Measuring anions and ammonium in bases and acids with ion chromatography (Second in a series)

Jian-Ge Chen and ManLi Wu, Because impurities in process chemicals can adversely affect semiconductor device surfaces, the ability to identify and measure such contaminants is important to the industry. As reported in the first article in this series, our laboratory recently developed a reliable and sensitive method for analyzing anionic contaminants at <10-ppb levels in 49% hydrofluoric acid using two-dimensional ion chromatography. However, the analysis of trace cationic impurities in concentrated acids and some anionic impurities in concentrated bases has been an especially difficult analytical challenge. Direct analysis of trace anions in concentrated bases has been limited by hydroxide concentration. When the behavior of anions in the presence of very large amounts of hydroxide was investigated, it was found that the analyte peaks and separation were changed as a result of hydroxide anion elution from the sample matrix. A similar problem is expected when analyzing cations in strong acids, where the hydronium ion matrix can interfere with the analysis. It has been reported elsewhere that low- to 30-ppb-level detection of trace anions and cations in concentrated bases and acids can be achieved with electrolytic sample pretreatment. A high-capacity electrochemical membrane suppressor electrolyzes water to create the acid or base required for neutralization of the concentrated base or acid. However, the literature indicated that high amounts of Cl can bleed out from the sulfonated membrane used in the neutralization unit and contribute to the background, which can increase the detection limits and affect the accuracy of trace-level analyses. It was also reported that, because of the diffusion of ammonia across the membrane during neutralization, the recovery of ammonium ions was only about 50%. In addition, the neutralizer unit could not be used with hydrochloric acid because chloride can be oxidized on the electrode surface, leading to the destruction of the neutralizer. This