Micromembrane suppressor conductivity

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 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. 

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. 

The CMMS micromembrane suppressor also allows the application of concentration gradients in combination with conductivity detection that is indispensable for cation detection. A mixture of hydrochloric acid and 2,3-diaminopropionic acid suitable for chemical suppression is used as the eluent. The gradient technique, however, plays a secondary role in cation analysis, since it can only be applied for the analysis of alkali and alkaline-earth metals as well as a number of short-chain aliphatic amines. It is definitely not suitable for the analysis of heavy and transition metals, where different... 

Figure 4.17. Separation of a mixture of inorganic and organic anions by gradient elution ion chromatography with conductivity detection using a micromembrane suppressor. A variable rate gradient from 0.5 mM to about 40 mM sodium hydroxide on an lonPac ASH column was used for the separation. Peak identification 1 = isopropylmethylphosphonate 2 = quinate 3 = fluoride 4 = acetate 5 = propionate 6 = formate 7 = methylsulfonate 8 = pyruvate 9 = chlorite 10 = valerate 11 - monochloroacetate 12 - bromate 13 = chloride 14 = nitrite 15 = trifluoroacetate 16 = bromide 17 = nitrate 18 = chlorate 19 = selenite 20 = carbonate 21 = malonate 22 = maleate 23 = sulfate 24 = oxalate 25 = ketomalonate 26 = tungstate 27 = phthalate 28 = phosphate 29 = chromate 30 = citrate 31 = tricarballylate 32 = isocitrate 33 = cis-aconitate and 34 = trans-aconitate. Each ion is at a concentration between 1 to 10 mg/1. (From ref. [417]. Marcel Dekker).

F-, cr, NO2-, Br-, NO3- HPO -, SeO -SeO -, SO - Organic compounds Water as absorption solution Suppressed IC with anion micromembrane suppressor and conductivity detector Shodex IC IF-424 anion exchange column... 

Suppressed IC was initially developed using an anion micromembrane suppressor and a conductivity... 

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