Keywords: PERCHLORATE, CONTAMINANT CANDIDATE LIST (CCL), DRINKING WATER,
An ion chromatograph and mass spectrometer were purchased by the Technical Support Center (TSC) of the Office of Ground Water and Drinking Water (OGWDW) in October 2002, and have been on loan to CERB for development of a low-level perchlorate method that should be completed and peer-reviewed by early FY05.
With a few modifications, the ion chromatography conditions used in EPA Method 314.0 have been used successfully in this work. The modifications have been to use microbore as opposed to standard bore columns so that a lower mobile phase flow rate can be used. This is necessary for optimum electrospray ionization sample introduction into the mass spectrometer. Other modifications that have been made have been to use an eluent generator, and an ultra clean conductivity suppressor with electrolytic suppression as opposed to chemical suppression. In chemical suppression, sulfuric acid is used as the regenerant. The small amount of sulfate that crosses the conductivity suppressor membrane, although not a problem for conductivity detection, results in high background counts at m/z 99 due to a minor sulfur isotope (HSO4-). High concentrations of sulfate in samples also presents problems by elevating the baseline at m/z 99. Although the predominant ion of perchlorate is m/z 99, based on the 75% abundance of the Cl-35 isotope, the m/z 101 ion has proved to be better for reporting purposes due to the better signal to noise for samples containing sulfate.
An oxygen-18 enriched isotopic perchlorate standard was found to be pure enough to use as a simple internal standard. As an internal standard it has improved ruggedness of the method tremendously. Without the internal standard, instrumental drift could cause percent recoveries of check standards to drift as much as 15% over the course of a day of continuous analysis of high ionic samples. With the internal standard, normalizing for instrumental drift and matrix effects, recovery and accuracy have been improved as well as the period over which a calibration is valid.
The method has been evaluated with respect to low-level calibration, detection limits, precision, accuracy, and ruggedness. The method is capable of low-level calibration from 0.01 - 1.0 ug/L (ppb) with an r^2 value of 0.999 or better. With the use of a weighted least squares linear regression model, the calibration range can be extended to 10 ppb. Detection limits in deionized water and in synthetic waters containing up to 1000 mg/L (ppm) each of chloride, sulfate and carbonate are between 0.02 - 0.06 ppb whether quantitating on mass 99 or mass 101. Precision of 1.0 ppb replicate injections at all the masses of interest has been <10% RSD. Accuracy as measured by analysis of an external QC sample is within 5% of the certified concentration. Fifteen hours of continuous analysis has been possible with the use of the enriched isotopic standard.
All the goals for this project have been met and the laboratory work for the method development effort is complete. A draft method has been completed and is undergoing internal review prior to being sent out for external peer-review. Efforts in the early part of FY05 will be focused on editing the method, conducting the peer review of the method and responding to comments. Also in FY05, publication of a holding time study of perchlorate in raw to finished drinking water and in biologically active surface waters is planned.
Perchlorate (ClO4-) is a drinking water contaminant originating from the dissolution of the salts of ammonium, potassium, magnesium, or sodium in water. It is used as an oxidant in solid propellant for rockets, missiles and fireworks, as a component in air bag inflators, an additive in lubricating oil, rubber, paints and dyes and it is used in many other industries. From accidental releases and disposal, perchlorate has become a contaminant in surface and ground waters where it is highly mobile and, due to its chemical stability, persists for decades. The primary human health effect is inhibition of iodide uptake by the thyroid gland. By disrupting thyroid hormone production, perchlorate interferes with metabolism and can affect brain development in fetuses and children, leading to mental impairment. When perchlorate was placed on the Contaminant Candidate List (CCL) in 1998, the U.S. EPA published provisional concentration levels in water believed to be protective of human health pending more health effects studies. Data from the Unregulated Contaminant Monitoring Rule (UCMR), health effects studies, water treatment options and more sensitive analytical methods will help the U.S. EPA make informed decisions on the possible regulation of perchlorate in drinking water.
In March 2000, the EPA published a rule specifying that EPA Method 314.0 was approved for perchlorate monitoring. EPA Method 314.0 is a relatively low-level method having a method detection limit of 0.53 ppb and a minimum reporting limit of 4.0 ppb. Although toxicological studies are still in progress, recent feeding studies on pregnant female rats showed changes in developmental brain morphology of the developing fetuses at the lowest dose tested. These results indicated that the concentration level of concern for perchlorate in drinking water may be less than 1 ppb, a concentration that is an order of magnitude below the original estimate of 18 ppb and below the minimum reporting level (4 ppb) of EPA Method 314.0.
The ability to quantify perchlorate at sub-ppb concentrations will be necessary should perchlorate be regulated or undergo a second occurrence survey under the UCMR in 2004. Ultimately, the intended impact of this research is to provide a method for the sub-ppb quantitation of perchlorate in drinking water so that the Office of Water, Agency risk assessors and managers, the scientific community at large and drinking water utilities will have an analytical method(s) that produces data of known quality for collection of chemical occurrence data or for regulatory requirements at concentration levels that are protective of human health.
Office of Ground Water and Drinking Water (Dave Munch, Hiba Shukairy, Valerie Blank, Kesha Forrest)