New Ion Chromatography Solutions for Routine Water Analysis

Anion and cation determination in all types of water is one of the domains of ion chromatography (IC). Laboratories that routinely analyze water require rugged, reliable, and easy-to-use instruments and solutions that allow the operator to achieve fast results with minimum effort and low cost. These requirements have driven innovations in IC over the last three decades. Users have enjoyed increasingly easy-to-use instruments and software solutions that reduce the time for data processing and generating reports. This article describes the latest technologies to simplify analyses for routine water laboratories.

Advantages of 2-mm column chemistries

Although traditional 4-mm columns provide the high capacity needed for separation of highly concentrated samples or matrices, they often lack the sensitivity required for trace determinations. Columns with 2-mm inner diameters have smaller cross-sectional areas, increasing the ratio of analyte concentration-to-eluent volume, thus increasing the sensitivity. These smaller columns also require only one-quarter of the eluent, regenerant, and sample required when using 4-mm columns. Cost-conscious laboratories use 2-mm-i.d. columns not only to save eluent cost and reduce waste, but also to reduce labor, since the operator has to make up reagents much less often and can operate the IC system four times longer compared to a system using 4-mm columns. Modern IC systems such as the ICS-900 (Dionex, Sunnyvale, CA) support 2- and 4-mm column chemistries.

The dual piston pump of the ICS-900 was designed for chromatographic performance at flow rates as low as 0.1 mL/min. Flow rates in this range allow applications to be performed on 2-mm-i.d. columns. Additional benefits of the system include its compact size, simple operation, easy accessibility for connections, and simple software control, all without the performance deficiencies characteristic of other basic systems.

Figure 1 - Chromeleon software monitors flow rate, pressure, injection valve position, eluent usage, and system connection. Pump and column usage are also monitored, and automated warnings inform users of maintenance requirements.

State-of-the-art Chromeleon® chromatography management software (Dionex) provides full instrument setup and control as well as data reduction and archiving. This powerful instrument control software package includes capabilities for instrument wellness monitoring (Figure 1) and automated instrument IQ and OQ procedures.

Chemical suppression

Ion chromatographers who choose chemically regenerated suppression require a fluidic delivery system for the regenerant. This has typically been achieved either by gravity feed or a secondary peristaltic pump. Both chemical regeneration methods have the disadvantage that the regenerant flow needs to be optimized and is not as constant and reproducible as the eluent flow. Gravity-fed flows can change over time and thus change the background conductivity level. Regenerant flow pulsation from a peristaltic pump can cause baseline noise. These effects reduce the sensitivity of analysis done on the IC.

Figure 2 - Flow diagram of displacement chemical regeneration.

Displacement chemical regeneration (DCR) has been available for some time, and is now integrated into the ICS-900 starter-line IC system.1 DCR utilizes a closed-loop regeneration cycle whereby the eluent stream is directed into a sealed regenerant reservoir. The eluent displaces the regenerant from the reservoir through the regeneration chambers of the suppressor (Figure 2).

DCR has several advantages over other chemical suppression techniques. First, because it requires only a single pump, it is mechanically simple and requires less maintenance and overall attention. When the eluent flow is turned off, so is the regenerant flow. Operational simplicity is also enhanced by the fact that when the eluent supply is exhausted, the regenerant is as well—the operator always replaces both eluent and regenerant simultaneously. Eliminating the regenerant pumping system also removes the need to calibrate the second pump and adjust the flow rate. Suppression power is adjusted by modifying the regenerant concentration as necessary.

Solutions to improve detection limits with carbonate eluents

Many laboratories, because of regulatory requirements, instrument limitations, or preference, still use carbonate-based eluents. The CRD 300 (Dionex) further suppresses carbonate eluents, providing users up to an order of magnitude improvement in signal-to-noise ratio when applied to the analysis of anions.2 The small device utilizes a special chemically derivatized membrane that is highly selective for the transport of carbon dioxide. Volatile CO2 is transported across the membrane, out of the eluent stream. The membrane is surrounded by a chamber that can be evacuated or flushed with a dilute hydroxide solution to remove the CO2 from the system.

Figure 3 - Comparison of anion separation with and without a CRD 300. Background conductance without the CRD is 23 μS/cm, and only 0.87 μS/cm with the CRD 300.

The CRD 300 efficiently removes carbon dioxide remaining in the eluent stream after suppression. This results in increased peak heights, lower background conductivity, and an accompanying lower baseline noise and reduced water dip (Figure 3). The device enables the ion chromatographer to achieve hydroxide-like background conductivity and improves the quantification of earlyeluting anions such as fluoride.

The special chemical selectivity of the membrane in the CRD 300 eliminates the possibility of the loss of volatile organic acids, which was characteristic of some older design carbonate removal devices. Since it is a simple, self-contained flow-through device, it can be added to any IC running carbonate, except those that are set up for eluent regeneration.

