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Dielectric barrier discharge molecular emission spectrometer as gas chromatographic detector for amines
Microchemical Journal 119 (2015) 108–113
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Dielectric barrier discharge molecular emission spectrometer as gas chromatographic detector for amines Chenghui Li a, Xue Jiang a, Xiandeng Hou a,b,⁎ 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 12 August 2014 Accepted 17 November 2014 Available online 22 November 2014 Keywords: Dielectric barrier discharge Molecular optical emission spectrometry Gas chromatographer Amine Fish sample
a b s t r a c t Nano-SiO2 was immobilized on the inner wall of a dielectric barrier discharge (DBD) tube, and it was coupled to a conventional gas chromatographer to investigate its performance as a molecular emission spectrometric detector for the determination of five volatile aliphatic amines. A charge-coupled device (CCD) was applied to observe the nano-SiO2-enhanced molecular emission spectra. The characteristic molecular emission bands of volatile aliphatic amines at 326.5 nm, 336.0 nm and 388.3 nm can be clearly resolved from the background emission spectra of carrier gas argon. The emission band of CN at 388.3 nm was used for quantitative detection of volatile aliphatic amines due to its high sensitivity. Nanomaterial catalysts including TiO2, MnO2, SiO2 and ZnO were tested to enhance the emission signal of amines, and SiO2 shows the best performance. The factors that influence the emission signal, such as discharge voltage, inner electrode length and carrier gas flow rate, were investigated in detail. The analytical performance of this method was evaluated by separation and detection of the mixture of five volatile aliphatic amines. Under the optimal experimental conditions, the limits of detection were found to be 4.4, 2.5, 2.2, 1.8 and 2.4 μg for dimethylamine, trimethylamine, n-butylamine, cyclohexylamine and ethylenediamine, respectively. This GC detector is not only sensitive but also fast in response to volatile aliphatic amines with good stability. Trimethylamine in a carp fish sample was monitored with storage time by the proposed method. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Volatile aliphatic amines like dimethylamine (DMA), trimethylamine (TMA), n-butylamine (BA), ethylenediamine and cyclohexylamine are important industrial chemicals with a wide range of applications as raw materials or intermediate products in the manufacturing of other chemicals, pharmaceuticals, polymers, pesticides, rubber, dyes, adhesives, solvents and corrosion inhibitors [1,2]. Volatile aliphatic amines are also well known as air pollutants, which are emitted to the atmosphere through a variety of channels, such as waste incineration, sewage treatment, vehicle exhaust gases, cigarette smoke and so on [3]. In addition, some volatile aliphatic amines are generated in the process of biodegradation of animal tissue like fish and meat. Trimethylamine (TMA) is infamous for its pungent and fishy smell, which is generated during the bacterial or enzymatic deterioration of trimethylamine oxide in dead fish [4,5]. An increase in concentration of TMA could be used as an indicator of the degree of spoilage of fish. Due to their pungent odorous and toxic characteristics, most of volatile aliphatic amines are sensitizers and irritants to the skin, the eyes and the respiratory and central nervous ⁎ Corresponding author at: College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. E-mail address: [email protected] (X. Hou).
http://dx.doi.org/10.1016/j.microc.2014.11.005 0026-265X/© 2014 Elsevier B.V. All rights reserved.
systems. Some amines can react with nitrite, forming the N-nitroso compounds that are potentially carcinogenic [6]. Therefore, it is necessary to monitor the levels of volatile aliphatic amines to protect human health, environment and infer food quality. There have been various research works carried out for the development of analytical methods for determining amines by using gas chromatography (GC) [7–9], high performance liquid chromatography (HPLC) [10,11], ion chromatography (IC) [12], capillary electrophoresis (CE) [13], sensors [14], spectrophotometry [15] or other methods. However, most of these methods need complicated pretreatments such as derivatization [16]. As an old but recently renewed plasma technology, dielectric barrier discharge (DBD) is a typical non-equilibrium atmospheric pressure ac gas discharge technology that was first reported by Siemens in 1857 [17]. It has at least one dielectric material (e.g., glass, quartz, ceramic or polymer layers) to block discharge channel, with a gap distance in the range of 0.1–10 mm between the two electrodes. When the electrode is powered by an alternating current voltage with a frequency ranging from a few Hz to MHz, the DBD could generate discharge for high energy electron of 1–10 eV [18,19]. These electrons will dissociate the surrounding gas molecular and produce plasma that contains free radicals, ions, atoms, molecules and so on. The unique features and characteristics of DBDs, such as simple configuration, low energy consumption, long lifetime, easy operation under ambient temperature and
C. Li et al. / Microchemical Journal 119 (2015) 108–113
Fig. 1. Schematic diagram of the experimental arrangement.
