DSpace at VNU: Automated dual capillary electrophoresis system with hydrodynamic injection for the concurrent determination of cations and anions

G Model ACA 233290 No of Pages Analytica Chimica Acta xxx (2014) xxx–xxx Contents lists available at ScienceDirect Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca Automated dual capillary electrophoresis system with hydrodynamic injection for the concurrent determination of cations and anions Thi Thanh Thuy Pham a,b , Thanh Duc Mai a,b , Thanh Dam Nguyen b , Jorge Sáiz c , Hung Viet Pham b, ** , Peter C Hauser a, * a University of Basel, Department of Chemistry, Spitalstrasse 51, Basel 4056, Switzerland Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, Nguyen Trai Street 334, Hanoi, Viet Nam c Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering – University of Alcalá, Ctra Madrid-Barcelona km 33.6, Alcalá de Henares, Madrid 28871, Spain b H I G H L I G H T S G R A P H I C A L A B S T R A C T  Concurrent determination of cations and anions was carried out by electrophoretic separation  Optimized conditions for each class of analystes was possible by using separate capillaries  Simultaneous hydrodynamic injection was carried out  Pneumatic actuation was used for flushing and sample handling  The denitrification of drinking water was successfully demonstrated A R T I C L E I N F O A B S T R A C T Article history: Received March 2014 Received in revised form 15 May 2014 Accepted 25 May 2014 Available online xxx The capillary electrophoresis instrument developed for the concurrent determination of cations and anions features two separate capillaries and individual detectors to allow independent optimization for each group of ions The capillaries are joined in a common injector block The sample is drawn into the injector with a small membrane pump and automated simultaneous injection into both capillaries is achieved by pressurization of the fluid with compressed air Flushing of the injector and of the capillaries with the background electrolyte is also carried out automatically by the same means The buffer consisted of 12 mM histidine and mM 18-crown-6 adjusted to pH with acetic acid and was suitable for the contactless conductivity detection employed The system was optimized for the determination of À cationic NH4+ and anionic NOÀ and NO2 , and linear calibration curves from about 20 mM up to about 1.5 mM were obtained for these ions In a test run over h, the reproducibility for the peak areas was within Ỉ7% For demonstration, the instrument was successfully applied to the concurrent monitoring of the concentrations of the three ions during the biological removal of ammonium from contaminated À groundwater in a sequencing batch reactor, where NOÀ and NO2 are formed as intermediate products ã 2014 Elsevier B.V All rights reserved Keywords: Dual-capillary electrophoresis Capacitively coupled contactless conductivity detection (C4D) Simultaneous separations Cations Anions * Corresponding author Tel.: +41 612671003; fax: +41 61 267 1013 ** Corresponding author Fax: +84 3858 8152 E-mail addresses: phamhungviet@hus.edu.vn (H.V Pham), Peter.Hauser@unibas.ch (P.C Hauser) Introduction Capillary electrophoresis (CE) is a relatively simple method as basically only a capillary and a high voltage power supply are http://dx.doi.org/10.1016/j.aca.2014.05.046 0003-2670/ ã 2014 Elsevier B.V All rights reserved Please cite this article in press as: T.T.T Pham, et al., Automated dual capillary electrophoresis system with hydrodynamic injection for the concurrent determination of cations and anions, Anal Chim Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.05.046 G Model ACA 233290 No of Pages T.T.T Pham et al / Analytica Chimica Acta xxx (2014) xxx–xxx needed for the separation of the analyte ions It is, therefore, possible to construct compact and inexpensive portable instruments for field analysis [1–3] Sample injection into the separation capillary can easily be automated by employing a flow-injection analysis (FIA) front end [4–6] The use of a sequential injection analysis (SIA) manifold as an alternative fluid handling method for capillary electrophoresis [7,8] is a bit more complex, but its higher degree of versatility allows, for example, the implementation of extended unattended monitoring [9], or automated preconcentration [10] In CE the separation of both, cations and anions is possible However, if both types of ions must be determined in the same sample this usually has to be done in two separate runs with opposite polarity of the applied voltage In order to simplify the analysis, i.e to enable concurrent separation of both types of ions, the method of dual opposite end injection has been developed [4,11–17] In this approach, analyte cations and anions migrate in opposite direction through the capillary The need to inject at both ends is, however, a complication It may be overcome by pumping a sample plug from one capillary end to the other