Safer drinking water through automated 2-D ion chromatography
Jeffrey Rohrer, Brian DeBorba and Richard Jack
Two-dimensional ion chromatography (2-D IC) provides improved detection limits and ease of analysis for trace ions in complex, high-salt liquids. Improved IC column chemistries are enabling highly specific and complementary first- and second-dimension separations of ions. IC systems and software that support automated valve switching make it possible to link the two dimensions immediately without manual intervention. The capabilities and broadening role of 2-D IC for trace ion analysis are illustrated in two pending EPA methods for determining bromate, a regulated carcinogen, and perchlorate, which inhibits iodide uptake by the thyroid gland.
Even trace concentrations of bromate or perchlorate in drinking water are associated with increased health risks. Unfortunately, regulatory limits on these contaminants have been constrained historically by the detection limits of available methods. Measurement of target ions at very low concentrations is especially challenging in the presence of high concentrations of other ions, as found in bottled mineral water and groundwater, where interferences may exceed analyte concentrations 100-fold or more.
A dual IC system with built-in signal enhancement can remove interfering ions and detect target analytes at sub-μg/L concentrations, achieving detection limits similar to or better than post-column methods and rivaling mass spectrometry at a lower cost. The new methods avoid the need for time-consuming off-line sample preparation and the potential of column overload seen with previous approaches.
Matrix Interference and Removal
Trace-level analysis in IC is typically performed using concentrator columns or large-volume injections. When high levels of interfering ions are present in the sample matrix, those ions may prevent the analyte of interest from remaining on a concentrator column and instead wash them into the effluent waste. For that reason, large-volume injections are preferred over concentration methods for high-salt matrices. Unfortunately, large-volume injections enhance not only the signal of trace components, but that of other matrix components as well. In some cases, the matrix components interfere with analysis by co-eluting or by behaving as an eluent, broadening the trace component peak and leading to poor detection. This is often the case with perchlorate and bromate determination.
Two-dimensional IC enables the use of large-volume injections by capturing the analyte-containing fraction after the first-dimension separation, then diverting the rest of the sample matrix to waste. The analyte fraction is focused on a concentrator column, and then eluted onto a second, smaller-dimension analytical column. This approach avoids the column overload that often results from high ionic-strength matrices by separating the ions twice: once on a high-capacity column to remove high-concentration interferences, then on a smaller-bore column to resolve trace-level analytes.
Instrumentation and Software
The ICS-3000 Reagent-Free IC system (Dionex Corp., Sunnyvale, CA) includes a dual-pump, dual-valve configuration and automation through Chromeleon software for easy implementation of 2-D IC (Figure 1). For both the first- and second-dimension separations, automated eluent generation (EG) ensures consistent eluent concentrations, minimizing baseline shift during step changes or gradients. The suppressors enhance the signal by neutralizing the eluent back to DI water before conductivity detection in Cells 1 and 2.
Figure 1. Diagram of a 2-D IC system Click to enlarge.
The first dimension uses a large-loop injection onto a 4-mm high-capacity column, allowing injection of more sample for increased detection of trace components. After partial separation of the analyte from matrix ions in the first dimension, the fraction of effluent containing the analyte is transferred to the concentrator column (lower right), while the remaining matrix is diverted to waste. The second pump and eluent generator are used to perform a higher-resolution separation of the analyte on a smaller 2-mm column. This column can use a different column chemistry that further separates co-eluting peaks and reduces the possibility of false positives.
The final analyte peak reaching detector Cell 2 is clearly defined from background and interfering ions, for a high-sensitivity determination. The entire process, including valve changes for matrix elimination, data collection and analysis, is automated using Chromeleon software.
The columns, valves and detectors are mounted inside the same compartment, so that the separations, matrix removal and concentration are all performed in a controlled environment and with very little intervening flow volume. This minimizes dispersion of the analyte between separations. A conventional IC system is usually equipped with a single injection valve.
Installing additional valves for 2-D separations may result in long flow paths, leading to peak diffusion and poor detection. For this reason, a system with built-in dual capabilities is best for 2-D IC methods.
Perchlorate Determination
Perchlorate is a relatively common groundwater contaminant originating from manmade and natural sources. It inhibits the normal uptake of iodide by the thyroid gland, which results in reduced thyroid hormone production, improper metabolic regulation and potentially the development of thyroid tumors. In the United States, perchlorate has been detected at nearly 400 sites, mostly in the western and southwestern regions. It is estimated that more than 11 million people have perchlorate in their drinking water supplies at concentrations of 4 μg/L (ppb) or greater.
Although there are no federal drinking water regulations for perchlorate, various states have adopted their own advisory levels that range from 1 to 18 μg/L. While perchlorate is typically analyzed in the μg/L range, the matrix ion concentrations in drinking water range in the hundreds to thousands of μg/L, requiring special methods to eliminate matrix interferences.
Earlier EPA Methods for perchlorate required an off-line matrix elimination step with solid-phase extraction cartridges. Later improvements used a trap column for matrix elimination, but required confirmation on a second column to reduce the likelihood of a false positive. The new EPA method, 314.2, simplifies the quantification of perchlorate in high salt matrices. This 2-D IC method can detect perchlorate to a limit of quantification (LOQ) of 0.06 μg/L and a method detection limit (MDL) of 0.016 μg/L.1
This method uses 2-D IC as an in-line approach, using a large-volume sample loop (4 mL) in the first dimension. The injection valve in the second dimension is fitted with a concentrator column that focuses the heart-cut analyte peak from the first dimension for further analysis, while the matrix is diverted to waste.
