer, the connection tubing was about 30 cm long (∼8 mL in volume), which may be the major reason for broadening of the chromatogram. Nevertheless, the chromatographic peaks are well resolved.

Figure 9. Chromatogram of formaldehyde (9 µg), ethanol (24 µg), and propanol (24 µg). GC conditions: injection temperature, 130 °C; column temperature, 90 °C; and N2 as the carrier gas at 12.5 mL min-1. Main experimental conditions for the DBD-CL: DBD discharge voltage, 1.7 kV; and N2 flow rate at 300 mL min-1; and luminol concentration and flow rate, 8 × 10-5 mol L-1 and 1.6 mL min-1, respectively.

thermal conductivity detector, other detectors only selectively respond to some specific compounds, and they are comparatively complicated and delicate in structure and require relatively stringent working environments. CL detectors has already been used in GC, such as the sulfur CL detector52,53 and the nitrogen or phosphorus CL detector.54 However, they are also only suitable for compounds containing a specific element, such as sulfur, nitrogen, or phosphorus. In contrast, the proposed DBD-CL as a GC detector has the following advantages: compactness, simplicity, low power, ease of hyphenation, mild working environment, and sensitive and rapid response to a wide range of organic compounds. In order to test its suitability as a GC detector, the DBD inlet was directly (52) Blomberg, J.; Riemersma, T.; van Zuijlen, M.; Chaabani, H. J. Chromatogr., A 2004, 1050, 77-84. (53) Hua, R. X.; Li, Y. Y.; Liu, W.; Zheng, J. C.; Wei, H. B.; Wang, J. H.; Lu, X.; Kong, H. W.; Xu, G. W. J. Chromatogr., A 2003, 1019, 101-109. (54) Grebel, J. E.; Suffet, I. H. Abstr. Pap. Am. Chem. Soc. 2004, 227: U1038U1038 (001-ENVR Part 1).

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CONCLUSIONS Atmospheric DBD-induced chemiluminescence was proposed as a new optical method for the sensitive detection of volatile organic species, with promising analytical applications such as a GC detector. It has many unique features: high sensitivity to a broad range of organic compounds, fast response, simplicity in fabrication, compactness in size, low power consumption, easy hyphenation to GC with good performance, and cost effectiveness. It is reported that catalyst-containing electrodes can enhance the DBD performance for pollutant degradation29 and synthesis gas conversion;55 thus, further work can be focused on the investigation of the influence on the DBD-CL system by coating the electrode with different catalysts or porous or nanosize materials. In addition, a screw-type electrode24 and a meshed-plate electrode25 can enhance the ozone generation efficiency of the DBD; hence, different electrode shapes can also be explored for this purpose. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of this work from the National Natural Science Foundation of China through Grants 20472059 and 20605013, and the Ministry of Education of China through Grant NCET-04-0869. Received for review February 14, 2007. Accepted April 4, 2007. AC070321U (55) Kim, S.-S.; Lee, H.; Na, B.-K.; Song, H. K. Catal. Today 2004, 89, 193-200.