Calculation of detection limits for a single-laboratory ion-chromatographic method to determine parts-per-trillion ions in ultrapure water

This paper addresses the calculation of detection limits (DLs) for ion-chromatographic data at low parts-per-trillion (w/w) levels. The main objectives are: (1) to explain two statistical techniques (the EPA or ''3 sigma'' approach and the Hubaux-Vos method), (2) to calculate DLs using each procedure, (3) to discuss the strengths and weaknesses of each statistical approach and (4) to decide if the analytical method is appropriate for quantifying anions at the 50-ppt level in deionized water. The analytes of interest are: fluoride, chloride, nitrite, bromide, nitrate, sulfate and phosphate. All work was performed on a Dionex DX500 microbore unit, using an As11 column. Results indicate that the H-V method gives a more realistic (and higher) DL than does the 3 sigma. Assuming false-negative and false-positives probabilities of 10% or less, this analytical method is not acceptable for quantifying anions at the desired level.