Reagent-Free™ IC systems with Eluent Regeneration (RFIC-ER™ systems)

Since 2003, with the introduction of eluent generation, Reagent-Free IC (RFIC™, Dionex) systems have become the standard for ultrahigh-performance IC. RFIC systems with eluent generation (RFIC-EG™ systems) provide the advantages of consistent and reproducible data and ease of use. RFIC-EG systems are based on electrolytic technology for eluent generation, purification, and suppression. More information about RFIC-EG can be found in Refs. 3–7.

The capabilities of RFIC have recently been extended by a new technology to provide the same advantages as RFIC for cost-conscious laboratories with systems dedicated to routine analyses. RFIC systems with eluent regeneration (RFIC-ER systems) can run continuously for up to 28 days using a single preparation of eluent.7,8

The electrolytic suppressor is used to reconstitute the suppressed eluent, and catalytic and purification columns recombine and clean up the returning eluent, ensuring pure eluent with a constant concentration. The chromatographic separation is unchanged, and all current isocratic IC methods using carbonate-based eluents for anion analysis or methanesulfonic acid for cation separations can be transferred easily to RFIC-ER systems.

Figure 4 - An anion calibration check standard was run daily on an RFIC-ER system for 28 days while the system was being used for the analysis of water samples. Peak areas for all seven anions remained within ±5%, demonstrating that the system remained calibrated for all four weeks of operation.

Because RFIC-ER systems form a closed loop, they can be operated in an “always on, always ready” state. In this mode, the eluent composition remains exactly the same for weeks at a time, producing highly reproducible elution times and peak area counts. In fact, separations remain so reproducible that only one calibration curve needs to be run per month, saving labor and time (Figure 4). In addition, all data for the month are linked to a single calibration curve, further enhancing time savings and strengthening data traceability and quality. As long as the system is left on, reequilibration is not necessary, saving operator labor and allowing more time for analysis. Waste is also reduced by up to an order of magnitude, since all effluent is recirculated and reused for up to a month.

Eluent regeneration is an option that can be added to existing and new IC systems and is suitable for customers who routinely run isocratic separations of anions and cations in water samples. RFIC-ER systems extend the capabilities of RFIC systems to routine isocratic separations, while RFIC-EG systems offer higher flexibility with gradient applications and ultimate sensitivity utilizing hydroxide eluents.

Conclusion

New developments in ion chromatography continue to address the needs of water analysis laboratories by increasing productivity, reducing down time, and cutting costs with user-friendly solutions that provide the flexibility to address tomorrow’s increasingly challenging application demands.

References

  1. ICS-900 Ion Chromatography System Brochure, LPN 2003; Dionex Corp.: Sunnyvale, CA, 2008; www.dionex.com/en-us/webdocs/4503-Bro_ICS_900_03Jun08_LPN2003_01.pdf.
  2. Carbonate Removal Device 300 (CRD 300) for Carbonate Eluents Data Sheet, LPN 1978-01; Dionex Corp.: Sunnyvale, CA, 2008; www.dionex.com/en-us/webdocs/65842-DS_CRD_300_IC_04Apr08_LPN1978_01.pdf.
  3. ICS-3000 System Brochure, LPN 1691-04; Dionex Corp.: Sunnyvale, CA, 2007; www.dionex.com/en-us/webdocs/86527-Bro-ICS-3000-06Feb09-LPN1691-05.pdf.
  4. ICS-3000 Ion Chromatography System Data Sheet, LPN 1683-07; Dionex Corp.: Sunnyvale, CA, 2007; www.dionex.com/en-us/webdocs/85525-DS_ICS-3000_19Sep2008_LPN1683_09.pdf.
  5. ICS-1000, -1500, -2000 Ion Chromatography System Brochure, LPN 1514-04; Dionex Corp.: Sunnyvale, CA, 2008; www1.dionex.com/en-us/literature/lp56198.html.
  6. ICS-2000 Ion Chromatography System Data Sheet, LPN 1500-06; Dionex Corp.: Sunnyvale, CA, 2008; www1.dionex.com/en-us/literature/lp56845.html.
  7. Reagent-Free Ion Chromatography (RFIC) System Capabilities Brochure, LPN 1819-03; Dionex Corp.: Sunnyvale, CA, 2008; www1.dionex.com/en-us/literature/lp56198.html.
  8. Reagent-Free Ion Chromatography System with Eluent Regeneration Data Sheet, LPN 2022-02; Dionex Corp.: Sunnyvale, CA, 2008; www1.dionex.com/en-us/literature/lp56845.html.

Dr. Sheldon is Product Manager Ion Chromatography Systems, and Dr. Hoefler is Product Line Manager Ion Chromatography Systems, Dionex Corp., 1228 Titan Way, Sunnyvale, CA 94086, U.S.A.; tel.: 408-481-4542; fax: 408-732-2007; e-mail: [email protected].

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