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Atmospheric Pressure Air Direct Current Glow Discharge Ionization Source for Ion Mobility Spectrometry Can Dong,†,‡ Weiguo Wang,† and Haiyang Li*,† Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China, and Graduate School, Chinese Academy of Sciences, Beijing 100039, People’s Republic of China A new atmospheric pressure air direct current glow discharge (DCGD) ionization source has been developed for ion mobility spectrometry (IMS) to overcome the regularity problems associated with the conventional 63Ni source and the instability of the negative corona discharge. Its general electrical characteristics were experimentally investigated. By equipping it to IMS, a higher sensitivity was obtained compared to that of a 63Ni source and corona discharge, and a linear dynamic range from 20 ppb to 20 ppm was obtained for m-xylene. Primary investigations showed that alkanes, such as pentane, which are nondetectable or insensitively detectable with 63 Ni-IMS, can be efficiently detected by DCGD-IMS and the detection limit of 10 ppb can be reached. The preliminary results have shown that the new DCGD ionization source has great potential applications in IMS, such as online monitoring of environment pollutants and halogenated compounds. Ion mobility spectrometry (IMS) is a gas-phase electrophoretic technique that allows for characterizing chemical substances on the basis of the velocity of gas-phase ions in an electric field.1,2 With the properties of high sensitivity, high-speed analyses, relatively low technical expenditure, portability, and applicability to numerous organic functionalities, it has found wide applications in military and security fields, industrial and environmental analyses, biological and medical uses, and so on.1,3 Generally, the IMS instrument is mainly composed of an ionization source, drift tube, and detector, of which the ionization source is a key part and to a great extent determines the performance and applications of IMS. Conventionally, a 10 mCi radioactive 63Ni foil is used as the ionization source due to its stability, low weight, low power requirements, simplicity, and noise-free operation. However, there is a significant administrative burden associated with the safety requirements of the radioactive ionization sources, such as the requirement for leak testing and * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: 86-411-84379517. † Dalian Institute of Chemical Physics. ‡ Graduate School, Chinese Academy of Sciences. (1) Eiceman, G. A.; Karpas, Z. Ion Mobility Spectrometry, 2nd ed.; CRC Press: Boca Raton, FL, 2005. (2) McDaniel, E. W.; Mason, E. A. The Mobility and Diffusion of Ions in Gases; John Wiley & Sons: New York, 1973. (3) Borsdorf, H.; Eiceman, G. A. Appl. Spectrosc. Rev. 2006, 41, 323–375. 10.1021/ac800197g CCC: $40.75 2008 American Chemical Society Published on Web 04/17/2008
the special safety regulations in manufacturing, storing, disposal, and transportation, which makes it less likely to be accepted in the marketplace. In addition, the ion and electron density produced by the 63Ni source is not high enough, resulting in weak signal and a low dynamic range. Therefore, it is urgent to search for a new ionization source to substitute the conventional radioactive source. In recent decades, many ionization sources have been investigated including corona discharge ionization,4,5 photoionization by ultraviolet light,6,7 laser ionization,8 thermal ionization,9 surface ionization,10 corona spray ionization,11 and electrospray ionization,12 of which corona discharge ionization received the most attention in terms of research and development efforts. The pulsed corona discharge ionization source was developed for IMS without ion gate.13,14 A continuous corona discharge ionization source was also designed and optimized.4 However, negative corona discharge is difficult to realize because of the instability of the discharge itself in the presence of electron-attaching compounds. In order to solve this problem, two novel designs of the IMS cell have been developed. In one design curtain gas is used to prevent the diffusion of sample into the discharge region, but pure nitrogen has to be used.15,16 In another one, reverse flow is applied to prevent the neutral species from entering the reaction region.17 Besides corona discharge, partial discharge has also been used in IMS.18,19 However, it causes intensive fragmentation of the ions. (4) Tabrizchi, M.; Khayamian, T.; Taj, N. Rev. Sci. Instrum. 