Determination of Trace Anions in High Purity Waters by High Volume/Direct Injection Ion Chromatography

INTRODUCTION There has been considerable interest in the determination of anions at trace levels by IC. For example, the Electric Power Research Institute (EPRI) has established IC as the analytical technique for determining sodium, chloride, and sulfate down to 0.25 μg/L (ppb) in power plant waters. For high-purity water used in semiconductor processing, the Semiconductor Equipment and Materials International (SEMI) recommends the use of IC for tracking trace ionic contaminants from 0.025 to 0.5 μg/L. To determine ions from mg/L (ppm) down to mid-μg/L (ppb) levels with IC, a sample size of 10 to 50 μL is sufficient. If lower levels of determination are required, then a preconcentration or trace enrichment technique has typically been utilized. With this method, the analytes of interest are preconcentrated on a small precolumn in order to “strip” ions from a measured sample volume. This process concentrates the desired species, resulting in lower detection limits. However, preconcentration has several disadvantages. Compared with a direct injection method, additional hardware is required, such as a concentrator column used to preconcentrate the ions of interest and a sample pump used for loading the sample. An additional valve is often required for switching the concentrator column in and out of line with the analytical column. Extra time is required for the preconcentration step. Also, analyte loading efficiency can be compromised when additives are present. This occurs because the matrix acts as an eluent for ions that have been retained on the concentrator column. This application note describes the use of a gradient separation using the IonPac AS11 column for the determination of trace anions by high volume/direct injection. This method is applicable to high-purity water as well as power plant high-purity waters containing corrosion inhibitors. The instrumentation, techniques, and representative applications of this method are discussed in this application note.