Ultrapure Water for Perchlorate Analysis of Environmental Samples

Perchlorate is considered an environmental pollutant because it could affect human health by interfering with the thyroid’s uptake of iodide, disrupting this gland’s function. Laboratories that analyze samples for the presence of perchlorate require perchlorate-free purified water. This study evaluates the efficiency of various water purification technologies to remove perchlorate from feed water.

Perchlorate as an environmental contaminant

Chemistry of perchlorate

Perchlorate salts (ClO₄^-) are stable and have excellent oxidizing properties (Figure 1). For this reason, they are widely used in explosives (including pyrotechnics), solid rocket fuel, matches, airbags and certain types of fertilizers. They can also be formed through the decomposition of chlorine-based products used for disinfection, such as sodium hypochlorite.

Perchlorate is chemically inert under most conditions. As a result, it was not previously considered to be a hazardous substance and was commonly disposed of in wastewater. Perchlorate is highly soluble in water and adheres poorly to both mineral surfaces and organic material. Therefore, it can remain in ground and surface water for extended periods.

Perchlorate contamination of food and drinking water

Following the development of a sensitive detection method for perchlorate in the late 1990s¹, it became evident that widespread perchlorate contamination of ground and surface waters existed in the southwestern United States. Perchlorate present in water can be taken up by plants and concentrated in their tissues. For example, it has been found in lettuce and fruits in areas irrigated with perchlorate-containing water. It is not metabolized by animals but has been found in cow and human milk.² Perchlorate now appears to be present in various foods and drinks in Europe, the Americas, Africa and Asia.³

Health effects and risks of perchlorate contamination

Perchlorate competitively inhibits the uptake of iodide by the thyroid gland. The health effects of perchlorate, which are dependent on iodine intake levels, are not understood completely. For those with diets poor in iodine, ingesting high levels of perchlorate may have deleterious effects on thyroid function. Improper thyroid function may affect metabolic functions, growth, and cardiovascular and central nervous systems. In pregnant women, thyroid malfunction may distress the fetus and result in behavioral changes, delayed development and decreased learning capabilities.⁴ Consequently, it has become a global health concern.

Regulatory limits for perchlorate

In 2005, the U.S. Environmental Protection Agency (EPA) established an official perchlorate reference dose (RfD) of 0.7 μg/kg/day. This translates to a Drinking Water Equivalent Level (DWEL) of 24.5 µg/L (ppb), assuming a person weighing 70 kg drinks two liters of water per day. As the COVID epidemic led to increased use of chlorine-based disinfectants, the European Commission set maximum levels for perchlorate in most foodstuff to 0.05 mg/kg.⁵ The EPA Method 314.0, “Determination of Perchlorate in Drinking Water Using Ion Chromatography,” has a perchlorate minimum reporting level (MRL) of 4 ppb.⁶ Recently, more sensitive methods have been developed for trace measurements of perchlorate⁷ and measurement in various matrices.

Removal of perchlorate from laboratory water

Need for perchlorate-free water in analytical laboratories

Water is the main component of ion chromatography mobile phases and is used in many steps of the analytical process, such as sample preparation, dilutions, standard preparation, blanks, and glassware and plasticware rinsing. An increasing number of laboratories are monitoring perchlorate levels in water and food, but in many areas the local tap water may contain perchlorate. Hence, it is critical to ensure that the water used in all these analytical workflow steps is perchlorate-free.

Experimental design for perchlorate analysis

Perchlorate can be removed using anion exchange, reverse osmosis (RO) or bioremediation.⁸ Milli-Q^® water purification systems use ion exchange and/or RO combined with other purification technologies. To address the needs of different users, this study evaluated four water purification systems for their ability to remove perchlorate from tap water:

• System 1: A Direct-Q^® UV system that combines RO, ultraviolet (UV) photooxidation, activated carbon and ion exchange to produce both pure and ultrapure water from tap water.

• System 2: A Milli-Q^® pure water system, like a Milli-Q^® IX system, that combines activated carbon, RO and Elix^® EDI to produce pure water from tap water.

• System 3: A Milli-Q^® ultrapure system, similar to the Milli-Q^® IQ 7000 system, that combines UV photooxidation, activated carbon and ion exchange to produce ultrapure water from pure water.

• System 4: A Simplicity^® UV system, that combines UV photooxidation, activated carbon and ion exchange to produce ultrapure water from pure water.

