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Determination of ultratrace nitrogen in pure argon gas by dielectric barrier discharge-molecular emission spectrometry
Microchemical Journal 99 (2011) 114–117
Contents lists available at ScienceDirect
Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
Determination of ultratrace nitrogen in pure argon gas by dielectric barrier discharge-molecular emission spectrometry Wei Li a, Xiaoming Jiang a, Kailai Xu a, Xiandeng Hou a,b,⁎, Chengbin Zheng a,⁎ a b
College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China
a r t i c l e
i n f o
Article history: Received 5 April 2011 Accepted 7 April 2011 Available online 15 April 2011 Keywords: Dielectric barrier discharge Nitrogen Argon Molecular emission spectrometry Portable instrumentation
a b s t r a c t A dielectric barrier discharge (DBD) was used as a new atmospheric optical emission detector for the determination of trace nitrogen in pure argon gas in this work. The whole system was composed of an ac ozone generation device for power supply, a six-way valve, a laboratory-built DBD device and a USB2000 charge coupled device (CCD). Trace nitrogen in argon was detected at nitrogen molecular emission line of 337 nm. This method features with several advantages: atmospheric working condition, low power consumption (≤12 W), simple and cheap instrumentation, fast response and high sensitivity and accuracy. Under the optimized conditions, the limits of detection can be down to 34 ppb. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Inert gasses are commonly used as protective atmosphere in high-technology industries [1]. Controlling trace impurities, such as nitrogen in argon, plays an important role in these fields. Some matured analytical methods for these trace impurities include atmospheric pressure ionization/mass spectrometry (API/MS) [2–6] and gas chromatography mass spectrometry (GC/MS) [7]. API/MS has high sensitivity for determination of gas phase contaminants such as oxygen and water. Because it is based on the energy transfer, however, it is not so efficient for detection of nitrogen in argon since the ionization energy of nitrogen is much higher than that of argon, this leads to unsatisfactory sensitivity. In addition, the maintenance (gas consumption etc.) of the MS-based system is usually expensive. Although GC/MS can detect trace nitrogen in argon, it also has similar problems in the determination of trace nitrogen in pure argon. Nevertheless, MS technique is the choice in real sample analysis for trace impurities in pure inert gas also because of its successful commercialization. Plasma optical emission spectrometry as tools for the determination of trace impurities in inert gasses have been explored since 1960s. The first plasma-based method used in this field is microwave induced plasma optical emission spectrometry. In the last couple of decades, it has been used for direct and continuous detection of impurities in argon [8–10], with limits of detection (LODs) of below 1 ppm. And, for trace nitrogen in argon with analytical emission detected at 337 nm, the LOD could be as low as 0.01 ppm. In 1997, a low pressure, capillary glow discharge method ⁎ Corresponding authors at: College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. E-mail addresses: [email protected] (X.D. Hou), [email protected] (C.B. Zheng). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.04.005
was used for the determination of trace nitrogen in argon [11]. It also used 337 nm as the analytical line of nitrogen and the LOD was impressively down to 0.1 ppb. These two methods solved the problem of sensitivity and expenses associated with API/MS and GC/MS for the determination of ultratrace nitrogen in pure argon. However, it should also be pointed out that these two tools require low pressure which makes the whole system and operation become complicated. In 2003, an atmospheric pulsed plasma was used as a simple, sensitive nitrogen analyzer [12], and was used for detection of trace nitrogen in helium at the emission line of 357 nm. The instrument consisted of a pulsed plasma source, an optical collection system, and a charge coupled device (CCD) detector. The LOD for nitrogen was 15 ppb which is satisfactory in many applications. DBD is a typical nonequilibrium high pressure ac gas discharge [13] and it can work at atmospheric pressure, which was firstly reported by Siemens in as early as 1857 [14]. The structure of DBD simply includes two electrodes and at least one is protected by dielectric medium. The discharge can occur between the two electrodes when supplying high ac voltage to the electrodes. DBD has many attractive advantages including atmospheric pressure working condition, simplicity in fabrication, fast response, high sensitivity, low power consumption, and low cost for construction and maintenance. All these make it widely used in industry and very promising for its applications in analytical spectroscopy. In recent years, in fact, DBD has been introduced into analytical areas such as chemiluminescence (CL), [15,16] ion mobility spectrometry (IMS) [17], mass spectrometry (MS) [18], and atomic spectrometry [19–24], which includes atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), and atomic emission spectrometry (AES). In this work, DBD was used as a new optical analyzer for the detection of trace nitrogen in argon at 337 nm. This is a new, simple, sensitive, and cost-effective method.
