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An exploiting of cost-effective direct current conductivity detector in gas diffusion flow injection system
Talanta 170 (2017) 298–305
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An exploiting of cost-effective direct current conductivity detector in gas diffusion flow injection system Wasin Somboota, Jaroon Jakmuneea,b, Tinakorn Kanyaneea,b, a b
MARK
⁎
Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Center of Excellence for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
A R T I C L E I N F O
A BS T RAC T
Keywords: Direct current conductometry Gas diffusion flow injection Dissolved inorganic carbon Natural water
In this work, a homemade direct current (DC) conductivity detector as an alternative cost-effective detection device has been fabricated and investigated to use in flow analysis system. Under the selected appropriate conditions of flow system, the electrolysis of a carrier stream at the conductivity detector was negligible and provides well-defined signal. The cost-effective DC conductivity detector was demonstrated to couple with gas diffusion flow injection system for determination of dissolved inorganic carbon (DIC) in water. The method is based on the conversion of DIC (dissolved CO2, HCO3- and CO32-) presented in the injected sample to carbon dioxide in an acidic donor stream and then CO2 gas diffuses through a hydrophobic porous membrane to an acceptor stream. As a result, the change of conductivity signal was observed corresponding to DIC concentration. A linear calibration range of DIC in 1.0–10 mmol L−1, with limit of detection of 70 µmol L−1, repeatability of < 3% RSD and 15 injections h−1 sample throughput can be obtained. This method was applied for DIC determination in natural water.
1. Introduction CO32-/HCO3-/CO2(g) in carbon dioxide cycle as total inorganic carbon(TIC) or dissolved inorganic carbon (DIC) in natural water system is important species which the environmental scientist needs to monitor. Total DIC is one major chemical component in geothermal water [1] and particular interest in environmental, biological, and geological studies [2]. Many analytical methods employ the conversion of analyte as gas convertible species in sample and liberate the gaseous product out from the complex matrices of sample through a gas diffusion unit. For example, a membrane based gas diffusion-flow injection (GD-FI) system with conductivity detector has been reported such as NH4+ determination from Kjedahl digestion [3]. Recently, Pheeraya et al. applied the liquid drop as micro membraneless gas absorber with miniature conductivity probe to measure the change in conductivity of sulfuric acid liquid drop due to the reaction with NH3 gas converted from NH4+ ion in natural water [4]. Several methods of DIC determination in geothermal or natural water usually employ the gas diffusion unit to convert dissolved carbonate to CO2. Since, the flow injection analysis system provides fast chemical analysis, it was utilized to couple with gas diffusion unit for determination of DIC with various detection techniques. Monser et al. employed the tungsten oxide wire as pH sensor in the GD-FI
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system for total inorganic carbon determination in water [5]. In addition, the simultaneously determination of CO2 and SO2 in wine was sensitive performed by GD-FI coupled to two detection units: a pH sensor and a spectrophotometer [6]. The pulse conductometric detector as a sensitive and simple technique is also used to couple with GDFI system for total inorganic carbon determination in water [7–10]. In addition, after the discovery of using contactless conductivity detector (CCD), it was applied to couple with GD-FI system for total dissolved inorganic carbon [11,12] and total organic carbon determination [13]. Moreover, spectrophotometric detectors with some colorimetric dyes [14,15], FT-IR [16,17], chemiluminescence [18] and bulk acoustic wave [19] coupled with GD-FI system have been used for DIC determination. The conductometry is one of simple and sensitive electrochemical techniques. Although low cost conventional conductivity detector as pulsed format system has been sold in market or can be built up inhouse as low cost pulse circuit equipment, the simpler pulseless circuitry would be an alternative detection system in budget limit laboratory. The principle of direct current (DC) conductivity detector or pulseless conductivity detector was also mentioned for chemical analysis applications [20]. Once the cell was designed to negligible polarization effect during the measurement and the generated gas bubble was removed from electrode in appropriate hydrodynamics
Corresponding author at: Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand. E-mail address: [email protected] (T. Kanyanee).
http://dx.doi.org/10.1016/j.talanta.2017.04.015 Received 31 January 2017; Accepted 7 April 2017 Available online 09 April 2017 0039-9140/ © 2017 Elsevier B.V. All rights reserved.
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conditions, the DC- conductometry is comparable to pulse conductivity measurement. The four electrodes cell design of using 540 V DC was applied through platinum electrode in titration cell with vigorously stirred solution and the voltage drop across pair of tungsten electrodes was monitored [21]. The conductance of solution was measured by using a high impedance voltmeter. Various chemical reactions including neutralization of weak/strong acid in a conductometric titration were investigated. With the stirred solution condition, the fluctuation of the measurement signal due to gas bubble evolved from electrolysis process was negligible. Moreover, the conductometric cell with two external non-polarizable redox electrode was separated from analytical solution compartment by using ion exchanged membrane [22]. The current passing through the cell depended on the applied potential and solution conductivity. The proposed cell was applied to use with acidbase conductometric titration system. Another design with two compartments of measuring solution and the reference solution connected together by using 2-cm capillary glass tube to decrease the polarization effect in a DC conductivity measurement [23]. The proposed conductivity cell was applied to test with mixed acid-base titration and Ca2++ Mg2+ titration with EDTA. In addition, the DC-conductivity detector was investigated to use in packed column liquid chromatography for separation of some organic acids [24] with 0.1 µL cell volume. The detector provides good signal to noise ratio without observed polarization effect with limit of detection of 0.67 µmol L−1 acetic acid. Qi et al. studied some characteristics of a DC-conductivity detector and utilized it in ion chromatography [25]. It was found that current signal depended on a reciprocal of cell resistance and an applied voltage, and the DC conductivity measurement provided the same performance as a conventional pulse conductivity detector. For on-site measurement, the miniature detector was required to be developed as a portable system. Although, the contactless conductivity detector [11] and OEM capacitance sensor board [26] were demonstrated to work as a detection unit in some portable flow analysis systems, the DC conductivity detector will provide the simpler circuitry and can be an alternative detection unit in flow analysis system for down-scaling and on-site measurement application. Although, the applied potential format of DC conductivity detector is akin of single potential amperometry, it does not require any reference electrode or expensive 3-electrodes based potentiostat/electrochemical analyzer. Although the direct applied voltage in DC conductivity measurement may cause electrochemical reactions at the detection cell, gas diffusion flow injection system provide benefit to select the interested component to be detected and would be fit with cost-effective DC conductivity detector. This work aims to utilize a simple, compact and low cost DC conductivity detector to couple with gas diffusion flow injection system for determination of some gas convertible ions. The gas diffusion unit with simple gravity/pressurized propelling flow injection analysis was employed with HCO3- as a model analyte to evaluate the proposed detector. Some characteristic in flow analysis system and potential for field based unit application was described.
Fig. 1. Circuitry of DC conductometers: a) using bridge circuit and operational amplifier, and b) using 78xx IC.
