High capacity suppressor

Small and co-workers [6] at Dow Chemical discovered that alkali and alkaline earth metals could be separated and determined analogously to anions with a cation exchange separation and an anion stripper now commonly known as a suppressor. With the latest developments in high efficiency columns and high capacity suppressors, it is possible to determine the common alkali and alkaline earth metals along with ammonia in a single injection run of less than 15min (Fig. 2.1). 

Aliphatic tricarboxylic acids such as citric acid, isocitric acid, tricarballylic acid, and aconitic acid exhibit a remarkably high affinity toward the stationary phase of carbonate-selective anion exchangers. Hence, low-ionic strength car-bonate/bicarbonate buffer solutions are not particularly suitable as eluents. However, when a sodium hydroxide solution at a comparatively high concentration (c 0.08 mol/L) is used, tricarboxylic acids may be eluted. When the detection of these compounds is carried out via electrical conductivity, a high-capacity suppressor system such as micromembrane (MMS) or self-regenerating suppressors must be used to reduce background conductivity. 

IC column anion guard anion separators in tandem packed with low capacity anion exchange resin anion suppressor packed with high capacity cation exchange resin. Other IC column resins may be used eluent 1.5 mm NaHC03. 

Shown in Fig. 2.8 is a schematic diagram of the Dionex ion exchange liquid chromatograph used by Smee et al. [40], The instrumentation consists essentially of a low capacity anion exchange column, the separator , a high capacity cation exchange column, the suppressor and the detection system, a high sensitivity conductivity meter and recorder. 

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 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... 

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. 

In the earlier instruments, the suppressor unit was a cation-exchange column of high capacity that had to be regenerated periodically. Newer suppressors contain ion-exchange membranes that can be regenerated continuously by flowing a solution of sulfuric acid over the outer membrane surface or by electrically generated acid. 

The suppressor column obviously will eventually become depleted and will have to be regenerated (with HCl for the cation exchanger and with NaOH for the anion exchanger). The suppressor column is usually a small-volume bed of a high-capacity resin to minimize band spreading in the colunm. Since microgram or smaller quantities of analytes are usually separated, a low-capacity analytical column is used coupled with relatively low eluent concentration (1 to 10 mM). 

In the early form of IC, columns of high-capacity, gel-type cation- and anion-exchanger resins were indeed used to remove the excess eluent they were placed immediately after the separator columns, which were packed with low-capacity, surface-functional exchangers. These high-capacity columns were called suppressor columns. They were clumsy, because they needed regeneration from time to time, and they caused additional band spreading. Today, their place has been taken by ion-exchange membranes. The evolution of suppressors is presented in Table 3. 

The use of suppressed conductivity detection is limited when high-capacity ion-exchange columns are used the concentrations of hydroxide- or carbonate-based eluates needed for reasonable elution times would be too high for continuously working membrane suppressors. 

In non-suppressed ion chromatography, mixtures of ethylenediamine and aliphatic carboxylic acids (such as tartaric acid or oxalic acid) are typically employed. In suppressed ion chromatography, the required reduction of background conductivity would only be possible by using high-capacity membrane-based suppressor systems. 

Two variants of this exist the anionic micromembrane suppression (AMMS) and the cationic micromembrane (CMMS) suppressor. The micromembrane suppressor consists of a low dead volume eluent flow path through alternating layers of high-capacity ion exchange screens and ultra-thin ion exchange membranes. Ion exchange sites in each screen provide a site-to-site pathway for eluent ions to transfer to the membrane for maximum chemical suppression. 

In the original scheme for the separation of anions a mixture of sodium hydroxide and sodium phenate was used in the eluent. The suppressor column was packed with a cation-exchange resin of high capacity. The suppressor column con-... 

The schematics and operation of the CMMS 300 micromembrane suppressor suitable for cation-exchange chromatography corresponds precisely to the AMMS 300 suppressor described in Section 3.6.3, which was developed for anion-exchange chromatography. The high capacity of this micromembrane... 

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