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OVERVIEW OF A NEW EPA METHOD: DETERMINATION OF PERCHLORATE IN DRINKING WATER, GROUNDWATER AND HIGH SALINITY WATER BY ION CHROMATOGRAPHY, SUPPRESSED CONDUCTIVITY WITH ELECTROSPRAY IONIZATION MASS SPECTROMETRIC DETECTION
Citation:
Hedrick, E J. OVERVIEW OF A NEW EPA METHOD: DETERMINATION OF PERCHLORATE IN DRINKING WATER, GROUNDWATER AND HIGH SALINITY WATER BY ION CHROMATOGRAPHY, SUPPRESSED CONDUCTIVITY WITH ELECTROSPRAY IONIZATION MASS SPECTROMETRIC DETECTION. Presented at Texas Commission on Environmental Quality, Environmental Trade Fair and Conference, Austin, TX, May 3-5, 2004.
Impact/Purpose:
The goal of this project is to develop a method for the analysis of perchlorate in drinking water with a method detection limit between 0.02 and 0.1 ug/L (ppb) and a minimum reporting limit between 0.1 and 0.5 ug/L (ppb). [Note: this effort focuses on development of a sensitive and accurate method for laboratory analysis of perchlorate in drinking water: a related NERL task under the RARE program, 12505, focuses on development of a screening method for use with source waters.]
Description:
In this presentation the analytical instrumentation and procedures necessary to qualitatively and quantitatively determine low levels of perchlorate (ClO4-) in drinking waters using ion chromatography with electrolytic conductivity suppression, electrospray ionization mass spectrometric detection (IC-ESI-MS) will be described. Using an anion exchange separator column, ClO4- is separated from constituent cations and anions in natural water samples using a potassium hydroxide mobile phase. During the time frame that ClO4- elutes from the column, the eluate is passed through an electrolytic conductivity suppressor to remove the K+ ions of the mobile phase and the analyte counter ions prior to the eluate entering the mass spectrometer. The masses of interest for ClO4- are 99 and 101, based on the 75.53% relative abundance of the chlorine-35 isotope and 24.47% abundance of the chlorine-37 isotope, respectively. Accurate quantitation is made possible by using an oxygen-18 enriched ClO4- internal standard which has a mass-to-charge ratio of 107. Qualitative identification of ClO4- is made by retention time and mass ratio confirmation (99:101) within predetermined quality control limits.
U.S. EPA type method detection limits are between 0.02 - 0.06 ppb whether quantifying on the mass 99 or mass 101 ion. The lowest concentration minimum reporting limit (LCMRL) using the mass 99 ion is 0.15 ppb. The LCMRL is defined as the lowest concentration for which the 99% prediction interval for results is within a predefined acceptance limit of 50-150% recovery. The method is applicable to waters containing up to 3,000 ppm total dissolved solids without sample dilution or sample pretreatment to remove the common anions chloride, sulfate or carbonate (Cl-, SO42-, CO32-). Of the common anions, Cl-, SO42-, CO32-, sulfate is the most problematic due to an isotope of sulfur contributing to the background at mass 99 (HSO4-). For samples containing more than 1,000 ppm sulfate, pretreatment to remove the sulfate is recommended to maintain optimal method performance. Analysis of perchlorate concentrations less than1.0 ppb is possible in high salinity samples (>30,000 ppm TDS) by sample dilution and in some cases sample pretreatment to reduce sulfate levels.