DSpace at VNU: Triple-channel portable capillary electrophoresis instrument with individual background electrolytes for the concurrent separations of anionic and cationic species

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Accepted Manuscript
Triple-channel portable capillary electrophoresis instrument with individual
background electrolytes for the concurrent separations of anionic and cationic species
Thanh Duc Mai, Minh Duc Le, Jorge Sáiz, Hong Anh Duong, Israel Joel Koenka,
Hung Viet Pham, Peter C. Hauser
PII:

S0003-2670(16)30117-9

DOI:

10.1016/j.aca.2016.01.029

Reference:

ACA 234369

To appear in:

Analytica Chimica Acta

Received Date: 30 November 2015
Revised Date:

12 January 2016

Accepted Date: 16 January 2016

Please cite this article as: T.D. Mai, M.D. Le, J. Sáiz, H.A. Duong, I.J. Koenka, H.V. Pham, P.C. Hauser,
Triple-channel portable capillary electrophoresis instrument with individual background electrolytes for
the concurrent separations of anionic and cationic species, Analytica Chimica Acta (2016), doi: 10.1016/

j.aca.2016.01.029.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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Triple-channel portable capillary electrophoresis instrument with individual

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background electrolytes for the concurrent separations of anionic and cationic species

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Thanh Duc Mai1, Minh Duc Le1, Jorge Sáiz2, Hong Anh Duong1, Israel Joel Koenka3,

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Hung Viet Pham1*, Peter C. Hauser3*

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University of Science, Nguyen Trai Street 334, Hanoi, Viet Nam

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Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi

Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering,

University of Alcalá, Ctra. Madrid-Barcelona km 33.6, Alcalá de Henares, Madrid, Spain

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University of Basel, Department of Chemistry, Spitalstrasse 51, 4056 Basel, Switzerland

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* Corresponding authors

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e-mail:

; Tel : ++41 61 267 1003; Fax: ++41 61 267 1013
; Fax: ++84 4 3858 8152

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Keywords: Capillary clectrophoresis, Capacitively coupled conductivity detection,

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Concurrent separations, Portable instrument

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Abstract

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The portable capillary electrophoresis instrument is automated and features three independent

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channels with different background electrolytes to allow the concurrent optimized

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determination of three different categories of charged analytes. The fluidic system is based on

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a miniature manifold which is based on mechanically milled channels for injection of samples

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and buffers. The planar manifold pattern was designed to minimize the number of electronic

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valves required for each channel. The system utilizes pneumatic pressurization to transport

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solutions at the grounded as well as the high voltage side of the separation capillaries. The

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instrument has a compact design, with all components arranged in a briefcase with dimensions

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of 45 (w) × 35 (d) × 15 cm (h) and a weight of about 15 kg. It can operate continuously for 8 h

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in the battery-powered mode if only one electrophoresis channel is in use, or for about 2.5 h in

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the case of simultaneous employment of all three channels. The different operations, i.e.

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capillary flushing, rinsing of the interfaces at both capillary ends, sample injection and

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electrophoretic separation, are activated automatically with a control program featuring a

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graphical user interface. For demonstration, the system was employed successfully for the

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concurrent separation of different inorganic cations and anions, organic preservatives,

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additives and artificial sweeteners in various beverage and food matrices.

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1. Introduction

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The concurrent separation of anionic and cationic species in capillary electrophoresis (CE) is

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desirable as it significantly enhances the analytical throughput. The time and effort otherwise

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needed for switching over the task, such as a change of background electrolyte (BGE) and

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capillary conditioning, are then not required. The different techniques for concurrent

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separations have been discussed in a recent review [1]. So far, this has most often been

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realized on a single capillary employing dual opposite end injections where hydrodynamic

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sample injection is realized into both ends of the capillary, with detection carried out near the

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middle of the capillary [1-4]. This technique, introduced by Kubáň and Karlberg [5] and

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Paderauskas and coworkers [6] in 1998, has repeatedly been reported for the simultaneous

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determination of cations and anions with conductometric detection [7-14]. Manual operations

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and the likelihood of peak overlaps when cations and anions appear on the same

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electropherogram are nevertheless some issues of consideration when using this technique. On

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the other hand, the employment of more than one capillary at the same time is not much of a

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complication because of the simplicity and fully electronic principle of CE. The only essential

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components for separation are a capillary and a high voltage power supply module. Dual-

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channel CE, where separations of cationic and anionic species were simultaneously realized

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on two independent capillaries, was first reported by Bachmann et al. in 1992 [15] and has

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more recently been communicated by several research groups [16-19]. The concurrent

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electrophoretic separation of inorganic anions and cations in two channels has also been

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reported for microchip platforms [20-22].