with DI water. Stock standard solution of S2- was prepared by dissolving 0.6503 g of Na2S (Loba Chemie, India) in 100 mL volumetric flask and adjusting volume to the mark with DI water. The exact concentration of a stock solution of standard bicarbonate was determined by titrating with a standard HCl solution [27]. The hot spring water samples were collected from the places around Chiang Mai province. The commercial mineral drinking water samples were also taken to test the method. The water samples were collected and kept in refrigerator at 4 °C. The KMnO4 was added to the sample to remove interference from sulfide/sulfite. The sample was cleaned through 0.22 µm pore size Nylon syringe filter (membrane-solutions) prior to injecting into the GD-FI system. 2.2. Experimental setup The DC conductivity detectors were fabricated in 2 formats, the first one by using a simple voltage regulator IC (7805 or 7815, bought from local electronics shop) and the second one by using bridge circuit with operational amplifier (TL072, Texas instrument) as illustrated in Fig. 1. The response signal from DC conductivity detector was acquired to a computer through a 22-bit A/D card (Emant300, Emant Pte) with 10 Hz sampling rate. The software was written in-house with Visual Basic 6 for data acquisition. The analytical signals of the fabricated DC conductivity detector were compared with a commercial pulse conductivity detector (712 conductometer, Metrohm, Switzerland or EP357 conductivity isoPOD™, EDAQ, www.edaq.com). The Metrohm conductivity detector provides the selectable gain amplification and analog signal out (0–2 V DC) was acquired to a computer through a 22bit A/D card. The conductivity cell was fabricated to test the DC conductivity as illustrated in Fig. 1b with cell constant of 20.8 cm−1. Two of 0.4 mm ID/0.6 mm OD stainless steel tubes each with 1 cm long were used as electrodes. The stainless steel tubes were inserted and fitted in 3 pieces of 1 cm length of Tygon or PEEK tube and let the electrode gap about
2. Experimental 2.1. Reagents and solutions Analytical grade reagents and deionised water were used throughout the experiment. Standard KCl was prepared as a stock solution by dissolving 0.3746 g of KCl (Merck, Germany) in 100 mL volumetric flask and adjusting volume to the mark with DI water. Standard HCO3was prepared by dissolving 0.4222 g of NaHCO3 (Merck, Germany) in 100 mL volumetric flask and adjusting the volume to the mark with DI water. Stock standard solution of NH3 was prepared by dissolving 0.1072 g of NH4Cl (Scharlab, Spain) in 100 mL volumetric flask and adjusting volume to the mark with DI water. Stock standard solution of SO32- was prepared by dissolving 0.6471 g of Na2SO3 (Carlo Erba, France) in 100 mL volumetric flask and adjusting volume to the mark 299
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Fig. 2. Schematic diagram of a) simple FI gas diffusion unit with DC-conductometer and b) conductivity cell.
standard deviation of baseline (S/SDb).
1 mm. The 0.8 mm copper wire from normal electrical cable was coiled on stainless steel tube electrodes. The parts were attached on glass slide and sealed together with epoxy glue. The 1/16″ OD Teflon tube was inserted to connect and fit in Tygon tube of the conductivity cell. Two identical conductivity cells were fabricated and setup as dual cell detectors connected in series in the flow system for comparison the performance of the proposed detector with a commercial pulse conductivity detector. The gas diffusion flow injection system for evaluating the proposed DC conductivity detector consisted of an acidic or basic donor stream to convert ion to gas and DI water as an acceptor stream. The gas diffusion unit was fabricated as described in the previous work [3]. The acrylic block was fabricated with dimension of W×L×T=3×16×4 cm as shown in Fig. A2 in Supporting information. The length of membrane is about 30 cm. This design of gas diffusion unit serves to use with a low cost Teflon tape membrane (which is normally used for plumbing and can buy in a local shop). The hydrophobic surface of Teflon tape separate the aqueous solutions in both sides but only the produced gaseous analyte (CO2 or NH3) can pass through the membrane to the acceptor line. The schematic diagram of flow analysis system is depicted in Fig. 2. The Teflon tube (0.5×1.6 mm IDxOD, IDEX Health & Science LLC) was used throughout the experiment. The 6-ports-2-position injection valve was used for sample/standard injection with using 40 µL injection loop and 20 cm mixing coil length. Two of 50 mL bottles (Scot Duran, Duran group) as of carrier/reagent reservoir were modified to hang on the stand to provide gravity driven propulsion system [28]. Flow rate of the reagent was adjusted according to the relative height of the solution level and the end tube orifice. The typical flow rate to use with this system is 1.0 mL min−1 of DI water stream. In some experiments, the pressurized propulsion system using constant pressure of nitrogen gas through adjustable pressure regulator was used to replace the gravity flow system. The flow rate of solution was controlled according to the pressure and size of tubing. The carrier/acceptor stream flow rates used in the manifold were optimized to provide the better ratio of signal/
3. Results and discussion 3.1. Some characteristic signal of the fabricated DC Conductometer in FI system This conductivity detector approach is based on the electrolytic reaction of water at the small electrodes with about 3–18 V DC applied voltage. The trace concentration of inorganic salt in water stream would make the change of conductivity of the DI-water and as a result increase the current flow in the electrolytic cell. Therefore the relative electrolytic current would relate to the inorganic salt concentration. At appropriate movement of measuring solution such as in a flow cell or vigorously stirred solution [21–23], the gases bubble from such electrolytic reaction can be removed from the conductivity cell efficiently. The direct current electrolytic cell can play a role as conductivity detector for some inorganic salts. Although, the true conductance signal can not be measured precisely, the change of relative current when analyte zone flowed through the conductivity cell would provide the relative peak signal for quantitative chemical analysis. The peak height signal of DC conductivity detector with operational amplifier depends on the applied DC potential to the electrodes as suggested and explained by Qi et al. [25]. Curiously, using the higher applied voltage, the higher peak current would be obtained but it could generate higher amounts of gas bubble due to electrochemical reaction occurring at the electrode surface. The gas bubble generation would make the fluctuation of response signal and standard deviation of baseline would increase. The limit of detection for quantitative chemical analysis application would be limited by the baseline signal fluctuation. Therefore, the ratio of signal/standard deviation of baseline (S/SDb ratio) was defined to evaluate or compare the experimental condition to accomplish the best response. In addition, a well-defined peak shape in FI-gram is also considered to compare different designs of the 300
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Table 1 Comparison of performance of fabricated detectors with 10–100 µM KCl injection in DI water stream. Detector
Linear relationship equation
DC bridge ± 5 V DC bridge ± 9 V 7805 IC 7815 IC Metrohm 712***
Y=2.31X+16.8, Y=8.39X+29.9, Y=0.09X+0.28, Y=0.65X+1.26, Y=10.1X+42.1,
r2=0.9995 r2=0.9914 r2=0.9983 r2=0.9963 r2=0.9997
Standard deviation of baseline signal (SDb*) (mV)
Peak height/SDb ratio**
0.5 0.7 0.1 0.2 2.8
83.3 98.0 12.3 38.3 50.9
± 2.2 ± 4.2 ± 0.1 ± 0.9 ± 0.8
*** The commercial pulse conductivity detector generates the analog signal corresponding to conductivity (0–2 V) and acquired to computer through external DAQ. The device allows user to change the factors to amplify an appropriate analog signal. * SDb is standard deviation of baseline signal of 200 data points. ** The ratio of peak height of 10 µM KCl and SD of baseline signal (average ± SD). The SD=standard deviation of triplicate injections.
conductivity detector. In this experiment, the DC conductivity detector was fabricated as two formats; i) using resistor bridge and operational amplifier (hereinafter call BOA) as suggested by Qi et al. [25] and ii) using the 78xx voltage regulator IC, as depicted the circuitry in Fig. 1. The BOA conductivity signal was amplified and converted to voltage according to feedback resistor of the FET input operational amplifier IC (TL072). Two applied voltages ( ± 5V DC and ± 9V DC) were tested and compared with a commercial pulse conductivity detector. In another format with using 7805 or 7815 regulator IC, the voltage drop at the 200 kΩ-resistor was measured without any further amplification and it would correspond to the current flow resulting from the change of solution conductance in the cell. The standard solution of KCl was injected into a stream of DI water. The conductivity signals in various conditions such as various applied voltages, and flow rates of carrier were investigated. All fabricated detectors provide the linear relationship of peak height versus concentration in the range of 10–100 µmol L−1 KCl and the S/SDb were compared as shown in Table 1. The BOA based DC conductivity with using ± 5V-DC gave a well-defined FI-gram as demonstrated in Fig. 3. Although, it provided lower peak height of KCl solution compared with using the ± 9V-DC as demonstrated in Supporting information (Fig. A3) and a model 712-Metrohm conductometer, it gave the best S/SDb. The circuit with using 7805 or 7815 IC is the simple setup and awell-defined FI peak can be obtained but peak height of KCl solution is not high as BOA based circuit. Therefore, the ± 5V-BOA DC conducitivity detector was selected for further experiment. With using ± 5V-BOA based DC conductivity circuit, the reproducibility of signal was evaluated by injecting the standard KCl 60 µmol L−1 in a single line FI system with flow rate of DI water carrier of 1.0 mL min−1. The relative standard deviation (%RSD) of the peak height obtained is about 3% (with 40 injections). Flow rate of the carrier affects to conductivity peak height signal due to it promotes the mass transfer of water/H+ to the electrode and possibility of removal of the gas bubble from the conductivity cell as shown the result in Fig. 4. By injecting 1, 10 and 100 µmol L−1 of KCl into a water stream with flow rate of 0.1–1.5 mL min−1, the flow rate 0.1 mL min−1 provides the highest signal meanwhile flow rate 1.5 mL min−1 provides the lower standard deviation of baseline signal since the entrapped gas can be removed efficiently at the higher flow rate. Moreover, the voltage regulator IC based circuit was compared with a miniatured commercial pulse conductivity detector (IsoPod™, EDAQ conductivity detector, with dimension: 10.8×5.8×3.5 cm). The 200 mVpp pulse was applied to the conductivity cell and the measuring signal was acquired to a computer through USB port and a vender software. By injecting 0.1–1.9 mmol L−1 HCO3- into a single line FI system of DI water as a carrier stream, it was found that EDAQ Conductivity detector provides S/SDb of 0.1 mmol L−1 HCO3- of 2.69 while the 7805 IC-based DC detector provides S/SDb of 14.10. After smooth curve with moving average function in Microsoft Excel software, the S/SDb of 7805 IC based conductivity circuit is better.