2000, 71, 2321– 2328. (5) Borsdorf, H.; Rudolph, M. Int. J. Mass Spectrom. 2001, 208, 67–72. (6) Baim, M. A.; Eatherton, R. L.; Hill, H. H. Anal. Chem. 1983, 55, 1761– 1766. (7) Leasure, C. S.; Fleischer, M. E.; Anderson, G. K.; Eiceman, G. A. Anal. Chem. 1986, 58, 2142–2147. (8) Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982, 54, 1546–1551. (9) Tabrizchi, M. Anal. Chem. 2003, 75, 3101–3106. (10) Wu, C.; Hill, H. H.; Rasulev, U. K.; Nazarov, E. G. Anal. Chem. 1999, 71, 273–278. (11) Shumate, C. B.; Hill, H. H. Anal. Chem. 1989, 61, 601–606. (12) Wittmer, D.; Chen, Y. H.; Luckenbill, B. K.; Hill, H. H. Anal. Chem. 1994, 66, 2348–2355. (13) Xu, J.; Whitten, W. B.; Ramsey, J. M. Anal. Chem. 2003, 75, 4206–4210. (14) An, Y. A.; Aliaga-Rossel, R.; Choi, P.; Gilles, J. P. Rev. Sci. Instrum. 2005, 76, 1–6. (15) Tabrizchi, M.; Abedi, A. Int. J. Mass Spectrom. 2002, 218, 75–85. (16) Khayamian, T.; Tabrizchi, M.; Jafari, M. T. Talanta 2003, 59, 327–333. (17) Ross, S. K.; Bell, A. J. Int. J. Mass Spectrom. 2002, 218, L1-L6. (18) Schmidt, H.; Baumbach, J. I.; Pilzecker, P.; Klockow, D. Int. J. Ion Mobility Spectrom. 2000, 3, 8–14. (19) Schmidt, H.; Baumbach, J. I.; Sielemann, S.; Wember, M.; Klockow, D. Int. J. Ion Mobility Spectrom. 2001, 4, 39–42.
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Figure 1. Schematic diagram of the DCGD-IMS apparatus.
High-density ions and electrons are often generated from gas molecules by various electrical discharges; among them, direct current glow discharge (DCGD) has engaged attention in recent years due to its stability at atmospheric pressure air condition. It was first demonstrated by Kruger and co-workers.20,21 Unlike the widely investigated atmospheric pressure glow discharge (APGD),22–24 DCGD needs neither a high-flux gas flow nor a special electrode design. It is a nonequilibrium gas plasma with high electron density (above 1012 cm-3) and relatively low gas temperature.21 In addition, dc operation enables easy control of the current and plasma properties, making it attractive for various applications, such as volatile organic compounds abatement.25,26 In this study, atmospheric pressure air DCGD is first investigated as an ionization source for IMS. On the basis of the investigation of its electrical characteristics, the capabilities of DCGD as a stable and reliable ionization source for IMS were demonstrated. EXPERIMENTAL SECTION The experiment setup, including the DCGD ionization source, the attached ion mobility spectrometer, and the sample introduction system, is schematically illustrated in Figure 1. The attached ion mobility spectrometer was similar to the conventional structure, and it mainly included a reaction region, ion gate, drift region, and detection region. Ions of the sample formed in the reaction region passed through the pulse opened ion gate and entered the drift region, where they were distinguished according to their drift velocities. Then, the ion current was collected by a Faraday cup and enlarged by an amplifier with an enlargement factor of 109 V/A. Finally, the signal was stored by an oscilloscope (Tektronix, TDS 2024) after averaging many times. The reaction region and drift region were a conventional discrete ring design, with the (20) Duten, X.; Packan, D.; Yu, L.; Laux, C. O.; Kruger, C. H. IEEE. Trans. Plasma Sci. 2002, 30, 178–179. (21) Machala, Z.; Marode, E.; Laux, C. O.; Kruger, C. H. J. Adv. Oxid. Technol. 2004, 7, 133–137. (22) Akishev, Y. S.; Deryugin, A. A.; Kochetov, I. V.; Napartovich, A. P.; Trushkin, N. I. J. Phys. D: Appl. Phys. 1993, 26, 1630–1637. (23) Goossens, O.; Callebaut, T.; Akishev, Y.; Napartovich, A.; Trushkin, N.; Leys, C. IEEE. Trans. Plasma Sci. 2002, 30, 176–177. (24) Temmerman, E.; Akishev, Y.; Trushkin, N.; Leys, C.; Verschuren, J. J. Phys. D: Appl. Phys. 2005, 38, 505–509. (25) Machala, Z.; Morvova, M.; Marode, E.; Morva, I. J. Phys. D: Appl. Phys. 2000, 33, 3198–3213.