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In the concurrent CE separation approaches reported so far, generally a common BGE was

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employed for electrophoresis of both cations and anions. However, the use of a common

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buffer is not always ideal or even possible. For example, an acidic BGE (pH ≤ 4) used for the

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separation of inorganic cations which may precipitate as hydroxides at higher pH values

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cannot be employed for the separation of carboxylate anions whose full deprotonation occurs

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only at pH ≥ 6 (pH higher than their pKa values). The use of different BGEs in a multi-

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channel CE system can solve this problem. Furthermore, this also allows the independent

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optimization of the electro-osmotic flow (EOF) for different classes of analytes, such as the

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use of a slow EOF for best separation of cations, or of a high EOF against the mobility of

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slowly migrating anions. While not readily possible for a conventional commercial

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instrument, the simplicity of CE nevertheless allows the easy extension to more independent

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channels; and the use of different buffers is not much of an additional complication. Such a

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CE setup, however, has to our knowledge not been reported.

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A further benefit of the straightforward nature of CE is the possibility of its implementation in

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portable instruments. On-site measurements eliminate complications with sample storage and

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transport and offer better cost effectiveness and rapid availability of results. Different portable

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CE instruments relying on the single-channel CE format have been reported in recent years by

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several research groups [23-31]. Earlier developments have been covered in two review

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articles [32, 33]. Recently, Sáiz et al. also reported a portable dual-capillary CE system

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employing a single BGE [19]. Thanks to its positive features of low power consumption, high

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versatility, high degree of miniaturization, lack of requirement for removal of the capillary

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coating to create an optical window, easy alignment and movement along the capillary, and

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ease in construction and operation, capacitively coupled contactless conductivity detection

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(C4D) has been the detection method of choice and can indeed be considered to have been an

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enabling factor in the development of these instruments. Detailed descriptions of the

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construction and working principles of C4D can be found elsewhere [34-41]. Readers can also

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find a variety of recent CE-C4D applications covered in several reviews [42-48]. Herein, to

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the best of our knowledge, for the first time a portable triple-channel CE instrument is

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reported. It employs individual BGEs for the concurrent separation of analytes belonging to

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different catagories.

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2. Experimental

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2.1. Chemicals and Materials

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All chemicals were of analytical or reagent grade and purchased from Fluka (Buchs,

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Switzerland) or Merck (Darmstadt, Germany). Stock solutions (10 mM) of chloride, nitrate,

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nitrite, sulfate and phosphate were used for the preparation of the standards of inorganic

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anions, using their respective sodium or potassium salts. Those of the inorganic cations (10

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mM) (ammonium, potassium, calcium, sodium, manganese, barium, zinc and magnesium)

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were prepared from the chloride salts. Cyclamic acid (cyclohexanesulfamic acid) sodium salt,

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saccharin sodium salt hydrate, aspartame and acesulfame-K were used to prepare standard

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solutions of the four artificial sweeteners. Histidine (His), acetic acid, lactic acid, 2-

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(cyclohexylamino)-ethanesulfonic acid (CHES), tris(hydroxymethyl)aminomethane (Tris), 2-

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(N-morpholino)ethanesulfonic acid (MES) and 18-crown-6 were used for preparation of BGE

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solutions. The separation of inorganic cations was done with a BGE composed of 12 mM

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histidine and 2 mM 18-crown-6 adjusted to pH 3.7 with acetic acid, using a fused-silica

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capillary of 25 µm I.D. (with a total length, Lt, of 60 cm and an effective length, Leff, of 44

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cm). The BGE for the inorganic anions was composed of 12 mM histidine adjusted to pH 4

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with lactic acid, employing a fused-silica capillary of 25 µm I.D. (Lt = 60 cm, Leff = 44 cm).

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For the artificial sweeteners, the separation was realized with a BGE of 100 mM Tris / 30 mM

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CHES, pH 9.1 and with a fused-silica capillary of 25 µm I.D. (Lt = 54 cm; Leff = 38 cm).