Fig. 3. The FI-gram of 10–100 µmol L−1 KCl with DC conductivity detector and pulse conductivity detector (712, Metrohm) which were serially arranged in a stream of DI water (injecting 30 µL with 1.0 mL min−1 flow rate).
Fig. 4. The relative peak height signal of KCl injection and standard deviation of baseline signal with various flow rates of DI water stream using ± 5V bridge-operational amplifier DC conductivity detector.
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Fig. 5. Typical FI-GD grams and calibration curves with using DC conductivity detector. a) 1.0–10 mmol L−1 HCO3- b) 0.1–1.0 mmol L−1 NH4+ c) 1–10 mmol L−1 S2- (40 µL injection with 20 cm long-mixing coil, 1.0 mL min−1 flow rate of both DI water as an acceptor line and 10 mmol L−1 H3PO4 as a donor line).
mode has been selected to test as a simple DC conductivity detector unit. The 10–100 µmol L−1 KCl was injected into the DI water stream. The peak signal can be obtained and the peak height relates to the concentration of KCl. Although, the sensitivity of this device is not high as the fabricated DC circuit due to the lower applied voltage from the ohmmeter probe, it can be applied for other applications such as timebased peak width measurement in flow injection system [29].
In addition, a digital multimeter has been tested as a simple DC conductivity detection unit. The digital multimeter in an ohmmeter mode has been designed to measure the resistance of solid materials by applying DC voltage and measuring the returned current that corresponds to the resistance of tested materials. The resistance which is the reciprocal of conductance could be acquired to a computer through RS232-PC interface without circuit modification and could be applied to be a compact detection unit in flow analysis system. In this experiment, the low cost digital multimeter with RS232 interface to PC system (Unit-T 60A, Uni-trend Technology) operated in ohmmeter
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Fig. 6. Optimization of an acceptor stream in GD-FI-DC conductometric system for HCO3- with using 1.0 mL min−1 of 0.1 mol L−1 H3PO4 as a donor stream: a) signal of 10 mmol L−1 HCO3- in 1.0 mL min−1 of various solution types of an acceptor stream, b) signal of 5 mmol L−1 HCO3- in various flow rates of DI-water acceptor stream (0.1 mol L−1 H3PO4 with 1.0 mL min−1 donor stream).
3.2. DC-conductivity detector coupled with gas diffusion flow injection system
Fig. 7. Optimization of acid donor stream in GD-FI-DC conductivity system for HCO3with 1.0 mL min−1 of DI water was used as an acceptor stream: a) Injection of 10 mmol L−1 HCO3- in various acid concentrations, b) Injection of 5 mmol L−1 HCO3in various flow rates of 0.1 mol L−1 H3PO4 as a donor stream.
The ± 5V-DC BOA based conductivity circuit was selected to couple with gas- diffusion flow injection (GD-FI) system as explained in an experimental section. The gas diffusion unit (GDU) composes of 2 lines; donor and acceptor lines as demonstrated in Fig. 2. Both lines were separated with hydrophobic membrane which lets only hydrophobic gas molecule to diffuse pass through it. Some gas convertible ions such as HCO3-/CO2, NH4+/NH3, S2-/H2S would be detected with this system. For HCO3- detection, the donor line is an acidic solution to convert the analyte to be CO2 gas. The H2SO4 or H3PO4 can be used. But the H3PO4 was reported as the lowest volatile acid [30] and was selected to use in this work. The produced CO2 in a donor line would diffuse pass through the hydrophobic Teflon tape membrane and as a result would be trapped in an acceptor line. The conductivity of solution in the acceptor line would be changed corresponding to the concentration of the injected HCO3-. By this concept, the slower flow rate of the acceptor line would provide higher signal as discussed in Section 3.3. With using DI water as an acceptor line for the GD-FI with DC conductivity detector, the well-defined FI-gram of HCO3- injection can be obtained. The linear relationship of DC conductivity signal vs concentration of 1.0–10 mmol L−1 of standard HCO3- can be obtained as depicted in Fig. 5a with standard calibration equation of y (mV) =6.67[HCO3-]+6.33, where y is expressed in mV and the concentration of HCO3- in mmol L−1, r2=0.996. The standard solution of S2- was also injected to the same GD-FI system with a stream of H3PO4 solution in a donor line and the acceptor line is DI-water. The signal of S2- can be
Table 2 The recovery of DIC determination in water samples with the proposed conductivity detector. Sample
HCO3- added
DIC Found* (mM)
% Recovery
Hot spring water 1
0.0 1.0 3.0
4.18 5.18 6.97
± 0.17 ± 0.14 ± 0.08
100 93
0.0 1.0 3.0
4.32 5.34 7.32
± 0.00 ± 0.11 ± 0.05
102 100
0.0 1.0 3.0
4.23 5.24 7.04
± 0.03 ± 0.05 ± 0.02
101 94
0.0 1.0 3.0
4.26 5.28 7.28
± 0.07 ± 0.05 ± 0.11
102 101
0.0 1.0 3.0
2.37 3.40 5.44
± 0.06 ± 0.10 ± 0.05
103 102
Hot spring water 2
Hot spring water 3
Hot spring water 4
Mineral water
*
303
The value is average ± SD of triplicate results.
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FI-GD coupled with DC conductometer for total DIC determination. The linear relationship of measuring signal vs. 1–10 mmol L−1 HCO3can be obtained with sample throughput of 15 injections h−1 and < 3% RSD. The limit of detection based on 3 times of standard deviation of blank is 70 µmol L−1. The proposed cost-effective DC conductivity detector would provide for determination of other gas convertible species such as ammonium as well. Typically, the hot spring water usually contains sulfide or sulfite which can be converted to be gas in acidic condition and can be significant candidate interferences. The sample was treated to eliminate sulfide or sulfite interference by adding KMnO4, Cr(IV), or H2O2 [7]. The successful removal of sulfide/sulfite by using 5 mmol L−1 KMnO4 is demonstrated in Fig. A6 in Supporting information. The DIC determination with sulfide/sulfite removal with KMnO4 adding into hot spring water samples is shown in Table 2. The validation of accuracy of the proposed system was tested by spiking the standard HCO3- into the water sample and recovery of HCO3- determination was calculated. In addition, the DIC determination by using the proposed detector was compared with using the pulsed conductivity detector as the standard method by serial connection the identical conductivity cells in a flow system as demonstrated in Supporting information (Fig. A1). The DIC results from both detectors were shown in Table 3. The linear range of calibration, limit of detection, %RSD, and sample throughput of the proposed system as compared with previous FI-GD system is shown in Table A1 in Supporting information. The proposed analysis system can be an alternative method for DIC determination in natural water.