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conducting rings made of stainless steal and the insulating rings made of high-temperature Teflon. The reaction region was 20 mm long with an i.d. of 14 mm, and the drift region was 110 mm long with an i.d. of 30 mm. The thicknesses of the conducting ring and the insulating ring are 1.0 and 8.0 mm, respectively. The conducting rings were connected by a series of precise 1 MΩ resistors to form a homogeneous drift field. The ion gate was a Bradbury-Nielson-type, which was made of two series of parallel wires biased to a potential, creating an orthogonal field relative to the drift field, to block ions into the drift tube. The injection pulse was 200 µs under our conditions. The DCGD ionization source with a point-to-grid geometry is composed of a discharge electrode of sharp tungsten needle and a target electrode of stainless steel grid, with an interelectrode distance of 2.0 mm except as indicated. Another electrode, called the extraction electrode, placed close to the target electrode, was used to extract ions into the reaction region. A homemade dc highvoltage power supply (10 kV, 20 mA) was employed with a ballast resistor for producing DCGD. The ballast resistor was used for stabilization and protection, which was typically between 10 kΩ and 1.5 MΩ, depending on the operating current, and 1 MΩ was used here. A bidirectional flow scheme was used in our IMS instrument. The drift gas was brought into the drift region from behind the ion collector to keep the drift tube free from contaminants, and the sample was introduced into the reaction region by directly bringing diluted headspace vapor of an 80 mL flask, inside of which there is a 2.0 mL vessel loaded with 1.0 mL of pure sample. The vessel was sealed by a capsule, and the concentration of the sample in the headspace of the flask was controlled by regulation of the capsule and could be estimated by a weighing method. It was kept at 1-10 ppm except as noted. Compressed air filtered with silica gel and activated carbon was used as the drift gas and carrier gas, with flow rates of 500 and 100 mL/min, respectively. The concentration of water in the gases was about 790 ppm, which was detected at a gas flow rate of 500 mL/min with a dew point instrument (DP300, CS Instrument GMH). (26) Machala, Z.; Marode, E.; Morvova, M.; Lukac, P. Plasma Process. Polym. 2005, 2, 152–161.
Figure 2. Photographs of the DCGD in ambient air at atmospheric pressure with an interelectrode distance of 2.0 mm: (a) photograph for positive DCGD; (b) photograph for negative DCGD.
Figure 3. Discharge current as a function of total voltage of the electrocircuit at several interelectrode distances.
RESULTS AND DISCUSSION Electrical Characteristics of the DCGD. Figure 2 shows typical photographs of both positive and negative DCGD in atmospheric pressure air. A stable continuous discharge regime can be seen, and the stratification into dark and bright layers typical of a low-pressure glow discharge can also be observed here. Negative glow, which is the brightest of the entire discharge and the most intense on the cathode side, forming close to the cathode (grid in positive DCGD and point in negative DCGD), can also been seen clearly in Figure 2. The interelectrode distance could be varied from a few hundred micrometers to a few centimeters, depending on the gas flow conditions and the current. Similar to subnormal glow discharge, the current of DCGD decreases as a function of a second-order exponential with the needle voltage,27 and such a voltage-current characteristic is different from that of a corona discharge, in which the current increases with the rising of the voltage.4 The current I versus the total voltage V of the electrocircuit at several interelectrode distances from 0.5 to 8 mm is plotted in Figure 3. Within the range of the total voltage V investigated, the behavior of I is divided into two regions, the microamp region and the milliamp region, and the microamp region is longer when the interelectrode distance is larger. For example, the microamp region changes (27) Druyvesteyn, M. J.; Penning, F. M. Rev. Mod. Phys. 1940, 12, 87–174.
