Technical Note pubs.acs.org/ac
Miniaturized Dielectric Barrier Discharge Carbon Atomic Emission Spectrometry with Online Microwave-Assisted Oxidation for Determination of Total Organic Carbon Bingjun Han,† Xiaoming Jiang,† Xiandeng Hou,*,†,§ and Chengbin Zheng*,§ †
Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China
§
S Supporting Information *
ABSTRACT: A simple, rapid, and portable system consisted of a laboratory-built miniaturized dielectric barrier discharge atomic emission spectrometer and a microwave-assisted persulfate oxidation reactor was developed for sensitive flow injection analysis or continuous monitoring of total organic carbon (TOC) in environmental water samples. The standard/sample solution together with persulfate was pumped to the reactor to convert organic compounds to CO2, which was separated from liquid phase and transported to the spectrometer for detection of the elemental specific carbon atomic emission at 193.0 nm. The experimental parameters were systematically investigated. A limit of detection of 0.01 mg L−1 (as C) was obtained based on a 10 mL sample injection volume, and the precision was better than 6.5% (relative standard deviation, RSD) at 0.1 mg L−1. The system was successfully applied for TOC analysis of real environmental water samples. The obtained TOC value of 30 test samples agreed well with those by the standard hightemperature combustion coupled nondispersive infrared absorption method. Most importantly, the system showed good capability of in situ continuous monitoring of total organic carbon in environmental water.
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methods and is thus widely used in TOC determination. However, several disadvantages associated with HTC include poor reproducibility and accuracy, incomplete separation of inorganic carbon, serious memory effect, time-consuming, and capillary blocking as working with water sample containing high concentration of salt.11 In addition, the method is handicapped by the difficulties in continuous and in situ monitoring of TOC. The produced CO 2 could be directly or indirectly determined by many methods, including nondispersive infrared absorption method (NDIR),12 thermal conductivity detector,13 CO2 sensor,14 or the method of electrolytic conductivity,7 gravimetry,8 ion chromatography,15 acid/base titration,16 and colorimetry after absorbing by alkaline,11 etc. However, more or less shortcomings remain in these methods: titrimetry and gravimetry are not sensitive enough; electrolytic conductivity and CO2 sensor are still not robust for practical applications because of low stability and serious interference from sample matrix; and NDIR, thermal conductivity, and ion exclusion chromatography typically require bulky and dedicated equipment. Moreover, it is worthwhile to note that the TOC values
ue to the increasing concern about organic pollution/ impurity in various environmental waters, many efforts have been made to develop accurate methods for determination of total organic compounds in water samples.1 Compared to biochemical oxygen demand (BOD) or chemical oxygen demand (COD), total organic carbon (TOC) is generally recognized as the more suitable and direct expression of the pollution status of water caused by organic pollutants because it is directly correlated with the carbon concentration regardless of the chemical forms and oxidation states of organic compounds on top of several other advantages.2 However, current analytical methods for TOC are usually used in laboratory and operated in batch mode because of the bulky equipment size and large power and gas consumption.3 Therefore, it is attractive to develop a portable, simple, fast, inexpensive, and sensitive method for TOC determination because of the increasing demand for field, in situ, and continuous monitor of TOC at trace level. In general, analytical procedures for TOC include the oxidation of organic compounds to CO2 and its subsequent quantification.3 For the oxidation, either chemical oxidation,4,5 photooxidation,6,7 or high-temperature combustion (HTC)8−10 can be used. HTC operated at temperatures ranging from 680 to 900 °C in the presence of platinum-based catalyst offers higher sample throughput and oxidation efficiency than other © XXXX American Chemical Society
Received: April 11, 2014 Accepted: May 26, 2014
