Reagent-Free Ion Chromatography Systems With Eluent Regeneration: RFIC-ER Systems

Since the introduction of ion chromatography (IC) in 1975, Dionex Corp. (Sunnyvale, CA) has continued to improve the performance of IC systems while developing new capabilities for the determination of anionic and cationic analytes in various sample matrices. The introduction of Reagent-Free™ Ion Chromatography (RFIC™) systems transformed the world of IC separations. The advent of RFIC systems fundamentally changed the routine operation of IC by making it an easy-to-use, powerful, and reliable analytical technique.

RFIC systems combine three core patented technologies. Electrolytic Eluent Generation produces acid, base, or salt eluents for IC separations. Self-Regenerating Electrolytic Suppression produces the regenerant ions necessary for eluent suppression and allows continuous operation with less maintenance. Finally, Continuously Regenerated Trap Columns (CR-TC) remove trace-level contaminants. RFIC systems with eluent generation (RFICEG™) make it possible to perform a wide range of ion chromatographic separations using only deionized water as the carrier. For many applications, RFICEG systems provide high performance with increased sensitivity and the flexibility to perform isocratic and gradient separations using alkali hydroxides (e.g., KOH), methanesulfonic acid, or carbonate/bicarbonate eluents. The RFIC-EG systems, including the ICS-2000 and ICS-3000, offer IC users the benefits of simplicity, ease of use, and improved reproducibility—day to day, operator to operator, and laboratory to laboratory.

To expand RFIC capabilities for frequent-use applications, Dionex is developing RFIC systems with Eluent Regeneration: RFIC-ER™ systems. These systems use the electrolytic suppressor to regenerate the starting eluent after suppression. They provide benefits similar to RFIC-EG systems, offering simplicity, ease of use, and high reproducibility. The two technologies differ in the additional advantages they offer for various user needs. While RFIC-EG systems provide isocratic and gradient method flexibility for a wide range of IC applications, RFIC-ER systems are designed specifically for a set of routine IC analyses, such as the determination of anions or cations in drinking water. The major advantages of RFIC-ER are continuous operation, reduced waste disposal, and lower cost of ownership.

Operation principles of RFIC-ER systems

Figure 1 - Electrochemical processes in an electrolytic suppressor for determination of anions (Na2CO3 eluent).

One key feature of an RFIC-ER system is the operation of the electrolytic suppressor in eluent regeneration mode. Figure 1 shows the electrochemical processes in an electrolytic suppressor for the determination of anions (Na2CO3 eluent) in an IC system. The combined effluent from the suppressor anode and cathode chambers is a mixture of hydrogen gas, oxygen gas, and the aqueous solution containing the eluent components as well as the ions from the injected sample.

Figure 2 - Block diagram of an RFIC-ER system.

RFIC-ER systems use novel and patented approaches to remove hydrogen and oxygen gases, sample ions, and other trace contaminants. They purify the suppressor regenerant effluent so that it is converted into the pure electrolyte solution for reuse as the ion chromatographic eluent. Figure 2 shows the basic components of an RFIC-ER system. The effluent from the electrolytic suppressor regenerant chamber is passed through the catalytic gas elimination column. The column is packed with a noble-metal catalyst that induces the reaction between hydrogen gas and oxygen gas to form water. This serves several important functions. First, it provides a means to conveniently eliminate the buildup of hydrogen and oxygen gases, and thus facilitates the operation of continuous eluent recycle. Second, the water-forming reaction of hydrogen and oxygen is stoichiometric in the column; the amount of water formed is expected to be in principle the same as the amount of water consumed originally to produce hydrogen and oxygen gases in the electrolytic operation of the suppressor. This avoids an increase in the concentration of eluent that would occur due to the consumption of water in the electrolytic operation of the suppressor.

In the RFIC-ER system, an analyte trap column is placed after the outlet of a conductivity detector to trap analyte ions. Additionally, eluent purification columns packed with appropriate ion exchange resins are placed in the RFIC-ER system to further purify the regenerated eluent for use in the separation process. This IC operation mode is compatible with IC separations using carbonate/bicarbonate and methanesulfonic acid (MSA) eluents.

An RFIC-ER system can operate for up to four weeks without any user intervention apart from loading samples. Under heavy workloads, the analyte trap and eluent purification columns as well as the eluent may require replacement or regeneration more frequently. Chromeleon® chromatography management software (Dionex) provides system wellness features to monitor usage of analyte trap and eluent purification columns, and eluent.

Applications of RFIC-ER systems

RFIC-ER systems are designed for the ion chromatographic determination of common anions and cations in drinking water samples. The systems can be used to perform isocratic separation of common anions using IonPac® 4-mm AS22 and AS23 anion separation columns (Dionex) and carbonate and bicarbonate eluents. The systems are also compatible with isocratic separation of common cations using IonPac 4-mm CS12A columns and methanesulfonic acid eluents. The systems are suitable for U.S. EPA Methods 300.0 Part A and 300.1 Part A, or equivalent applications.

Figure 3 - Separation of common anions in drinking water spiked with 10 mg/L bromide, using an RFIC-ER system with a 4-mm × 250- mm IonPac AS22 column and suppressed conductivity detection: 1) fluoride, 2) chloride, 3) bromide, 4) sulfate.

RFIC-ER systems have been evaluated for long-term performance in the determination of common anions and cations in water samples. In one study, drinking water samples collected from Fremont, CA, and spiked with 10 ppm bromide were analyzed repeatedly. The initial 2 L of eluent, containing 3.5 mM Na2CO3 and 1.0 mM NaHCO3, were regenerated over a period of 33 days. Figure 3 shows representative chromatograms obtained over the course of 2000 injections. The results demonstrate that the RFIC-ER system provided highly reproducible separation of common anions in drinking water samples over an extended period. The percent retention time change ranged from 0.2% for fluoride to 3.1% for sulfate over the 33-day period. In a second study, an RFIC-ER system was used to separate common cations on a 4-mm CS12A column. The initial 2 L of 20 mM MSA eluent were regenerated over a period of 52 days. Figure 4 shows representative chromatograms. The results demonstrate that the RFIC-ER system can provide highly reproducible separation of six target cationic analytes over 2000 injections.

Figure 4 - Separation of six common cations on a 4-mm × 250-mm IonPac CS12A column, using an RFIC-ER system: 1) lithium, 0.5 mg/L; 2) sodium, 2 mg/L; 3) ammonium, 4 mg/L; 4) potassium, 2 mg/L; 5) magnesium, 2 mg/L; 6) calcium, 10 mg/L.

Conclusion

RFIC-ER systems address the challenges faced by routine IC users such as contract laboratories and municipalities analyzing drinking water samples. The systems offer several advantages when compared to conventional IC systems. Eluent preparation time and waste disposal costs are significantly reduced; a single bottle of eluent can last up to four weeks. Routine IC operation is greatly simplified. The operator is free to focus on key analytical tasks rather than eluent preparation. The systems can operate continuously for an extended period of time and thus ensure that the system is always equilibrated and ready for sample analysis. They offer enhanced retention time reproducibility, since the same eluent can serve many injections.

RFIC-ER systems improve the ease of use and performance of IC methods for routine anion and cation analysis in drinking water. RFIC-EG systems are recommended for those users with more complex sample matrices and who may require more flexibility in eluent type and gradient capability.

The authors are with Dionex Corp., 1228 Titan Way, Sunnyvale, CA 94086, U.S.A.; tel.: 408-481-4278; fax: 408-735-9413; e-mail: [email protected].

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