Suppressor membrane

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. 

Figure 4.8—Membrane and electrochemically regenerated suppressors. Two types of membrane exist those that allow the permeation of cations (H+ and Na+) and those that allow the permeation of anions (OH and X ). a) The microporous cationic membrane model is adapted to the elution of an anion. Only cations can migrate through the membrane (corresponding to a polyanionic wall that repulses the anion in the solution) b) Anionic membrane suppressor placed after a cationic column and in which ions are regenerated by the electrolysis of water. Note in both cases the counter-current movement between the eluted phase and the solution of the suppressor c) Separation of cations illustrating situation b).

Wallschlager, D. and R. Roehl. 2001. Determination of inorganic selenium speciation in waters by ion chromatography-inductively coupled plasma-mass spectrometry using eluant elimination with a membrane suppressor. J. Anal. At. Spectrom. 16 922-925. 

Dionex AMS, preproduction prototype, Membrane suppressor 0.10ml... 

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... 

The introduction of the IonPac AS5 separator column significantly facilitated the analysis of polarizable anions. Reducing the hydrophobicity of the functional groups bonded to the latex beads makes it possible to elute polarizable anions using a standard mixture of sodium bicarbonate and sodium carbonate. To minimize adsorption phenomena, some p-cyanophenol is added to the eluent mixture. The influence of this species on the peak form is evident in Fig. 3-18. The peak broadening could also be greatly reduced because of the compatibility of the eluent with commercial membrane suppressors and the reduction in the void volume. 

The detection of aliphatic carboxylic acids usually involves the measurement of the electrical conductivity. In ion-exclusion chromatography, therefore, suppressor systems are used to chemically reduce the background conductivity of the acid that acts as the eluent. Due to a lack of modern membrane suppressors in the past, suppressor columns... 

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. 

Fig. 5-26 shows the separation of mono-, di-, and triethylamine, accomplished by using octanesulfonic acid as the ion-pair reagent. The less-hydrophobic hexanesulfonic acid is used in combination with boric add as the eluent for the separation of ethanol-amines, as shown in Fig. 5-27. These compounds are detected by measuring the electrical conductance, thus the background conductance is generally lowered with a membrane suppressor. The addition of boric acid to both the eluent and the regenerent serves to enhance the sensitivity for di- and triethanolamine.

Figure 14.3 Separation of inorganic anions in rain water [after W. Shotyk, J. Chromatogr., 640, 309 (1993)]. Conditions stationary phase, AS4A mobile phase, 1.8 mM Na2CO3 + 1.7 mM NaHCOg conductivity detector after membrane suppressor. Concentrations (in ngg ) chloride, 17 nitrite, 51 bromide, 5 nitrate, 1329 hydrogenephosphate, 50 sulfate, 519.

In 1992 Dionex introduced a commercial electrochemical suppressor called a Self Regeneration Suppressor, or SRS [6]. The internal design is similar to the membrane suppressor, but the regenerating ion for anion chromatography) is produced by electrolysis of water. This allows the use of very low flow rates for regenerant water and avoids the use of independent chemical feed needed for earlier suppression devices. 

Membrane suppressors that can be regenerated continuously have also been developed for the ion-exclusion chromatography of weak acids. In IE, the primary contributor to the high eluent conductance is the hydrogen cation from the acid eluent. Membrane suppressors can reduce this background by exchanging the hydronium ion for tetrabutylammonium ion (TBA). This is shown in Table 8.2. 

What is the regenerate compound for cation membrane suppressors ... 

Figure 3.25 Schematic diagram of a membrane suppressor for use in anion-exchange chromatography (ions replace the Na ions yielding water as the background electrolyte which has low conductivity).

Figure 3,25 Schematic diagram of a membrane suppressor for use in anion-exchange...

In order to ensure the column effluents are compatible with a mass selective system, a micromembrane suppressor for trapping nonvolatile ion-pairing agents used in LC-MS with a moving-belt interface is described by Escott et al. Both cationic and anionic membrane suppressors for LC-MS with an ion-spray interface were reported by Conboy et al., whereas Forngren et al. " removed nonvolatile mobile phase ingredients with an ion exchanger placed between the separation column and MS interface. 

Figure 5.20. Schematic representation of the operation of a membrane suppressor for suppressed conductivity detection of anions and cations. (From ref. [168] Elsevier).

For suppressed conductivity detection, the end of the separation column is connected to a tubular ion exchange membrane suppressor surrounded by a reservoir of regenerant solution [512,513]. The electrodes for conductivity detection are located in a separate capillary downstream of the suppressor. The high voltage electrode for the separation is located in the regenerant reservoir. In this way, the detector is decoupled from the electric field for the separation, and the electroosmotic flow generated in the separation column is used to drive the electrolyte solution through the suppressor and detector. The function of the suppressor (see section 5.7.4.1) is to neutralize electrolyte solution ions. 

The employment of an electrode system to generate H or OH on demand in the amounts required to operate a self-regenerating membrane suppressor system led to the idea of using a similar generator to also produce the desired concentrations of these... 

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