pressure, offer a wide range of analytical applications. Up to now, DBDs have been introduced into analytical chemistry as detectors for gas chromatography (GC) [20–22], as ionization sources for ion mobility spectrometry (IMS) [23] and mass spectrometry (MS) [24], as atomizers for atomic absorption spectrometry (AAS) [25] and atomic fluorescence spectrometry (AFS) [26], as excitation sources for atomic emission spectrometry (AES) [27] and molecular emission spectrometry [20]. In order to expand the application scope of DBD, extensive research has been conducted on the combination of a DBD and a catalyst to enhance its excitation capability. DBD-catalysis technique has the advantages of high selectivity from catalysis and fast response from DBD technique. The combination of a DBD and a catalyst creates a synergistic effect because both plasma and catalysis take place simultaneously and interact with each other [28]. The DBD-catalysis technique is mostly applied for environmental protection such as pollutants abatement, but much less for spectrochemical analysis. Micro-plasma-based optical emission spectrometry (OES) has proven to be an effective detection method for chemical analysis [29,30]. This technique was first used as a GC detector by McCormack et al. [31]. The molecule was fragmented and excited by argon microwaveinduced plasma (MIP) and observed through molecular emission
109
bands (e.g., C2, CH and CN). Since then, the combination of microplasma OES with gas chromatograph, by the analysis of highresolution rotational bands in the emission spectra of suitable electronically excited molecular species present in the discharge, for example, C2, CN, CH, NH, OH, F, Cl, Br [32], has attracted growing interest in the past decades due to its good linearity and sensitivity [33–35]. DBDs have recently received much attention for GC-MES due to their beneficial analytical properties [20], such as a low gas temperature for weak background radiation and high electron temperatures for excitation of molecular/radical emission. Our group has also established DBD molecular/radical emission spectrometric (MES) detectors using characteristic emission bands of analytes [21,22,36]. Li et al. [20] reported a multi-channel GC detector for the detection of VOCs, by use of DBD MES with a linear CCD. Compared to the traditional elemental detectors such as MS or ICP-AES/MS, a DBD-based spectrometric detector possesses some attractive characteristics such as multi-channel, fast respond, high sensitivity, compactness and simplicity and low-cost in construction and maintenance. In the present work, we propose nano-SiO2-assisted DBD-MES as a GC detector to determinate five volatile aliphatic amines by use of CN radical emission at 388.3 nm. Compared with other nanomaterials like TiO2, MnO2 and ZnO, SiO2 can significantly enhance the intensity of the molecular emission. The approach is successfully applied to monitor the biodegradation of a spoiled fish sample. The results show that GC detector is not only sensitive but also fast in response to volatile aliphatic amines with high stability.
2. Experimental section 2.1. Reagents All chemicals used in this experiment were of analytical reagent grade. As analytes, dimethylamine and trimethylamine were purchased from Aladdin Reagent Company (Shanghai, China), while n-butylamine, ethylenediamine and cyclohexylamine were purchased from Kelong Reagent Company (Chengdu, China). High purity argon (Ar, 99.999%) (Qiaoyuan Gas Co. Ltd., Chengdu, China) was used as the discharge and carrier gas. The carp was obtained from a local market. The chemicals used for synthetic nanomaterials (MnO2, TiO2, Al2O3, SiO2 and ZnO) received from the supplier (Kelong Reagent Company, Chengdu, China) expect Triton X-114, which was purchased from
Fig. 2. Molecular/radical emission of amines. Experimental conditions: input voltage, 195 V; argon discharge gas flow rate, 600 mL min−1; and integration time, 100 ms.
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Fluka Company (France). The shapes of synthesized MnO2, TiO2, SiO2 and ZnO were layered, rodlike, mesoporous and spherical, respectively. 2.2. Coating procedure Using nano-SiO2 as an example to introduce the catalyst coating procedure: mix 0.25 g nano-SiO2 with 85 μL water and 8 μL acetyl acetone, then gradually add 340 μL water. Keep grinding, then add 5 μL triton X-114 and mix well. Coat the catalyst material on the inner wall of the DBD tube, air dry the tube at room temperature and then calcinate in a muffle furnace at 200 °C for 3 hours to complete the coating. 2.3. Instrumentation The schematic diagram of the experimental setup was shown in Fig. 1. The instrumental setup consisted of a GC unit and a DBD-MES detector using a miniaturized CCD spectrometer. The DBD-MES detector was a cylindrical laboratory-built DBD device consisting of a 2.0 mm i.d. × 4.0 mm o.d. × 50 mm long quartz tube and two copper electrodes. One of the electrodes (1.2 mm in diameter) was inserted into the tube as the inner electrode; the other (1 mm in diameter) was tightly wrapped around the outer side of the quartz tube as the outer electrode. An ac ozone generation power supply (YG. BP105P, Electronic Equipment Factory of Guangzhou Salvage, Guangzhou, China; 6 cm long × 4 cm wide × 3 cm high, with a rated output of 4 kV, 20 kHz, and 12 W at 220 V, 50 Hz input) was applied to the two electrodes of the DBD to provide the necessary voltage for generation of the DBD plasma. A transformer (TDGC2-1, Tianzheng Electronic Equipment Ltd. Co., Tianjin, China) was connected to the ozone generation power supply for adjusting the discharge power. The optical emission from the DBD plasma was collected onto the entrance slit of a commercial hand-held CCD spectrometer (Maya 2000 Pro; Ocean Optics Inc., Dunedin, FL). As a GC detector, this CCD based DBD-MES was connected to a Techcomp GC 7890 F GC instrument (Techcomp Ltd., Shanghai, China) equipped with an Rtx-Wax capillary column (30 m × 0.25 mm i.d, 0.50 μm polyethylene, Restek Co., Bellefonte, PA, USA). The operation parameters of GC were as follows: injection temperature, 170 °C; oven temperature program, 30 °C initial temperature, 2 min hold, and 25 °C min− 1 to 160 °C, 2 min hold; and carrier gas flow rate: Ar, 2 mL min−1. The working parameters of the DBD plasma: input voltage, 195 V; Ar gas flow rate, 600 mL min− 1; and integration time, 100 ms. The peak area of the analytical line at 388.3 nm was used for quantification throughout the work.