There are several advantages of the 2-D matrix diversion approach. The first-dimension 4-mm column allows a large sample injection volume, due to the high capacity of the analytical column and higher selectivity for perchlorate relative to the matrix ions. Second, the perchlorate peak that is partially resolved in the first dimension is focused on the concentrator column. The suppressed effluent from the first dimension is water, which provides the ideal environment for ion exchange retention and focusing. Third, the second dimension uses a column with a smaller cross-sectional area relative to the first dimension, increasing sensitivity. Finally, this approach allows the combination of two different chemistries in two dimensions, thereby enabling a selectivity not possible using only a single chemistry dimension.
Figure 2. 2-D separation and determination of perchlorate Click to enlarge.
Figure 2a shows the analysis of a sample in the first dimension, using a 4 mm IonPac AS20 column, KOH eluent, and suppressed conductivity detection. The sample included 5 ppb perchlorate in the presence of 1000 ppm matrix ions (chloride, bicarbonate and sulfate). As can be seen in this figure, the matrix interferes with detection of the perchlorate peak, which is broadened and difficult to quantify. The perchlorate-containing fraction of this same sample was concentrated then analyzed using the 2-mm IonPac AS16 column in the second dimension. As shown in Figure 2b, perchlorate (peak 1) is well resolved and free from matrix effects. This 2-D IC approach yields a sensitivity improvement proportional to the cross-sectional area ratio of the first dimension versus the second dimension: a four-fold gain. Due to the physical proximity of the two dimensions in the system and minimal delay volume, it was possible to achieve excellent peak shape and recovery.
Bromate Determination
Bromate is a disinfection byproduct, produced during the ozonation of drinking waters contaminated with bromide. Bromate has been identified as an animal carcinogen and a potential human carcinogen by the International Agency for Research on Cancer. The U.S. EPA and FDA have established a regulatory maximum contaminant level (MCL) of 10 μg/L bromate in drinking water and bottled water. More recently, the European Commission set a lower MCL of 3 μg/L bromate for natural mineral waters and spring waters treated by ozonation. However, these limits were based on the feasibility of detection and removal. Studies suggest concentrations lower than 1 μg/L pose increased lifetime cancer risks. The U.S. EPA has estimated a potential cancer risk of 1 in 105 for lifetime exposure to drinking water containing 0.5 μg/L bromate.
Traditionally, IC with suppressed conductivity detection has been used to determine chlorite, bromate and chlorate in environmental waters, with initial MDLs of 20 μg/L. Later methods reduced the bromate MDL to 1.4 μg/L, using large-volume injection followed by suppressed conductivity detection. Sample pretreatment and preconcentration can further reduce the bromate detection limit to <1 μg/L. However, these methods are subject to matrix interferences.
Alternatively, postcolumn derivatization methods can also quantify bromate at sub-μg/L concentrations. Initial methods used suppressed conductivity and the postcolumn addition of o-dianisidine (ODA). However, because ODA is a potential human carcinogen, more recent methods use a postcolumn reaction that generates hydroiodic acid (HI) in situ, that combines with bromate from the column effluent to form the triiodide anion (I3-) detected by absorbance. Although postcolumn reaction methods do not suffer from interferences by common anions, column overloading with high ionic-strength samples can cause peak broadening and an associated loss of response. Pending EPA method 302.0 eliminates the need for postcolumn reagents by using 2-D IC. It achieves an MDL of 0.036 μg/L.2
Figure 3. 2-D separation and determination of bromate. Click to enlarge.
The first dimension in this method uses a high capacity 4-mm IonPac AS19 column to resolve the bromate from the matrix ions. The matrix ions are diverted to waste while a 2-mL plug (cut volume) containing the bromate is transferred to the second dimension for analysis. Bromate is well resolved in the second dimension using a 2-mm IonPac AS24 column. The fully automated 2-D IC method avoids column overload during analysis of high-ionic-strength matrices, eliminates the cost and disposal of the chemicals required for postcolumn configurations and simplifies the experimental setup.
Conclusion
Two-dimensional IC methods can provide lower detection limits and improved recoveries of perchlorate and bromate while eliminating matrix interferences in a wide variety of high-ionic-strength matrices. The ICS-3000 system provides an effective platform for 2-D IC. The close proximity of the valves to the columns and detector cells in this system design provides a low dispersion platform for multidimensional sample preparation and analysis. Samples can be injected directly without requiring the use of pretreatment cartridges, sample dilution or sample degassing for carbonate removal prior to analysis. The elimination of off-line sample preparation and automation of two-dimensional analysis saves time and improves consistency between analysts and laboratories.
References
1. Application Note 178: Improved Determination of Trace Concentrations of Perchlorate in Drinking Water Using Preconcentration with Two-Dimensional Ion Chromatography and Suppressed Conductivity Detection. Dionex Corporation, 2007.
2. Application Note 187: Determination of Sub-μg/L Bromate in Municipal and Natural Mineral Waters Using Preconcentration with Two-Dimensional Ion Chromatography and Suppressed Conductivity Detection. Dionex Corporation, 2007.
Jeffrey Rohrer is the Director of Corporate Applications Development, Brian DeBorba is the Corporate Applications Chemist and Richard Jack is the Director of Corporate Market Development, Environmental, all from Dionex Corp. They may be reached at ChromatographyTechniques@advantagemedia.com.
