Conductivity and indirect UV absorption detection of inorganic cations in non-suppressed ion chromatography using aromatic bases as eluents : I. Principles of operation

Abstract

An equation is developed to describe the retention behaviour of inorganic cations in non-suppressed ion chromatography and it is shown that a linear relationship exists between the logarithm of the concentration of the eluting species in the eluent and the logarithm of the solute capacity factor. Moreover, the negative slope of this relationship is equal to the ratio of the charges on the solute and eluent ions, provided only one eluent species is responsible for elution. The conductivity change accompanying elution of a solute cation can be positive or negative, depending on whether the eluent cation has a high or low limiting equivalent ionic conductance. When an aqueous solution of an aromatic base is used as eluent, the direction of the conductivity signal when a solute cation elutes is dependent on the relative eluting roles of hydrogen ion and the protonated base cation. When both of these species contribute to the elution of a solute, detection sensitivity is decreased in comparison to the situation where only one species is responsible for elution. An equation for the conductivity change occurring on elution of a solute cation is presented. Indirect UV absorption detection is also possible when aromatic bases are used as eluents, provided the role of hydrogen ion as an eluting species is minimised. This can be achieved using an eluent comprising an aqueous solution of an aromatic base with p K a value in excess of 7.5, and by maintaining the eluent pH in the neutral to alkaline region. Benzylamine at pH 7.15 is shown to be a suitable eluent for indirect UV absorption and conductivity detection and using this eluent, detection limits in the low parts per billion range (for an injection volume of 10 μl) were obtained for lithium, sodium, ammonium and potassium, with both detection methods.