Determination of inorganic anions by high performance liquid chromatography

Chromatographic techniques, and in particular ion-exchange methods, have long been used for the analysis of inorganic ions using diverse monitoring techniques such as conductivity, polarography and spectrophotometry. A major advance in this field occurred in 1975, when Small et al. described a novel system for the chromatographic determination of inorganic ions, in which a specially developed pellicular resin was used to effect the separation and a second ion-exchange column to reduce the background conductivity of the eluent in order to improve the detection limits for the eluted analyte ions. This system, which is described in detail later, illustrated the enormous potential of chromatography for inorganic ion analysis and deservedly received considerable research attention for some years. An important side effect of this development was to stimulate greatly research into alternative chromatographic methods suitable for inorganic ions, and over the past 7 years a considerable volume of material has been published. It is the purpose of this paper to review these alternative methods, and therefore those articles which employ the same or a similar technique to that used by Small et al. will be discussed only briefly in order to provide a basis for comparison with other methods. Further to the above specific restriction, this review will be confined to a discussion of methods that are chiefly applicable to anion analysis, because until very recently these methods have far outnumbered those for cation analysis. The preponderance of research into chromatographic methods for anions strongly reflects the paucity of alternative analytical procedures. This situation does not apply to cations, for which a number of excellent rapid and sensitive spectroscopic techniques (such as atomic-absorption spectrometry and inductively coupled plasma atomic-emission spectrometry) and electrochemical methods (such as polarography and anodic-stripping voltammetry) are available. Moreover, many of these are multi-element techniques and so duplicate one of the chief attractions of chromatography for inorganic cation analysis. Despite this, much of the experience gained from the intensive development of chromatographic approaches to inorganic anion analysis has recently been applied successfully to inorganic cations. At this stage, however, it is fair to say that these chromatographic methods for cations are generally inferior to the existing spectroscopic and electrochemical methods mentioned above. In contrast, the use of chromatography for anion analysis has proved so successful that chromatographic methods are among the best available and have been applied to a wide range of inorganic species. This success can be attributed to concurrent advances in separation technology and detection methods, and in this review these two aspects will be described separately.