Fig. 3. The catalytic performance of different nanomaterials for amine detection by the DBD from 0.1 μL mixed amines. Experimental conditions: input voltage, 195 V; argon discharge gas flow rate, 600 mL min−1; and integration time, 100 ms.
resolved from the Ar emission spectra. For amines in the atmospheric DBD, the following processes might contribute to the formation of CN in plasma [37]: þ
Ar þ e þ R‐NH2 → multiple redical=ion species
ð1Þ
2C þ N2 → 2CN
ð2Þ
2C þ 2NH → 2CN þ H2 ðor 2HÞ
ð3Þ
CH þ N2 → CN þ NH
ð4Þ
NH þ CH → CN þ H2 ðor 2HÞ
ð5Þ
Because of the high emission intensity and sensitivity, the characteristic line of CN at 388.3 nm was used as the analytical line for the followed experiments.
3. Results and discussion 3.1. DBD optical emission from amines on the surface of different nano-catalysts 3.1.1. Emission spectra The characteristic emission bands of amines in the DBD were discussed in this section. Fig. 2 shows the emission spectra of the analytes amines in atmospheric pressure for the wavelength region between 175 and 400 nm. From the argon plasma background, typical emission line and significant molecular bands of OH (283 and 309 nm, A2Σ+ → X2Π+) and N2 (316, 358 and 380 nm, C3Πu → B3Πg) were observed due to the residue in the 99.99% pure argon. In order to demonstrate the feasibility of the DBD as an MES detector for amines, five volatile aliphatic amines were chosen as model analytes to observe the specific molecular emission bands. Compared to the emission spectra of the pure argon DBD plasma, the OH molecular band at 309 nm decreased significantly, and obvious specific molecular emission bands were found when these amines were direct introduced into the DBD plasma. As shown in Fig. 2, emission bands of CN (B2Σ+ → A2Π+) at 383–388 nm and NH (A3Π → X3Σ) at 326.5 and 336.0 nm were clearly
Fig. 4. Effect of input voltage on amine signal intensity from 0.1 μL amines. Experimental conditions: input voltage, 195 V; argon discharge gas flow rate, 600 mL min−1; and integration time, 100 ms.
C. Li et al. / Microchemical Journal 119 (2015) 108–113
Fig. 5. Effect of carrier gas flow rate on amine signal intensity from 0.1 μL amines. Experimental conditions: input voltage, 195 V; argon discharge gas; and integration time, 100 ms.
3.1.2. Effect of nano-catalysts It is well known that nano-catalysts located in the discharge may have a complex effect on the discharge operation, such as shortening the discharge gap, changing dielectric effects or altering discharge nature, depending on their physical and chemical properties. The plasma discharge could change the status of gas phase reactants for catalytic reactions, and it could modify the surface properties of catalysts. The type of catalysts and the loading amount also greatly influence the retention time and concentration. Fig. 3 shows the catalytic performance of different nanomaterials for amine detection by the DBD. Comparison among MnO2, TiO2, SiO2 and ZnO, SiO2 shows the best performance to enhance the emission signal of amines. One reason is that the short living active species are more easily formed in the pore volume of porous SiO2 when exposed to plasma [38]. At the same time, the holes have a significant adsorption capacity for amines, and it prolongs the analytes retention time on the catalyst's surface and in the DBD. The high collision between amines and active species enhances the emission signal. SiO2 was thus chosen as the catalyst for the following experiments.
Fig. 6. Effect of electrode length on amine signal intensity from 0.1 μL amines. Experimental conditions: input voltage, 195 V; argon discharge gas flow rate, 600 mL min−1; and integration time, 100 ms.
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Fig. 7. Chromatogram obtained by using GC-DBD-MES for five amines: (1) trimethylamine, (2) dimethylamine, (3) n-butylamine, (4) cyclohexylamine and (5) ethylenediamine.
3.2. Optimization of experimental conditions 3.2.1. Effect of input voltage To generate and maintain homogeneous and stable plasma in the DBD cell, an appropriate discharge voltage should be applied between the two copper electrodes. The discharge voltage was controlled by adjusting the input voltage of the ac ozone generation power supply through a transformer. The maximum signal appeared when the input voltage ranged from 180 to 200 V. As the voltage increased, the plasma became brighter and the dissociation efficiency was improved, and this was proved by the higher emission signal. At higher input voltage, more radicals or ions were probably generated by more active and effective collision between carrier and analyte gas molecules and high energy electrons. However, when the input voltage exceeded 195 V, the plasma became inhomogeneous with observable flicker as well as the deteriorated stability and intensity of the signal. As shown in Fig. 4, an input voltage of 195 V ac was the best choice.
Fig. 8. Stability of the GC-DBD-MES for trimethylamine detection.
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3.2.2. Effect of argon flow rate Here argon played two roles in the DBD. One is to carry the volatile amines into the discharge cell and the other is to serve as discharge gas. Therefore, the argon flow rate at the range of 400 to 800 mL min−1 was optimized in order to obtain the best emission signal. Fig. 5 shows the emission signal intensity of amines in dependence on argon flow rate. The maximum signals of amines were achieved at the flow rate of 600 mL min−1. Lower argon gas flow rate results in inefficient dissociation of amines in the plasma; and higher argon gas flow rate may cause the dilution of amines in the discharge cell. The argon flow rate of 600 mL min−1 was chosen for the further experiments.
