Analysis of Perchlorate in Water by Non-Suppressed Ion Chromatography-Mass Spectrometry

Introduction

Perchlorate is a chemical substance that occurs both naturally and as a manufactured compound, and is used in a wide range of applications. It is a common accelerant in rocket engines and an explosive in the pyrotechnics industry. From the standpoint of health, however, it is now suspected to be associated with hypothyroidism. Thus, in 2005, the U.S. Environmental Protection Agency (EPA) set the reference dose (RfD) of perchlorate to 0.0007 mg/kg body weight/day. This reference dose is reflected in the limit of 24.5 ppb in drinking water. In Japan, perchlorate is a required test item as specified in the Waterworks Law. For perchlorate analysis, the EPA method specifies that following separation by reversed phase or anion- exchange chromatography, detection must be conducted by mass spectrometry. For anion-exchange chromatography in the non-suppressed mode as specified in Method 331.0, a 100–200 mmol/L volatile salt solution is used as the eluent. Despite its volatility, highly concentrated salt eluent is a source of contamination which can adversely affect ionization efficiency or contaminate an instrument, so the amount used must be kept to a minimum. Here, we introduce an example of analysis of perchlorate using the Shim-pack IC-A3 (S) anion- exchange column. This column permits the analysis of perchlorate using a low-concentration volatile salt eluent, which is suitable for the mass spectrometer. We checked the analytical conditions using an eluent consisting of an aqueous solution of 25 mmol/L ammonium acetate containing 20 % acetonitrile, and the perchlorate ion eluted in about 16 minutes. To prevent the anions (F^-, Cl^-, NO₂^-, Br^-, NO₃^-, PO₄^3-, SO₄^2-), etc. from being introduced into the mass spectrometer before the later eluting perchlorate ion, the portion of the eluate from 0–11.5 minutes was diverted from the column to waste. During that time, a solution with the same composition as the eluent was directed into the mass spectrometer using a separate pump to maintain liquid flow into the source and prevent a shock to the interface due to a sudden change from zero to constant flow.

February 29, 2012 GMT