Micromembrane Suppressors

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

Anions of weak acids can be problematic for detection in suppressed IEC because weak ionization results in low conductivity and poor sensitivity. Converting such acids back to the sodium salt form may overcome this limitation. Caliamanis et al. have described the use of a second micromembrane suppressor to do this, and have applied the approach to the boric acid/sodium borate system, using sodium salt solutions of EDTA.88 Varying the pH and EDTA concentration allowed optimal detection. Another approach for analysis of weak acids is indirect suppressed conductivity IEC, which chemically separates high- and low-conductance analytes. This technique has potential for detection of weak mono- and dianions as well as amino acids.89 As an alternative to conductivity detection, ultraviolet and fluorescence derivatization reagents have been explored 90 this approach offers a means of enhancing sensitivity (typically into the low femtomoles range) as well as selectivity. 

Dionex, Anion Micromembrane Suppressor III—Cation Micromembrane Suppressor III, Product Manual, Document No. 031727, Dionex Corporation, 2004. (http //wwwl.dionex.com/ en-us/webdocs/4366 31727-03 MMS Combined V21. pdf). 

Fibre or micromembrane suppressors of high ionic capacity have now taken over from chemical suppressors. With dead volumes in the order of 50 pi, they allow gradient elution with negligible drift in the baseline. Figure 4.8a shows the passage of an anion A- in solution in a typical electrolyte used for anionic columns through a membrane suppressor. 

Some typical separations of these anions achieved are shown in Fig. 2.18. Fig. 2.18(a),(b) illustrates ion chromatographic separations of mono and diprotic organic acids by ion exchange using anodic AMMS and CMMS micromembrane suppressors. 

A further development is the Dionex HPIC AS5A-5p analytical anion separator column. This offers separation efficiency previously unattainable in ion chromatography. When combined with a gradient pump and an anion micromembrane suppressor the AS5A-5p provides... 

Micromembrane suppressors introduced in 1985 use thin, fiat ion-exchange membranes to enhance ion transport while maintaining a very low dead volume, providing a high suppression capacity, with low dispersion. 

A gas chromatograph (Yanaco G-3810) was equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Molecular Sieve 5A and Porapak Q were used for CO and Hj analysis in the TCD and CH4 and C2H4 analysis in the FID, respectively. Soluble products such as CH3OH, CH3CHO, and CjHjOH were analyzed by the FID after electrolysis for 5 h. Formate ions and other anions in the solution were analyzed by means of an ion chromatograph (Dionex DX-lOO) equipped with an anion exchange column (lonPac ICE-ASl), an anion exchange micromembrane suppressor, and a conductivity detector module. 

Sodium hydroxide at a maximum concentration of c = 0.1 mol/L (100 pequiv/mL) and with a flow rate of 2 mL/min can be used as eluent with a micromembrane suppressor, resulting in a suppressor capacity of about 200 pequiv/min. 

The high capacity of a micromembrane suppressor and the ability to provide continuous suppression enable the number of possible eluents to be significantly enlarged. In general, any weak acid can be used as eluent as long as it exists in an anionic form above pH 8 and in neutral form between pH 5 and 8. Above all, this includes the... 

Fig. 3-41. Enlarged view of an eluent chamber in a micromembrane suppressor.

Fig. 3-42. Dependence of the micromembrane suppressor capacity on the length of the eluent chamber at a constant width of 1 cm.

The high capacity of a micromembrane suppressor also allows the use of high ionic strength eluents, which significantly reduces the analysis times. The low void volume of the suppressor that hardly affects the separation efficiency also contributes to the sensitivity increase achieved therewith. 

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. 

Fig. 3-43. Flow diagram for continuous operation of a micromembrane suppressor.

These problems have been circumvented by both the introduction of modern micromembrane suppressors and the development of short clean-up columns for eliminating inorganic impurities in the eluent [130]. Therefore, the gradient elution technique in ion chromatography today is as common as in the area of conventional HPLC. 

A micromembrane suppressor (see Section 3.4.3) combined with a mixture of 2,3-diaminopropionic acid (DAP) and hydrochloric acid is now used for the separation of alkaline-earth metals. The advantage of this eluent is the possibility to adjust the elution power via the dissociation equilibrium of... 

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