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Microwave Oven Wiring Company, Foshan, China), a cooling bath, a quartz gas liquid separator (GLS), and a dryer. Two holes (3.0 mm i.d.) were made through the top of the microwave oven for inserting and hanging the PTFE container in the microwave oven. It should be noted that the container must aim at the magnetron of microwave oven to ensure the microwave irradiation to focus on the reaction solution because microwave distribution in the oven is spatially dependent. One of the ends of the container connects with the injection valve and another one connects to a three-port valve wherein an argon carrier flow is introduced to flush the reaction solution to the cooling bath. The cooling bath is used to cool the reaction solution from microwave-assisted oxidation reactor, minimizing the condensed liquid droplets that are transported to the dryer during the gas−liquid separation. The dryer filled with CaCl2 was connected with the GLS and the DBD-C-AES, respectively, to further ensure that minimal liquid vapor is transported to the DBD-C-AES. The holes made on the body of microwave oven were sealed tightly with aluminum foil, which was served to protect the operator from microwave irradiation. The six-port injection valve was removed only when the system was used for in situ continuous monitor of TOC. The DBD-C-AES detector is similar to that reported in previous work24 and was briefly described together with other information on this microwaveassisted oxidation DBD-C-AES in Section 1 of the Supporting Information. A HTC-NDIR (1020A, OI Analytical Company) was used to analyze the tested samples and validate the applicability of the proposed method. This detection system was optimized independently, and the typical operating conditions for TOC analysis were listed as follow: furnace temperature, 680 °C; sample volume, 100 μL; and oxygen gas flow rate, 550 mL min−1. Reagents. All solutions were prepared using 18 MΩ cm deionized water (DIW) produced by a water purification system (Chengdu Ultrapure Technology Company, Ltd., China) and boiled to remove CO2 prior to use. TOC standard solutions were prepared daily by dissolution potassium hydrogen phthalate (KHP, C8 H5 KO 4 , Kelong Reagent Company, Chengdu, China). Other used chemicals were of at least analytical-reagent grade and also purchased from Kelong Reagent Company. Argon (99.99%, Qiaoyuan Gas Company, Chengdu, China) was used as both discharge gas and carrier gas. Sample Collection. A total of 30 water samples were used to evaluate the accuracy of the proposed method. Twenty-four water samples were collected from four rivers (Sha river, Fu river, Nan river, and Funan river) of Chengdu City, Sichuan Province of China; and other 6 water samples were collected from Nandu River, Meishe River, and Hongcheng Lake of Haikou city, Hainan Province of China. The samples were stored in an ice box using 500 mL glass bottles and analyzed immediately after transport to the laboratory. The sampling map and sites of the rivers of Chengdu city are shown in Figure S1 (see Section 2 of the Supporting Information). All containers used in sample collection were soaked in 0.2% v/v nitric acid for 12 h, successively rinsed with deionized water, and the corresponding river water prior to use. Analytical Procedure. In order to determine TOC, the total inorganic carbon (TIC) must be removed from the sample prior to analysis.3,7 To this end, several methods have been proposed, the most common one being the addition of acid to sample to transform inorganic carbon to CO2 and remove it by
obtained by these techniques are not by directly measuring the elemental specific signal of carbon atom and are thus frequently influenced by similar molecules.3 Although Todoli et al.17 have found that the precision was significantly improved by directly determining the atomic emission intensity of carbon at 193.090 nm by inductively coupled plasma atomic emission spectrometry (ICP-AES), the large and dedicated equipment was still needed and could not be used in field and continuous monitoring. Therefore, the aim of this work is to develop a new system to meet all of the requirements of in situ and continuous monitoring, elemental specificity, portability, high sensitivity and simplicity as well as high sample throughput and costeffectiveness. As a typical nonequilibrium and low temperature plasma, dielectric barrier discharge (DBD) offers important advantages of simple setup, low power and gas consumption, fieldportability, and convenient operation compared to ICP and other conventional plasmas.18,19 Thus, it has become an important research topic and been extensively applied in various fields of analytical chemistry.20−23 Most recently, we have found that DBD can also be used as a low power compact excitation source for generation of specific carbon atomic emission lines and applied to determine volatile carboncontaining compounds after gas chromatographic separation.24 In this work, therefore, we present a novel portable system for measuring TOC in real environmental water samples by dielectric barrier discharge-carbon atomic emission spectrometry (DBD-C-AES). In order to improve sample throughput and achieve in situ continuous monitoring of TOC, an online microwave-assisted persulfate (Na2S2O8) oxidation method was used with the DBD-C-AES to improve the conversion efficiency to CO2. To the best of our knowledge, this is the first report of use of DBD-C-AES for TOC analysis.
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EXPERIMENTAL SECTION Instrumentation. Figure 1 provides a schematic of the flow injection microwave-assisted persulfate oxidation DBD-C-AES
Figure 1. Schematic diagram of the instrumental setup. GLS: gas liquid separator.