W. Li et al. / Microchemical Journal 99 (2011) 114–117
115
Fig. 1. Schematic diagram of the experimental setup. A: a photograph of the DBD plasma.
2. Experimental section 2.1. Instrumentation The schematic diagram of the DBD–CCD optical emission system is shown in Fig. 1. This system includes three main parts, a six-way valve, a laboratory-built DBD device and a USB2000 charge coupled device (Ocean Optics, USA). The six-way valve comes from a Techcomp GC7890F GC instrument (Techcomp Ltd., Shanghai, China). It consists of a quantitative ring which can contain 3 mL of sample. First, the sample fully fills the quantitative ring, while the carrier gas argon passes through another way to the DBD directly; then, the carrier gas is switched back to the sampling channel to sweep the sample into the DBD for detection. The laboratory-built DBD device consisted of a 3.0 mm i.d. × 5.0 mm o.d. × 50 mm long quartz tube and two copper electrodes. One of the copper wires was inserted into the quartz tube as the inner electrode, and the other wrapped on the outside of the tube as another electrode. Both of them were 1 mm in diameter. The power supply consisted of an ac ozone generation device (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) and a transformer (TPGC2J-1, Shanghai Pafe Electronic Equipment Ltd. Co., Shanghai, China). The optical signal generated from the DBD plasma was detected by the CCD. 2.2. Reagents High purity Argon (99.999%, QiaoYuan Gas Co. Ltd., Chengdu, China) was used as both carrier gas and discharge gas. Argon gasses containing 5 ppm and 20 ppm nitrogen (Testing Technology Institute of China, Chengdu, China) were used as standards. The samples were prepared by mixing the high pure argon and the standards. Both flow rates were controlled by a mass flow controller within 1% precision. 3. Results and discussion 3.1. Emission spectrum Different carrier gasses give varied emission spectrum backgrounds. In this work, we used the characteristic line of nitrogen at 337 nm as its analytical line for quantitative detection. As shown in Fig. 2, trace A represented the background of high pure argon. The weak peak at 337 nm and 357 nm is most probably due to the trace nitrogen impurities in the high pure argon gas. When adding 5 ppm nitrogen into the high pure argon, the spectral peaks at 337 nm and 357 nm increased sharply as shown in trace B of Fig. 2. Therefore, this specific emission line of nitrogen can be used for quantification of trace nitrogen in argon, as also reported in Refs. [8–12]. 3.2. Optimization of experimental conditions 3.2.1. Effect of flow rate of discharge gas Argon was used as both carrier gas and discharge gas. Fig. 3 shows the signal and S/N (signal to noise) ratio of nitrogen at different argon gas flow rate. As can be seen from this figure, in the flow rate range of 200–1000 mL min−1 of high pure argon, signal increased as the increasing of the gas flow rate. When the flow rate reached 500 mL min−1, the signal came to the highest point and then decreased with the increasing of gas flow rate. Taking this into consideration, 500 mL min−1 of argon flow rate was chosen for use.
Fig. 2. Spectral emission from trace nitrogen in argon gas. Integrated time, 300 ms; gas flow rate, 1000 mL min−1; discharge voltage, 2.95 kV. (A) High purity argon and (B) argon emission spectrum with 5 ppm nitrogen added. The four nitrogen emission lines are at 337 nm, 357 nm, 379 nm and 405 nm.
3.2.2. Optimization of discharge voltage The effect of discharge voltage ranging from 2.21 to 3.25 kV was examined in this work. As shown in Fig. 4, the discharge voltage determined the signal to a certain extent because higher voltage could
116
W. Li et al. / Microchemical Journal 99 (2011) 114–117
1400 1200
Intensity
1000 800 600 400 200 0 0
10
20
30
40
50
60
Time(s) Fig. 3. Signal and S/N ratio of trace nitrogen in argon at different gas flow rates. The error bars stand for ±SD of three replicates. Injection volume, 3 mL of sample (argon containing 5 ppm nitrogen); and discharge voltage, 2.95 kV.
supply higher power to generate more radicals and the like in the DBD device. But when the power reached 200 V, the signal tended to be very unstable and the S/N ratio increased sharply. Thus, the discharge voltage of 2.95 kV was the best choice.