before separation [18] Nevertheless, these methods require careful optimization in order to avoid peak overlaps arising from the opposite movement of cations and anions Alternatively, concurrent separation is carried out in two separate capillaries, following simultaneous injection from the same sample container This was demonstrated by Bächmann et al in 1992 [19] employing two fluorescence detectors and simultaneous manual hydrostatic sample injection More recently, Huang et al [20] reported a dual capillary system for the determination of inorganic cations and anions in an aerosol sample However, this system was improvised and was based on two completely separate injections into the two capillaries Gaudry et al [21] reported an automated dual capillary system connected to a manifold based on a peristaltic pump and a miniature piston pump The former was used for flushing of the system, including the capillaries, with background electrolyte, and the latter for sample aspiration followed by concurrent electrokinetic injections into both capillaries from the same sample plug The application of the instrument to the monitoring of inorganic cations and anions in industrial and municipal water samples was demonstrated Contactless conductivity detection (C4D) was employed for both of these systems Among other advantages, one of the features of this detection method is low cost, so that the need for two detectors in dual capillary electrophoresis is not a limitation For fundamental aspects of C4D see, for example, [22–28] Applications of C4D for CE have been described in several reviews [29–32] The alternative automated dual CE system reported herein is also based on contactless conductivity detection, uses a simple pneumatic mechanism for the pumping of background electrolyte and a small membrane pump for sample aspiration The pneumatic pressurization system also allowed the implementation of hydrodynamic injection This is generally preferred to the electrokinetic injection employed by Gaudry et al in their dual capillary system [21] Electrokinetic injection is easier to implement than hydrodynamic injection but suffers from a sampling bias The system was successfully applied to the simultaneous monitoring of the concentrations of NHỵ , NO3 and NO2 during the biological removal of ammonium from contaminated groundwater in Hanoi, Vietnam Experimental 2.1 Chemicals and materials All chemicals were of analytical or reagent grade and purchased from Fluka (Buchs, Switzerland) or Merck (Darmstadt, Germany) For the preparation of the stock solutions (10 mM) of chloride, nitrate, sulfate and nitrite their sodium or potassium salts were used Similarly, those of the inorganic cations (NH4+, Na+, Ca2+, Mg2+, K+, Li+) were prepared from the chloride salts The separation buffer consisted of 12 mM L-histidine (His) and mM 18-crown-6 adjusted to pH with acetic acid Before use, the capillaries were preconditioned with M NaOH for 15 min, 0.1 M NaOH for and deionised water for 10 prior to flushing with the buffer The groundwater contaminated with ammonium was collected from Van Phuc village (Hanoi, Vietnam) Deionised water purified using a system from Millipore (Bedford, MA, USA) was used for the preparation of all solutions and for sample dilution if required 2.2 Instrumentation The solenoid valves were purchased from NResearch (116T021 and 116T031, Gümligen, Switzerland) and the micro-graduated needle valve from Idex (P-470, Oak Harbor, WA, USA) The membrane pump for sample aspiration was obtained from KNF (NF-5-DCB, Balterswil, Switzerland) All fluidic connections were made with 0.02 in i.d and 1/16 in o.d Teflon PFA tubing unless otherwise stated and with 1/4-280 UNF fittings (Idex) The interface accommodating the capillaries and the ground electrode was machined in a PMMA (poly(methyl methacrylate)) block (3 cm  cm  cm) and is a modification of the split injector reported  et al [5] Pneumatic pressurization was achieved with a by Kubán standard cylinder of compressed nitrogen at 200 bar The outlet pressure was adjusted to bar with a regulator The electrophoresis section was based on two dual polarity high voltage power supplies (Spellman CZE2000, Pulborough, UK) with Ỉ30 kV maximum output The high voltage electrodes were contained in insulated cages fitted with safety switches Polyimide coated fused silica capillaries of 50 mm i.d and 365 mm o.d (from Polymicro, Phoenix, AZ, USA) were used for the separations The high voltage ends of the capillaries were isolated with safety cages made from PMMA, which were equipped with microswitches to interrupt the high voltage on opening Detection was carried out with two miniaturized high-voltage C4D cells built in-house For excitation, a sine wave of 400 kHz and 20 Vp–p was produced with a function generator integrated circuit (XR2206, Exar, Fremont CA, USA) This was boosted to 200 Vp–p using purpose-built transformers made from two E 13/7/4, N87 ferrite cores with matching E 