Determination Of Organic Acids And Inorganic Anions In Semiconductor-Industry Water

摘要

Compared with traditional water-usage and water-discharge systems, treating semiconductor-industry reclaim water for reuse is more cost-effective and can produce a higher-quality product. There is less wear-and-tear on processing equipment, since reclaim water is typically cleaner than city water. Additionally, some municipalities are beginning to restrict water usage, resulting in a practical need for reprocessing procedures. However, knowing what contaminants are in the reclaim water is necessary for the efficient development of a repurification program. The organic contamination in water-purification process systems typically has been evaluated solely via total-organic-carbon (TOC) monitoring. These inline monitors can measure values sub-ppb and provide information on the overall "health" of the DIW system. However, with the advent of water reuse, this "bulk-property" number is not sufficient, and more details on the identity and level of each constituent are desired. A step in this direction has been the introduction of liquid chromatography coupled with organic-carbon detection (LC-OCD), a technique developed by Stefan Huber. This analysis provides quantitative information of classes of organics (e.g., humics, organic acids, polysaccharides) at sub-ppm levels. In some situations, identities and amounts of specific compounds are needed. One class of compounds that is of growing interest is organic acids (e.g., acetic, formic, lactic, oxalic). Water-purification systems can tolerate high levels of these ionizable organics, but not neutral species such as alcohols and ketones. Knowing which species are present lets the water-system owner know exactly what will be sent to the equipment. To help address this situation, a rapid ion-chromatographic method has been developed to quantify eight organic acids (as well as seven common anions) that typically are seen in reclaim waters. Calibration results, detection the sample was reanalyzed to identify and quantify the various component(s). This second analysis is shown in Figure 6. Both acetate and formate were found to be present, at 1 ± 1 ppb for acetate and 5 ± 2 ppb, respectively; the uncertainties are at the 95% confidence level. Another example is the analysis of a point-of-use (POU) sample. The chromatogram (front-end portion) for the ppt-level-anion screen is shown in Figure 7. The organic-acid analysis revealed the presence of three organic acids: acetate, formate, and propionate (see Figure 8). Concentrations were 15 ± 1 ppb, 44 ± 20 ppb, and 2 ± 1 ppb, respectively (confidence level = 95%). It should be noted that the sample had to be diluted 1-to-10 in order to quantify formate, since its concentration exceeded the upper limit (i.e., 32 ppb) of the calibration curve. Consequently, the uncertainty had to be multiplied by the dilution factor of ten.