3.3. System stability At atmospheric pressure, when supplying an ac high voltage to the electrodes, the discharge can occur between the two electrodes. For studying the stability of the system, the background was continuously collected 3 s per point with nitrogen concentration of 5 ppm. Fig. 5 showed that when feeding the gas sample, the intensity of nitrogen emission line at 337 nm maintained stable, with a relative standard deviation (RSD) of 0.9%.
3.4. Analytical characteristics and potential applications The analytical figures of merit of the proposed method for nitrogen determination were obtained under optimal experimental conditions. The peak height was used for quantitative measurement throughout this work. Under these conditions, the calibration curves had good linearity, with linear regression coefficients (R) of N0.99. The LOD was
Fig. 5. Stability of the system. The emission intensity fluctuation of nitrogen at 337 nm with a concentration of 5 ppm during 60 s. Integrated time, 300 ms; gas flow rate, 1000 ml min−1; and discharge voltage, 2.95 kV. RSD= 0.9%.
calculated as LOD = 3 standard deviation/slope of the calibration curve. All of these analytical figures of merit were summarized in Table 1. In order to fully verify the applicability of the proposed method in real sample analysis, we attempted to determine trace nitrogen impurities in industrial gas samples, with analytical results listed in Table 2. The analytical results showed that all of these gas samples had a much lower concentration of nitrogen than it is labeled. For each sample, the analysis time was less than 2 min, securing a really fast screen method good for in site analysis of industrial argon gas samples. 3.5. Discussion of emission mechanism The optical emission of nitrogen usually has two pathways. One way is the direct emission of nitrogen, and the other is through energy transferring from argon to nitrogen. The latter was useful when detecting trace nitrogen. In the discharge progress, the following reaction may occur in DBD [25]. Ar discharge→Ar⁎→Ar + hυ
ð1Þ
Ar discharge→Ar⁎⁎→Ar + hυ
ð2Þ
N2 discharge→N2 ⁎→N2 + hυ
ð3Þ
½Ar N N ½N2 Ar⁎⁎ + N2 →N2 ⁎ + Ar→N2 + Ar + hυ
ð4Þ
Where h and υ are the Plank constant and the optical emission frequency, respectively. In the above equations, Ar* and N2* denote the excited state of argon and nitrogen, and Ar** the metastable excited argon atom. The energy of Ar** is equivalent to the energy for excitation of molecular nitrogen. Eq. (3) displays the direct emission of nitrogen, but when the concentration of nitrogen in DBD is too low to emit directly, the energy will be transferred from Ar** to N2, as depicted in Eq. (4).
Table 1 The analytical characteristics of ultratrace nitrogen in argon by the proposed method. Analyte Fig. 4. Signal and S/N ratio of trace nitrogen in argon at different discharge voltage. The error bars stand for ±SD of three replicates. Injection volume, 3 mL of sample (argon containing 5 ppm nitrogen); and gas flow rate, 500 mL min−1.
Calibration equation (ppm) Concentration range (ppm) R
nitrogen y = 1223x − 155
0.24 − 2.5
LOD (ppb)
0.997 34
W. Li et al. / Microchemical Journal 99 (2011) 114–117
117
Table 2 The analysis of real argon samples for trace nitrogen impurity. Samples
Manufacturer
Purity specification
Production date
Nitrogen impurity labeled (ppm)
Nitrogen found (ppm)
Sample Sample Sample Sample
Qiao Qiao Qiao Qiao
99.99% 99.99% 99.99% 99.99%
April 13, 2010 April 15, 2010 May 7, 2010 May 26, 2010
≤5 ≤5 ≤5 ≤5
0.22 0.41 0.27 0.26
1 2 3 4
Yuan Yuan Yuan Yuan
4. Conclusions The atmospheric pressure DBD plasma is a new promising optical detector for monitoring (especially field screening) trace nitrogen in high purity argon gas samples. This work expanded the analytical applications of DBD to molecular emission spectrometry. There are many attractive characteristics of this method: atmospheric pressure working condition, fast response, simple and portable instrumentation, high sensitivity, low cost and power consumption.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgments [17]
The authors acknowledge the financial support from the National Natural Science Foundation of China (no. 20835003). We thank Mr. Xi Wu of Analytical & Testing Center for technical help with this project.
