Fiber Suppressors

Figure 9.12. Alternative forms of suppressor column for ion chromatography (a) hollow fiber, (b) membrane sandwich. Reprinted by permission of Dionex Corporation, Sunnyvale, CA.

Suppressor devices include packed column suppressors, hollow-fiber membrane suppressors, micromembrane suppressors, suspension postcolumn reaction suppressors, autoregenerated electrochemical suppressors, and so forth. 

Some of the drawbacks that packed column suppressors have were eliminated when hollow-fiber membrane suppressors were introduced in 1981. These were found to be even more convenient and efficient, with low dead volume and high capacity, and they are dynamically regenerated. Eluent passes through the... 

In contrast to conventional suppressor columns, hollow fiber suppressors are continuously regenerated, and thus do not require an additional pump system. The reactions that occur across the membrane wall are shown in Fig. 3-37. Since fiber suppressors suited for anion exchange chromatography act as cation exchangers, the eluent cations are exchanged with protons in the regenerent solution. The driving force for the diffusion of protons across the membrane is provided by their subsequent reaction with... 

Fig. 3-37. Schematic of a hollow fiber suppressor for anion exchange chromatography.

An expended region where the eluent enters the fiber suppressor (cation exchanger in the sodium form). 

Fig. 3-38. Comparison of detection sensitivities for the application of a conventional suppressor column and a hollow fiber suppressor. — Separator column IonPac AS4 eluent 0.0028 mol/L NaHC03 + 0.0022 mol/L Na2C03 flow rate 2 mL/min detection conductivity with a) ASC-1, and b) hollow fiber suppressor AFS-1 regenerent 0.0125 mol/L H2S04 injection 50 pL anion standard.

The regeneration of a micromembrane suppressor occurs in much the same way as for a hollow fiber suppressor. The regenerent is delivered pneumatically from the reservoir, since the required flow rate cannot be obtained simply via gravity feed. While a sulfuric acid concentration of c = 0.01 mol/L suffices for isocratic operation, a twofold regenerent concentration is recommended for gradient techniques. The flow rate should be adjusted to ensure a sufficiently low background conductivity when the maximum eluent conentration is reached. Maintaining these conditions, one can then switch to the initial eluent concentration. 

Eluents and regenerents suitable for the analysis of organic acids when applying an AFS-2 hollow fiber membrane suppressor are listed in Table 4-2. For the analysis of borate and carbonate with octanesulfonic acid as the eluent, an ammonium hydroxide solution with a concentration c = 0.01 mol/L can also be used as the regenerent. 

The CFS hollow fiber suppressor (see Section 3.4.3) that was developed for cation exchange chromatography can also be applied to cation analysis via ion-pair chromatography. It features good solvent stability and sufficient membrane transport properties for the anionic ion-pair reagent. This suppressor is regenerated with tetramethylam-monium hydroxide using a concentration of c = 0.04 mol/L. 

As described in Sections 3.3.3 and 3.4.3, hollow fiber suppressors no longer represent the state-of-the-art. Thus, a micromembrane suppressor was introduced under the trade name AMMS-MPIC for ion-pair chromatography of anions. Its structure corresponds to the systems developed for ion-exchange and ion-exclusion chromatography. Like the AFS-2, the AMMS-MPIC micromembrane suppressor contains a solvent-resistant membrane that is permeable to quaternary ammonium bases. Regarding the exchange... 

T. S. Stevens, J. C Davis and H. Small, Hollow fiber ion exchange suppressor for ion chromatography, Anal. Chem., 53,1488,1981. 

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