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Organic anions were electrophoretically separated in a fused silica capillary of the same

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internal diameter (Lt = 70 cm; Leff = 54 cm) with the BGE composed of 90 mM His / MES

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and 20 µM CTAB, pH 6.1. Voltages of 15 kV with appropriate polarities were used for the

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separations. Before use, the fused silica capillaries were preconditioned with 1 M NaOH for

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10 min and deionized water for 10 min prior to flushing with buffer. The capillaries were then

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used continuously for successive analyses. Deionized water purified using a system from

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Millipore (Bedford, MA, USA) was used for the preparation of all solutions and for sample

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dilution if required. Carbonated soft drink, fruit juice, beer, wine, tea (with milk aded) and

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fish sauce were purchased from local shops in Hanoi, Vietnam. The beverage and fish sauce

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samples were prepared by filtering with a 0.02-mm PTFE membrane filter (Chromafil O-

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20/15 MS, Macherey-Nagel, Oensingen, Switzerland), then diluted with deionized water and

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ultra-sonicated for 10 min prior to CE-C4D measurements.

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2.2. Instrumentation

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The microfluidic manifolds for each separation channel were machined in poly(methyl

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methacrylate) (PMMA) plates with the dimensions of 10 cm (w) x 15 cm (l) x 1.5 cm (h). The

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fluidic channels were milled using a high frequency spindle (R30) from Ray Ltd. (Nänikon,

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Switzerland) at 30'000 rpm with a carbide cutter (Graphograph, Murten, Switzerland) under

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application of a lubricating oil (Rubin G-8 from Neoval, Hofstetten, Switzerland). The cutting

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tool had been modified to yield trapezoidal channels with a bottom width of 0.2 mm, wall

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angles of 18° and a depth of approximately 0.2 mm. The channels were sealed with a second

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PMMA plate of identical dimensions fixed tightly with screws. The valves and pumps were

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mounted on top of the cover plate. Access to the fluidic channels was via perpendicular holes

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in the cover. Connections to tubings were made via female ¼-28 UNF threads. The solenoid

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valves (LFVA0030000C-LFVA1230113H and LFRA0030000C-LFRA1230110H) were

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purchased from the Lee Company (Westbrook, CT, USA). The check valves (CV-3315) were

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obtained from Upchurch Scientific (Oak Harbor, WA, USA). The miniature peristaltic pumps

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(RP-Q1-SP45A) were purchased from Takasago Fluidic Systems (Nagoya, Japan). Fluidic

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connections external to the planar manifold were made with 0.02 inch I.D. and 1/16 inch O.D.

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teflon tubing and with polyether ether ketone (PEEK) flangeless ¼-28 nuts (P-235) and

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ferrules (P-221) (Upchurch Scientific). The polyimide-coated fused silica capillaries (25 µm

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I.D. and 365 µm O.D., from Polymicro, Phoenix, AZ, USA) were fitted to the microfluidic

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manifolds using adaptor sleeves (F-242x, Upchurch Scientific). Pneumatic pressurization was

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achieved with a standard cylinder of compressed air, or with an air pump and a reservoir for

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field measurements [29]. The outlet pressure of either system was adjusted to 1 bar with a

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regulator.

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The electrophoresis section is based on miniature high voltage units (UM20*4) with

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dimensions of 12 cm (l) x 3.8 cm (w) x 2.5 cm (h) and a weight of 200 g each (Spellman

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Pulborough, UK) providing a maximum of 20 kV of pre-selected polarities. The high voltage

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ends of the capillaries were isolated in safety cages made from PMMA, which were equipped

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with microswitches to interrupt the high voltage on opening. The high voltage interfaces were

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machined in PMMA blocks of 2 cm (l) × 2 cm (w) × 3 cm (h) dimensions, and pressurized

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Falcon tubes (Fisher Scientific, Reinach, Switzerland) served for automated buffer

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replenishment. Detection was carried out with three miniaturized C4D cells built in-house

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according to a design reported previously [18, 49]. The resulting signals were recorded with a

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12 V-powered E-corder 201 data acquisition system (eDAQ, Denistone East, NSW, Australia)

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connected to the USB-port of a personal computer. For powering the electrophoretic and

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fluidic parts, a lithium battery pack of 14.8 V and with a capacity of 6.6 Ah (CGR 18650CG

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4S3P, Contrel, Hünenberg, Switzerland) fitted with a voltage regulator was used to provide a

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12 V output. A separate pair of smaller Li-ion batteries with a capacity of 2.8 Ah each (CGR

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18659CG 4S1P, Contrel), which was fitted with positive and negative 12 V regulators,

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provided the split ±12 V supply required for the C4D circuitry.

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2.3. System control

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The system was controlled with a personal computer using a USB (Universal Serial Bus)

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connection to an Arduino Nano microcontroller board (Gravitech, Minden, NV, USA).