13/7/4 coil formers These components were obtained from EPCOS (Munich, Germany) (product nos B66305-G-X187 and B66306-C1010-T1) The amplifiers on the pick-up side (OPA602 and OPA2227) were obtained from Texas Instruments (Austin, TX, USA), and the synchronous detectors (AD630) from Analog Devices (Norwood, MA, USA) The resulting signals were recorded with an e-corder 401 data acquisition system (eDAQ, Denistone East, NSW, Australia) connected to the USB-port of a personal computer 2.3 System control The system was controlled with a personal computer via its parallel port A purpose-built electronic interface allowed switching of the stop-valves, of the 3-port valves and of the high voltage, as well as triggering of the recording of electropherograms The solenoid valves were controlled via a special driver board obtained from the supplier of the valves (CoolDrive, 116D5X12, NResearch) The Forth programming package ProForth for Windows (MicroProcessor Engineering Limited, Southampton, UK) was used to write the control code Different modules were written to independently carry out tasks including flushing of the interface and capillaries, sample delivery, hydrodynamic injection and electrophoretic separations All modules were then assembled together to produce the instruction protocol for the entire analytical method Please cite this article in press as: T.T.T Pham, et al., Automated dual capillary electrophoresis system with hydrodynamic injection for the concurrent determination of cations and anions, Anal Chim Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.05.046 G Model ACA 233290 No of Pages T.T.T Pham et al / Analytica Chimica Acta xxx (2014) xxx–xxx Fig Schematic drawing of the dual-capillary electrophoresis system HV, high voltage; GND, electrical ground; V1, V2, electrically actuated 3-port valves; V3, V4, electrically actuated stop valves; Pt: platinum electrodes 2.4 Biological removal of ammonium from contaminated groundwater A 30 cm (width)  30 cm (depth)  60 cm (height) PMMA sequencing batch reactor (SBR) for biological nitrogen removal with a holding capacity of 54 L was constructed according to the design by Lee et al [33] Aeration was carried out with an aquarium air pump The seed sludge used in this study was taken from an urban wastewater treatment plant in Hanoi and was first cultivated for 15 days in tap-water to which sugar, and NPK fertilizer was added prior to the ammonium removal experiments Each sequence of ammonium treatment in the SBR lasted for h First, the reactor was filled with 30 L of ammonium-contaminated groundwater Subsequently, aeration of the solution inside the reactor was implemented for h using an air pump After this period, brown sugar was added to the reactor to provide an endogenous organic carbon source as recommended by Guo et al [34] Sedimentation was then carried out for one more hour under an anoxic condition Every hour during this 6-h treatment process, a mL aliquot was withdrawn from the reactor, filtered through a 0.45-mm membrane and analyzed without dilution Results and discussion 3.1 System design and operation A schematic drawing of the system is given in Fig The fluid propulsion and handling system was adopted from an earlier design [35] It is based on pneumatic pumping (pressurization of a reservoir of background electrolyte with compressed air) and twoand three-port valves to direct the flow Sample is aspirated into a sample loop, located between two 3-port valves, by a small membrane pump (with dimensions of approximately cm  cm  cm) This is then transported to a split injector block made from PMMA where the ends of both separation capillaries are located Some of the sample plug is pushed into the capillaries hydrodynamically by closing a valve at the exit of the interface to create a backpressure for a controlled length of time The desired backpressure is set with an adjustable needle valve More details can be found in the earlier publication [35] Both capillaries share a common electrical ground electrode for the application of the electrophoresis voltage, which is also located in the injector block The separation voltages are applied at the detection ends of the two capillaries, using two high voltage modules set to either negative or positive polarity for the separation of cations and anions, respectively The capillary ends are placed in buffer vials together with the high voltage electrodes Note that the electrolysis occurring at the electrodes leads to a slow change of the composition of the buffer in these containers This tends to affect the baseline due to the migration of ions into the capillary from the far end For this reason, the electrolyte in these containers needs to be exchanged occasionally This operation has not been automated in the current system For safety, the vials with the high voltage electrodes are enclosed in PMMA cages which are fitted with microswitches to interrupt the power on opening The fact that the high