[18] [19] [20] [21]
References [1] [2] [3] [4] [5]
J.W. Mitchell, Solid State Technol. (1985, March) 131–137. J.L. Briesacher, M. Nakamura, T. Ohmi, J. Electrochem. Soc. 138 (1991) 3717–3723. K. Siefering, H. Berger, J. Electrochem. Soc. 139 (1992) 1442–1445. K. Siefering, W. Whitlock, H. Berger, J. Electrochem. Soc. 140 (1993) 1165–1168. K. Siefering, H. Berger, W. Whitlock, J. Vac. Sci. Technol. A 11 (1993) 1593–1597.
[22] [23] [24] [25]
E.J. Hunter, A.R. Homyak, S.N. Ketkar, J. Vac. Sci. Technol. A 16 (1998) 3127–3130. J. Hille, J. Chromatogr. A 502 (1990) 265–274. H.E. Taylor, J.H. Gibson, R.K. Skogerboe, Anal. Chem. 42 (1970) 876–881. T.H. Ramsey, M.D. Nelson, Anal. Chem. 54 (1982) 826–828. E. Denkhaus, A. Golloch, H.-M. Kuss, Fresenius, J. Anal. Chem. 353 (1995) 156–161. H. Ogino, T. Seki, Anal. Chem. 69 (1997) 3636. Z. Jin, Y.X. Duan, Rev. Sci. Instrum. 74 (2003) 5156–5160. X.J. Xu, Thin Solid Films 390 (2001) 237–242. I. Weeks, in: G. Svehla (Ed.), Chemiluminescence Immunoassay Wilson and Wilson's Comprehensive Analytical Chemistry, Vol. 29, Elsevier: Amsterdam, 1992. Y.H. He, Y. Lv, Y.M. Li, H.R. Tang, L. Tang, X. Wu, X.D. Hou, Anal. Chem. 79 (2007) 4674–4680. Y.M. Li, J. Hu, L. Tang, Y.H. He, X. Wu, X.D. Hou, Y. Lv, J. Chromatogr. A 1192 (2008) 194–197. A. Michels, S. Tombrink, W. Vautz, M. Miclea, J. Franzke, Spectrochimi. Acta. B 62 (2007) 1208–1215. T. Zoltan, J.M. Wiseman, R.G. Cooks, J. Mass Spectrom. 40 (2005) 1261–1275. M. Miclea, K. Kunze, G. Musa, J. Franzke, K. Niemax, Spectrochimi. Acta B 56 (2001) 37–43. Z.L. Zhu, S.C. Zhang, Y. Lv, X.R. Zhang, Anal. Chem. 78 (2006) 865–872. Z.L. Zhu, J.X. Liu, S.C. Zhang, X. Na, X.R. Zhang, Spectrochimi. Acta B 63 (2008) 431–436. Z.L. Zhu, J.X. Liu, S.C. Zhang, X. Na, X.R. Zhang, Anal. Chim. Acta 607 (2008) 136–141. Y.L. Yu, Z. Du, M.L. Chen, J.H. Wang, Angew. Chem. Int. Ed. 47 (2008) 7909–7912. Z.L. Zhu, G.C.-Y. Chan, S.J. Ray, X.R. Zhang, G.M. Hieftje, Anal. Chem. 80 (2008) 8622–8627. H. Fay, P.H. Mohr, G.A. Cook, Anal. Chem. 34 (1962) 1254–1260.
