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  • High-Pressure Electrolytic Carbonate Eluent Generation Devices and Their Applications in Ion Chromatography Systems

    Yan Liu, Zhongqing Lu, and Chris Pohl, Thermo Fisher Scientific, Sunnyvale, CA USA

    Conclusion  The development of electrolytic eluent generation technology and RFIC

    systems has fundamentally changed the practice of ion chromatography

     RFIC systems offer significant benefits in terms of ease of use and

    improved performance of IC methods

     The combined use of RFIC systems fitted new high-pressure EGC

    cartridges and new IC columns packed with resins of smaller particle sizes

    (e.g., 4 µm) provides new opportunities to perform fast and high resolution

    IC separations.

    Overview Purpose: In this work, new high-pressure electrolytic potassium carbonate and

    potassium bicarbonate eluent generation devices are developed.

    Methods: The new electrolytic eluent generation devices utilize the

    electrochemical processes involving the electrolysis of water and charge-

    selective transport of ions across ion exchange media to generate high-purity

    potassium carbonate and potassium bicarbonate solutions as eluents for use in

    ion chromatography systems.

    Results: The combined use of Reagent-Free™ Ion Chromatography (RFIC™)

    systems fitted with new high-pressure eluent generation devices and new IC

    columns packed with resins of smaller particle sizes (e.g., 4 µm) provides new

    opportunities to perform fast and high resolution IC separations.

    Introduction Ion chromatography (IC) is a widely used analytical technique for determination

    of anionic and cationic analytes in various sample matrices. In modern IC

    systems, high purity acid, base, or salt eluents are generated electrolytically

    using deionized water as the carrier. The Reagent-Free Ion Chromatography

    (RFIC) systems with electrolytic eluent generation make it possible to perform a

    wide range of ion chromatographic separations using only deionized water as the

    carrier. For many applications, the RFIC systems provide improved performance

    with increased sensitivity and the flexibility to perform isocratic and gradient

    separations. In addition to saving time, labor, and operating costs, the RFIC

    systems eliminate errors and problems associated with manual eluent

    preparation and offer users the benefits of simplicity, ease of use, and improved

    method reproducibility.

    There is growing interest in the development and application of new IC columns

    packed with resins of smaller particle sizes (e.g., 4 µm or smaller) since these

    columns bring out new opportunities to perform fast and high-resolution IC

    separations. The new smaller-particle-size IC columns typically yield pressures

    higher than 3000 psi, which is the maximum operating pressure of the current

    generation of electrolytic eluent generators. Therefore, there is a need to develop

    electrolytic eluent generators capable of operating at elevated pressures. Here,

    the authors describe the development of a new generation of electrolytic devices

    for generating high-purity potassium carbonate and potassium bicarbonate using

    deionized water as the carrier stream. The new electrolytic eluent generators can

    be operated under pressures up to 5000 psi. We will describe the principles and

    operation of the new electrolytic eluent generators, and demonstrate the

    advantages of using these devices to achieve fast and high resolution ion

    chromatographic separations.

    FIGURE 3. Block diagram of a typical RFIC system using Dionex EGC

    500 carbonate eluent generator and Dionex EPM 500 electrolytic pH

    modifier.

    FIGURE 1. Electrolytic generation of K2CO3 eluents using a high-

    pressure Dionex EGC 500 K2CO3 cartridge.

    FIGURE 2. Electrolytic Generation of K2CO3/KHCO3 eluents using a

    Dionex EGC 500 K2CO3 cartridge and Dionex EPM 500 modifier.

    FIGURE 4. Separation of seven common anions on a 4 mm Dionex

    IonPac AS22-Fast-4 µm column.

    FIGURE 5. Separation of seven common anions on a 4 mm Dionex

    IonPac AS22-Fast-4µm column Using an Dionex EGC 500 K2CO3

    cartridge and an Dionex EPM 500 modifier.

