Suppressor void volume

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. 

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. 

In HPLC peak dispersion is significantly affected by the particle size of the resin. Thus, the band broadening in a suppressor column decreases as the particle diameter of the employed resin decreases. Similarly, the void volume of the suppressor column should be as small as possible to reduce band broadening effects. An optimal suppressor column would therefore have a very low volume and would contain an exchange resin with a very small particle diameter. Both requirements are inconsistent with the analytical practice, which calls for a suppressor column that can be employed at least for one working day without regeneration. This requirement can only be met by suppressor columns with a higher volume. 

FIGURE 28 23 A micromembrane suppressor. Eluent flows through a narrow channel that contains a plastic screen that reduces the void volume and appeeirs to increase mass-transfer rates. The eluent is separated from the suppressor solution by 50-pm exchange membranes. Regenerant flow is in Ihe direction opposite to eluent flow (Courtesy ol Dionex Corporation. Sunnyvale. CA.)... 

The particle size of the resin and the void volume of the suppressor column affect the quality of the separation, because both parameters determine the peak broadening. The total volume of the suppressor column should be as small as possible to prevent mixing of the already separated signals. For the resulting suppression capacity, however, the total volume of the suppressor column should be as large as possible. These two requirements are incompatible, so the dimensioning of packed-bed suppressors is always a compromise between the suppression capacity and the peak broadening caused by the void volume. Therefore, it is advisable to use suppressor columns only in combination with separator columns packed with resins that have a particle diameter of more than 15 pm. Under these conditions, the contribution to the total peak variance is small. 

This setup represents a certain improvement over conventional packed bed suppressors, because the void volume of the cartridges is very small. However, as a consequence, the suppression capacity of the MSM is also very small, so that this kind of packed bed suppressor can only be used for low ionic strength eluants (see Fig. 3-18 in Section 3.4.1). High-capacity anion exchangers as well as gradient elution techniques cannot be utilized with this suppressor module. 

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