3.2.3. Effect of length of electrode The electrode is an important part of the DBD because its shape, material and arrangement have significant effects on energy density of the DBD cell. Although the length of electrode also influence the discharge efficiency as shown in Fig. 6, the signal varied at different length ratio of the two electrodes (inner electrode length /outer electrode length) from 0.25 to 1.25. As the ratio increased, the emission signal decreased due to the continuous reduction of the discharge zone, leading to a loss of dissociation and excitation efficiency. Although the maximum emission signal achieved at the ratio of zero, plasma became instable with loud noise. As a compromise, the ratio of 0.25 was selected for use.
3.3. DBD-MES as GC detector For the determination of trace volatile aliphatic amines by GC, here nano-SiO2-assisted DBD-MES was used as a sensitive detector. To reduce the dead volume, the end of the GC column was inserted directly into the discharge cell of the DBD device to ensure that analytes well mix with the plasma gas as quickly as possible. The performance of this GC detector was evaluated by separation and detection of the mixture of dimethylamine, trimethylamine, n-butylamine, ethylenediamine and cyclohexylamine. Under the optimal experimental conditions, the chromatogram obtained by the GC-DBD-MES setup indicates that this detector provides sensitive signals of these five amines with satisfactory chromatographic resolution, as shown in Fig. 7. The stability of the detector was examined by five injections of 0.06 μL TMA into GC. From Fig. 8, it is clear that the emission intensity thus obtained shows high stability of the system with a relative standard deviation (RSD) of 3.0%. The analytical performance of the GC-DBD-MES was evaluated under optimal experimental conditions, with 388.3 nm as the analytical line for peak area measurement. Besides its stability and specificity, it provided a wide linear dynamic range of two to three orders of magnitude. Linear correlation coefficients for calibration curves of these five amines are all better than 0.99. The limits of detection (LODs) were calculated using LOD = 3 N/S (where N is the standard deviation of 11 measurements of a blank solution and S is the slope of the calibration curve). The LOD for dimethylamine, trimethylamine, n-butylamine, ethylenediamine and cyclohexylamine were 4.4, 2.5, 2.2, 1.8 and 2.4 μg, respectively. All of these analytical figures of merit were summarized in Table 1. This detector can provide low limits of detection, high sensitivity and a wide linear dynamic range.
Table 1 The analytical figures of merit of GC-DBD-MES for five amines. Amine
Calibration equation (μg)
LDR (μg)
R
LOD (μg)
Dimethylamine Trimethylamine n-Butylamine Cyclohexylamine Ethylendiamine
Y Y Y Y Y
13.6–680 13.2–660 7.3–1100 8.6–1290 9.0–1347
0.999 0.996 0.994 0.999 0.999
4.4 2.5 2.2 1.8 2.4
= = = = =
468.9X + 129.7 813.1X + 6134.5 949.5X + 468.0 1133.9X + 680.3 5278.4X + 464.1
Fig. 9. The proposed method for detection of TMA in a carp kept at room temperature as a function of storage time.
3.4. Preliminary application in real sample analysis The nano-SiO2-assisted DBD-MES detector for GC was used to monitor trimethylamine (TMA) content in a carp fish sample. The fish sample was kept aerated at room temperature and TMA was extracted according to an AOAC method [39], and the gas was directly injected into the GC for analysis. As can be seen in Fig. 9, the emission intensity obtained in this manner experiences a continuous increase in 45 hours as reported previously [40], and this is due to the deterioration of the fish after death. This detector would provide a promising method for instant test of fish freshness. 4. Conclusions The nano-SiO2-assisted DBD-MES as a GC detector could sensitively detect amines, such as monitoring trimethylamine in a spoiled fish with storage time. The emission intensity could be greatly enhanced by nano-SiO2 because nano-SiO2 increased the retention time of amines in discharge zone and changed the discharge characteristics of the DBD. It is a simple, fast and green method to detect amines without complicated pretreatments such as derivatization. This MES detector has many other advantages such as minimal dead volume, high sensitivity, fast response, simplicity, low power consumption and easy hyphenation to GC with good analytical performance. Acknowledgments We thank the National Natural Science Foundation of China (grant no. 20835003) for financial support for this project. References [1] M. Abalos, J.M. Bayona, F. Ventura, Development of a solid-phase microextraction GC-NPD procedure for the determination of free volatile amines in wastewater and sewage-polluted waters, Anal. Chem. 71 (1999) 3531–3537. [2] B. Sahasrabuddhey, A. Jain, K.K. Verma, Determination of ammonia and aliphatic amines in environmental aqueous samples utilizing pre-column derivatization to their phenylthioureas and high performance liquid chromatography, Analyst 124 (1999) 1017–1021. [3] J. Namiesnik, A. Jastrzebska, B. Zygmunt, Determination of volatile aliphatic amines in air by solid-phase microextraction coupled with gas chromatography with flame ionization detection, J. Chromatogr. A 1016 (2003) 1–9. [4] Z.Y. Zhang, K. Xu, Z. Xing, X.R. Zhang, A nanosized Y2O3-based catalytic chemiluminescent sensor for trimethylamine, Talanta 65 (2005) 913–917. [5] A.K. Anderson, Biogenic and volatile amine-related qualities of three popular fish species sold at Kuwait fish markets, Food Chem. 107 (2008) 761–767. [6] H. Greim, D. Bury, H.J. Klimisch, M. Oeben-Negele, Z. Zeigler-Skylakakis, Toxicity of aliphatic amines: structure–activity relationship, Chemosphere 36 (1998) 271–295.
