Determination of Perchlorate in Water

Perchlorate is both a naturally occurring and man made chemical. Naturally occurring perchlorate is used in fertilizers; man-made perchlorate is used in applications such as: tanning and leather finishing, rubber manufacture, paint and enamel production, additives in lubricating oils and as a primary ingredient of solid rocket propellant.

Usage and improper disposal of the highly water soluble perchlorate migrates through the aquifers and surface water, contaminating soil and drinking and irrigation water. Thirty five states have detected perchlorate in drinking water at levels higher than expected; the same water is used for crop irrigation. As a result, there is concern for the food safety of fruits, vegetables and grain and their use as animal feed in the U.S. Southwest.

Today, Maryland, Massachusetts and New Mexico have established a 1 ppb perchlorate action limit. California and Texas have a 4 ppb limit; action limits are higher in Nevada and Arizona. In February 2005, the Environmental Protection Agency (EPA) established an official reference dose of 0.0007 mg/kg/day of perchlorate.

Method

EPA method 314.1, “The Determination of Perchlorate in Drinking Water Using Ion Chromatography,” 2004, is the approved method for perchlorate in drinking water. This ion chromatography method is capable of detecting sub-ppb ClO 4, but is limited by sample ionic strength, mainly chloride and sulfate concentrations, adversely limiting detection. Other common anions (thiosulfate, thiocyanate and iodide) elute in the same region, for analyte identification, an alternative, simpler method is required.

The key to this application is the resolution of <0.5 ppb perchlorate from mid-ppm levels of chloride and sulfate. Sulfate is the main interferent with method 314.1, and the 34 S isotope as H 34SO – 4, at 4.2 percent abundance, has the same MS/MS transition as perchlorate. It is critical to remove SO 4 or provide baseline resolution from perchlorate. By combining chromatographic selectivity and mass spectrometry selectivity and sensitivity, perchlorate can be determined in high total dissolved solids (HTDS) water at the sub-ppb levels. HTDS is a synthetic solution defined as 1,000 mg/L each of bicarbonate, chloride and sulfate prepared in drinking water.

Using increased concentrations of organic solvent, acetonitrile (AcCN) in this case because of low back pressure, perchlorate can be chromatographically positioned after high chloride and before high sulfate. Use of ammonium bicarbonate eluent allows for the sub-ppb determination of perchlorate within 15 minutes without the need for sample prep. This is demonstrated in Figure 1.

Ammonium bicarbonate is also a volatile buffer conducive for use with mass spectrometry. Any sodium (Na) or potassium (K) based eluents requires chemical suppression prior to MS. Chemical suppression converts KOH to H 2O. Chemical suppression is not needed for this application.

To quantitatively meet the anticipated 1 ppb action limit requires a method detection limit of less than 0.5 ppb perchlorate, using either 3x signal to noise (S/N) or EPA protocols.

This LC/MS/MS method meets the requirements. Lower detection may be achieved by using a larger injection volume. The use of MRM mass spectrometry to monitor the transition of perchlorate to chlorate, 99.1>82.7 (loss of 1 oxygen) is used for perchlorate quantification. Positive perchlorate confirmation is achieved using a second transition of 101.1>84.7 for the 37 Cl isotope and the ion ratio of the 35 Cl and 37 Cl isotopes.

Although H 34SO – 4 sulfate may give a 99>83 response, it will not yield the required Cl isotope ratio. See Figure 2.

The linearity of this method was evaluated between 0.25 and 100 ppb perchlorate containing 10 ppb internal standard using a 100 µL injection. Similar data is observed with calibration standards prepared in HTDS solution and drinking water.