    When the Dionex EGC 500 K2CO3 cartridge is combined with a Thermo

    Scientific Dionex EPM 500 electrolytic pH modifier, eluents of potassium

    carbonate and bicarbonate can be generated electrolytically. As shown in

    Figure 2, the Dionex EPM 500 modifier consists of a cation-exchange bed that

    is fitted with an anode at its outlet. The inlet end of the device is connected to a

    cathode through the cation exchange connector. A DC current is applied to the

    Dionex EPM 500 modifier to remove a controlled amount of potassium ions

    which are forced to migrate across the cation-exchange connector. The

    displaced potassium ions move toward the cathode and combine with

    hydroxide ions to form a solution of potassium hydroxide, which is directed to

    waste. In the meantime, hydronium ions generated at the anode converts

    carbonate into bicarbonate. Therefore, by controlling the applied current, the

    pH of the incoming potassium carbonate eluent can be modified to form a

    potassium carbonate and bicarbonate solution for use as the eluent in IC

    separations.

    FIGURE 6. Fast separation of seven common anions on a 4 mm Dionex

    IonPac AS22-Fast-4µm column Using an Dionex EGC 500 K2CO3 cargridge

    and a Dionex EPM 500 modifier.

    FIGURE 7. Determination of inorganic anions in a drinking water

    sample.

    All trademarks are the property of Thermo Fisher Scientific and its subsidiaries.

    This information is not intended to encourage use of these products in any manners that might infringe the

    intellectual property rights of others.

    The Dionex ICS-5000+ RFIC systems using Dionex EGC 500 K2CO3 cartridge and

    Dionex EPM 500 modifier are capable of providing highly reproducible separation

    of target analytes that are difficult to achieve using standard IC systems. Figure 5

    shows an overlay of 100 consecutive high-resolution separations of seven common

    anions obtained using a a 4 mm Dionex IonPac AS22-Fast-4µm column. We are

    to achieve highly reproducible results with retention time RSD ranging from 0.03%

    for phosphate to 0.06% for fluoride over the 100 consecutive runs.

    potassium ions in the electrolyte reservoir migrate across the cation-exchange

    connector and combine with the hydroxide ions produced at the cathode through

    the reduction of water to form a KOH solution. In the meantime, carbonate ions

    migrate across the anion exchange connector and combine with H+ ions produced

    at the anode through the oxidation of water to form a carbonic acid solution. The

    KOH solution reacts with the carbonic acid solution to form a K2CO3 solution,

    which can be used as the eluent in ion chromatography. The concentration of

    K2CO3 formed is directly proportional to the applied DC current and inversely

    proportional to the flow rate of DI water going through the eluent generation

    chamber. The Dionex EGC 500 K2CO3 cartridge can be operated under pressures

    up to 5000 psi.

    Figure 3 shows the block diagram of key components in a typical RFIC system

    using Thermo Scientific Dionex EGC 500 carbonate eluent generator and Dionex

    EPM 500 electrolytic pH modifier. A high-pressure pump is used to deliver a

    stream of deionized water into the Dionex EGC 500 K2CO3 eluent generator

    cartridge where the high-purity K2CO3 eluent is generated electrolytically. A

    Dionex EPM 500 electrolytic pH modifier is the used to convert a controlled

    amount of carbonate into bicarbonate in the eluent. A high-pressure degasser

    containing a gas permeable tubing is used to remove hydrogen or oxygen gas

    formed electrolytically. There are several other downstream system components

    including a sample injector, a separation column, and an electrolytic suppressor.

    In the system shown, the conductivity detector effluent is routed through the

    regenerant chambers of the electrolytic suppressor, the Dionex EPM 500 modifier,

    and the high-pressure degasser assembly before going to waste. Therefore, the

    RFIC system makes it possible to perform the entire IC separation process using

    deionized water as the carrier.

    Experimental All experiments were performed using Thermo Scientific™ Dionex™ ICS-5000+

    RFIC™ systems with electrolytic eluent generation. A typical Dionex ICS-5000+

    system consists of a dual pump module (DP), an eluent generator (EG) module,

    and a detector/chromatography module (DC). The modular design of the Dionex

    ICS-5000+ system allows users to quickly configure and customize components

    for a wide range of applications. The system can be configured as a dual-channel

    capillary RFIC system, a dual-channel conventional RFIC system, or a dual-

    channel RFIC system supporting both conventional and capillary-scale IC

    separations. The Dionex ICS-5000+ RFIC systems are fully supported by Thermo

    Scientific™ Dionex™ Chromeleon™ Chromatography Data System (CDS) 7

    software.

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