Micromembranes

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. 

A similar micromembrane (Figure 6.30) formed by TBAB on Pd/C was able to selectively synthesize biphenyls from halobenzenes using a reducing... 

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... 

Fig. 2.18 Ion chromatograms obtained with Dionex instrument using (anodic) AMMS and CMMS micromembrane suppression (a) monoprotic organic acids by anion exchange (b) diprotic organic acids by anion exchange...

Franz AJ, Schmidt MA, Jensen KF, and Firebaugh S. Integrated palladium-based micromembranes for hydrogen separation and hydrogenation/dehydrogenation reactions. US Patent 6,541,676, Apr 2003. 

The benefits of the use of micromembranes for the selective removal of one or more products during reaction have been demonstrated for equdibrium-limited reactions [289]. For example, the performance of hydrophilic ZSM-5 and NaA membranes over multichannel microreactors prepared from electro-discharge micromachining of commercial porous stainless steel plates was studied by Yeung et al. in the Knoevenagel condensation [290,291] and andine oxidation to azoxybenzene [292]. For such kind of reactions, the zeolite micromembrane role consists of the selective removal of water, which indeed yields higher conversions, better product purity, and a reduction in catalyst deactivation in comparison to the traditional packed bed reactor. 

Mateo E, Lahoz R, Puente G, Paniagua A, Coronas J, and Santamarfa J. Preparation of silicalite-1 micromembranes on laser-perforated stainless steel sheets. Chem Mat 2004 16(24) 4847-4850. 

Leung YLA and Yeung KL. Microfabricated ZSM-5 zeolite micromembranes. Chem Eng Sci 2004 59 4809-4817. 

Membranes fabricated using the MEMS technology are finding an increasing number of applications in sensors, actuators, and other sophisticated electronic device. However, the new area of application of MEMS is creating new materials demands that traditional silicon cannot fulfill [43]. Polymeric materials, also in this case, are the optimal solution for many applications. Microfabrication of polymeric films with specific transport properties, or micromembranes, already exists, and much work is in progress [44-50]. 

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

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. 

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