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Different outputs of the Arduino Nano board allowed switching of the solenoid valves, of the

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high voltage, and triggering of the recording of electropherograms with the help of a purpose

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built electronic interface. This also allowed the monitoring of the high voltages and

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electrophoretic currents. A versatile graphical user interface (GUI) written in the Python

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programming language was used to control the triple channel CE system. More details on our

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software approach (Instrumentino) can be found in recent publications [50, 51].

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3. Results and Discussion

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3.1. System design and operation

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A simplified schematic drawing of a single channel of the triple-CE system is shown in Fig. 1.

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The three identical channels are based on standard capillaries. For each capillary a separate

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purpose made manifold serves for sample injection and capillary flushing. The injection ends

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of the capillaries are grounded, while the separation voltages are applied at the detector ends,

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which is not a problem with C4D. The high voltage ends of the capillaries are contained in

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PMMA cages to prevent accidental exposure to the separation voltage and also provides the

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necessary insulating air space to prevent spurious discharges. A specially designed interface

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allows automated flushing of the BGE at the capillary outlet. Propulsion of liquids through the

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miniature planar injection manifolds and the high voltage interface was carried out by

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pressurization of the individual BGE reservoirs at both ends of the capillaries. Three miniature

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peristaltic pumps serve to pass the sample to the manifolds for subsequent injection into the

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capillaries. As the three channels are completely separate, the set-up has full flexibility.

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Usually the same sample solution is aspirated concurrently into the three channels, but it is

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also possible to draw different solutions. This feature may be employed if the sample has to be

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pre-diluted differently for different analytes, but it would also be possible to work on different

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samples in parallel using identical conditions on each channel. The detector cells are mounted

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outside the high voltage cages and the capillaries can be moved freely for optimization of

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separation time and efficiency as it is not necessary to remove the polyimide coating of the

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fused silica capillaries.

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A more detailed diagram of the fluidic connections of the injection block for a single channel

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is shown in Fig. 2. For automated injection and flushing two pressure levels (low for sample

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injection, high for capillary flushing) are necessary with an associated number of valves and

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fluidic connections. In order to achieve a triple-channel configuration in a compact and

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portable design with limited power consumption it was desirable to reduce the number of

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electromechanical components required for each CE channel compared to the set-up in our

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earlier systems [18, 29-31]. This was achieved by employing two passive check valves as well

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as a length of narrow tubing to obtain the reduced pressure for controlled sample injection.

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For robustness and in order to limit the number of required tubing connections a planar

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microfluidic manifold was employed, as illustrated in Fig. 2, onto which the different

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components, including separation capillary and ground electrode, were mounted. Two power-

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consuming valves were required for the injection side of each channel, i.e. one stop valve and

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one 3-port valve (a third valve is required for the flushing of the high voltage interface). A

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miniature peristaltic pump was employed for sample aspiration into the injection block.

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Compared to microchip electrophoresis, the ‘marriage’ between microfluidic manifolds and

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standard capillaries for separation offers some advantageous features, including high

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flexibility in separation length, and better separation and detection performance.

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Manufacturing microfluidic manifolds for such a hybrid configuration is easier than CE

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microchip fabrication as the larger dimensions of the manifold allow conventional mechanical

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machining. The entire fluidic block can easily be removed from the system for maintenance if

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required.

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Frequent renewal of the BGE at the HV end of the capillary is important in order to minimize

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effects on the stability of baselines caused by the electrolysis-induced modification of the

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buffer during electrophoresis. The automated buffer replacement at the HV end is not trivial

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because absolute electrical isolation between the HV and other electronic components must be

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guaranteed in order to avoid any possible (air-humidity dependent) arcing which may even

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lead to destruction of part of the electronic instrumentation. So far this advanced option has

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been communicated twice [52, 53]. Horstkotte et al. [52] utilized an auxiliary BGE bearing

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tubing from which the buffer was transferred into the container at the high voltage end of the

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capillary by letting it drop through an air space for insulation. As in our hands this approach

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led to a catastrophic failure due to arcing we previously used an approach in which the

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flushing was achieved through the capillary itself [53]. The limitation of this method was the

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long time (~ 15 min) required to pass a sufficient amount of BGE through the capillary. To

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overcome these problems, a pneumatic method was employed now, using gas pressurization

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to push the fresh buffer into the HV-end for buffer replenishment after each CE separation.

