Ion Chromatography Systems With Eluent Regeneration

Users who perform routine analyses of anions and cations in simple matrices, such as drinking water, can benefit from Reagent-Free™ Ion Chromatography systems with Eluent Regeneration (RFICER™ systems) (Dionex Corp., Sunnyvale, CA). The systems were developed to adapt RFIC™ system capabilities for routine, frequent-use applications. The systems use the electrolytic suppression and regeneration technology first developed for RFIC systems with Eluent Generation (RFIC-EG™ systems) to allow continuous operation for up to four weeks.

Mixing eluents and regenerants for ion chromatography (IC) is a time-consuming, repetitive task. It can also be a source of error, if different operators follow slightly different procedures. Since most eluents and regenerants are caustic or corrosive, mixing these chemicals also exposes operators to hazards. Reagent-Free systems were developed in 2003 in an effort to make IC easier to use and more reproducible. The systems incorporate electrolytic eluent and regenerant production, reducing labor while increasing reproducibility and control.

In an RFIC-ER system, the eluent suppressor serves double duty, concurrently suppressing the eluent used for separation and regenerating returning suppressed eluent. Regenerated eluent is circulated through trap and purification columns, back to the eluent reservoir, creating a closed-loop system that allows continuous operation and produces less waste, lowering cost of ownership. While RFIC-EG systems are designed to be highly adaptable, offering ease of use and reproducibility for a variety of applications, RFIC-ER systems are designed for dedicated routine, repetitive isocratic IC analyses, such as monitoring anions or cations in drinking water.

Comparison of RFIC-EG and RFIC-ER

The RFIC-EG systems utilize three patented electrolytic technologies to automate chromatography: eluent generation, Self-Regenerating Suppressors® (SRS®), and continuously regenerated trap columns (CR-TC). Electrolytic eluent generation uses electrolysis to generate controlled concentrations of eluent ions from the source water itself. The eluent cartridge provides counterions to the eluent from an electrolyte reservoir through an ion-exchange membrane. Precise control of current and flow rate regulates the concentration of the base, acid, or salt eluent. The SRS uses electrolysis to produce the regenerant ions necessary to reduce the background conductance of the eluent prior to conductivity detection. The CR-TC removes trace impurities from the system, reducing potential interferences and increasing sensitivity. These three technologies support a wide variety of separations using only deionized water as the carrier. Chromeleon® chromatography management software (Dionex) coordinates and controls these devices, providing effortless automation.

RFIC-EG systems, including the ICS-2000 and ICS-3000, give operators the flexibility to perform isocratic or gradient separations using hydroxide or methanesulfonic acid eluents, and isocratic separations using carbonate or carbonate/bicarbonate eluents. Because of the precise control afforded by the electrolytic eluent generation and suppression, the systems improve reproducibility and sensitivity while simplifying operation. While RFIC-EG systems are powerful, adaptable, and easy to use, they do require periodic operator attention. Waste, consisting of eluent and analytes, must be emptied periodically. Eluent cartridges must be replaced as they are exhausted. If the system has remained idle, or if the eluent is changed, the system must be reequilibrated.

RFIC-ER systems exploit the electrolytic suppressor by using it in eluent regeneration mode to allow prolonged, uninterrupted operation. In this mode, the suppressor uses the suppressed eluent from the detector cell as a source of water for electrolytic suppression. The electrolytic suppression of a sodium carbonate eluent (Na2CO3) produces an effluent consisting of Na2CO3, hydrogen and oxygen gases, water, and analyte ions. In an RFIC-EG system, the gases would be vented and the effluent would be routed to waste. An RFIC-ER system uses patented technologies to remove analyte ions from the detector effluent and catalytically recombine the hydrogen and oxygen to form water. The suppressor regenerant effluent is then returned to the eluent reservoir as pure electrolyte solution for reuse as eluent.

Figure 1 - Diagram of RFIC-ER system operation.

Figure 1 illustrates the operation of an RFIC-ER system. Eluent from the reservoir is pumped through the analytical column, suppressor, and detector, separating and determining the analytes. Suppressed eluent from the detector is routed through a trap column that removes the analyte ions. The positioning of the trap in the suppressed eluent stream increases its efficacy and extends its life since carbonic acid is a weak eluent. The suppressed eluent then passes through the suppressor regenerant chambers, supplying water for the electrolytic suppression of the eluent and reclaiming the eluent ions as well as the oxygen and hydrogen generated by the electrolysis. The effluent passes through a noble-metal treated catalyst column, which recombines the hydrogen and oxygen. This not only eliminates the buildup of gases, allowing continuous operation, but also stoichiometrically converts the gases back into water. Converting the gases back into water prevents the increase in eluent concentration that would occur over time due to the consumption of water by the electrolytic suppression of eluent. The eluent is returned to the eluent reservoir, and is further purified by a specialized ion-exchange column immediately before its use in chromatographic separations.