Table 3 Comparison of dissolved inorganic carbon determination by using proposed detector and pulsed conductivity detector. Samples
Hot spring water-1 Hot spring water-2 Hot spring water-3 Hot spring water-4 Hot spring water-5 Hot spring water-6 Mineral water-1 Mineral water-2 *
Dissolved inorganic carbon (mM)* DC conductivity detector
Pulsed conductivity detector
7.26 7.72 7.76 8.55 9.21 9.58 4.35 5.81
6.94 7.41 7.35 8.56 9.24 9.60 4.59 6.11
± 0.30 ± 0.05 ± 0.15 ± 0.04 ± 0.20 ± 0.19 ± 0.13 ± 0.19
± 0.20 ± 0.08 ± 0.18 ± 0.36 ± 0.19 ± 0.12 ± 0.11 ± 0.13
The value is average +/− SD of triplicate results.
detected with DC conductivity detector with y=3.92[S2-]+5.76, where y is expressed in mV and the concentration of sulfide in mmol L−1, r2=0.986 as shown in Fig. 5c. Moreover, the proposed detector with GD-FI system has been tested with 0.1–1.0 mmol L−1 of standard NH4+ injection. By changing the donor stream to be 0.1 mol L−1 NaOH, NH4+ ion can be converted to NH3 gas with DI-water as an acceptor. Without optimization of flow rate in both lines, the good peak signal can be obtained as demonstrated in Fig. 5b with y=95.04[NH4+]+5.76, where y is expressed in mV and the concentration of ammonium in mmol L−1, r2=0.999. Hence, the results confirmed that the GD-FI system coupled with DC conductivity detector could be used for HCO3-, S2-, and NH4+ detection. In addition, for HCO3-/CO2 in GD-FI system, although the DI water was successfully used as an acceptor solution, the NaOH has been reported to use in an acceptor line to provide a better signal [10]. The mild basic solution of 10 or 25 µmol L−1 NaOH was investigated to use in an acceptor line. DC-conductivity signals of 10 mmol L−1 HCO3- in GD-FI system with water and NaOH as the acceptor solution were compared as depicted in Fig. 6a. The higher concentration of NaOH, the higher amount of gas bubble generated due to the higher conductivity of NaOH solution, the easier the electrochemical reaction to occur and producing the gas bubble. The system requires higher flow rate of the carrier stream to remove the bubble from the electrodes. As a result, the peak signal of HCO3-/CO2 is not sharp and lower signal. Moreover, the CO2 free boiled DI water was recommended to use as the acceptor solution for HCO3-/CO2 in GD-FI system. The signal of using an acceptor stream with/without boiled DI water was also compared. The relative peak signals from both types of waters were not significantly different. Curiously, the tap water in a laboratory was also tested to use as an acceptor line. The baseline signal was too much fluctuation due to the high level of dissolved chloride and can not provide a good signal. Therefore, DI-water is the best acceptor solution for this detector.
3.4. Benefit of using the proposed detector The proposed DC conductivity detection system is extremely low cost detection unit about 120$ USD (including data acquisition unit), and it is easily fabricated without handle with frequency generator and pulse rectifier circuits. In addition, a 9 V-battery (model Energizer Max) can be employed to replace desktop power supply. The detector shows low energy consumption as indicated by continuous operation of the system for 8 h, the battery voltage decreased about 0.8 V. (9.6– 8.8 V.) with repeatability of conductivity signal of KCl about 4.5%RSD (Fig. A5). This would support availability for a portable unit with a simple circuit. The data acquisition board in this work can be changed to be others that can provides the Bluetooth signal to connect with a smart phone or tablet computer. As a result, the proposed system provides the potential to develop for on-site based equipment. The forthcoming work as a portable unit with DC conductivity detector is in progress.
4. Conclusion The fabricated DC conductivity detector shows good performance to use in flow injection analysis coupled with gas diffusion unit for dissolved inorganic carbon determination in natural water. This conductivity detector could be an alternative detection system with simple pulseless circuitry fabrication, low energy consumption, extremely low cost and compact conductivity detector, and possible for portable measuring system. This detector with gas diffusion flow injection system would be applicable to other gas convertible compounds such as NH4+/NH3.
3.3. Application of the proposed system for DIC determination in natural water The dissolved inorganic carbon (DIC) was selected as a model analyte since the typical amount of DIC appears in some natural waters in the range of 1–10 mmol L−1 [10] which matches with detection ability of the proposed detector. The parameters affecting to the performance of GD-FI coupled with ± 5V BOA based DC conductivity system, such as concentration of acid in a donor line, donor line flow rate and acceptor line flow rate, were studied and the results are shown in Fig. 7. Using 0.1 M H3PO4 or H2SO4 is enough to convert HCO3- to be CO2. From Fig. 7b, the slower donor line flow rate provides longer time of analyte zone in the GDU and the higher signal can be obtained. Therefore, 0.1 mol L−1 H3PO4 with flow rate of 0.5 mL min−1 and DI water without boiling with the flow rate of 1 mL min−1 was selected as the optimized condition for donor and acceptor lines, respectively, for
Acknowledgements The authors would like to thank the Research Fund for DPST Graduate with First Placement (IPST), International Foundation for Science (IFS) (Grant No. W-5094-1), Thailand Research Fund (TRF), and Center of Excellence for Innovation in Chemistry, Chiang Mai University for financial support. 304
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(2009) 245–250. [14] M. Sanada, M. Oshima, S. Motomizu, Assembly of a new gas-diffusion unit and its application to the determination of total carbonate and ammoniacal nitrogen by FIA, Bunseki Kagaku 42 (1993) T123–T128. [15] M. Oshima, Y. Wei, M. Yamamoto, H. Tanaka, T. Takayanagi, S. Motomizu, Highly sensitive determination method for total carbonate in water samples by flow injection analysis coupled with gas-diffusion separation, Anal. Sci. 17 (2001) 1285–1290. [16] A. Perez-Ponce, S. Garrigues, M. Guardia, Microwave-assisted vapour-generation Fourier transform infrared spectrometric determination of carbonate in waters, Anal. Chim. Acta 358 (1998) 235–243. [17] A. Perez-Ponce, S. Garrigues, M. Guardia, Determination of carbonates in waters by on-line vapor generation FTIR, Vib. Spectrosc. 16 (1998) 61–67. [18] S.L. Fan, F. Qu, L. Zhao, J.M. Lin, Flow-injection analysis for the determination of total inorganic carbon and total organic carbon in water using the H2O2–luminol– uranine chemiluminescent reaction, Anal. Bioanal. Chem. 386 (2006) 2175–2182. [19] X.L. Su, L.H. Nie, S.Z. Yao, A novel gas-diffusion/flow-injection system coupled with a bulk acoustic wave impedance sensor for total inorganic carbonate and its application to determination of total inorganic and total organic carbon in waters, Anal. Chim. Acta 349 (1997) 143–151. [20] D.T. Sawyer, J.L. Roberts, Experimental Electrochemistry for Chemists, John Wiley & Sons, New York, 1979, pp. 156–157. [21] R.P. Taylor, N.H. Furman, Constant current conductance, Anal. Chem. 24 (1952) 1931–1933. [22] L. Kekedy, C. Muzsnay, Direct current conductometry with non-polarizable external electrodes, Fresenius Z. Anal. Chem. 199 (1964) 340–348. [23] O. Hello, Improved capillary direct current cell suitable for conductometric titrations, Anal. Chem. 44 (1972) 646–648. [24] D. Kourilova, K. Slais, M. Krejci, A conductivity detector for liquid chromatography with a cell volume of 0.1 µL, Czech. Chem. Commun. 48 (1983) 1129–1136. [25] D. Qi, T. Okada, P.K. Dasgupta, Direct current conductivity detection in ion chromatography, Anal. Chem. 61 (1989) 1383–1387. [26] I.K. Kiplagat, P. Kubán, P. Pelcová, V. Kubán, Portable, lightweight, low power, ion chromatographic system with open tubular capillary columns, J. Chromatogr. A 1217 (2010) 5116–5123. [27] G.H. Jeffery, J. Bassett, J. Mendham, R.C. Denney, Vogel's Textbook of Quantitative Chemical Analysis, Fifth ed., John Wiley & Sons, New York, 1989, p. 299. [28] K. Grudpan, P. Sritharathikhun, J. Jakmunee, Cost-effective flow injection analysis systems for acetic acid, Lab. Robot. Autom. 