Figure 4. Signal intensity of the RIP with the DCGD ionization source as a function of Ud at different drift fields.
from 4 to 6 µA for the interelectrode distance of 2 mm, which is much smaller than that for the interelectrode distance of 8 mm, from 1 to 101 µA. The microamp region for an interelectrode distance of 8 mm is zoomed out and inset in Figure 3. A voltage-current characteristic of a corona discharge is obtained for the total voltage less than 9 kV, while after that the discharge becomes a normal glow discharge (last three points). It demonstrates that DCGD is initiated in the process of a streamer developing into a spark, but the ballast resistor immediately limits the spark current. The current does not fall to zero after the spark pulse; it rests at a certain value in the order of milliamps.25 At the same time, a new streamer cannot develop after this, because the discharge potential is too weak. No more pulses appear, and the discharge remains stable. The region where the current rises sharply (last three points) maybe corresponds to APGD,23 which is unstable due to the low flux of the gas flow and the configuration of the discharge electrodes in this study. Performance of the DCGD-IMS. Ion Mobility Spectra. Ion mobility spectra of a single run of air using DCGD as the ionization source in both positive and negative mode show only one main peak at drift times of 19.70 and 20.50 ms, respectively. From the positive reactant ion peak (RIP), the following parameters were derived: reduced mobility, K0 ) 2.48 cm2/V · s; peak width, w1/2 ) 0.61 ms; amplitude, I ) 1.03 nA; resolving power, R ) 32.30. The intensity amplitude of the spectrum is much higher than that obtained by conventional 63Ni-IMS, and the resolving power is similar. For negative mode, O2 and NO2 molecules were introduced into the discharge region, and no change was observed in the spectrum for O2, while a small increase of the peak height was observed for NO2, implying that the peak probably corresponds to NO2-(H2O)n. The stability of the DCGD-IMS was studied by measuring the height of the RIP every 1 min in a total time of 85 min in both positive mode and negative mode. The relative standard deviations were about 2.06% and 1.14%, respectively, which show that the stability and reliability of DCGD are relatively high. Sensitivity. The effect of electric field force on the efficiency of ions entering the reaction region from the discharge region has been investigated. The electric field force was produced by the electric potential difference (Ud) between the target electrode and the extraction electrode. Figure 4 shows the signal intensity Analytical Chemistry, Vol. 80, No. 10, May 15, 2008
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Figure 5. Comparison of the signal intensities of the RIP without sample as a function of the drift field with DCGD, corona discharge, and 63Ni as ionization sources.
of the RIP as a function of Ud at different drift fields. The signal is weak and shows almost no growth when Ud increases from 0 to 100 V, because in this region the electric field force is weak and the ions entering the reaction region are mostly attributed to diffusion. When Ud is higher than 100 V, the signal intensity starts to increase exponentially with the increase of Ud, which indicates that the electric field force plays a more and more important role, and as a result more ions enter reaction region. When Ud is higher than 600 V, the signal intensity nearly keeps constant as Ud increases further, indicating that almost all the ions have entered the reaction region. Similar results was observed in the corona discharge ionization source.15 With Ud of 1000 V, the ion signal intensities of the RIP for IMS with 15 mCi 63Ni, corona discharge, and DCGD ionization sources as functions of the drift field are compared and plotted in Figure 5. The geometry of the corona discharge was similar to that of the DCGD, except that the ballast resistor was 100 MΩ. It was optimized, and the electric potential difference (Ud) between the target electrode and the extraction electrode was also kept at 1000 V. The cylinder of the 63Ni source (Beijing Atom High-Tech Co., Ltd.) is 10 mm long with an i.d. of 10 mm. The data for corona discharge is only given as drift field up to 250 V/cm as the power supply used here is not high enough and can not create enough electric potential difference Ud. As can be seen from Figure 5, the ion signal intensities obtained with the three ionization sources all increase with the drift field getting stronger. But the increase is the most remarkable for the DCGD; for example, the signal intensity of the DCGD is about 2 times that of the corona discharge in a drift field of 250 V/cm and 7 times that of 63Ni in a drift field of 350 V/cm. The high ion current of the DCGD, indicating a better sensitivity and a higher signal-to-noise ratio, can be ascribed to the high electron densities (above 1012 cm-3).21 Linear and Working Ranges. The application of IMS for quantitative analysis is limited because of its restricted linear range and its narrow working range. Commonly, the linear dynamic ranges of only 10-100 are reported for IMS, and the working ranges can be near or less than 1000.1 For DCGD-IMS, the working range and linear dynamic range were also studied, and the results are shown in Figure 6, in which the ion signal intensity is plotted against the concentration (v/v) of m-xylene from 10 ppb 3928
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Figure 6. Ion signal intensity of m-xylene as a function of the concentration of m-xylene.
