EPA compliant perchlorate determination with Metrohm

It is like trying to find a needle in a haystack: less than one microgram per liter of perchlorate in the presence of large amounts – and here we are talking about grams per liter – of chloride, carbonate and sulphate.

Perchlorate is an anion that has been at the center of public interest for several years. Even at very low concentrations it can affect the function of the thyroid gland as perchlorate prevents the uptake of iodine. In the worst case this could cause cancer or embryonic damage. Perchlorate is a constituent of many explosives in which it is used as an oxidizing agent. From airbags to US space shuttle propellant – perchlorate is used everywhere. However, the problem has less to do with its use in explosives but far more with its manufacture. In the USA this takes place chiefly in the south-west of the country near Las Vegas (Nevada). For many years the producers dumped their perchlorate waste indiscriminately or discharged it untreated into rivers, particularly the Colorado. Today unnaturally high perchlorate concentrations are found in more than 20 states in the USA and more than 15 million people are exposed to drinking water with an increased perchlorate content. In addition to drinking water, perchlorate also accumulates in fruits and vegetables with a high water content and in this way enters the food chain. The regulations of the EPA (US Environmental Protection Agency) require a detection limit of less than 1 ppb perchlorate (1 ppb = 1 µg/kg » 1 µg/L). This places very high demands on the analysis as regards its sensitivity and selectivity.

What is Ion Chromatography (IC)?

Chromatography is a method for separating mixtures of substances using two phases, one of which is stationary and the other mobile moving in a particular direction. Chromatography techniques are divided up according to the physical states of the two participating phases. The term Ion Exchange Chromatography or Ion Chromatography (IC) is a subdivision of High Performance Liquid Chromatography (HPLC).

A general definition of ion chromatography can be applied as follows;" ion chromatography includes all rapid liquid chromatography separations of ions in columns coupled online with detection and quantification in a flow-through detector". A stoichiometric chemical reaction occurs between ions in a solution and a solid substance carrying functional groups that can fix ions as a result of electrostatic forces. For anion chromatography these are quaternary ammonium groups. In theory ions with the same charge can be exchanged completely reversibly between the two phases. The process of ion exchange leads to a condition of equilibrium, the side to which the equilibrium lies depends on the affinity of the participating ions to the functional groups of the stationary phases.

The instrumental solution: tandem IC

With a classical ion chromatography system it is possible to detect 10 ppb perchlorate in the presence of 1 g/L each of chloride, carbonate and sulfate. However, the system is working at its limit. The required detection of 1 ppb perchlorate is not possible. Nonetheless, if a switch is made from a standard IC system to a more complex tandem IC system then 0.5 ppb can be reliably detected. One system is used to separate the matrix and only those parts of the sample that contain perchlorate reach the analytical column of the second system, on which the fine separation takes place. An intermediate detector is optional.

Detection of 0.5 ppb perchlorate in ultrapure water using a tandem IC system. Only that part of the sample that elutes between 15 and 26 min reaches the preconcentration column in the injection valve of system 2.

Separation column: Metrosep A Supp 5 – 150 (6.1006.520)

Eluent: 10 mmol/L Na2CO3, 15% acetone

Flow rate: 0.8 mL/min

Sample volume: 3.5 mL

The Metrohm tandem IC system is reliable and easy to handle; however, in comparison with a standard IC system it is somewhat complex and, as it practically consists of two independent systems, it is also relatively expensive. A monolithic column Rapid developments in the separation column sector have now permitted a return to a simpler IC system without having to accept any lack of performance. The column of choice is the Metrosep Anion Dual 4 – 50. What is basically new with this type of column is the material on which the analytes are separated. Whereas classical IC columns contain small polymer spheres with a diameter of, for example, 5 µm (for Metrosep A Supp 5 columns), the new Metrosep separation columns contain a so-called monolithic material. This is a solid based on silica gel that is functionalized with the ion exchanger material in a very complex process. The solid is characterized both by its high porosity and its high capacity. As the counterpressure of monolithic columns is very low, they can be used at very high flow rates of 1 to 5 mL/min. This makes very rapid separation possible, which for the perchlorate determination translates into a short retention time and a narrow peak. The excellent sensitivity and selectivity of the method described here are made possible by the high number of theoretical plates and the high capacity of the Dual 4 columns. The fact that the application is not limited to standards but also functions with real samples is shown in the above chromatogram. In drinking water from Houston it was possible to detect 1.75 ppb perchlorate. Thanks to the high capacity and outstanding separation performance of the monolithic Dual 4 column it is possible to work with a classical IC system. With the high-performance conductivity detector of the Advanced IC System from Metrohm a perchlorate detection limit of 0.1 ppb can be achieved. Metrohm has extended the range of columns for ion chromatography by three monolithic separation columns:

• Metrosep Anion Dual 4 – 25 (6.1016.010) for rapid separations
• Metrosep Anion Dual 4 – 50 (6.1016.020) with higher capacity, e.g. for the perchlorate application
• Metrosep Anion Dual 4 – 100 (6.1016.030) with very high capacity, for complex separating problems

The new monolithic column Metrosep Anion Dual 4 is available in three different lengths.

More perchlorate application solutions

If, for example, fruits or some types of vegetable are analyzed by IC then coelution of the perchlorate with organic acids could occur. In this case the combined use of ion chromatography and mass spectrometry is absolutely essential. In close cooperation with Agilent, Metrohm can also offer an optimal solution for such applications – see Metrohm Information 2/2004 and 1/2005, downloadable from www.metrohm.info.