12 (2000) 129–132. [29] J.S. Rhee, P.K. Dasgupta, Determination of acids, bases, metal ions and redox species by peak width measurement flow injection analysis with potentiometric, conductometric, fluorometric and spectrophotometric detection, Mikrochim. Acta III (1985) 107–122. [30] K. Toda, H. Kuwahara, S. Ohira, On-site measurement of trace-level sulfide in natural waters by vapor generation and microchannel collection, Environ. Sci. Technol. 45 (2011) (2011) 5622–5628.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.talanta.2017.04.015. References [1] R. Cioni, B. Gambardella, L. Marini, Field determination of total dissolved inorganic carbon (TDIC) in natural waters using an IR analyzer I. Preliminary laboratory tests, Geothermics 36 (2007) 47–62. [2] R. Geldern, M.P. Verma, M.C. Carvalho, F. Grassa, A.D. Huertas, G. Monvoisin, J.A.C. Barth, Stable carbon isotope analysis of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in natural waters – results from a worldwide proficiency test, Rapid Commun. Mass Spectrom. 27 (2013) 2099–2107. [3] J. Junsomboon, J. Jakmunee, Flow injection conductometric system with gas diffusion separation for the determination of Kjeldahl nitrogen in milk and chicken meat, Anal. Chim. Acta 627 (2008) 232–238. [4] P. Jaikang, K. Grudpan, T. Kanyanee, Conductometric determination of ammonium ion with a mobile drop, Talanta 132 (2015) 884–888. [5] L. Monser, N. Adhoum, S. Sadok, Gas diffusion–flow injection determination of total inorganic carbon in water using tungsten oxide electrode, Talanta 62 (2004) 389–394. [6] P. Linares, M.D.L. Castro, M. Valcarcel, Simultaneous determination of carbon dioxide and sulphur dioxide in wine by gas-diffusion/flow-injection analysis, Anal. Chim. Acta 225 (1989) 443–448. [7] T. Aoki, Y. Fujimaru, Y. Oka, K. Fujie, Continuous-flow method for the determination of total inorganic carbonate in water, Anal. Chim. Acta 284 (1993) 167–171. [8] F.V. Almeida, J.R. Guimaraes, W.F. Jardim, Measuring the CO2 flux at the air/ water interface in lakes using flow injection analysis, J. Environ. Monit. 3 (2001) 317–321. [9] V. Martinotti, M. Balordi, G. Ciceri, A flow injection analyser conductometric coupled system for the field analysis of free dissolved CO2 and total dissolved inorganic carbon in natural waters, Anal. Bioanal. Chem. 403 (2012) 1083–1093. [10] C. Henríquez, B. Horstkotte, V. Cerdà, A highly reproducible solenoid micro pump system for the analysis of total inorganic carbon and ammonium using gasdiffusion with conductimetric detection, Talanta 118 (2014) 186–194. [11] Z. Hohercakova, F. Opekar, A contactless conductivity detection cell for flow injection analysis: determination of total inorganic carbon, Anal. Chim. Acta 551 (2005) 132–136. [12] S. Pencharee, P.A. Faber, P.S. Ellis, P. Cook, J. Intaraprasert, K. Grudpan, I.D. McKelvie, Underway determination of dissolved inorganic carbon in estuarine waters by gas-diffusion flow analysis with C4D detection, Anal. Method 4 (2012) 1278–1283. [13] K. Tian, P.K. Dasgupta, A permeable membrane capacitance sensor for inorganic gases application to the measurement of total organic carbon, Anal. Chim. Acta 652
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An exploiting of cost-effective direct current conductivity detector in gas diffusion flow injection system Wasin Somboota, Jaroon Jakmuneea,b, Tinakorn Kanyaneea,b, a b
MARK
⁎
Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Center of Excellence for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
A R T I C L E I N F O
A BS T RAC T
Keywords: Direct current conductometry Gas diffusion flow injection Dissolved inorganic carbon Natural water
In this work, a homemade direct current (DC) conductivity detector as an alternative cost-effective detection device has been fabricated and investigated to use in flow analysis system. Under the selected appropriate conditions of flow system, the electrolysis of a carrier stream at the conductivity detector was negligible and provides well-defined signal. The cost-effective DC conductivity detector was demonstrated to couple with gas diffusion flow injection system for determination of dissolved inorganic carbon (DIC) in water. The method is based on the conversion of DIC (dissolved CO2, HCO3- and CO32-) presented in the injected sample to carbon dioxide in an acidic donor stream and then CO2 gas diffuses through a hydrophobic porous membrane to an acceptor stream. As a result, the change of conductivity signal was observed corresponding to DIC concentration. A linear calibration range of DIC in 1.0–10 mmol L−1, with limit of detection of 70 µmol L−1, repeatability of < 3% RSD and 15 injections h−1 sample throughput can be obtained. This method was applied for DIC determination in natural water.
1. Introduction CO32-/HCO3-/CO2(g) in carbon dioxide cycle as total inorganic carbon(TIC) or dissolved inorganic carbon (DIC) in natural water system is important species which the environmental scientist needs to monitor. Total DIC is one major chemical component in geothermal water [1] and particular interest in environmental, biological, and geological studies [2]. Many analytical methods employ the conversion of analyte as gas convertible species in sample and liberate the gaseous product out from the complex matrices of sample through a gas diffusion unit. For example, a membrane based gas diffusion-flow injection (GD-FI) system with conductivity detector has been reported such as NH4+ determination from Kjedahl digestion [3]. Recently, Pheeraya et al. applied the liquid drop as micro membraneless gas absorber with miniature conductivity probe to measure the change in conductivity of sulfuric acid liquid drop due to the reaction with NH3 gas converted from NH4+ ion in natural water [4]. Several methods of DIC determination in geothermal or natural water usually employ the gas diffusion unit to convert dissolved carbonate to CO2. Since, the flow injection analysis system provides fast chemical analysis, it was utilized to couple with gas diffusion unit for determination of DIC with various detection techniques. Monser et al. employed the tungsten oxide wire as pH sensor in the GD-FI
⁎
system for total inorganic carbon determination in water [5]. In addition, the simultaneously determination of CO2 and SO2 in wine was sensitive performed by GD-FI coupled to two detection units: a pH sensor and a spectrophotometer [6]. The pulse conductometric detector as a sensitive and simple technique is also used to couple with GDFI system for total inorganic carbon determination in water [7–10]. In addition, after the discovery of using contactless conductivity detector (CCD), it was applied to couple with GD-FI system for total dissolved inorganic carbon [11,12] and total organic carbon determination [13]. Moreover, spectrophotometric detectors with some colorimetric dyes [14,15], FT-IR [16,17], chemiluminescence [18] and bulk acoustic wave [19] coupled with GD-FI system have been used for DIC determination. The conductometry is one of simple and sensitive electrochemical techniques. Although low cost conventional conductivity detector as pulsed format system has been sold in market or can be built up inhouse as low cost pulse circuit equipment, the simpler pulseless circuitry would be an alternative detection system in budget limit laboratory. The principle of direct current (DC) conductivity detector or pulseless conductivity detector was also mentioned for chemical analysis applications [20]. Once the cell was designed to negligible polarization effect during the measurement and the generated gas bubble was removed from electrode in appropriate hydrodynamics
Corresponding author at: Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand. E-mail address: [email protected] (T. Kanyanee).
http://dx.doi.org/10.1016/j.talanta.2017.04.015 Received 31 January 2017; Accepted 7 April 2017 Available online 09 April 2017 0039-9140/ © 2017 Elsevier B.V. All rights reserved.