system, which was constructed with a flow injection microwaveassisted oxidation reactor and a miniaturized DBD-C-AES detector using a commercial hand-held charge coupled device (CCD) spectrometer (Maya2000 Pro, Ocean Optics Inc., Dunedin, FL) with 0.4 nm of spectral resolution. The flow injection microwave-assisted oxidation reactor consists of a four channel peristaltic pump (BT100-02, Baoding Qili Precision Pump Company, Ltd., Baoding, China), a six-port injection valve, a homemade coil polytetrafluoroethylene (PTFE) container (500 cm length × 2.4 mm o.d. × 2.0 mm i.d) placed inside a commercial microwave oven (WP800 T, Galanz B
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10.0 mg L−1 (as C) TOC standard solution, with results summarized in Section 4 of the Supporting Information. The oxidation efficiency on different organic compounds is the most important factor, which affected the accuracy of the method.7,17 In order to investigate the efficiency of oxidation processes of different organic molecules in our system, 17 chemical compounds with different functional groups, bond structures, and chain lengths were tested under the optimized oxidation conditions. Their oxidation efficiencies are summarized in Figure 2. The resultant efficiencies are relative to the
use of a carrier gas stream before chemical oxidation of water sample.12 However, the purgeable organic carbon (POC) was also removed in this procedure, and thus the determined TOC by these methods was nonpurgeable organic carbon (NPOC) actually. Fortunately, the POC is negligible in many surface and ground waters and then the NPOC is substituted for TOC in these samples analysis.3 In this work, the TOC was directly determined by the microwave-assisted persulfate oxidation DBD-C-AES. The TOC standard or sample solutions after removing TIC by adding 10% phosphoric acid were initially pumped to a 10 mL sample loop through the six port injection valve. The valve was activated to pass DIW carrier solution so as to flush the standard or sample solution to mix with sodium persulfate (Na2S2O8). Then, the mixture was transported to the chemical oxidation reactor for microwave irradiation to efficiently convert all organic compounds to CO2. In the case of continuous monitoring, the standard or sample solution together with Na2S2O8 was directly pumped to the reactor for microwave irradiation. After cooling, the reaction solution was swept to the GLS from which CO2 was transported to the dryer to remove any water vapor. Finally, CO2 was swept to the DBD microplasma for atomization and excitation, and the optical emission of carbon atom at 193.0 nm was measured by the CCD spectrometer. Consequently, the TOC can be calculated from the intensity of the carbon atomic emission.
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RESULTS AND DISCUSSION Preliminary Studies and Optimization of Operation Parameters for DBD-C-AES. Although direct determination of TOC could be accomplished via monitoring the intensity of specific carbon atomic emission at 193.0 nm excited by ICP,17 the equipment is large and highly power consuming, and then could not be used in field and continuous monitoring. The DBD has the advantages for portable instrumentation. However, due to its low temperature and low power consumption, DBD has only been used to generate atomic emission of mercury or molecular emission of halogen molecules22,23,25−27 before our very recent work that DBD could also be used to generate specific carbon atomic emission lines from carbon-containing compounds. Therefore, initial experiments were taken to prove the feasibility and practicability of DBD as a carbon elemental specific detector for TOC analysis. When 10 mL 10 mg L−1 (as C) of TOC standard solution (KHP) was transported to the microwaveassisted persulfate oxidation reactor, specific carbon atomic emission lines with bandwidth less than subnanometer at 193.0 and 247.8 nm (those belonged to carbon atomic emission in previous works using ICP or microwave induced plasma, MIP)28−30 were observed in the emission spectrum of Ar DBD plasma, as shown in Figure S2 (see Section 3) of the Supporting Information. Moreover, the linear coefficient of the typical calibration curve of the intensity of carbon atomic emission versus TOC is better than 0.99. These support the feasibility of this method for TOC analysis. The operation parameters of DBD-C-AES including carrier gas and discharge voltage were investigated in detail (see Section 3 of the Supporting Information). Optimization of Microwave-Assisted Persulfate Oxidation and Oxidation Efficiencies. The effects of microwave power, the flow rate of sample solution and the concentration of Na2S2O8 on response were also carefully investigated using
Figure 2. Relative oxidation efficiencies of different organic compounds relative to KHP TOC standard. Experimental conditions: microwave power, 700 W; carrier solution flow rate, 6 mL min−1; Ar flow, 200 mL min−1; and the concentration of Na2S2O8, 300 g L−1. THAM, tris(hydroxymethyl)-aminomethane; EDTA, ethylene diamine tetraacetic acid.
