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Procedure

The IC method used a constant eluent flow rate of 0.8 mL/min and a constant column/oven temperature of 55°C. The MSM stepping interval was 10 min and the conductivity detector was set at 2.3% per °C. At these conditions, the back pressure was 12 ± 0.4 MPa. The pump start-up time was at 45–60 min during the equilibration of the instrument. Calibrations for the anion analytes were established by injecting quadruplicates of 1, 5, 10, 25, 50, 100, and 200 μg/L standard mixture. The upper limit of quantification (LOQ) for Cl, $NO3−$, $SO42−$, $ClO4−$, and Cr (VI) was 10,000 μg/L (10 mg/L). For other analytes, the upper LOQ was in the range of 1000–9000 μg/L.

EPA Method 7196A was used to quantify Cr (VI) in a contaminated surface water sample and compare the concentrations obtained by the IC method. Cr (VI) concentration was determined colorimetrically at 540 nm using the diphenylcarbazide method (US EPA, 1992). In brief, 0.1 mL of sample or standard was added to a 10 mL test tube followed by addition of 1 mL each of 10% H2SO4 and 10% H3PO4. Then, 0.1 mL of 5 g/L diphenylcarbazide in acetone was added to the test tube. The mixture was then vortexed and incubated at room temperature for 5 min. Absorbance of the magenta color was analyzed using a Varian Cary 50 UV-Vis spectrophotometer (Agilent, Santa Clare, CA) at 540 nm. The spectrophotometer was calibrated using the standard Cr (VI) solution. The calibration range for the colorimetry method was 0.5–75 mg/L Cr (VI) and the detection limit was 0.25 mg/L.

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