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Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Dielectric barrier discharge molecular emission spectrometer as gas chromatographic detector for amines Chenghui Li a, Xue Jiang a, Xiandeng Hou a,b,⁎ 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
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Article history: Received 12 August 2014 Accepted 17 November 2014 Available online 22 November 2014 Keywords: Dielectric barrier discharge Molecular optical emission spectrometry Gas chromatographer Amine Fish sample
a b s t r a c t Nano-SiO2 was immobilized on the inner wall of a dielectric barrier discharge (DBD) tube, and it was coupled to a conventional gas chromatographer to investigate its performance as a molecular emission spectrometric detector for the determination of five volatile aliphatic amines. A charge-coupled device (CCD) was applied to observe the nano-SiO2-enhanced molecular emission spectra. The characteristic molecular emission bands of volatile aliphatic amines at 326.5 nm, 336.0 nm and 388.3 nm can be clearly resolved from the background emission spectra of carrier gas argon. The emission band of CN at 388.3 nm was used for quantitative detection of volatile aliphatic amines due to its high sensitivity. Nanomaterial catalysts including TiO2, MnO2, SiO2 and ZnO were tested to enhance the emission signal of amines, and SiO2 shows the best performance. The factors that influence the emission signal, such as discharge voltage, inner electrode length and carrier gas flow rate, were investigated in detail. The analytical performance of this method was evaluated by separation and detection of the mixture of five volatile aliphatic amines. Under the optimal experimental conditions, the limits of detection were found to be 4.4, 2.5, 2.2, 1.8 and 2.4 μg for dimethylamine, trimethylamine, n-butylamine, cyclohexylamine and ethylenediamine, respectively. This GC detector is not only sensitive but also fast in response to volatile aliphatic amines with good stability. Trimethylamine in a carp fish sample was monitored with storage time by the proposed method. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Volatile aliphatic amines like dimethylamine (DMA), trimethylamine (TMA), n-butylamine (BA), ethylenediamine and cyclohexylamine are important industrial chemicals with a wide range of applications as raw materials or intermediate products in the manufacturing of other chemicals, pharmaceuticals, polymers, pesticides, rubber, dyes, adhesives, solvents and corrosion inhibitors [1,2]. Volatile aliphatic amines are also well known as air pollutants, which are emitted to the atmosphere through a variety of channels, such as waste incineration, sewage treatment, vehicle exhaust gases, cigarette smoke and so on [3]. In addition, some volatile aliphatic amines are generated in the process of biodegradation of animal tissue like fish and meat. Trimethylamine (TMA) is infamous for its pungent and fishy smell, which is generated during the bacterial or enzymatic deterioration of trimethylamine oxide in dead fish [4,5]. An increase in concentration of TMA could be used as an indicator of the degree of spoilage of fish. Due to their pungent odorous and toxic characteristics, most of volatile aliphatic amines are sensitizers and irritants to the skin, the eyes and the respiratory and central nervous ⁎ Corresponding author at: College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. E-mail address: [email protected] (X. Hou).
http://dx.doi.org/10.1016/j.microc.2014.11.005 0026-265X/© 2014 Elsevier B.V. All rights reserved.
systems. Some amines can react with nitrite, forming the N-nitroso compounds that are potentially carcinogenic [6]. Therefore, it is necessary to monitor the levels of volatile aliphatic amines to protect human health, environment and infer food quality. There have been various research works carried out for the development of analytical methods for determining amines by using gas chromatography (GC) [7–9], high performance liquid chromatography (HPLC) [10,11], ion chromatography (IC) [12], capillary electrophoresis (CE) [13], sensors [14], spectrophotometry [15] or other methods. However, most of these methods need complicated pretreatments such as derivatization [16]. As an old but recently renewed plasma technology, dielectric barrier discharge (DBD) is a typical non-equilibrium atmospheric pressure ac gas discharge technology that was first reported by Siemens in 1857 [17]. It has at least one dielectric material (e.g., glass, quartz, ceramic or polymer layers) to block discharge channel, with a gap distance in the range of 0.1–10 mm between the two electrodes. When the electrode is powered by an alternating current voltage with a frequency ranging from a few Hz to MHz, the DBD could generate discharge for high energy electron of 1–10 eV [18,19]. These electrons will dissociate the surrounding gas molecular and produce plasma that contains free radicals, ions, atoms, molecules and so on. The unique features and characteristics of DBDs, such as simple configuration, low energy consumption, long lifetime, easy operation under ambient temperature and
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Fig. 1. Schematic diagram of the experimental arrangement.