Contents lists available at ScienceDirect
Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
Determination of ultratrace nitrogen in pure argon gas by dielectric barrier discharge-molecular emission spectrometry Wei Li a, Xiaoming Jiang a, Kailai Xu a, Xiandeng Hou a,b,⁎, Chengbin Zheng a,⁎ a b
College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China
a r t i c l e
i n f o
Article history: Received 5 April 2011 Accepted 7 April 2011 Available online 15 April 2011 Keywords: Dielectric barrier discharge Nitrogen Argon Molecular emission spectrometry Portable instrumentation
a b s t r a c t A dielectric barrier discharge (DBD) was used as a new atmospheric optical emission detector for the determination of trace nitrogen in pure argon gas in this work. The whole system was composed of an ac ozone generation device for power supply, a six-way valve, a laboratory-built DBD device and a USB2000 charge coupled device (CCD). Trace nitrogen in argon was detected at nitrogen molecular emission line of 337 nm. This method features with several advantages: atmospheric working condition, low power consumption (≤12 W), simple and cheap instrumentation, fast response and high sensitivity and accuracy. Under the optimized conditions, the limits of detection can be down to 34 ppb. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Inert gasses are commonly used as protective atmosphere in high-technology industries [1]. Controlling trace impurities, such as nitrogen in argon, plays an important role in these fields. Some matured analytical methods for these trace impurities include atmospheric pressure ionization/mass spectrometry (API/MS) [2–6] and gas chromatography mass spectrometry (GC/MS) [7]. API/MS has high sensitivity for determination of gas phase contaminants such as oxygen and water. Because it is based on the energy transfer, however, it is not so efficient for detection of nitrogen in argon since the ionization energy of nitrogen is much higher than that of argon, this leads to unsatisfactory sensitivity. In addition, the maintenance (gas consumption etc.) of the MS-based system is usually expensive. Although GC/MS can detect trace nitrogen in argon, it also has similar problems in the determination of trace nitrogen in pure argon. Nevertheless, MS technique is the choice in real sample analysis for trace impurities in pure inert gas also because of its successful commercialization. Plasma optical emission spectrometry as tools for the determination of trace impurities in inert gasses have been explored since 1960s. The first plasma-based method used in this field is microwave induced plasma optical emission spectrometry. In the last couple of decades, it has been used for direct and continuous detection of impurities in argon [8–10], with limits of detection (LODs) of below 1 ppm. And, for trace nitrogen in argon with analytical emission detected at 337 nm, the LOD could be as low as 0.01 ppm. In 1997, a low pressure, capillary glow discharge method ⁎ Corresponding authors at: College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. E-mail addresses: [email protected] (X.D. Hou), [email protected] (C.B. Zheng). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.04.005
was used for the determination of trace nitrogen in argon [11]. It also used 337 nm as the analytical line of nitrogen and the LOD was impressively down to 0.1 ppb. These two methods solved the problem of sensitivity and expenses associated with API/MS and GC/MS for the determination of ultratrace nitrogen in pure argon. However, it should also be pointed out that these two tools require low pressure which makes the whole system and operation become complicated. In 2003, an atmospheric pulsed plasma was used as a simple, sensitive nitrogen analyzer [12], and was used for detection of trace nitrogen in helium at the emission line of 357 nm. The instrument consisted of a pulsed plasma source, an optical collection system, and a charge coupled device (CCD) detector. The LOD for nitrogen was 15 ppb which is satisfactory in many applications. DBD is a typical nonequilibrium high pressure ac gas discharge [13] and it can work at atmospheric pressure, which was firstly reported by Siemens in as early as 1857 [14]. The structure of DBD simply includes two electrodes and at least one is protected by dielectric medium. The discharge can occur between the two electrodes when supplying high ac voltage to the electrodes. DBD has many attractive advantages including atmospheric pressure working condition, simplicity in fabrication, fast response, high sensitivity, low power consumption, and low cost for construction and maintenance. All these make it widely used in industry and very promising for its applications in analytical spectroscopy. In recent years, in fact, DBD has been introduced into analytical areas such as chemiluminescence (CL), [15,16] ion mobility spectrometry (IMS) [17], mass spectrometry (MS) [18], and atomic spectrometry [19–24], which includes atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), and atomic emission spectrometry (AES). In this work, DBD was used as a new optical analyzer for the detection of trace nitrogen in argon at 337 nm. This is a new, simple, sensitive, and cost-effective method.
W. Li et al. / Microchemical Journal 99 (2011) 114–117
115
Fig. 1. Schematic diagram of the experimental setup. A: a photograph of the DBD plasma.
