Detection limit. A sensitive chromatographic method was developed to measure sub-part per-billion levels of the disinfectant by-products iodate (IO 3 ), chlorite (ClO 2 ), and bromate (BrO 3 ) in drinking water. As the oxyhalides emerge from the column, they
react with Br to make Br 3, which is measured by its strong absorption at 267 nm. For example, each mole of bromate makes 3 mol of Br 3 by the reaction
BrO3- + 8Br- + 6H+ †’ 3Br3- + 3H2O
Bromate near its detection limit gave the following chromatographic peak heights and standard deviations (s). For each concentration, estimate the detection limit. Find the mean detection limit. The blank is 0 because chromatographic peak height is measured from the baseline adjacent to the peak. Because blank 0, relative standard deviation applies to both peak height and concentration, which are proportional to each other. Detection limit is 3s for peak height or concentration.

Bromate Peak height Relative Number of measurements concentration (arbitrary (Hg/L) 0.2 0.5 1.0 2.0 units) 17 31 56 standard deviation(%) 14.4 6.8 3.2 1.9