voltages are applied at the detection ends of the capillaries (rather than the injection end as is usually the case) is not a problem with C4D The two detector cells can be positioned freely on the respective capillaries for independent optimization as it is not necessary to remove the polyimide coating at the detection point, as would be necessary for optical detection The detectors were built in-house and are a more compact and less expensive modification of our proven design [36,37] Mechanically, the arrangement has been borrowed from Francisco and Lago [38] and is based on a stack of printed circuit boards which hold the circuitry as well as the tubular electrodes and act as Faradaic shield between the two half cells More details on the mechanical cell setup can be seen in previous publications [39,40] A block diagram of the circuitry is given in Fig and consists of sine wave generator, booster, tubular electrode pair, pick-up amplifier, rectifier, low pass and offset circuitry This has now been implemented completely in surface mount technology, and the cells feature a built-in miniature transformer to boost the excitation voltage to 200 V peak-to-peak (400 kHz) for a high signal-to-noise ratio An entire detector could be housed in a small case of 10 cm length  cm width  cm depth The previous arrangement required a much larger case to contain the electronic circuitry which needed to be separate from the cell containing the electrodes The performance of the new device was found to be comparable to that of our earlier design The operational sequence for the instrument is given in Table The protocol starts with the rinsing of the interface by allowing the flow of the pressurized separation buffer through the sample loop Fig Simplified electronic circuit diagram of the miniaturized high-voltage C4D Please cite this article in press as: T.T.T Pham, et al., Automated dual capillary electrophoresis system with hydrodynamic injection for the concurrent determination of cations and anions, Anal Chim Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.05.046 G Model ACA 233290 No of Pages T.T.T Pham et al / Analytica Chimica Acta xxx (2014) xxx–xxx Table Typical operation sequence Step Operation Duration Flushing the interface Flushing the capillaries Aspiration of sample into the loop Hydrodynamic injection Flushing the interface after hydrodynamic injection Concurrent separations Flushing the interface after electrophoretic separations Flushing the capillaries after separation 15 s 180 s 4s Variable 3s 12 15 s 180 s and the injector block while both stop valves at the outlet are open (designated as V3 and V4) Then the electrolyte is forced through the capillaries for flushing by closing both stop valves at the outlet of the injector block Subsequently, valves V1 and V2 are turned and sample is aspirated into the sample loop by activation of the membrane pump Valves V1 and V2 are then turned back to the original position, and the sample plug is pushed into the injection interface by the pressurized buffer A split injection into both capillaries is then performed by turning on the required backpressure, which is set with the needle valve, by closing only V3 while leaving V4 open The interface is then flushed again to replace the sample by background electrolyte before commencement of the separation Both high voltage power supplies are turned on at the same time for the concurrent separations of the anionic and cationic analytes in the respective capillaries The common electrode in the interface remains grounded at all times Operation V1, V2 V3 V4 High voltage Membrane pump 1 1 1 Open Closed Open Closed Open Open Open Closed Open Closed Open Open Open Open Open Closed Off Off Off Off Off On Off Off Off Off On Off Off Off Off Off À À 3.3 Monitoring of the concentrations of NHỵ , NO3 and NO2 during biological removal of ammonium from contaminated groundwater In Hanoi, groundwater, which is an important source for drinking water, is often contaminated by ammonium [41] Chronic consumption of this contaminated groundwater results in ammonium accumulation in the body, which in turn can lead to the problems of methemoglobinemia in infants and the formation of carcinogenic nitrosamines Biological removal of ammonium from contaminated water using a sequencing batch reactor is a reliable, inexpensive, and simple method [34,42,43], which has been practiced in Hanoi The treatment process consists of two main steps In the first step, NH4+ is microbially oxidized to NOÀ and NOÀ under aerobic conditions In the second step, denitrification of these ions to molecular nitrogen occurs under anoxic conditions A schematic drawing of the small experimental reactor 3.2 Performance A slightly acidic background electrolyte (pH 4), which was based on histidine, acetic acid and 18-crown-6 and had been used successfully for the separation of inorganic cations as well as anions by CE-C4D [9,35], was employed to investigate the