Suppressor Systems in Ion-Pair Chromatography

In analogy to ion-exchange and ion-exclusion chromatography, the background conductance was chemically decreased in ion-pair chromatography to enable a sensitive conductivity detection. Conventional suppressor columns such as the ASC-1 (for anion determinations) and CSC-1 (for cation determinations) were initially used. After an average operation of about six hours, these columns had to be regenerated by the procedure described in Section 3.3.3. 

The CFS hollow fiber suppressor (see Section 3.4.3) that was developed for cation exchange chromatography can also be applied to cation analysis via ion-pair chromatography. It features good solvent stability and sufficient membrane transport properties for the anionic ion-pair reagent. This suppressor is regenerated with tetramethylam-monium hydroxide using a concentration of c = 0.04 mol/L. 

As described in Sections 3.3.3 and 3.4.3, hollow fiber suppressors no longer represent the state-of-the-art. Thus, a micromembrane suppressor was introduced under the trade name AMMS-MPIC for ion-pair chromatography of anions. Its structure corresponds to the systems developed for ion-exchange and ion-exclusion chromatography. Like the AFS-2, the AMMS-MPIC micromembrane suppressor contains a solvent-resistant membrane that is permeable to quaternary ammonium bases. Regarding the exchange 

the CMMS micromembrane suppressor, introduced in Section 3.4.3, is employed for ion-pair chromatography of cations. Its solvent stability is sufficient, and it is regenerated with tetrabutylammonium hydroxide, also for ion-pair chromatographic applications. The regenerent concentration adheres to the concentration of the ion-pair reagent. A reagent concentration of c = 0.002 mol/L, for example, requires a TBAOH concentration of c = 0.04 mol/L. 

The hollow fiber suppressors, developed for ion-exchange chromatography at the beginning of the 1980s, could not be employed in ion-pair chromatography, because the occasionally high concentrations of organic solvents in the mobile phase were 

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Ion-pair chromatography is also suited for the analysis of metal complexes. For their chromatographic separation, the complexes must be thermodynamically and kinetically stable. This means that complex formation must be thermodynamically possible and furthermore an irreversible process. Metal-ETDA and metal-DTPA complexes exhibit a corresponding high stability. To separate the Gd-DTPA complex (Fig. 5-21), which is of great relevance in the pharmaceutical industry, TBAOH was used as the ion-pair reagent [36], Detection was carried out by measuring the electrical conductivity in combination with a suppressor system. 

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