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conditions, the DC- conductometry is comparable to pulse conductivity measurement. The four electrodes cell design of using 540 V DC was applied through platinum electrode in titration cell with vigorously stirred solution and the voltage drop across pair of tungsten electrodes was monitored [21]. The conductance of solution was measured by using a high impedance voltmeter. Various chemical reactions including neutralization of weak/strong acid in a conductometric titration were investigated. With the stirred solution condition, the fluctuation of the measurement signal due to gas bubble evolved from electrolysis process was negligible. Moreover, the conductometric cell with two external non-polarizable redox electrode was separated from analytical solution compartment by using ion exchanged membrane [22]. The current passing through the cell depended on the applied potential and solution conductivity. The proposed cell was applied to use with acidbase conductometric titration system. Another design with two compartments of measuring solution and the reference solution connected together by using 2-cm capillary glass tube to decrease the polarization effect in a DC conductivity measurement [23]. The proposed conductivity cell was applied to test with mixed acid-base titration and Ca2++ Mg2+ titration with EDTA. In addition, the DC-conductivity detector was investigated to use in packed column liquid chromatography for separation of some organic acids [24] with 0.1 µL cell volume. The detector provides good signal to noise ratio without observed polarization effect with limit of detection of 0.67 µmol L−1 acetic acid. Qi et al. studied some characteristics of a DC-conductivity detector and utilized it in ion chromatography [25]. It was found that current signal depended on a reciprocal of cell resistance and an applied voltage, and the DC conductivity measurement provided the same performance as a conventional pulse conductivity detector. For on-site measurement, the miniature detector was required to be developed as a portable system. Although, the contactless conductivity detector [11] and OEM capacitance sensor board [26] were demonstrated to work as a detection unit in some portable flow analysis systems, the DC conductivity detector will provide the simpler circuitry and can be an alternative detection unit in flow analysis system for down-scaling and on-site measurement application. Although, the applied potential format of DC conductivity detector is akin of single potential amperometry, it does not require any reference electrode or expensive 3-electrodes based potentiostat/electrochemical analyzer. Although the direct applied voltage in DC conductivity measurement may cause electrochemical reactions at the detection cell, gas diffusion flow injection system provide benefit to select the interested component to be detected and would be fit with cost-effective DC conductivity detector. This work aims to utilize a simple, compact and low cost DC conductivity detector to couple with gas diffusion flow injection system for determination of some gas convertible ions. The gas diffusion unit with simple gravity/pressurized propelling flow injection analysis was employed with HCO3- as a model analyte to evaluate the proposed detector. Some characteristic in flow analysis system and potential for field based unit application was described.
Fig. 1. Circuitry of DC conductometers: a) using bridge circuit and operational amplifier, and b) using 78xx IC.
with DI water. Stock standard solution of S2- was prepared by dissolving 0.6503 g of Na2S (Loba Chemie, India) in 100 mL volumetric flask and adjusting volume to the mark with DI water. The exact concentration of a stock solution of standard bicarbonate was determined by titrating with a standard HCl solution [27]. The hot spring water samples were collected from the places around Chiang Mai province. The commercial mineral drinking water samples were also taken to test the method. The water samples were collected and kept in refrigerator at 4 °C. The KMnO4 was added to the sample to remove interference from sulfide/sulfite. The sample was cleaned through 0.22 µm pore size Nylon syringe filter (membrane-solutions) prior to injecting into the GD-FI system. 2.2. Experimental setup The DC conductivity detectors were fabricated in 2 formats, the first one by using a simple voltage regulator IC (7805 or 7815, bought from local electronics shop) and the second one by using bridge circuit with operational amplifier (TL072, Texas instrument) as illustrated in Fig. 1. The response signal from DC conductivity detector was acquired to a computer through a 22-bit A/D card (Emant300, Emant Pte) with 10 Hz sampling rate. The software was written in-house with Visual Basic 6 for data acquisition. The analytical signals of the fabricated DC conductivity detector were compared with a commercial pulse conductivity detector (712 conductometer, Metrohm, Switzerland or EP357 conductivity isoPOD™, EDAQ, www.edaq.com). The Metrohm conductivity detector provides the selectable gain amplification and analog signal out (0–2 V DC) was acquired to a computer through a 22bit A/D card. The conductivity cell was fabricated to test the DC conductivity as illustrated in Fig. 1b with cell constant of 20.8 cm−1. Two of 0.4 mm ID/0.6 mm OD stainless steel tubes each with 1 cm long were used as electrodes. The stainless steel tubes were inserted and fitted in 3 pieces of 1 cm length of Tygon or PEEK tube and let the electrode gap about
2. Experimental 2.1. Reagents and solutions Analytical grade reagents and deionised water were used throughout the experiment. Standard KCl was prepared as a stock solution by dissolving 0.3746 g of KCl (Merck, Germany) in 100 mL volumetric flask and adjusting volume to the mark with DI water. Standard HCO3was prepared by dissolving 0.4222 g of NaHCO3 (Merck, Germany) in 100 mL volumetric flask and adjusting the volume to the mark with DI water. Stock standard solution of NH3 was prepared by dissolving 0.1072 g of NH4Cl (Scharlab, Spain) in 100 mL volumetric flask and adjusting volume to the mark with DI water. Stock standard solution of SO32- was prepared by dissolving 0.6471 g of Na2SO3 (Carlo Erba, France) in 100 mL volumetric flask and adjusting volume to the mark 299
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Fig. 2. Schematic diagram of a) simple FI gas diffusion unit with DC-conductometer and b) conductivity cell.
standard deviation of baseline (S/SDb).
1 mm. The 0.8 mm copper wire from normal electrical cable was coiled on stainless steel tube electrodes. The parts were attached on glass slide and sealed together with epoxy glue. The 1/16″ OD Teflon tube was inserted to connect and fit in Tygon tube of the conductivity cell. Two identical conductivity cells were fabricated and setup as dual cell detectors connected in series in the flow system for comparison the performance of the proposed detector with a commercial pulse conductivity detector. The gas diffusion flow injection system for evaluating the proposed DC conductivity detector consisted of an acidic or basic donor stream to convert ion to gas and DI water as an acceptor stream. The gas diffusion unit was fabricated as described in the previous work [3]. The acrylic block was fabricated with dimension of W×L×T=3×16×4 cm as shown in Fig. A2 in Supporting information. The length of membrane is about 30 cm. This design of gas diffusion unit serves to use with a low cost Teflon tape membrane (which is normally used for plumbing and can buy in a local shop). The hydrophobic surface of Teflon tape separate the aqueous solutions in both sides but only the produced gaseous analyte (CO2 or NH3) can pass through the membrane to the acceptor line. The schematic diagram of flow analysis system is depicted in Fig. 2. The Teflon tube (0.5×1.6 mm IDxOD, IDEX Health & Science LLC) was used throughout the experiment. The 6-ports-2-position injection valve was used for sample/standard injection with using 40 µL injection loop and 20 cm mixing coil length. Two of 50 mL bottles (Scot Duran, Duran group) as of carrier/reagent reservoir were modified to hang on the stand to provide gravity driven propulsion system [28]. Flow rate of the reagent was adjusted according to the relative height of the solution level and the end tube orifice. The typical flow rate to use with this system is 1.0 mL min−1 of DI water stream. In some experiments, the pressurized propulsion system using constant pressure of nitrogen gas through adjustable pressure regulator was used to replace the gravity flow system. The flow rate of solution was controlled according to the pressure and size of tubing. The carrier/acceptor stream flow rates used in the manifold were optimized to provide the better ratio of signal/
3. Results and discussion 3.1. Some characteristic signal of the fabricated DC Conductometer in FI system This conductivity detector approach is based on the electrolytic reaction of water at the small electrodes with about 3–18 V DC applied voltage. The trace concentration of inorganic salt in water stream would make the change of conductivity of the DI-water and as a result increase the current flow in the electrolytic cell. Therefore the relative electrolytic current would relate to the inorganic salt concentration. At appropriate movement of measuring solution such as in a flow cell or vigorously stirred solution [21–23], the gases bubble from such electrolytic reaction can be removed from the conductivity cell efficiently. The direct current electrolytic cell can play a role as conductivity detector for some inorganic salts. Although, the true conductance signal can not be measured precisely, the change of relative current when analyte zone flowed through the conductivity cell would provide the relative peak signal for quantitative chemical analysis. The peak height signal of DC conductivity detector with operational amplifier depends on the applied DC potential to the electrodes as suggested and explained by Qi et al. [25]. Curiously, using the higher applied voltage, the higher peak current would be obtained but it could generate higher amounts of gas bubble due to electrochemical reaction occurring at the electrode surface. The gas bubble generation would make the fluctuation of response signal and standard deviation of baseline would increase. The limit of detection for quantitative chemical analysis application would be limited by the baseline signal fluctuation. Therefore, the ratio of signal/standard deviation of baseline (S/SDb ratio) was defined to evaluate or compare the experimental condition to accomplish the best response. In addition, a well-defined peak shape in FI-gram is also considered to compare different designs of the 300
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Table 1 Comparison of performance of fabricated detectors with 10–100 µM KCl injection in DI water stream. Detector
Linear relationship equation
DC bridge ± 5 V DC bridge ± 9 V 7805 IC 7815 IC Metrohm 712***
Y=2.31X+16.8, Y=8.39X+29.9, Y=0.09X+0.28, Y=0.65X+1.26, Y=10.1X+42.1,
r2=0.9995 r2=0.9914 r2=0.9983 r2=0.9963 r2=0.9997
Standard deviation of baseline signal (SDb*) (mV)
Peak height/SDb ratio**
0.5 0.7 0.1 0.2 2.8
83.3 98.0 12.3 38.3 50.9
± 2.2 ± 4.2 ± 0.1 ± 0.9 ± 0.8
*** The commercial pulse conductivity detector generates the analog signal corresponding to conductivity (0–2 V) and acquired to computer through external DAQ. The device allows user to change the factors to amplify an appropriate analog signal. * SDb is standard deviation of baseline signal of 200 data points. ** The ratio of peak height of 10 µM KCl and SD of baseline signal (average ± SD). The SD=standard deviation of triplicate injections.