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The integrity of the high voltage safety cage is maintained in this approach, as a separate BGE

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reservoir is contained inside the cage and the gas filled tubing leading into the cage provides

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good electrical insulation. The operation can be realized within 10 seconds and is controlled

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automatically via a separate valve mounted outside the safety cage.

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All components were integrated into an aluminum briefcase with the dimensions of 35 (w) x

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45 (d) x 15 cm (h), and the system had a weight of about 15 kg. The electronic parts were

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positioned underneath a PMMA panel and the fluidic parts were arranged on top for ease of

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operation. The system was divided into three sections. A section on the left contains some

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manual switches, LED indicators and connectors mounted on a panel while the

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microcontroller board, its associated electronics and the rechargeable batteries are located

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underneath. In the middle, the three separated CE channels are arranged one next to another.

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On the right are the pressurized buffer containers and the PMMA safety cages which contain

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the HV interfaces and on which the three C4D cells are located. The internal batteries were

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found to provide sufficient power for typically about 8 h of operation (when one channel was

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in use) or 2.5 h (using all three channels) before recharging was necessary. Direct operation

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from mains power is also possible when available.

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An overview of a typical sequence of the operations of one CE channel is given in Table 1.

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Please consult Figs. 1 and 2 for details of the manifolds. The protocol starts with the rinsing of

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the microfluidic manifolds with electrolyte solution on the opening of V2 and the setting of

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V3 to position 2. Then the capillary is flushed by pushing the solution into the injection

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manifold from both directions on the opening of V2 and switching of V3 to position 1.

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Rinsing of the HV interface is then carried out by pressurizing the HV buffer container inside

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the safety cage on turning V1 to position 2. Upon switching back V3 to position 1, the gas

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flow to the HV buffer container is blocked and the excess gas inside it is released, thus

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stopping the buffer flow to the HV interface. Subsequently, the sample is aspirated through

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the check valve CV1 into the microfluidic manifold by activation of the peristaltic pump

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while V3 remains at position 2. Excess sample is diverted to waste. This process is followed

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by hydrodynamic injection at reduced pressure. The gas pressure is applied onto the solution

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inside the injection manifold while V2 is blocked and V3 is at position 1. Pressure reduction

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is achieved with a piece of narrow tubing positioned right after CV2. The desired injection

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pressure can be achieved by adjusting the diameter and length of this pressure-reducing

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tubing. In our case, a piece of fused silica capillary of 25 µm I.D. and 10 cm length was found

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optimal. During hydrodynamic injection, the sample plug is confined in a closed compartment

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between V2 (at the closed state), V3 (at position 1), CV1 and CV2. The injection manifold is

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then flushed again with BGE before turning on the separation voltage. To prevent any

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pressure induced during electrophoresis, V2 is closed and V3 stays at position 2. The

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electrode in the interface remains grounded at all times.

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3.2. Performance

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To evaluate the performance of the triple-channel CE system, different categories of ionic

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species were determined simultaneously on its three channels. The tested standards include i)

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inorganic cations (channel 1), ii) inorganic anions or artificial sweeteners (channel 2) and iii)

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carboxylate anions (channel 3). Inorganic cations and anions can be analyzed with CE-C4D at

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acidic conditions [29, 53]. A BGE containing 12 mM histidine and 2 mM 18-crown-6

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adjusted to pH 3.7 with acetic acid offered good separations of inorganic cations. 18-crown-6

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was included to facilitate baseline separation of K+ and NH4+. For the separation of inorganic

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anions, a BGE composed of 12 mM histidine adjusted to pH 4 with lactic acid was selected.

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At this relatively low pH-value the EOF is suppressed; therefore no EOF modification is

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needed. The separation of weak organic anions (carboxylates) with CE on the other hand has

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to be implemented at a relatively high pH to assure complete dissociation. Under those

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conditions the EOF is strong, and an EOF modifier is usually added to obtain parallel

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electrophoretic and electro-osmotic movements. The BGE for the CE-C4D separation of weak

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organic anions was adapted from that reported by Mai et al. [54], and is composed of 90 mM

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His / MES (pH 6.1) and 20 µM CTAB (a dynamic EOF modifier). For separation of four

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artificial sweeteners, namely aspartame, cyclamate, saccharin and acesulfame K, a strong EOF

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at high pH could be employed to sweep these weak anionic species towards the cathode by the

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electroosmotic flow against their electrophoretic mobility [55]. The optimized BGE for

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determination of these artificial sweeteners, adapted from [55], was composed of 100 mM

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Tris / 30 mM CHES (pH 9.1). Examples for the separation of standard mixtures of the four

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classes of species are shown in Fig. 3 and the calibration data for the ions of interest is given

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in Table 2. The correlation coefficients obtained were better than 0.992 for any category of

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tested ionic standards. The reproducibilities of the measurements of peak areas and migration

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times were better than 6 % and around 2 % respectively, which is comparable to the

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performance obtained with the conventional single-channel CE approach.