efficiency of the TOC standard (set to 100%) and range from 90 to 102%, suggesting that there should be no significant difference on the oxidation efficiency between the tested organic compounds and the TOC standard. Therefore, this methodology can be used for accurate determination of TOC in water samples, even if it contains varied complex organic compounds. Interferences. One of the major shortcomings associated with conventional TOC or COD analytical methods using chemical oxidation techniques is the serious interference arising from the coexisting anions, particularly from the chloride ion.31 Consequently, the effects of several anions including NO3−, PO43−, SO42−, and Cl− on the determination of TOC by the proposed method were carefully studied. TOC standard solutions containing 10.0 mg L−1 KHP (as C) were used with addition of 100.0 g L−1 of NO3−, PO43−, SO42−, and Cl−, respectively. Signal intensities for the TOC measured in these solutions were normalized to the intensity measured in a pure 10.0 mg L−1 (as C) of KHP solution. The results are summarized in Figure S4a (see Section 5) of the Supporting Information and demonstrate that no obvious interferences from 100.0 g L−1 NO3−, PO43−, and SO42−, but Cl− caused serious interference with the proposed method and recovery was only about 60% at the concentration of 100.0 g L−1 Cl−. C
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The test TOC standard solutions at 10.0 mg L−1 (as C) were added by different amounts of Cl− to further investigate the effect of chloride, with results shown in Figure S4b (see Section 5) of the Supporting Information). The recoveries are better than 90% when the added Cl− is lower than 10.0 g L−1, which meets the requirements of routine analysis of TOC in normal surface and river water samples. Moreover, the effect of 12 cations [K+, Ca2+, Na+, Mg2+, Fe3+, Al3+, Cu2+, Zn2+, Cd2+, Cr3+, Mn2+, and As(III)] on the response from 10 mg L−1 of TOC standard solutions was also investigated, and no obvious interferences (response change < ± 15%) from these tested elements were detected, even at cation concentrations as high as 500 mg L−1 (Figure S4c of the Supporting Information). Analytical Performance. The analytical performance of our system was evaluated under the optimal experimental conditions. Figure 3b shows that the intensity of carbon atomic
Table 1. LOD for TOC by DBD-C-AES in Comparison with Those Reported in Literaturea method UV photooxidation, electrolytic conductivity wet−dry combustion, gravimetric method HTC-NDIR HTC-ion chromatography HTC-isotope ratio mass spectrometer H2O2, alkaline discolors at 550 nm UV photooxidation-NDIR UV spectrophotometric sensor direct ICP-AES microwave-assisted persulfate oxidation DBD-C-AES a
symbol
LOD (mg L−1) (as C)
TOC
0.002
2
TOC
0.050
3
TOC TOC TOC
0.024 0.002 0.480
4 5 6
TIC DOC DOC TOC TOC
0.036 0.010 0.360 0.070 0.010
7 8 9 10 DBD-CAES
ref
DOC, dissolved organic carbon.
analytical results summarized in Table 2. The t test showed that all the analytical results produced by the proposed method were not significantly different from those results obtained by HTCNDIR at the confidence level of 95%. Three of the samples were analyzed to further validate the accuracy of the proposed Table 2. Analytical Results of TOC in Environmental Water Samples sampling location
Figure 3. (a) Effect of the TOC concentration on carbon atomic emission intensity. (b) Plots of the carbon atomic emission intensity as a function of TOC concentration from 0.1 to 1000 mg L−1, showing a good linearity in the range from 0.1 to 20 mg L−1. Experimental conditions: microwave power, 700 W; carrier solution flow rate, 6 mL min−1; Ar flow, 200 mL min−1; and the concentration of Na2S2O8, 300 g L−1.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
emission at 193.0 nm gradually increased as TOC concentration increased. The effect of TOC concentration on the atomic emission intensity of carbon was finally investigated in the range of 0.1−20 mg L−1 (based on a 10 mL sample volume), with results shown in Figure 3a. A typical calibration curve of the intensity of carbon atomic emission versus TOC concentration (Figure 3b, inset) can be characterized by the following calibration function: I = 311.48 CTOC + 53.39, where C is the concentration of TOC (milligram per inverse liter). The linear coefficient is better than 0.999. The linear range could be extended to higher than 200 mg L−1, when a 1 mL sampling loop was used. The limit of detection (LOD), defined as the analyte concentration equivalent to 3S (standard deviation) of 11 repeated measurements of a blank solution (DIW), is 0.01 mg L−1 by using a 10 mL sample volume. The precision, expressed as the relative standard deviation (RSD) for replicate (n = 11) injection of 10 mL of 0.1 mg L−1 TOC standard solution was better than 6.5%. The RSD for real water sample was 3.8% and 4.5% at the 5.0 and 10.0 mg L−1 level, respectively. The analytical performance compares favorably with that of similar analytical methods, as summarized in Table 1. Analysis of Samples. The applicability of our system was first validated using the standard TOC analysis method (HTCNDIR). The 30 real water samples were analyzed by the developed method and HTC-NDIR, respectively, and with
