The CRD 200 and CRD 300 Carbonate Removal Devices consist of a silicone-coated gas permeable membrane selective for carbon dioxide. Carbon dioxide removal decreases sample carbonate peaks and reduces background signal. The CRD 200 or CRD 300 is typically installed between the suppressor and the detector. The regenerant channel that encloses the CRD is flushed with base solution or evacuated via vacuum pump to aid in the removal of carbon dioxide.
CRD 200
The net effect of using a CRD 200 in conjunction with hydroxide eluent is a significant reduction in the sample carbonate peak, which may elute very near the sulfate peak. Sulfate quantitation is improved using RFIC-EGTM systems with hydroxide eluent. Installed after the suppressor, the carbon dioxide transfer is aided by a countercurrent flow of basic solution from the suppressor regenerant waste. No external regenerant is required.
CRD 200 Analytical Challenges
Samples and eluents can become contaminated with carbonate through simple expossure to air. In some samples, depending on the column and separation conditions, the presence of high levels of carbonate originating from dissolved carbon dioxide interferes with the accurate determination of analytes of interest, such as sulfate and nitrite. Under the above conditions, minimizing carbonate using the CRD leads to improved quantitation and improved peak integration for analytes such as sulfate and nitrite.
CRD 300
The CRD 300 Carbonate Removal Device improves detection when using carbonate eluents to achieve ‘hydroxide-like’ detection. Installed after the suppressor, the carbon dioxide transfer is aided by a countercurrent flow of basic solution from an external source (chemical regeneration mode), or evacuation of the regenerant chamber via a vacuum pump with an air bleed (vacuum mode).
CRD 300 Analytical Challenges
Since the introduction of suppressed conductivity in 1975, carbonate eluents have been extensively employed in anion separations because they are easily suppressed to relatively low-conductivity carbonic acid. However, the residual conductivity of the carbonic acid has several undesirable effects on the performance of ion chromatography (IC) employing carbonate eluent. At trace levels, carbonic acid can mask small peaks, decreasing the limit of detection (LOD). In a background of carbonic acid, the linear range of most species is limited. The residual background conductivity creates a large void dip which can adversely effect the integration of early eluting species, such as fluoride, acetate, and formate. The CRD 300 is a device used to remove the carbonic acid from the suppressed eluent, providing hydroxide-like detection to systems using carbonate eluents.
What Is a CRD?
A CRD consists of an extremely thin film of carbon dioxide-permeable coating on the exterior surface of a narrow-bore capillary membrane tube. The device is installed after the suppressor and the suppressed eluent is routed inside the coated capillary membrane tube. A base sweeps the exterior of the coated capillary membrane; the chamber surrounding the capillary membrane is evacuated. In operation, the CRD 200 removes the carbon dioxide originating from carbonate in the sample when using hydroxide eluents, while the CRD 300 improves detection when using carbonate eluents to achieve ‘hydroxide-like’ detection. In chemical regeneration mode the carbon dioxide stripped from the mobile phase is quickly converted to carbonate—due to the presence of base in the exterior chamber—and then swept to waste. In Vacuum Pump mode the carbon dioxide is pumped away via a vacuum pump. Figures 1 and 2 (below) show details of typical CRD 200 Plumbing.
Figure 1
Figure 2
A CRD 200 can be installed easily in any Dionex ion chromatograph equipped with an eluent generator, CR-ATC, and ASRS® 300 suppressor. The CRD 200 requires no reagents or software control. The CRD 200 is a low-dispersion device designed specifically for use with hydroxide-based and borate-based chemistries using the Dionex IonPac® AS19, AS18, AS17, AS16, AS15, AS14, AS4A, AS11, and AS11HC columns.
The CRD 300 can be installed easily in any Dionex IC system equipped with an ASRS 300 or AMMS 300 suppressor and requires no software control. If base is pumped through the regenerant channel (chemical regeneration mode), then an external pump is required. Dionex provides a two-channel peristaltic pump for this application. If the regenerant channel is to be evacuated with an air bleed (vacuum mode), an external vacuum pump is required. Dionex provides a vacuum pump with bleed tube assembly for this application.
Applications
The CRD 200 is ideal for analysis of drinking water, groundwater, wastewater, ultrapure water, and caustic solutions where carbonate is a major component in the samples, especially when pursuing large-volume injections or preconcentration techniques.
Figure 3 (below) shows the expected reduction of carbonate interferance after the CRD is installed, facilitating improved sulfate quantification.
Figure 3
The CRD 300 is recommended for both routine and trace level work with carbonate and carbonate/bicarbonate eluents. Figure 4 (below) compares the background signals of anion separations using carbonate eluents, with and without using a CRD 300. The background signal is more than an order of magnitude lower with the CRD 300, and the void dip is virtually eliminated. Fluoride analysis (peak 1) is simplified when void dip interference is minimized.
Figure 4
