Summary
This report provides a description of the analytical method and quality-control protocols for the determination of haloacetic acid (HAA) formation potential used by the U.S. Geological Survey California Water Science Center, Sacramento, laboratory. HAA formation potential is defined as the amount of HAA produced by chlorination of a water sample under specified, standard conditions. Nine HAA compounds are measured: bromochloroacetic acid, bromodichloroacetic acid, dibromochloroacetic acid, dibromoacetic acid, dichloroacetic acid, monobromoacetic acid, monochloroacetic acid, tribromoacetic acid, and trichloroacetic acid.
The analytical method includes producing the HAA, extracting and methylating the HAA, and then analyzing the methylated compounds by gas chromatography. The HAA are formed from dissolved organic carbon in water samples by dosing filtered water samples with chlorine in the form of sodium hypochlorite under specified conditions of pH (8.3), temperature (25°C), chlorine contact time (7 days), residual-free chlorine at the end (2–4 mg/L), and darkness. A surrogate compound, 2-bromopropionic acid, is added to the samples after the residual-free chlorine is quenched. Samples then are acidified, sulphate salts are added, and the HAA compounds are extracted from the aqueous solution with methyl tert-butyl ether (MTBE). The compounds then are methylated by adding acidified methanol to the MTBE extracts and heating the mixture. The MTBE-methanol mixture then is extracted with dilute sodium hydroxide to yield the final MTBE extract containing the methylated HAA and surrogate. An internal standard (IS), 2-bromo-1-chloropropane, is added to the final extracts, and the extracts are analyzed by gas chromatography. Chromatographic separation between the nine methylated HAA, the methylated surrogate, and the IS compounds is achieved with an Rtx-5 column and the analytes are detected with an electron-capture detector. HAA concentrations in the samples are quantified using a standard curve constructed from aqueous solutions extracted and methylated by the same procedure. The calibration ranges for the nine HAA are 0.2–100 μg/L for bromochloroacetic acid, bromodichloroacetic acid, and monobromoacetic acid; 0.5–250 μg/L for dibromochloroacetic acid; 0.1–50 μg/L for dibromoacetic acid and trichloroacetic acid; 0.3–150 μg/L for dichloroacetic acid and monochloroacetic acid; and 1–500 μg/L for trichloroacetic acid. The accuracy, precision, and method detection limit (MDL) for the method were determined. The MDL for the nine HAA ranged from 0.11 to 0.45 μg/L. Accuracy and precision were assessed using matrix spike experiments with natural water samples. Mean percent recovery of spike concentrations of the nine HAA ranged from 99 to 117 percent, with percent standard deviation of the means of 17 to 28 percent. Quality-control, data storage, instrument maintenance, and corrective action protocols were described. Quality-control protocols entail regular analysis of a variety of quality-control samples: instrument, extraction, and full-procedural blanks; continuing calibration verification standards; duplicate samples; matrix spikes; and independent quality-control standards. The use of a surrogate and an IS also are quality-control measures. The HAAFP data and associated quality-control information are stored in the California Water Science Center’s Laboratory Information Management System (LIMS), and also may be entered into the USGS National Water Information System database. Original chromatograms are archived and linked to the LIMS. Instrument maintenance and corrective actions are undertaken promptly.