Comparison of Suppressed to Nonsuppressed Conductivity Detection for the Determination of Common Inorganic Cations

INTRODUCTION In 1975, Small and coworkers first introduced the concept of ion chromatography (IC) that allowed the sensitive detection of ions using suppressed conductivity detection. A significant portion of this work was dedicated to cation analysis. The original components described by Small et al. for the separation of cations included a low-capacity, sulfonated polystyrene/ divinylbenzene (PS/DVB) column followed by a packed-bed suppressor in the hydroxide form and a conductivity detector. The primary purpose of the suppressor was to achieve sensitive detection of the ionic species by chemically modifying the eluent. This detection is accomplished by converting the mineral acid eluent to water and thereby achieving a very low background signal and low noise, while converting the analyte to its base form. Although mineral acid eluents are sufficient to elute alkali metals and ammonium, the low affinity of hydronium ions for sulfonated resins required a stronger eluting component, m-phenylenediamine, to elute the more retained alkaline earth metals. However, the concentrations of m-phenylenediamine required to separate the alkaline earth metals resulted in the alkali metals coeluting in the void volume. In addition to requiring two eluent systems for this analysis, the difficulty in converting the column from the mphenylenediamine to the hydronium form essentially required a separate column dedicated for the analysis of alkaline earth metals. Another major drawback of this system was the requirement for periodic regeneration of the suppressor column. Today, suppressor technology has improved considerably and the chemical regeneration requirement is a distant memory. Figure 1 shows a historical timeline of suppressor development. In 1979, a conductometric method for the determination of inorganic anions without a suppressor was first reported. This method was later commercialized and is known by various names, such as single-column IC, direct conductivity, and nonsuppressed conductivity detection. To achieve a lower background signal and therefore lower noise, nonsuppressed conductivity methods required low-capacity resins with dilute eluents. At higher conductivity levels, the influence of temperature changes become more significant, resulting in an increase in the baseline noise. Therefore, the low background requirement precludes the use of highcapacity columns that require high acid concentrations to elute the cationic species within a reasonable time. As with suppressed conductivity applications, sulfonated resins were also commonly used for nonsuppressed cation analysis, and a stronger eluting component—such as ethylenediamine—was required to separate the highly retained alkaline earth metals.