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Water for public consumption must be purified prior to distribution or bottling. Chlorine is commonly used for disinfection but may create hazardous disinfection byproducts depending on the residual organic content of the water supply. Treatment with ozone is another disinfection alternative. While this method is effective, ozonolysis can also convert bromide (Br-), a natural component of many waters, into bromate (BrO3-), a carcinogen. Therefore, the need exists to measure bromate in drinking waters, which means that it must be measured separately from other forms of bromine.
In addition, each form is usually present at low concentrations. Current methods for measuring bromate and bromide involve separating the bromine-containing components by liquid chromatography (HPLC) and using inductively coupled plasma mass spectrometry (ICP-MS) as a detector. This is the protocol stated in U.S. Environmental Protection Agency (EPA) drinking water method 321.8. Bromate at 10 µg/L (parts per billion) has been regulated in drinking water in the U.S. since 1999. The Food and Drug Administration has adopted the same standard for bottled water exposed to ozonation for disinfection. Similar levels are regulated in Europe for a variety of water types.
Primary drinking water contaminants are generally measured by a single technique, such as HPLC or gas chromatography for organic components and ICP-MS for inorganic components. The use of a combined technique for inorganic speciation has only recently become routine and is rapidly advancing in developments devoted to speed and ruggedness.
Development of a speciation method requires two types of expertise. The chromatographer is best skilled for development of the separation portion of the procedure, while the inorganic analytical chemist is best suited for optimizing the detection system—in this case, ICP-MS. Each area of expertise requires a separate language and knowledge of aspects that are important in its portion of the analysis.
Bridging the gap is key to creating the best method for analysis. For example, chromatographers must consider minimizing the use of glass, organic solvents and the total dissolved solids (TDS) content of the mobile phase. All of these areas do not need to be considered for normal HPLC analyses but can cause problems for inorganic detection. For example, glass is dirty with respect to inorganic elements, and glass components can therefore contaminate a system, leading to false high results. ICP-MS systems do not handle highly organic solvents very well under normal sample introduction conditions, although modifications can be made to accommodate samples with high organic content. Finally, high TDS can deposit on various components within an ICP-MS, leading to drift over time and frequent maintenance.
The inorganic chemist must also have proper expectations about HPLC method development, which is more complex and time-consuming than development of an ICP-MS method. Important variables include the mobile phase composition and various column characteristics, such as packing material, particle size and length. The method development involves real chemistry and may take weeks to complete.
Since the development of EPA method 321.8 in 1998, advances in speciation have taken place, allowing the method to be improved for ruggedness and speed. Figure 1 shows a chromatogram of 1 µg/L of bromate, demonstrating that even at low concentrations it can be distinguished from background levels. Note that time for analysis is less than three minutes, whereas the method 321.8 chromatographic separation takes eight minutes. Figure 2 shows overlaid chromatograms of a Chinese bottled water analyzed 49 times over the course of several hours to examine the short-term precision, which is better than 2%.
For a few samples with asymmetrical peaks and high bromate values, additional method development was done to ensure the analyte of interest was separated adequately from components that might mistakenly contribute to bromate measurement. A gradient HPLC method, although slightly longer than the isocratic method developed for routine use, showed several peaks overlapping the bromate peak in some samples. Advanced interference correction on the ICP-MS showed that the additional peaks did, in fact, contain bromine. The identification was not investigated further.
Figure 3 (page 17) shows the chromatogram for a bottled water purchased in Thailand, which contains an additional peak, uncovered with the gradient separation method. Table 1 (page 16) shows the concentrations of bromide and bromate for a variety of bottled and tap water samples. The samples with suspect peaks or bromate concentrations above the regulatory limit were confirmed with the developed gradient HPLC separation to ensure that only one peak was present at the retention time expected for bromate.
Although few elements are regulated by their species, there are many whose form influences their toxicity. For example, chromium VI is known to be much more hazardous than chromium IIII. So in the future, as inorganic speciated analysis becomes more common, it is likely that measurements will yield more information on water toxicity than at present.
Speciation development continues in this improvement of existing methodology. The separations were accomplished in less than three minutes and proved repeatable from injection to injection and over several days. For those waters containing additional bromine-containing species, a gradient HPLC method was established. Taken together, the isocratic separation scheme can serve as a rapid screening method; those samples that contain additional bromine species can then be confirmed using the longer gradient method.
It is interesting to note that not all countries regulate or enforce bromate at levels considered safe in the U.S. Although bottled water may contain fewer bacteria than tap water outside the U.S., it may not contain safe levels of disinfection byproducts, such as bromate.
The future may bring additional needs for speciated inorganic analysis, for additional toxicity information and perhaps for compliance with new regulations.