Suppressor dead volume

In summary, a chemical suppressor containing an anionic resin (e.g. ArCH2(NR)3OH) is associated to a cationic separation column (ArS03H) in order to neutralise the mobile phase. The limitation of this type of suppressor lies in its very large dead volume that diminishes separation efficiency by remixing ions before their detection. It must be periodically regenerated and can only be used in the isocratic mode. 

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. 

The first approach uses a suppressor device which is located between the analytical column and the detector cell. This device chemically removes the mobile-phase buffer counterions, thus reducing the background conductivity. This type of detector increases postcolumn dead volume and puts... 

The packed column suppressor, originally introduced by Small et al., suffers from a number of drawbacks, such as time shifts due to Donnan exclusion effects, band broadening (due to a large dead volume and high dispersion), and oxidation of nitrite, which is easily oxidized to nityrate, due to the formation of nitrous acid in the suppressor. Because of these limitations, they were only practical for isocratic elution. However, the main disadvantage of the method is the necessity for periodical regeneration of the suppressor (also called stripper) to restore its ion-exchange capacity. 

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... 

Micromembrane suppressors introduced in 1985 use thin, fiat ion-exchange membranes to enhance ion transport while maintaining a very low dead volume, providing a high suppression capacity, with low dispersion. 

These had some major drawbacks. To contain enough resin for continuous operation, the suppressors had a very large dead volume that caused considerable peak dispersion and broadening. Regeneration of the resin bed was another serious problem. After several hours of operation, the ion exchange bed became expanded and had to be regenerated. This was done offline with sulfuric acid (for anion chromatography) it was flushed with water, then placed back on-line. 

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. 

A flat membrane suppressor from Dionex, known as the Micro-Membrane Suppressor (MMS), had a much higher capacity and lower dead volume than previous devices and was able to operate around the clock with minimal attention. 

The introduction of the nanobead-agglomerated lonPac ASS many years ago significantly facilitated the analysis of polarizable anions. The hydrophobicity of the functional groups bonded to the nanobeads was lowered, so that polarizable anions could be eluted with a standard carbonate/bicarbonate eluent. To minimize adsorption effects, some / -cyanophenol was added to this eluent. The influence of / -cyanophenol on the peak shape is illustrated in Figure 3.55 (see Section 3.4.1.4). Due to the compatibility of this eluent with commercial membrane suppressors and the subsequent decrease in dead volume, peak broadening was significantly reduced. 

Both methods use a low-capacity cation exchanger as a stationary phase and a dilute mineral acid such as hydrochloric or nitric acid as a mobile phase. Although stationary phases and eluents have changed over the years, the principal difference between the methods is the same up to the present day. For his hypothetical experiments. Small kept constant the volume of the stationary phase, the ion-exchange capacity of the separator colunm, the selectivity coefficients for sodium and potassium relative to the hydronium ion, and the injection volume. With these values and the known acid concentration in the mobile phase, it is possible to calculate the elution volumes of sodium and potassium. To further simplify the calculation of the elution profiles, the chromatographic peaks are assumed to be symmetrical, so that they can be described by a Gaussian curve. One can further assume that the membrane-based suppressor system exhibits a very small dead volume and, therefore, subtracts negligibly from the efficiency of the separator column, which is estimated to be 3000 theoretical plates. 

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