to 100 ppm. The ion signal intensity in the concentration of 20-100 ppm is zoomed out and inset in the figure. As can be seen, the ion signal intensity rises linearly with the concentration of m-xylene increasing from 20 ppb to 20 ppm, and the relative standard deviation is 0.998. At concentrations from 20 to 100 ppm, the ion signal intensity rises from 1794 to 3517 mV, although the rise is not linear with the concentration of m-xylene. So the working range is about 4 orders of magnitude, and the linear dynamic range is about 3 orders of magnitude, which have been extended at least 1 order of magnitude compared with the conventional 63Ni-IMS. The reason is that the form of product ions may be due to the primary ionization processes in DCGD, including charge-transfer reaction, hydrogen abstraction, Penning ionization, and vacuum ultraviolet (VUV) photoionization, which are caused by the high concentration of energetic species, such as various active ions (H2O)nNO+, (H2O)nO2+, H2O+, metastable molecules, and VUV photons. Applications of the DCGD-IMS. Primary applications of the DCGD-IMS were also performed, and the results are shown in Figures 7–9. First, alkanes, which are nondetectable or insensitively detectable in 63Ni-IMS, were found to be easily detected by DCGD-IMS. In this study, the drift and carrier gases were filtered with silica gel and activated carbon, and the water concentration was 790 ppm. It is much higher than that in the investigation performed by Karasek et al., where the gases were passed through individual metal traps of 2.25 L capacity packed with Linde Molecular Sieve 13X, and the water concentration was 10 ppm.28 Figure 7a shows the positive ion mobility spectrum of n-pentane with concentration of 1 ppm. As can be seen, there are two wellresolved peaks at drift times of 22.70 and 25.35 ms, corresponding to K0 values of 2.15 and 1.93 cm2/V · s, which probably formed by hydrogen abstraction and clustering with water. The ion mobility spectrum of n-pentane with a concentration of 10 ppb is shown in Figure 7b. For comparison, the ion mobility spectrum without n-pentane is also shown. Only one peak at a drift time of 25.03 ms can be observed for n-pentane. So the reduced mobilities of the produced ions depend on the concentration of the alkanes, and similar phenomena were reported previously.29 The detection limit for n-pentane is about 10 ppb, at least 2 orders of magnitude (28) Karasek, F. W.; Denney, D. W.; DeDecker, E. H. Anal. Chem. 1974, 46, 970–973.
Figure 9. Negative ion mobility spectra of several halogenated compounds, CCl4, CH2Br2, and CH3I, with the DCGD ionization source.
Figure 7. Positive ion mobility spectra of n-pentane with the DCGD ionization source: (a) 1 ppm; (b) 10 ppb.