RFIC-ER systems are compatible with carbonate/bicarbonate and methanesulfonic acid (MSA) eluents. After the initial eluent is prepared, the systems can operate continuously for up to four weeks without user intervention, other than loading samples. Heavy workloads or samples with high-strength matrices may necessitate more frequent replacement or regeneration of analyte trap columns and eluent. Chromeleon software system wellness features monitor usage of these components, alerting the operator when maintenance is required.

Reproducibility of RFIC-ER system separations

RFIC-ER systems are designed to operate continuously for routine ion chromatographic determinations. The benefit of these systems is that they are always on, and always ready for analysis. Because the eluent is recirculated rather than sent to waste, the system can continue to run around the clock, remaining equilibrated and calibrated without operator intervention.

A primary application for RFIC-ER systems is the monitoring of drinking water samples for common anions and cations. Isocratic separations of anions using carbonate/bicarbonate eluents can be performed using IonPac® AS22 or AS23 4-mm columns (Dionex). Isocratic separations of cations using MSA eluents can be performed using an IonPac CS12A 4-mm or CS16 5-mm column. RFIC-ER systems are designed to comply with U.S. EPA Methods 300.0 Part A and 300.1 Part A, ASTM Methods D 4327-03 and D 6581-00 (2005), and equivalent methods for monitoring water purity. 

Figure 2 - Percent peak area changes for a seven-anion calibration check standard over 28 days.

In one reproducibility study, an IonPac AS22 4-mm column was used for the isocratic separation of anions in drinking water samples. After calibration with six different concentrations of a seven-anion standard (F, Cl, NO2, Br, NO3, PO4, and SO4), one of the lower-concentration standards was tested daily as a continuing calibration check standard. Analyte peak areas remained within 4.2% for 28 days, as shown in Figure 2. The highest variances occurred in the biologically active NO3 and PO4. Peak areas of chloride, which is less prone to biological consumption, remained within 2%. This demonstrates that the system can remain stable enough for continuous use over four weeks without recalibration.

Figure 3 - Overlay of chromatograms of the anion calibration check standard taken over the course of a week using an RFIC-ER system with an IonPac AS22 4 × 250 mm column and suppressed conductivity detection. Peaks: 1) fluoride, 2) chloride, 3) nitrite, 4) bromide, 5) nitrate, 6) phosphate, and 7) sulfate.

RFIC-ER system consistency is also apparent in the retention time stability. Figure 3 shows an overlay of the chromatograms of the continuing calibration check standard run from day 10 through day 16. The retention times were very stable throughout the course of the week.

In a similar study, an RFIC-ER system was used to separate common cations in drinking water using an IonPac 4-mm CS12A column and 2 L of regenerated 20 mM MSA eluent. Figure 4 is an overlay of 100 injections of drinking water from Sunnyvale, CA, spiked with 1 ppm potassium. The retention time percent RSD for 100 injections ranged from 0.09% for potassium to 0.1% for calcium. The peak area percent RSD ranged from 0.14% for calcium to 0.51% for ammonium. Figure 5 shows representative chromatograms of a six-cation standard injected over a period of 32 days in a third study. Again, the peak retention times are very stable throughout the experiment. These three studies demonstrate the reproducibility of RFIC-ER systems over extended periods of continuous use. 

Figure 4 - Overlay of 100 injections of tap water spiked with 1 ppm potassium and determined using an RFIC-ER system with an IonPac CS12A 4 × 250 mm column and suppressed conductivity detection. Peaks: 1) sodium, 2) ammonium, 3) potassium, 4) magnesium, and 5) calcium.

Figure 5 - Overlay of representative chromatograms of a six-cation standard determined using an RFIC-ER system with an IonPac CS12A 4 × 250 mm column and suppressed conductivity detection. Peaks: 1) lithium, 2) sodium, 3) ammonium, 4) potassium, 5) magnesium, and 6) calcium.

Conclusion

RFIC-ER systems provide the reproducibility and ease of use necessary for laboratories with high throughput of routine samples, such as contract laboratories and drinking water providers. Because it can run continuously, the always-on, always-ready RFIC-ER system allows operators to analyze samples during time they would otherwise spend mixing eluent or equilibrating and calibrating a conventional IC system. The system greatly simplifies operation, allowing operators to spend more time on analytical tasks. Waste disposal is decreased, leading to a lower cost of ownership and less operator exposure to hazardous chemicals. Reproducibility is also increased because the same eluent is used for an extended period of time, minimizing inconsistency caused by variations in eluent preparation.

RFIC-ER systems are well suited for users who perform routine analyses of anions and cations in simple matrices, such as drinking water. RFIC-EG systems are still recommended for users who analyze a wider variety of samples, including samples with more complex matrices, or users requiring a wider choice of eluents, gradient capability, or sensitivity.

The authors are with Dionex Corp., 1228 Titan Way, Sunnyvale, CA 94086, U.S.A.; tel.: 408-481-4210; fax: 408-730-9403; e-mail: john.madden@dionex.com.

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