30 a
D
samples Fu River water Fu River water Fu River water Fu River water Fu River water Nan River water Nan River water Nan River water Nan River water Nan River water Nan River water Nan River water Nan River water Nan River water Funan River water Funan River water Funan River water Sha River water Sha River water Sha River water Sha River water Sha River water Sha River water Sha River water Nandu River water Nandu River water Meishe River water Meishe River water Hongcheng Lake water Hongcheng Lake water
DBD-C-AES (mg L−1) 50.4 50.1 50.9 51.9 52.6 37.1 38.1 39.7 40.2 41.4 41.4 41.5 41.6 44.3 49.1 50.1 49.2 29.4 30.9 33.5 34.4 35.4 35.6 37.3 12.1 14.6 21.6 22.7 25.4
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
1.3 1.2 2.9 1.9 2.5 1.9 0.6 1.7 1.3 1.1 1.5 1.8 0.9 2.6 0.8 1.2 0.7 1.5 2.4 0.6 1.3 1.2 1.7 1.0 0.6 1.1 0.8 0.6 0.8
24.9 ± 1.1
HTC-NDIR (mg L−1)a 50.2 49.8 50.1 52.6 51.8 36.8 37.4 40.8 39.7 40.4 42.3 40.3 40.9 44.9 49.8 49.8 50.3 28.3 31.9 32.8 33.6 35.0 35.1 37.0 11.7 14.3 21.3 23.1 24.8
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.9 0.9 1.4 1.9 1.4 1.2 0.3 0.7 1.0 1.1 0.8 1.9 0.7 0.8 1.0 0.9 0.5 0.2 1.5 1.4 0.8 1.0 0.8 0.3 0.4 0.7 0.5 0.2 0.5
25.2 ± 0.7
Average ± standard deviation of three trials. dx.doi.org/10.1021/ac501272m | Anal. Chem. XXXX, XXX, XXX−XXX
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CONCLUSIONS We have constructed and evaluated a new portable system using online microwave-assisted persulfate oxidation and a miniaturized DBD-C-AES for both flow injection analysis and in situ continuous flow monitoring of TOC. This system provides many advantages, including elemental specificity, portability, simplicity, high-sample throughput, low power consumption, high sensitivity and accuracy, and in situ continuous flow monitoring for TOC analysis, in comparison to the traditional TOC analysis methods such as HTC-based technique and other sensors. Further, the atomic or molecular emission spectrum of nitrogen, sulfur, chlorine, bromine, and iodine from their volatile species were also found to be detectable by DBD-AES or DBD molecular emission spectrometry (DBD-MES).24 Therefore, we believe that in situ continuous monitoring of some other environmental pollutants, such as total nitrogen, nitrate nitrogen, nitrite nitrogen, total sulfur, and so on will be accomplished by a similar system in the near future.
method via evaluation of the recoveries of the spiked TOC because no certified values of TOC in these samples are available. The results are summarized in Table 3. Satisfactory spike recoveries in the range of 90−105% could be achieved for all the tested samples, further supporting the suitability of the proposed method for TOC analysis. Table 3. Analytical Results of TOC in Environmental Water Samples samples Fu River water 4
Nan River water 9
Sha River water 24
a
added (mg L−1) 0 5.0 10.0 0 5.0 10.0 0 5.0 10.0
found (mg L−1a)
recovery (%)
± ± ± ± ± ± ± ± ±
− 90 97 − 98 105 − 98 103
53.6 58.1 63.3 40.3 46.2 50.8 37.5 42.4 47.8
0.5 0.6 1.2 0.2 1.1 1.2 2.5 0.9 3.0
Technical Note
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Average ± standard deviation of three trials.
ASSOCIATED CONTENT
* Supporting Information S
The most major problem associated with conventional TOC analysis methods, particularly HTC based techniques, is that they are handicapped by the difficulties in in situ continuous monitoring of TOC in water samples. The potential of in situ continuous monitoring of TOC by the proposed system was evaluated using a real water sample flow that was gradually increased with TOC via addition of KHP, as shown in Figure 4.
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Fax: +86 28 85412907. Tel: +8628-85415503. *E-mail:
[email protected]. Fax and Tel: +86-28-85410518. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS X. D. Hou gratefully acknowledges the National Nature Science Foundation of China (Grant 21275103) for financial support. C. B. Zheng is grateful for the financial support by Ministry of Education of China through Grant NCET-11-0361.
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REFERENCES
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Figure 4. Dynamic transient carbon atomic emission intensity of the proposed system to TOC standard (KHP, 0.0−50.0 mg L−1 as C). Experimental conditions: microwave power, 700 W; carrier solution flow rate, 6 mL min−1; Ar flow, 200 mL min−1; and the concentration of Na2S2O8, 300 g L−1.
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