The Determination of Perchlorate in Water using LC-MS/MS

This application note demonstrates the determination of perchlorate in water using LC-MS/MS.

Perchlorate is both a naturally occurring and man-made chemical. Naturally occurring perchlorate is a contaminant in fertilizers.

Man-made perchlorate is used in many applications, including tanning and leather finishing, rubber manufacture and paint and enamel production. Perchlorate is also used as an additive in lubricating oils, and is a primary ingredient of solid rocket propellant. Usage and improper disposal of the highly water soluble perchlorate migrates through the underground aquifers and surface water sources, contaminating soil, drinking water and irrigation water.

Thirty-five states have detected perchlorate in drinking water at higher levels 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 southwestern U.S.

Today, Maryland, Massachusetts and New Mexico have established a one part per billion (ppb) perchlorate action limit. California and Texas have established 4 ppb. Action limits are higher in Nevada and Arizona. In February 2005, the EPA established an official reference dose (RfD) of 0.0007 mg/kg per day of perchlorate. However, the creation of sub-ppb detection limits is desirable.

Method

US EPA method 314.1, “The Determination of Perchlorate in Drinking Water Using Ion Chromatography,” 2004, is the current approved method for detecting perchlorate in drinking water. This ion chromatography method is capable of detecting sub-ppb ClO₄, but is limited by sample ionic strength, mainly chloride and sulfate concentrations, thus adversely limiting detection. Other common anions (thiosulfate, thiocyanate and iodide) elute in the same region. 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 ³⁴S isotope as H₃₄SO-₄, at 4.2% abundance, has the same MS/MS transition as perchlorate. It is critical to remove SO₄ 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 1000 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. (See Figure 1).

Ammonium bicarbonate is also a volatile buffer conducive for use with mass spectrometry. Any sodium (Na) or potassium (K) based eluent requires chemical suppression prior to MS. Chemical suppression converts KOH to H₂O. Chemical suppression is not needed for this application.

To quantitatively measure sub-ppb perchlorate requires a method detection limit of less than 0.5 ppb perchlorate, using either 3x signal to noise (S/N) or EPA protocols. Figure 2 shows a 0.5 ppb perchlorate standard prepared in reagent water, drinking water and HTDS solutions. Detection limit is approximately <0.1 ppb, defined as 3x S/N, and limit of quantification is 0.2 ppb, at 10 times S/N.

This LC-MS/MS method meets the requirements. Lower detection may be achieved by using a larger injection volume.

The use of multiple reaction monitoring (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 ³⁷Cl isotope and the ion ratio of the ³⁵Cl and ³⁷Cl isotopes. Although H₃₄SO-₄ sulfate may give a 99>83 response, it will not yield the required Cl isotope ratio (See Figure 3).

The linearity of this method was evaluated between 0.25 and 100 ppb perchlorate containing 10 ppb internal standard using a 100 μL injection. The calibration curve, using 1/x weighting, Figure 4 shows good linearity between 0.25 and 10 ppb. Similar data is observed with calibration standards prepared in HTDS solution and drinking water.

Figure 5 shows the LOD and LOQ for perchlorate quantification. LOD = 0.02 ppb, LOQ = 0.05 ppb by EPA definition of method detection limit (MDL).

This work demonstrates the feasibility of using Waters IC-Pak Anion/HR chemistry with the Waters Alliance HPLC System and the Quattro micro API Mass Spectrometer for the selective analysis of perchlorate in environmental waters.

Chromatographic selectivity of the polymethacrylate-based IC-Pak A/HR Column using ammonium bicarbonate and AcCN, eliminates the interference from sulfate and the need for sample prep to remove chloride and sulfate.

LC-MS/MS is capable of detecting <100 ppt in a high totally dissolved solids water, such as 0.3% brackish water or soils leachate, within 15 minutes injection to injection.

The 50% AcCN eliminates any build-up neutral TOC effecting column performance, normally observed with all aqueous eluents.

This chromatographic method has not been fully validated and represents feasibility. It is the responsibility of the user of this method to provide an initial demonstration of performance on the expected matrices before quantitative work can begin. However, draft EPA method 331.0 (2005) has validated the MS/MS acquisition and processing for perchlorate in drinking and waste water using a similar column and eluent.