pressure, offer a wide range of analytical applications. Up to now, DBDs have been introduced into analytical chemistry as detectors for gas chromatography (GC) [20–22], as ionization sources for ion mobility spectrometry (IMS) [23] and mass spectrometry (MS) [24], as atomizers for atomic absorption spectrometry (AAS) [25] and atomic fluorescence spectrometry (AFS) [26], as excitation sources for atomic emission spectrometry (AES) [27] and molecular emission spectrometry [20]. In order to expand the application scope of DBD, extensive research has been conducted on the combination of a DBD and a catalyst to enhance its excitation capability. DBD-catalysis technique has the advantages of high selectivity from catalysis and fast response from DBD technique. The combination of a DBD and a catalyst creates a synergistic effect because both plasma and catalysis take place simultaneously and interact with each other [28]. The DBD-catalysis technique is mostly applied for environmental protection such as pollutants abatement, but much less for spectrochemical analysis. Micro-plasma-based optical emission spectrometry (OES) has proven to be an effective detection method for chemical analysis [29,30]. This technique was first used as a GC detector by McCormack et al. [31]. The molecule was fragmented and excited by argon microwaveinduced plasma (MIP) and observed through molecular emission
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bands (e.g., C2, CH and CN). Since then, the combination of microplasma OES with gas chromatograph, by the analysis of highresolution rotational bands in the emission spectra of suitable electronically excited molecular species present in the discharge, for example, C2, CN, CH, NH, OH, F, Cl, Br [32], has attracted growing interest in the past decades due to its good linearity and sensitivity [33–35]. DBDs have recently received much attention for GC-MES due to their beneficial analytical properties [20], such as a low gas temperature for weak background radiation and high electron temperatures for excitation of molecular/radical emission. Our group has also established DBD molecular/radical emission spectrometric (MES) detectors using characteristic emission bands of analytes [21,22,36]. Li et al. [20] reported a multi-channel GC detector for the detection of VOCs, by use of DBD MES with a linear CCD. Compared to the traditional elemental detectors such as MS or ICP-AES/MS, a DBD-based spectrometric detector possesses some attractive characteristics such as multi-channel, fast respond, high sensitivity, compactness and simplicity and low-cost in construction and maintenance. In the present work, we propose nano-SiO2-assisted DBD-MES as a GC detector to determinate five volatile aliphatic amines by use of CN radical emission at 388.3 nm. Compared with other nanomaterials like TiO2, MnO2 and ZnO, SiO2 can significantly enhance the intensity of the molecular emission. The approach is successfully applied to monitor the biodegradation of a spoiled fish sample. The results show that GC detector is not only sensitive but also fast in response to volatile aliphatic amines with high stability.
2. Experimental section 2.1. Reagents All chemicals used in this experiment were of analytical reagent grade. As analytes, dimethylamine and trimethylamine were purchased from Aladdin Reagent Company (Shanghai, China), while n-butylamine, ethylenediamine and cyclohexylamine were purchased from Kelong Reagent Company (Chengdu, China). High purity argon (Ar, 99.999%) (Qiaoyuan Gas Co. Ltd., Chengdu, China) was used as the discharge and carrier gas. The carp was obtained from a local market. The chemicals used for synthetic nanomaterials (MnO2, TiO2, Al2O3, SiO2 and ZnO) received from the supplier (Kelong Reagent Company, Chengdu, China) expect Triton X-114, which was purchased from
Fig. 2. Molecular/radical emission of amines. Experimental conditions: input voltage, 195 V; argon discharge gas flow rate, 600 mL min−1; and integration time, 100 ms.
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Fluka Company (France). The shapes of synthesized MnO2, TiO2, SiO2 and ZnO were layered, rodlike, mesoporous and spherical, respectively. 2.2. Coating procedure Using nano-SiO2 as an example to introduce the catalyst coating procedure: mix 0.25 g nano-SiO2 with 85 μL water and 8 μL acetyl acetone, then gradually add 340 μL water. Keep grinding, then add 5 μL triton X-114 and mix well. Coat the catalyst material on the inner wall of the DBD tube, air dry the tube at room temperature and then calcinate in a muffle furnace at 200 °C for 3 hours to complete the coating. 2.3. Instrumentation The schematic diagram of the experimental setup was shown in Fig. 1. The instrumental setup consisted of a GC unit and a DBD-MES detector using a miniaturized CCD spectrometer. The DBD-MES detector was a cylindrical laboratory-built DBD device consisting of a 2.0 mm i.d. × 4.0 mm o.d. × 50 mm long quartz tube and two copper electrodes. One of the electrodes (1.2 mm in diameter) was inserted into the tube as the inner electrode; the other (1 mm in diameter) was tightly wrapped around the outer side of the quartz tube as the outer electrode. An ac ozone generation power supply (YG. BP105P, Electronic Equipment Factory of Guangzhou Salvage, Guangzhou, China; 6 cm long × 4 cm wide × 3 cm high, with a rated output of 4 kV, 20 kHz, and 12 W at 220 V, 50 Hz input) was applied to the two electrodes of the DBD to provide the necessary voltage for generation of the DBD plasma. A transformer (TDGC2-1, Tianzheng Electronic Equipment Ltd. Co., Tianjin, China) was connected to the ozone generation power supply for adjusting the discharge power. The optical emission from the DBD plasma was collected onto the entrance slit of a commercial hand-held CCD spectrometer (Maya 2000 Pro; Ocean Optics Inc., Dunedin, FL). As a GC detector, this CCD based DBD-MES was connected to a Techcomp GC 7890 F GC instrument (Techcomp Ltd., Shanghai, China) equipped with an Rtx-Wax capillary column (30 m × 0.25 mm i.d, 0.50 μm polyethylene, Restek Co., Bellefonte, PA, USA). The operation parameters of GC were as follows: injection temperature, 170 °C; oven temperature program, 30 °C initial temperature, 2 min hold, and 25 °C min− 1 to 160 °C, 2 min hold; and carrier gas flow rate: Ar, 2 mL min−1. The working parameters of the DBD plasma: input voltage, 195 V; Ar gas flow rate, 600 mL min− 1; and integration time, 100 ms. The peak area of the analytical line at 388.3 nm was used for quantification throughout the work.