Figure 8. Positive ion mobility spectra of benzene, toluene, mxylenes, and MTBE with the DCGD ionization source.
lower than that obtained by corona discharge.29 The high detection sensitivity for alkanes with DCGD-IMS at high water concentration may attribute to the existence of various active ions, such as (H2O)nNO+, (H2O)nO2+, H2O+, which can react with alkanes via charge-transfer reaction and hydrogen abstraction to form alkanes ions.30,31 Meanwhile, Penning ionization and VUV photoionization, which may exist in DCGD, can also result in the (29) Borsdorf, H.; Schelhorn, H.; Flachowsky, J.; Doring, H. R.; Stach, J. Anal. Chim. Acta 2000, 403, 235–242.
ionization of alkanes, whereas the dominant ionization mechanism in the 63Ni source is the proton-transfer reaction, which can hardly ionize alkanes, with proton affinity near or less than that of water. Second, the detection of some environment pollutants, like benzene, toluene, xylenes (BTX), and methyl tert-butyl ether (MTBE) were primarily studied, and the results are shown in Figure 8. As demonstrated in the spectra, each compound creates one characteristic peak, for example, 24.24 ms for benzene, 25.30 ms for toluene, 22.01 ms for m-xylene, and 25.80 ms for MTBE, with K0 values of 2.01, 1.93, 2.22, and 1.89 cm2/V · s, respectively. These results for benzene and toluene correspond well with the results obtained by using UV-IMS and 63Ni-IMS.32,33 The higher K0 value for m-xylene possibly results from the fragment ions of m-xylene, and the lower K0 value for MTBE may be due to the form of clusters with water. Finally, three halogenated compounds, CCl4, CH2Br2, and CH3I, were investigated using the negative mode of DCGD, and the results are shown in Figure 9. As can be seen, two obvious product ions are observed for each compound, with K0 values of 2.38, 2.04 cm2/V · s for CCl4, 2.36, 2.28 cm2/V · s for CH2Br2, and 2.39, 1.98 cm2/V · s for CH3I. This result is different from the literature, in which the K0 values were 2.92, 2.63, and 2.53 cm2/V · s for CCl4, CH2Br2, and CH3I, respectively.34 It may be due to the form of clusters and dimers as the drift tube was at room temperature in our experiment, whereas the temperature in the literature was 130 °C. In experiment, we found that when halogenated hydrocarbons were introduced into the discharge region, the discharge current decreased by some degree, less than 100 µA, depending on the electron affinity and the concentration of the compounds. As the current only decreased less than 2% of the total discharge current (4.8 mA), it influences neither the signal intensity very much nor the stability of the discharge, whereas in corona discharge, the total discharge current is only about 100 µA (as (30) Bell, S. E.; Ewing, R. G.; Eiceman, G. A.; Karpas, Z. J. Am. Soc. Mass Spectrom. 1994, 5, 177–185. (31) Sˇpanı`l, P.; Smith, D. Int. J. Mass Spectrom. 1998, 181, 1–10. (32) Baumbach, J. I.; Sielemann, S.; Xie, Z.; Schmidt, H. Anal. Chem. 2003, 75, 1483–1490. (33) Wessel, M. D.; Sutter, J. M.; Jurs, P. C. Anal. Chem. 1996, 68, 4237–4243. (34) Karasek, F. W.; Hill, H. H.; Kim, S. H. J. Chromatogr. 1977, 135, 329–339.
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seen in the inset of Figure 3), so the discharge becomes unstable or even quenched when the electronegative compounds are introduced into the discharge region.
future, we plan to combine DCGD with a mass spectrometer to clarify the ionization processes in DCGD. ACKNOWLEDGMENT
CONCLUSIONS A DCGD ionization source for IMS in atmospheric pressure air has been demonstrated. The linear range of 3 orders of magnitude was obtained for m-xylene from 20 ppb to 20 ppm, and the sensitivity is about 7 times higher than that of a 15 mCi 63Ni ionization source. Primary investigations have been performed for detection of several samples, such as alkanes, aromatics, and halogenated compounds, and the results show that DCGD is a good atmospheric pressure ionization source for IMS. In the
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This work was supported by the National Natural Science Foundation of China (Nos. 20573111 and 40637036), the National High-Tech Research and Development Plan (No. 2007AA061503), and the Major Subject of The Chinese Academy of Sciences (No. Y2005005). Received for review January 26, 2008. Accepted March 26, 2008. AC800197G