conductivity detector. In this experiment, the DC conductivity detector was fabricated as two formats; i) using resistor bridge and operational amplifier (hereinafter call BOA) as suggested by Qi et al. [25] and ii) using the 78xx voltage regulator IC, as depicted the circuitry in Fig. 1. The BOA conductivity signal was amplified and converted to voltage according to feedback resistor of the FET input operational amplifier IC (TL072). Two applied voltages ( ± 5V DC and ± 9V DC) were tested and compared with a commercial pulse conductivity detector. In another format with using 7805 or 7815 regulator IC, the voltage drop at the 200 kΩ-resistor was measured without any further amplification and it would correspond to the current flow resulting from the change of solution conductance in the cell. The standard solution of KCl was injected into a stream of DI water. The conductivity signals in various conditions such as various applied voltages, and flow rates of carrier were investigated. All fabricated detectors provide the linear relationship of peak height versus concentration in the range of 10–100 µmol L−1 KCl and the S/SDb were compared as shown in Table 1. The BOA based DC conductivity with using ± 5V-DC gave a well-defined FI-gram as demonstrated in Fig. 3. Although, it provided lower peak height of KCl solution compared with using the ± 9V-DC as demonstrated in Supporting information (Fig. A3) and a model 712-Metrohm conductometer, it gave the best S/SDb. The circuit with using 7805 or 7815 IC is the simple setup and awell-defined FI peak can be obtained but peak height of KCl solution is not high as BOA based circuit. Therefore, the ± 5V-BOA DC conducitivity detector was selected for further experiment. With using ± 5V-BOA based DC conductivity circuit, the reproducibility of signal was evaluated by injecting the standard KCl 60 µmol L−1 in a single line FI system with flow rate of DI water carrier of 1.0 mL min−1. The relative standard deviation (%RSD) of the peak height obtained is about 3% (with 40 injections). Flow rate of the carrier affects to conductivity peak height signal due to it promotes the mass transfer of water/H+ to the electrode and possibility of removal of the gas bubble from the conductivity cell as shown the result in Fig. 4. By injecting 1, 10 and 100 µmol L−1 of KCl into a water stream with flow rate of 0.1–1.5 mL min−1, the flow rate 0.1 mL min−1 provides the highest signal meanwhile flow rate 1.5 mL min−1 provides the lower standard deviation of baseline signal since the entrapped gas can be removed efficiently at the higher flow rate. Moreover, the voltage regulator IC based circuit was compared with a miniatured commercial pulse conductivity detector (IsoPod™, EDAQ conductivity detector, with dimension: 10.8×5.8×3.5 cm). The 200 mVpp pulse was applied to the conductivity cell and the measuring signal was acquired to a computer through USB port and a vender software. By injecting 0.1–1.9 mmol L−1 HCO3- into a single line FI system of DI water as a carrier stream, it was found that EDAQ Conductivity detector provides S/SDb of 0.1 mmol L−1 HCO3- of 2.69 while the 7805 IC-based DC detector provides S/SDb of 14.10. After smooth curve with moving average function in Microsoft Excel software, the S/SDb of 7805 IC based conductivity circuit is better.
Fig. 3. The FI-gram of 10–100 µmol L−1 KCl with DC conductivity detector and pulse conductivity detector (712, Metrohm) which were serially arranged in a stream of DI water (injecting 30 µL with 1.0 mL min−1 flow rate).
Fig. 4. The relative peak height signal of KCl injection and standard deviation of baseline signal with various flow rates of DI water stream using ± 5V bridge-operational amplifier DC conductivity detector.
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Fig. 5. Typical FI-GD grams and calibration curves with using DC conductivity detector. a) 1.0–10 mmol L−1 HCO3- b) 0.1–1.0 mmol L−1 NH4+ c) 1–10 mmol L−1 S2- (40 µL injection with 20 cm long-mixing coil, 1.0 mL min−1 flow rate of both DI water as an acceptor line and 10 mmol L−1 H3PO4 as a donor line).
mode has been selected to test as a simple DC conductivity detector unit. The 10–100 µmol L−1 KCl was injected into the DI water stream. The peak signal can be obtained and the peak height relates to the concentration of KCl. Although, the sensitivity of this device is not high as the fabricated DC circuit due to the lower applied voltage from the ohmmeter probe, it can be applied for other applications such as timebased peak width measurement in flow injection system [29].
In addition, a digital multimeter has been tested as a simple DC conductivity detection unit. The digital multimeter in an ohmmeter mode has been designed to measure the resistance of solid materials by applying DC voltage and measuring the returned current that corresponds to the resistance of tested materials. The resistance which is the reciprocal of conductance could be acquired to a computer through RS232-PC interface without circuit modification and could be applied to be a compact detection unit in flow analysis system. In this experiment, the low cost digital multimeter with RS232 interface to PC system (Unit-T 60A, Uni-trend Technology) operated in ohmmeter
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Fig. 6. Optimization of an acceptor stream in GD-FI-DC conductometric system for HCO3- with using 1.0 mL min−1 of 0.1 mol L−1 H3PO4 as a donor stream: a) signal of 10 mmol L−1 HCO3- in 1.0 mL min−1 of various solution types of an acceptor stream, b) signal of 5 mmol L−1 HCO3- in various flow rates of DI-water acceptor stream (0.1 mol L−1 H3PO4 with 1.0 mL min−1 donor stream).
3.2. DC-conductivity detector coupled with gas diffusion flow injection system
Fig. 7. Optimization of acid donor stream in GD-FI-DC conductivity system for HCO3with 1.0 mL min−1 of DI water was used as an acceptor stream: a) Injection of 10 mmol L−1 HCO3- in various acid concentrations, b) Injection of 5 mmol L−1 HCO3in various flow rates of 0.1 mol L−1 H3PO4 as a donor stream.