Fig. 3. The catalytic performance of different nanomaterials for amine detection by the DBD from 0.1 μL mixed amines. Experimental conditions: input voltage, 195 V; argon discharge gas flow rate, 600 mL min−1; and integration time, 100 ms.
resolved from the Ar emission spectra. For amines in the atmospheric DBD, the following processes might contribute to the formation of CN in plasma [37]: þ
Ar þ e þ R‐NH2 → multiple redical=ion species
ð1Þ
2C þ N2 → 2CN
ð2Þ
2C þ 2NH → 2CN þ H2 ðor 2HÞ
ð3Þ
CH þ N2 → CN þ NH
ð4Þ
NH þ CH → CN þ H2 ðor 2HÞ
ð5Þ
Because of the high emission intensity and sensitivity, the characteristic line of CN at 388.3 nm was used as the analytical line for the followed experiments.
3. Results and discussion 3.1. DBD optical emission from amines on the surface of different nano-catalysts 3.1.1. Emission spectra The characteristic emission bands of amines in the DBD were discussed in this section. Fig. 2 shows the emission spectra of the analytes amines in atmospheric pressure for the wavelength region between 175 and 400 nm. From the argon plasma background, typical emission line and significant molecular bands of OH (283 and 309 nm, A2Σ+ → X2Π+) and N2 (316, 358 and 380 nm, C3Πu → B3Πg) were observed due to the residue in the 99.99% pure argon. In order to demonstrate the feasibility of the DBD as an MES detector for amines, five volatile aliphatic amines were chosen as model analytes to observe the specific molecular emission bands. Compared to the emission spectra of the pure argon DBD plasma, the OH molecular band at 309 nm decreased significantly, and obvious specific molecular emission bands were found when these amines were direct introduced into the DBD plasma. As shown in Fig. 2, emission bands of CN (B2Σ+ → A2Π+) at 383–388 nm and NH (A3Π → X3Σ) at 326.5 and 336.0 nm were clearly
Fig. 4. Effect of input voltage on amine signal intensity from 0.1 μL amines. Experimental conditions: input voltage, 195 V; argon discharge gas flow rate, 600 mL min−1; and integration time, 100 ms.
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Fig. 5. Effect of carrier gas flow rate on amine signal intensity from 0.1 μL amines. Experimental conditions: input voltage, 195 V; argon discharge gas; and integration time, 100 ms.
3.1.2. Effect of nano-catalysts It is well known that nano-catalysts located in the discharge may have a complex effect on the discharge operation, such as shortening the discharge gap, changing dielectric effects or altering discharge nature, depending on their physical and chemical properties. The plasma discharge could change the status of gas phase reactants for catalytic reactions, and it could modify the surface properties of catalysts. The type of catalysts and the loading amount also greatly influence the retention time and concentration. Fig. 3 shows the catalytic performance of different nanomaterials for amine detection by the DBD. Comparison among MnO2, TiO2, SiO2 and ZnO, SiO2 shows the best performance to enhance the emission signal of amines. One reason is that the short living active species are more easily formed in the pore volume of porous SiO2 when exposed to plasma [38]. At the same time, the holes have a significant adsorption capacity for amines, and it prolongs the analytes retention time on the catalyst's surface and in the DBD. The high collision between amines and active species enhances the emission signal. SiO2 was thus chosen as the catalyst for the following experiments.
Fig. 6. Effect of electrode length on amine signal intensity from 0.1 μL amines. Experimental conditions: input voltage, 195 V; argon discharge gas flow rate, 600 mL min−1; and integration time, 100 ms.
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Fig. 7. Chromatogram obtained by using GC-DBD-MES for five amines: (1) trimethylamine, (2) dimethylamine, (3) n-butylamine, (4) cyclohexylamine and (5) ethylenediamine.
3.2. Optimization of experimental conditions 3.2.1. Effect of input voltage To generate and maintain homogeneous and stable plasma in the DBD cell, an appropriate discharge voltage should be applied between the two copper electrodes. The discharge voltage was controlled by adjusting the input voltage of the ac ozone generation power supply through a transformer. The maximum signal appeared when the input voltage ranged from 180 to 200 V. As the voltage increased, the plasma became brighter and the dissociation efficiency was improved, and this was proved by the higher emission signal. At higher input voltage, more radicals or ions were probably generated by more active and effective collision between carrier and analyte gas molecules and high energy electrons. However, when the input voltage exceeded 195 V, the plasma became inhomogeneous with observable flicker as well as the deteriorated stability and intensity of the signal. As shown in Fig. 4, an input voltage of 195 V ac was the best choice.
Fig. 8. Stability of the GC-DBD-MES for trimethylamine detection.
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3.2.2. Effect of argon flow rate Here argon played two roles in the DBD. One is to carry the volatile amines into the discharge cell and the other is to serve as discharge gas. Therefore, the argon flow rate at the range of 400 to 800 mL min−1 was optimized in order to obtain the best emission signal. Fig. 5 shows the emission signal intensity of amines in dependence on argon flow rate. The maximum signals of amines were achieved at the flow rate of 600 mL min−1. Lower argon gas flow rate results in inefficient dissociation of amines in the plasma; and higher argon gas flow rate may cause the dilution of amines in the discharge cell. The argon flow rate of 600 mL min−1 was chosen for the further experiments.