performance of the dual CE system At the relatively low pH, the electro-osmotic flow (EOF) is suppressed; therefore, no EOF modification is needed 18-crown-6 was included to facilitate baseline separation of K+ and NH4+ An example of the concurrent analysis of a standard mixture of cations and anions in the two capillaries is shown in Fig As discussed, for example, in [35], a CE-C4D system may be optimized either for fast separations, for low limits of detection or for high separation efficiency, and compromises have to be made In view of the application example discussed below, the system was set up for high separation efficiency by injecting relatively short plugs of sample The calibration data for the three ions of interest (ammonium, nitrite and nitrate) is given in Table For NH4+ linearity up to 2000 mM was achieved For the anions nitrite and nitrate, the linear ranges were somewhat shorter (up to 1500 mM) The correlation coefficients obtained were better than 0.999 for all three ions The reproducibilities of the measurements of peak areas and migration times were better than 5% and around 1%, respectively The system was then set up for a supervised test run over a period of h, during which repeated measurements of the standard mixture were carried out automatically at intervals of 15 The membrane pump also enabled automatic aspiration of the sample for each measurement The results for peak areas are shown in Fig The maximum deviations are less than Ỉ7%, which is deemed acceptable considering that these are due to the accumulation of the errors of all operations, i.e sample loading, delivery, injection, separation and temperature fluctuations A drift in peak areas is not evident from the data for this h run, which demonstrates the suitability of the system for unattended operation Fig Concurrent separations of inorganic anions and cations (A) Cations: NH4+, 50 mM; K+, 75 mM; Ca2+, 75 mM; Na+, 225 mM; Mg2+, 75 mM; Li+, 50 mM (B) Anions: À 2À ClÀ (400 mM); NOÀ (75 mM) Electrolyte: 12 mM (75 mM); NO2 (75 mM); SO4 histidine and mM 18-crown-6 adjusted to pH with CH3COOH Capillaries: fused silica, 50 mm i.d., 40 cm effective length and 55 cm total length Sample loop: 50 mL Gas pressure: 0.8 bar Separation voltage: +15 kV for anion- and –15 kV for cation-separation Please cite this article in press as: T.T.T Pham, et al., Automated dual capillary electrophoresis system with hydrodynamic injection for the concurrent determination of cations and anions, Anal Chim Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.05.046 G Model ACA 233290 No of Pages T.T.T Pham et al / Analytica Chimica Acta xxx (2014) xxx–xxx Table À À Calibration ranges, limits of detection and reproducibilities for the concurrent determination of NHỵ , NO3 and NO2 Electrolyte: 12 mM histidine and mM 18-crown-6 adjusted to pH with acetic acid Ion Linear range (mM)a Correlation coefficient, r2 Limit of detectionb (mM) Reproducibility of peak area (RSD%)c Reproducibility of migration time (RSD%)c NOÀ NOÀ NHỵ 151500 251500 152000 0.9995 0.9997 0.9991 6.0 7.5 5.0 4.5 3.9 3.3 1.1 1.2 1.2 a b c Concentrations Concentrations corresponding to peak heights of times the baseline noise Relative standard deviation in %, n = used for this study is shown in Fig Activated sludge and groundwater were first added to the tank To create the aerobic conditions, air was passed in with a small aquarium pump, and this also led to an effective mixing During the second step, when aeration had stopped, the sludge slowly settled (within about 15 min.), and the supernatant clean water could then be drawn off To monitor the process, the concentrations of the indicative À À nitrogen-ions, namely, NHỵ , NO3 and NO2 , need to be determined À À periodically Analyses of NO3 and NO2 have mostly been carried out with ion chromatography, whereas NH4+ has frequently been determined by spectrophotometry using Nesslers reagent These methods, while working well for discrete samples, are costly and laborious if frequent sampling and determination of these positive and negative ions are needed Here, we propose a simple and inexpensive method for the concurrent determination of NHỵ 4, NOÀ and NO2 using the developed dual-channel CE system The raw groundwater sample and those withdrawn from the biological reactor were filtered and fed into the dual CE system without Fig Stability test The concentrations of the ions and other conditions were as for Fig dilution Note that while the instrument is capable of automated sample aspiration and operation (see Section 3.2), this feature was not made use of for this demonstration because of the high burden with suspended solids during most of the process It would have been fairly difficult to set up reliable on-line filtering NHỵ was determined in one channel and NO and NO2 in the other Electropherograms of groundwater samples taken before, during and after the biological ammonium removal process are shown in Fig As can be seen, the