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3.3. Applications

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The triple-channel CE instrument was then demonstrated for the separation of different

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primary quality-indicating ionic species in wine, beer and tea samples. The taste of these

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drinks is much dependent on their contents of inorganic ions and weak carboxylates. Indeed,

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low molecular weight organic anions are the major ionic components in several beverages

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[56-58]. Electropherograms for the concurrent separations of inorganic cations (channel 1),

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inorganic anions (channel 2) and organic anions (channel 3) in these samples are shown in

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Fig. 4. The major inorganic cations K+, Na+, Mg2+ and Ca2+ and the major inorganic anions

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Cl- and SO42- were found in all samples. In addition, NH4+ and NO3- were found in the wine

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and beer samples respectively. Different carboxylates were detected, with lactate present in all

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samples. Tartrate, a carboxylate typically found in wine, was also found in the tea sample.

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Pyruvate, which is a frequently found compound in wine and beer [56, 57], was also detected

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in these samples. The number of anions present in the tea sample is significantly lower than

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that in the other beverages.

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The triple-channel CE system was further demonstrated for content control of artificial

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sweeteners used in other drink and in fish sauce samples. As the sweeteners are prepared by

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chemical synthesis, their presence in food and beverage is the cause of extensive consumer

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mistrust [59]. In Vietnam, these compounds may be added deliberately to the drinks or fish

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sauce instead of the normally used natural sugar to reach the desired sweet taste. Aspartame,

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cyclamate, saccharin and acesulfame K are most frequently used, especially in combination in

326

order to mask undesired aftertastes, such as bitterness [60]. The exact composition used in

327

these mixtures is important in order to correctly balance the tastes. Together with inorganic

328

cations and organic anions, the artificial sweeteners in cola beverage, mango juice and fish

329

sauce samples were determined with the triple CE-C4D system. Electropherograms obtained

330

from each CE channel are shown in Fig. 5. Saccharin was found in the fish sauce and cola

331

samples, with its content in the former being much higher than in the latter. Cyclamate was

332

used in the cola sample, whereas acesulfame K was the artificial sweetener that had been

333

added to the mango juice. Many other organic anions, including malate, succinate, citrate,

334

acetate, ascorbate and lactate were also detected, with succinate found in all samples. Very

335

high concentrations of Na+ and Ca2+ in the fish sauce, as visualized with CE-C4D, are due to

336

the fact that it was based on the extract from salt water fish with a large content of salt. Note

337

that in all these analyses, appropriate dilutions were carried out to avoid overloading.

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4. Conclusions

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A portable triple-channel CE system using independent background electrolytes was

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constructed for the first time and successfully demonstrated for the concurrent monitoring of

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different anionic and cationic species. The instrument is cost-effective, simple in construction

343

and operation and could therefore be assembled in-house with a limited budget. The high

344

flexibility of the developed system to ion analyses thanks to the employment of different

345

buffers in parallel allows easy adaptation to various tasks. Depending on applications, one,

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two or all three CE channels can be used simultaneously and independently. With the

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possibility of battery-powered operation and compact size, the system has the potential for

348

mobile deployment.

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Acknowledgements

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The authors are grateful for the financial support by the National Foundation for Science and

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Technology Development of Vietnam (NAFOSTED) (grant No. 104.04-2013.70) and the

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Swiss National Science Foundation (grant No. 20020-149068). 3SAnalysis JSC

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(www.3SAnalysis.vn) is acknowledged for partial instrumental support. We would also like to

355

thank Assoc. Prof. Dr. Thi Thao Ta and Dr. Thi Anh Huong Nguyen (Faculty of Chemistry,

356

Hanoi University of Science, Vietnam National University) for sharing with us their

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experimental experience in food control applications.

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530

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Table 1. Typical operation protocol for one CE channel of the triple-channel system

542

2
3
4
5
6
7
8
9

Duration

Flushing of the injection
interface
Flushing of the capillary
Flushing of the HV
interface
Aspiration of sample into
the injection interface
Hydrodynamic injection
Flushing of the injection
interface
Electrophoretic separation
Flushing of the injection
interface
Flushing of the capillary

Settings
High
V3
Voltage