AWWA WQTC59004

U.S. Environmental Protection Agency (EPA) Method 314 (Hautman et al. 1999) was developed as an improvement to the 1997 perchlorate (ClO₄) analytical method established by the California Department of Health Services. The minimum reporting limit (MRL) for Method 314 was set at 4 ug/L (with an MDL of <1 ug/L) and was shown to be consistently achievable, even in high TDS matrices. This MRL has been used for both the California and Federal Unregulated Contaminant Monitoring Rules (UCMR) for ClO₄. However, following the promulgation of these regulations, both EPA and California determined that the level of interest for ClO₄ could well be below 4 ug/L and possibly as low as 1 ug/L. In response to this increased level of interest the authors have evaluated some of the options that have been proposed for reducing the reporting limit to sub-ug/L levels. Of particular interest were modifications to Method 314 that could be performed to reduce the MRL without significantly increasing the cost of the analysis in lieu of going to other instrumental methods. Methods based on LC-MS or IC-MS have been investigated by others as highly sensitive techniques for trace level ClO₄ determination, but would require additional instrumentation not readily available in most labs. Because of the high visibility of ClO₄ as an environmental contaminant, it is critical that methods be able to reliably and accurately quantify ClO₄ to at least 1 ug/L. Several options exist for potentially reducing ClO₄ reporting limits. Because MWH Labs analyzes in excess of 100 samples per week for ClO₄, reflecting a variety of different aqueous matrices, it was critical to ensure that the selected methods could withstand continued use in a high production laboratory. Potential methods evaluated included: increasing the signal-to-noise ratio by reducing the background signal and noise through the use of various suppressors or suppressor operating configurations, increasing the sample size without exceeding column capacities, looking at alternative pretreatment techniques to remove interferences, and performing on-line pre-concentration with selective "heart cutting" (Tian et al, 2003). It was also important to ensure that the method modifications could still meet the quality control criteria of Method 314, which is considered the "gold standard" for ClO₄ monitoring. One way this was done was to consistently include standards at the ppb/sub-ppb level in analytical runs under various conditions to monitor response on an ongoing basis. This paper provides an evaluation of each approach that was tested and make recommendations for future developments. Includes 4 references, tables, figures.