2. Experimental section 2.1. Instrumentation The schematic diagram of the DBD–CCD optical emission system is shown in Fig. 1. This system includes three main parts, a six-way valve, a laboratory-built DBD device and a USB2000 charge coupled device (Ocean Optics, USA). The six-way valve comes from a Techcomp GC7890F GC instrument (Techcomp Ltd., Shanghai, China). It consists of a quantitative ring which can contain 3 mL of sample. First, the sample fully fills the quantitative ring, while the carrier gas argon passes through another way to the DBD directly; then, the carrier gas is switched back to the sampling channel to sweep the sample into the DBD for detection. The laboratory-built DBD device consisted of a 3.0 mm i.d. × 5.0 mm o.d. × 50 mm long quartz tube and two copper electrodes. One of the copper wires was inserted into the quartz tube as the inner electrode, and the other wrapped on the outside of the tube as another electrode. Both of them were 1 mm in diameter. The power supply consisted of an ac ozone generation device (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) and a transformer (TPGC2J-1, Shanghai Pafe Electronic Equipment Ltd. Co., Shanghai, China). The optical signal generated from the DBD plasma was detected by the CCD. 2.2. Reagents High purity Argon (99.999%, QiaoYuan Gas Co. Ltd., Chengdu, China) was used as both carrier gas and discharge gas. Argon gasses containing 5 ppm and 20 ppm nitrogen (Testing Technology Institute of China, Chengdu, China) were used as standards. The samples were prepared by mixing the high pure argon and the standards. Both flow rates were controlled by a mass flow controller within 1% precision. 3. Results and discussion 3.1. Emission spectrum Different carrier gasses give varied emission spectrum backgrounds. In this work, we used the characteristic line of nitrogen at 337 nm as its analytical line for quantitative detection. As shown in Fig. 2, trace A represented the background of high pure argon. The weak peak at 337 nm and 357 nm is most probably due to the trace nitrogen impurities in the high pure argon gas. When adding 5 ppm nitrogen into the high pure argon, the spectral peaks at 337 nm and 357 nm increased sharply as shown in trace B of Fig. 2. Therefore, this specific emission line of nitrogen can be used for quantification of trace nitrogen in argon, as also reported in Refs. [8–12]. 3.2. Optimization of experimental conditions 3.2.1. Effect of flow rate of discharge gas Argon was used as both carrier gas and discharge gas. Fig. 3 shows the signal and S/N (signal to noise) ratio of nitrogen at different argon gas flow rate. As can be seen from this figure, in the flow rate range of 200–1000 mL min−1 of high pure argon, signal increased as the increasing of the gas flow rate. When the flow rate reached 500 mL min−1, the signal came to the highest point and then decreased with the increasing of gas flow rate. Taking this into consideration, 500 mL min−1 of argon flow rate was chosen for use.
Fig. 2. Spectral emission from trace nitrogen in argon gas. Integrated time, 300 ms; gas flow rate, 1000 mL min−1; discharge voltage, 2.95 kV. (A) High purity argon and (B) argon emission spectrum with 5 ppm nitrogen added. The four nitrogen emission lines are at 337 nm, 357 nm, 379 nm and 405 nm.
3.2.2. Optimization of discharge voltage The effect of discharge voltage ranging from 2.21 to 3.25 kV was examined in this work. As shown in Fig. 4, the discharge voltage determined the signal to a certain extent because higher voltage could
116
W. Li et al. / Microchemical Journal 99 (2011) 114–117
1400 1200
Intensity
1000 800 600 400 200 0 0
10
20
30
40
50
60
Time(s) Fig. 3. Signal and S/N ratio of trace nitrogen in argon at different gas flow rates. The error bars stand for ±SD of three replicates. Injection volume, 3 mL of sample (argon containing 5 ppm nitrogen); and discharge voltage, 2.95 kV.
supply higher power to generate more radicals and the like in the DBD device. But when the power reached 200 V, the signal tended to be very unstable and the S/N ratio increased sharply. Thus, the discharge voltage of 2.95 kV was the best choice.