3.2.3. Effect of length of electrode The electrode is an important part of the DBD because its shape, material and arrangement have significant effects on energy density of the DBD cell. Although the length of electrode also influence the discharge efficiency as shown in Fig. 6, the signal varied at different length ratio of the two electrodes (inner electrode length /outer electrode length) from 0.25 to 1.25. As the ratio increased, the emission signal decreased due to the continuous reduction of the discharge zone, leading to a loss of dissociation and excitation efficiency. Although the maximum emission signal achieved at the ratio of zero, plasma became instable with loud noise. As a compromise, the ratio of 0.25 was selected for use.
3.3. DBD-MES as GC detector For the determination of trace volatile aliphatic amines by GC, here nano-SiO2-assisted DBD-MES was used as a sensitive detector. To reduce the dead volume, the end of the GC column was inserted directly into the discharge cell of the DBD device to ensure that analytes well mix with the plasma gas as quickly as possible. The performance of this GC detector was evaluated by separation and detection of the mixture of dimethylamine, trimethylamine, n-butylamine, ethylenediamine and cyclohexylamine. Under the optimal experimental conditions, the chromatogram obtained by the GC-DBD-MES setup indicates that this detector provides sensitive signals of these five amines with satisfactory chromatographic resolution, as shown in Fig. 7. The stability of the detector was examined by five injections of 0.06 μL TMA into GC. From Fig. 8, it is clear that the emission intensity thus obtained shows high stability of the system with a relative standard deviation (RSD) of 3.0%. The analytical performance of the GC-DBD-MES was evaluated under optimal experimental conditions, with 388.3 nm as the analytical line for peak area measurement. Besides its stability and specificity, it provided a wide linear dynamic range of two to three orders of magnitude. Linear correlation coefficients for calibration curves of these five amines are all better than 0.99. The limits of detection (LODs) were calculated using LOD = 3 N/S (where N is the standard deviation of 11 measurements of a blank solution and S is the slope of the calibration curve). The LOD for dimethylamine, trimethylamine, n-butylamine, ethylenediamine and cyclohexylamine were 4.4, 2.5, 2.2, 1.8 and 2.4 μg, respectively. All of these analytical figures of merit were summarized in Table 1. This detector can provide low limits of detection, high sensitivity and a wide linear dynamic range.
Table 1 The analytical figures of merit of GC-DBD-MES for five amines. Amine
Calibration equation (μg)
LDR (μg)
R
LOD (μg)
Dimethylamine Trimethylamine n-Butylamine Cyclohexylamine Ethylendiamine
Y Y Y Y Y
13.6–680 13.2–660 7.3–1100 8.6–1290 9.0–1347
0.999 0.996 0.994 0.999 0.999
4.4 2.5 2.2 1.8 2.4
= = = = =
468.9X + 129.7 813.1X + 6134.5 949.5X + 468.0 1133.9X + 680.3 5278.4X + 464.1
Fig. 9. The proposed method for detection of TMA in a carp kept at room temperature as a function of storage time.
3.4. Preliminary application in real sample analysis The nano-SiO2-assisted DBD-MES detector for GC was used to monitor trimethylamine (TMA) content in a carp fish sample. The fish sample was kept aerated at room temperature and TMA was extracted according to an AOAC method [39], and the gas was directly injected into the GC for analysis. As can be seen in Fig. 9, the emission intensity obtained in this manner experiences a continuous increase in 45 hours as reported previously [40], and this is due to the deterioration of the fish after death. This detector would provide a promising method for instant test of fish freshness. 4. Conclusions The nano-SiO2-assisted DBD-MES as a GC detector could sensitively detect amines, such as monitoring trimethylamine in a spoiled fish with storage time. The emission intensity could be greatly enhanced by nano-SiO2 because nano-SiO2 increased the retention time of amines in discharge zone and changed the discharge characteristics of the DBD. It is a simple, fast and green method to detect amines without complicated pretreatments such as derivatization. This MES detector has many other advantages such as minimal dead volume, high sensitivity, fast response, simplicity, low power consumption and easy hyphenation to GC with good analytical performance. Acknowledgments We thank the National Natural Science Foundation of China (grant no. 20835003) for financial support for this project. References [1] M. Abalos, J.M. Bayona, F. Ventura, Development of a solid-phase microextraction GC-NPD procedure for the determination of free volatile amines in wastewater and sewage-polluted waters, Anal. Chem. 71 (1999) 3531–3537. [2] B. Sahasrabuddhey, A. Jain, K.K. Verma, Determination of ammonia and aliphatic amines in environmental aqueous samples utilizing pre-column derivatization to their phenylthioureas and high performance liquid chromatography, Analyst 124 (1999) 1017–1021. [3] J. Namiesnik, A. Jastrzebska, B. Zygmunt, Determination of volatile aliphatic amines in air by solid-phase microextraction coupled with gas chromatography with flame ionization detection, J. Chromatogr. A 1016 (2003) 1–9. [4] Z.Y. Zhang, K. Xu, Z. Xing, X.R. Zhang, A nanosized Y2O3-based catalytic chemiluminescent sensor for trimethylamine, Talanta 65 (2005) 913–917. [5] A.K. Anderson, Biogenic and volatile amine-related qualities of three popular fish species sold at Kuwait fish markets, Food Chem. 107 (2008) 761–767. [6] H. Greim, D. Bury, H.J. Klimisch, M. Oeben-Negele, Z. Zeigler-Skylakakis, Toxicity of aliphatic amines: structure–activity relationship, Chemosphere 36 (1998) 271–295.
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