In this study, tap water feeding the water purification systems was free of perchlorate. Therefore, large amounts of perchlorate (83-84 ppm) were added to the tap water before purification with the Milli-Q^® water purification systems (Figure 2). For the systems fed with pure (Type 2) water, a lower concentration of perchlorate was used (84-117 ppb). Perchloric acid, 1 N was used to spike the water. Potassium hydroxide was added for neutralization. Water was analyzed for perchlorate at the inlet and after each of the four systems’ main purification steps.

Analytical method for determination of perchlorate ions

An ion-chromatography method was developed to measure perchlorate in water. On-column pre-concentration was used to provide greater sensitivity. The retention time of the perchlorate peak was 32.5 minutes. The limit of detection (LOD) and limit of quantitation (LOQ) were calculated using the signal-to-noise ratio (S/N) of 3 and 10, respectively. The LOD and LOQ of the method were 0.005 ppb and 0.016 ppb of perchlorate, respectively.

• Ion chromatography instrument: Thermo Scientific™ Dionex™ DX-500 system with a GP50 Gradient Pump, AS40 Autosampler, LC20 Module, CD20 Conductivity Detector
• Columns: IonPac^® TAC-2 (3 x 35 mm) preconcentration column, AG19 (2 x 50 mm) precolumn and AS19 (2 x 250 mm) column
• Eluent suppressor: ASRS^® Ultra II 2 mm, 100 mA suppressor current
• Eluent: EG 40 Eluent Generator generating KOH gradient 0.5-100 mM, flow rate: 0.25 mL/min. The eluent gradient was: t₀= 0.5 mM, t₂₀ = 35 mM, t₄₀ = 100 mM, t₄₅= 100 mM, t_45.1 = 0.5 mM, t₅₅ = 0.5 mM
• Standards: Potassium perchlorate standard, 1000 mg/L diluted as needed with water from a Milli-Q^® ultrapure water system
• Sample injection: All samples were collected in a well-rinsed polypropylene container. Tap water samples were diluted (100x) before injection. Depending on the expected ion level in the water samples, one of two injection methods was used:
  • Direct injection (10 μL)
  • On-column pre-concentration (10 mL of the sample) before injection

For each system, ion chromatography was used to measure perchlorate levels of feed water and purified water following each of the main purification steps (Figure 3). For demonstration purposes, the quantity of perchlorate added to each system’s feed water was much higher than the levels found in tap water or in contaminated water, where it is typically less than 25 ppb.

Perchlorate removal by water purification systems

All four Milli-Q^® water purification systems tested removed high levels of perchlorate from water, yielding perchlorate-free water (Figure 4 and Table 1).

• The combination of activated carbon and RO removed more than 97% of the perchlorate present in the feed water (chromatogram B, Figure 4A). This is comparable to the ionic rejection levels commonly observed for anions (from 95% for Cl^- to 99% for SO₄^2-) and cations (from 90% for Na⁺ to 99% for Ca²⁺).
• When RO was coupled with Elix^® EDI, perchlorate levels were below the detection limit of 0.005 ppb (chromatogram C, Figure 4A).
• Perchlorate was undetectable in ultrapure water produced by the ultrapure Milli-Q^® water purification systems combining ion exchange with other purification technologies even when large amounts of perchlorate was spiked into the pure water feeding them (Chromatograms F’ and F’’, Figure 4B).
• UV photooxidation did not affect perchlorate and did not generate any degradation by-products (chromatogram E in Figure 4B).

In an additional test, the polishing cartridge of the Simplicity^® system (containing a combination of activated carbon and ion exchange resins) was replaced with an experimental cartridge containing only ion exchange resins. In both cases, the resulting water was free of perchlorate (data not shown). This suggests that ion exchange resins can remove perchlorate efficiently without using activated carbon.

Selecting the optimal water purification system for perchlorate analyses by ion chromatography

Perchlorate is removed efficiently from water by deionization technologies such as ion exchange or Elix^® EDI. RO can also remove large amounts of perchlorate ions from water (over 97 %), but to produce perchlorate-free water for analytical use, RO should be used in combination with other purification technologies. Laboratories testing for perchlorate need a purification system that not only provides appropriate water quality for their analytical procedures but also removes perchlorate ions efficiently. While activated carbon and UV photooxidation are not required for perchlorate removal, they are needed to remove organic impurities that may interfere with ion chromatography. All ultrapure Milli-Q^® water purification systems, such as the Milli-Q^® IQ 7000 ultrapure system, are suitable for perchlorate analyses when combined with good quality pretreatment technologies. Alternatively, an all-in-one system that combines both pretreatment and polishing steps can be used, such as the Milli-Q^® IQ 7003/05/10/15 pure and ultrapure water system.