groundwater contained abundant 2+ concentrations of ClÀ, SO2À and Na+ which, of course, stayed , Ca constant during the biological ammonium treatment The centrations of NHỵ , NO3 and NO2 on the other hand varied considerably during the treatment process Plots of the concentrations of these ions over time are given in Fig At the beginning (t = 0), an extremely high concentration of ammonium (1300 mM) was recorded, whereas only a minor amount of nitrite was found The nitrate concentration was below the detection limit As the collected groundwater was under an anoxic environment, nitrogen species in this groundwater should be present in the most reduced form, which is NH4+ rather than the oxidized products NOÀ and + NOÀ A rapid decrease in NH4 concentrations during the first h of the treatment when aeration took place was clearly observed At the same time, the concentration of NOÀ increased accordingly, reflecting the occurrence of nitrification The maximum concentration of NOÀ was observed at 240 mM at the end of the aeration process The concentration of NOÀ , the intermediate product when Fig Schematic drawing of the activated sludge reactor During the treatment process, samples were drawn from tap 2, and at the end, after the sludge had settled, clean water was taken from tap Please cite this article in press as: T.T.T Pham, et al., Automated dual capillary electrophoresis system with hydrodynamic injection for the concurrent determination of cations and anions, Anal Chim Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.05.046 G Model ACA 233290 No of Pages T.T.T Pham et al / Analytica Chimica Acta xxx (2014) xxx–xxx Fig Concentration proles of NHỵ , NO3 and NO2 monitored by CE during the biological treatment of ammonium-contaminated groundwater Other conditions as for Fig Conclusions A dual-channel CE system for the concurrent determination of cations and anions was constructed and successfully demonstrated for the monitoring of biological nitrogen removal from ammonium contaminated groundwater The instrument is inexpensive, simple in construction and can therefore be assembled with little effort The state-of-the-art contactless conductivity detectors can be built with modest expertise in electronics The only item that required engineering workshop facilities was the injection block, but it should be possible to substitute this with commercial capillary connectors [21] Pneumatic actuation proved to be a facile approach to the implementation of hydrodynamic injection, which is essential in order to avoid a bias which otherwise occurs when samples of varying background conductivity are to be analysed Further integration and miniaturization in order to obtain an readily portable instrument and battery operation are possible Acknowledgements Fig Electropherograms of ammonium-contaminated groundwater during treatment (A) Cations; (B) anions The labels on the electropherograms refer to the time delay from the commencement of the treatment The electrophoresis conditions were as for Fig ammonium is oxidized to nitrate, sharply increased in the first hour, but then slightly diminished when the treatment further proceeded In the last hour, when the environment inside the reactor was switched to anaerobic conditions, the NH4+ content À remained almost unchanged while those of NOÀ and NO2 decreased This is because denitrification of the generated NOÀ and NOÀ leads to their conversion to gaseous nitrogen After h of treatment, the ammonium content was decreased by 88%, 73% of which was converted into gaseous nitrogen (calculated based on the molar ratio of decreasing NH4+ to oxidized nitrogen ions NOÀ and NOÀ ) The nitrogen removal efficiency was slightly lower than that achieved with a step-feed sequencing batch reactor in which real-time control of the pH, oxidation reduction potential and dissolved oxygen was implemented in order to optimize ammonium elimination [34] The authors would like to thank the Swiss National Science Foundation (Grant No 200020-137676/1) and 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96 (2005) 7–14 Please cite this article in press as: T.T.T Pham, et al., Automated dual capillary electrophoresis system with hydrodynamic injection for the concurrent determination of cations and anions, Anal Chim Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.05.046 ... Off Off Off Off Off On Off Off Off Off On Off Off Off Off Off À À 3.3 Monitoring of the concentrations of NHỵ , NO3 and NO2 during biological removal of ammonium from contaminated groundwater... not evident from the data for this h run, which demonstrates the suitability of the system for unattended operation Fig Concurrent separations of inorganic anions and cations (A) Cations: NH4+,... in press as: T.T.T Pham, et al., Automated dual capillary electrophoresis system with hydrodynamic injection for the concurrent determination of cations and anions, Anal Chim Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.05.046