3.3. System stability At atmospheric pressure, when supplying an ac high voltage to the electrodes, the discharge can occur between the two electrodes. For studying the stability of the system, the background was continuously collected 3 s per point with nitrogen concentration of 5 ppm. Fig. 5 showed that when feeding the gas sample, the intensity of nitrogen emission line at 337 nm maintained stable, with a relative standard deviation (RSD) of 0.9%.
3.4. Analytical characteristics and potential applications The analytical figures of merit of the proposed method for nitrogen determination were obtained under optimal experimental conditions. The peak height was used for quantitative measurement throughout this work. Under these conditions, the calibration curves had good linearity, with linear regression coefficients (R) of N0.99. The LOD was
Fig. 5. Stability of the system. The emission intensity fluctuation of nitrogen at 337 nm with a concentration of 5 ppm during 60 s. Integrated time, 300 ms; gas flow rate, 1000 ml min−1; and discharge voltage, 2.95 kV. RSD= 0.9%.
calculated as LOD = 3 standard deviation/slope of the calibration curve. All of these analytical figures of merit were summarized in Table 1. In order to fully verify the applicability of the proposed method in real sample analysis, we attempted to determine trace nitrogen impurities in industrial gas samples, with analytical results listed in Table 2. The analytical results showed that all of these gas samples had a much lower concentration of nitrogen than it is labeled. For each sample, the analysis time was less than 2 min, securing a really fast screen method good for in site analysis of industrial argon gas samples. 3.5. Discussion of emission mechanism The optical emission of nitrogen usually has two pathways. One way is the direct emission of nitrogen, and the other is through energy transferring from argon to nitrogen. The latter was useful when detecting trace nitrogen. In the discharge progress, the following reaction may occur in DBD [25]. Ar discharge→Ar⁎→Ar + hυ
ð1Þ
Ar discharge→Ar⁎⁎→Ar + hυ
ð2Þ
N2 discharge→N2 ⁎→N2 + hυ
ð3Þ
½Ar N N ½N2 Ar⁎⁎ + N2 →N2 ⁎ + Ar→N2 + Ar + hυ
ð4Þ
Where h and υ are the Plank constant and the optical emission frequency, respectively. In the above equations, Ar* and N2* denote the excited state of argon and nitrogen, and Ar** the metastable excited argon atom. The energy of Ar** is equivalent to the energy for excitation of molecular nitrogen. Eq. (3) displays the direct emission of nitrogen, but when the concentration of nitrogen in DBD is too low to emit directly, the energy will be transferred from Ar** to N2, as depicted in Eq. (4).
Table 1 The analytical characteristics of ultratrace nitrogen in argon by the proposed method. Analyte Fig. 4. Signal and S/N ratio of trace nitrogen in argon at different discharge voltage. The error bars stand for ±SD of three replicates. Injection volume, 3 mL of sample (argon containing 5 ppm nitrogen); and gas flow rate, 500 mL min−1.
Calibration equation (ppm) Concentration range (ppm) R
nitrogen y = 1223x − 155
0.24 − 2.5
LOD (ppb)
0.997 34
W. Li et al. / Microchemical Journal 99 (2011) 114–117
117
Table 2 The analysis of real argon samples for trace nitrogen impurity. Samples
Manufacturer
Purity specification
Production date
Nitrogen impurity labeled (ppm)
Nitrogen found (ppm)
Sample Sample Sample Sample
Qiao Qiao Qiao Qiao
99.99% 99.99% 99.99% 99.99%
April 13, 2010 April 15, 2010 May 7, 2010 May 26, 2010
≤5 ≤5 ≤5 ≤5
0.22 0.41 0.27 0.26
1 2 3 4
Yuan Yuan Yuan Yuan
4. Conclusions The atmospheric pressure DBD plasma is a new promising optical detector for monitoring (especially field screening) trace nitrogen in high purity argon gas samples. This work expanded the analytical applications of DBD to molecular emission spectrometry. There are many attractive characteristics of this method: atmospheric pressure working condition, fast response, simple and portable instrumentation, high sensitivity, low cost and power consumption.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgments [17]
The authors acknowledge the financial support from the National Natural Science Foundation of China (no. 20835003). We thank Mr. Xi Wu of Analytical & Testing Center for technical help with this project.
[18] [19] [20] [21]
References [1] [2] [3] [4] [5]
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