The ± 5V-DC BOA based conductivity circuit was selected to couple with gas- diffusion flow injection (GD-FI) system as explained in an experimental section. The gas diffusion unit (GDU) composes of 2 lines; donor and acceptor lines as demonstrated in Fig. 2. Both lines were separated with hydrophobic membrane which lets only hydrophobic gas molecule to diffuse pass through it. Some gas convertible ions such as HCO3-/CO2, NH4+/NH3, S2-/H2S would be detected with this system. For HCO3- detection, the donor line is an acidic solution to convert the analyte to be CO2 gas. The H2SO4 or H3PO4 can be used. But the H3PO4 was reported as the lowest volatile acid [30] and was selected to use in this work. The produced CO2 in a donor line would diffuse pass through the hydrophobic Teflon tape membrane and as a result would be trapped in an acceptor line. The conductivity of solution in the acceptor line would be changed corresponding to the concentration of the injected HCO3-. By this concept, the slower flow rate of the acceptor line would provide higher signal as discussed in Section 3.3. With using DI water as an acceptor line for the GD-FI with DC conductivity detector, the well-defined FI-gram of HCO3- injection can be obtained. The linear relationship of DC conductivity signal vs concentration of 1.0–10 mmol L−1 of standard HCO3- can be obtained as depicted in Fig. 5a with standard calibration equation of y (mV) =6.67[HCO3-]+6.33, where y is expressed in mV and the concentration of HCO3- in mmol L−1, r2=0.996. The standard solution of S2- was also injected to the same GD-FI system with a stream of H3PO4 solution in a donor line and the acceptor line is DI-water. The signal of S2- can be
Table 2 The recovery of DIC determination in water samples with the proposed conductivity detector. Sample
HCO3- added
DIC Found* (mM)
% Recovery
Hot spring water 1
0.0 1.0 3.0
4.18 5.18 6.97
± 0.17 ± 0.14 ± 0.08
100 93
0.0 1.0 3.0
4.32 5.34 7.32
± 0.00 ± 0.11 ± 0.05
102 100
0.0 1.0 3.0
4.23 5.24 7.04
± 0.03 ± 0.05 ± 0.02
101 94
0.0 1.0 3.0
4.26 5.28 7.28
± 0.07 ± 0.05 ± 0.11
102 101
0.0 1.0 3.0
2.37 3.40 5.44
± 0.06 ± 0.10 ± 0.05
103 102
Hot spring water 2
Hot spring water 3
Hot spring water 4
Mineral water
*
303
The value is average ± SD of triplicate results.
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FI-GD coupled with DC conductometer for total DIC determination. The linear relationship of measuring signal vs. 1–10 mmol L−1 HCO3can be obtained with sample throughput of 15 injections h−1 and < 3% RSD. The limit of detection based on 3 times of standard deviation of blank is 70 µmol L−1. The proposed cost-effective DC conductivity detector would provide for determination of other gas convertible species such as ammonium as well. Typically, the hot spring water usually contains sulfide or sulfite which can be converted to be gas in acidic condition and can be significant candidate interferences. The sample was treated to eliminate sulfide or sulfite interference by adding KMnO4, Cr(IV), or H2O2 [7]. The successful removal of sulfide/sulfite by using 5 mmol L−1 KMnO4 is demonstrated in Fig. A6 in Supporting information. The DIC determination with sulfide/sulfite removal with KMnO4 adding into hot spring water samples is shown in Table 2. The validation of accuracy of the proposed system was tested by spiking the standard HCO3- into the water sample and recovery of HCO3- determination was calculated. In addition, the DIC determination by using the proposed detector was compared with using the pulsed conductivity detector as the standard method by serial connection the identical conductivity cells in a flow system as demonstrated in Supporting information (Fig. A1). The DIC results from both detectors were shown in Table 3. The linear range of calibration, limit of detection, %RSD, and sample throughput of the proposed system as compared with previous FI-GD system is shown in Table A1 in Supporting information. The proposed analysis system can be an alternative method for DIC determination in natural water.
Table 3 Comparison of dissolved inorganic carbon determination by using proposed detector and pulsed conductivity detector. Samples
Hot spring water-1 Hot spring water-2 Hot spring water-3 Hot spring water-4 Hot spring water-5 Hot spring water-6 Mineral water-1 Mineral water-2 *
Dissolved inorganic carbon (mM)* DC conductivity detector
Pulsed conductivity detector
7.26 7.72 7.76 8.55 9.21 9.58 4.35 5.81
6.94 7.41 7.35 8.56 9.24 9.60 4.59 6.11
± 0.30 ± 0.05 ± 0.15 ± 0.04 ± 0.20 ± 0.19 ± 0.13 ± 0.19
± 0.20 ± 0.08 ± 0.18 ± 0.36 ± 0.19 ± 0.12 ± 0.11 ± 0.13
The value is average +/− SD of triplicate results.
detected with DC conductivity detector with y=3.92[S2-]+5.76, where y is expressed in mV and the concentration of sulfide in mmol L−1, r2=0.986 as shown in Fig. 5c. Moreover, the proposed detector with GD-FI system has been tested with 0.1–1.0 mmol L−1 of standard NH4+ injection. By changing the donor stream to be 0.1 mol L−1 NaOH, NH4+ ion can be converted to NH3 gas with DI-water as an acceptor. Without optimization of flow rate in both lines, the good peak signal can be obtained as demonstrated in Fig. 5b with y=95.04[NH4+]+5.76, where y is expressed in mV and the concentration of ammonium in mmol L−1, r2=0.999. Hence, the results confirmed that the GD-FI system coupled with DC conductivity detector could be used for HCO3-, S2-, and NH4+ detection. In addition, for HCO3-/CO2 in GD-FI system, although the DI water was successfully used as an acceptor solution, the NaOH has been reported to use in an acceptor line to provide a better signal [10]. The mild basic solution of 10 or 25 µmol L−1 NaOH was investigated to use in an acceptor line. DC-conductivity signals of 10 mmol L−1 HCO3- in GD-FI system with water and NaOH as the acceptor solution were compared as depicted in Fig. 6a. The higher concentration of NaOH, the higher amount of gas bubble generated due to the higher conductivity of NaOH solution, the easier the electrochemical reaction to occur and producing the gas bubble. The system requires higher flow rate of the carrier stream to remove the bubble from the electrodes. As a result, the peak signal of HCO3-/CO2 is not sharp and lower signal. Moreover, the CO2 free boiled DI water was recommended to use as the acceptor solution for HCO3-/CO2 in GD-FI system. The signal of using an acceptor stream with/without boiled DI water was also compared. The relative peak signals from both types of waters were not significantly different. Curiously, the tap water in a laboratory was also tested to use as an acceptor line. The baseline signal was too much fluctuation due to the high level of dissolved chloride and can not provide a good signal. Therefore, DI-water is the best acceptor solution for this detector.
3.4. Benefit of using the proposed detector The proposed DC conductivity detection system is extremely low cost detection unit about 120$ USD (including data acquisition unit), and it is easily fabricated without handle with frequency generator and pulse rectifier circuits. In addition, a 9 V-battery (model Energizer Max) can be employed to replace desktop power supply. The detector shows low energy consumption as indicated by continuous operation of the system for 8 h, the battery voltage decreased about 0.8 V. (9.6– 8.8 V.) with repeatability of conductivity signal of KCl about 4.5%RSD (Fig. A5). This would support availability for a portable unit with a simple circuit. The data acquisition board in this work can be changed to be others that can provides the Bluetooth signal to connect with a smart phone or tablet computer. As a result, the proposed system provides the potential to develop for on-site based equipment. The forthcoming work as a portable unit with DC conductivity detector is in progress.
4. Conclusion The fabricated DC conductivity detector shows good performance to use in flow injection analysis coupled with gas diffusion unit for dissolved inorganic carbon determination in natural water. This conductivity detector could be an alternative detection system with simple pulseless circuitry fabrication, low energy consumption, extremely low cost and compact conductivity detector, and possible for portable measuring system. This detector with gas diffusion flow injection system would be applicable to other gas convertible compounds such as NH4+/NH3.
3.3. Application of the proposed system for DIC determination in natural water The dissolved inorganic carbon (DIC) was selected as a model analyte since the typical amount of DIC appears in some natural waters in the range of 1–10 mmol L−1 [10] which matches with detection ability of the proposed detector. The parameters affecting to the performance of GD-FI coupled with ± 5V BOA based DC conductivity system, such as concentration of acid in a donor line, donor line flow rate and acceptor line flow rate, were studied and the results are shown in Fig. 7. Using 0.1 M H3PO4 or H2SO4 is enough to convert HCO3- to be CO2. From Fig. 7b, the slower donor line flow rate provides longer time of analyte zone in the GDU and the higher signal can be obtained. Therefore, 0.1 mol L−1 H3PO4 with flow rate of 0.5 mL min−1 and DI water without boiling with the flow rate of 1 mL min−1 was selected as the optimized condition for donor and acceptor lines, respectively, for
Acknowledgements The authors would like to thank the Research Fund for DPST Graduate with First Placement (IPST), International Foundation for Science (IFS) (Grant No. W-5094-1), Thailand Research Fund (TRF), and Center of Excellence for Innovation in Chemistry, Chiang Mai University for financial support. 304
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