System peaks Suppressor

The term system peak refers to signals that may not be attributed to solutes. System peaks are characteristic for ion chromatographic systems that have have no suppressor system when weak organic acids are used as the eluent. Despite numerous publications concerning this subject [72-76], system peaks were often the reason of misinterpretations. However, some facts about the thermodynamic and kinetic processes that occur within the separator column may be inferred [77,78] from their occurrence and help in understanding the chromatographic processes. 

Eluent dips or system peaks and their causes were first described by Gjerde and Fritz [13]. Stevens et al. [3] described the effect of the system peak in suppressed ion chromatography. Called a carbonate dip, the system peak was said to be the absent peak (from the injection) of the carbonic acid that is retained by the unexhausted portion of suppressor column. 

In his pioneering paper, Small suggested silver nitrate as the eluent for alkaline-earth metals [3], which also exhibits a high affinity toward the stationary phase. However, this system required a suppressor column in the chloride form to precipitate the silver as insoluble silver nitrate. Application of this eluent quickly proved to be disadvantageous, since peak broadening and high backpressure from the suppressor column is associated with the precipitation. 

Hence, the proportionality between peak area and concentration still exists. Suppressor systems cannot be used in this case because they would convert the anions mentioned above into the corresponding acids, which are not suitable for conductivity detection because of their weak dissociation. Although sodium hydroxide fulfills all requirements for indirect conductivity detection, its elution power is in many cases not sufficient to elute anions of practical relevance. Retention times are significantly shortened by adding small amounts of sodium benzoate to the sodium hydroxide solution, which barely affects the conductivity difference between eluent and analyte ions. However, shorter retention times improve the peak shape, thus increasing the sensitivity of the method. Figure 3.145 shows such separation. 

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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