Ion Chromatography in Environmental Applications

Cr (VI) Analysis in Drinking Water, Groundwater and Surface Water Samples in Accordance with U.S. EPA Standard Method 218.7 Chromium is one of the essential trace nutrients required by living cells to convert fat into energy. In this context, while Cr (III) compounds pose no risk to humans, hexavalent chromium - Cr (VI) - a powerful reducing agent, is classified as a toxic substance that damages DNA due to its high toxicity as well as its carcinogenic and mutagenic effects. Acute Cr (VI) poisoning leads to liver and kidney damage, while chronic hexavalent chromium exposure causes complications in the gastrointestinal system and directly leads to lung cancer when inhaled. Today, chromium is used in various industries. In the steel industry, it is added to provide high hardness and corrosion protection, while in the electroplating industry it is used in surface treatments (e.g. plating and passive films), in leather production in tanning, and in paints and coatings as a pigment. Chromium was also previously used as a wood preservation agent. Limestone and clay, which are raw materials for cement production, also contain chromium in Cr (III) form, and a portion of this content can be oxidized to Cr (VI) form during production processes. Within the scope of this review, we aim to present the details of a high-sensitivity Ion Chromatographic detection method that is part of today's U.S. EPA Method 218.7 standard and is used for Cr (VI) in drinking water, groundwater and surface water analyses. Limits Related to Hexavalent Chromium The U.S. Environmental Protection Agency (U.S. EPA) drinking water standard sets a maximum limit of 100 μg/L or 100 ppb for total chromium concentration (including Cr (III), Cr (VI) and other forms). Since 31 December 2010, the state of California has been applying a stricter limit of 10 μg/L Cr (VI) and has defined its public health goal as 0.02 μg/L Cr (VI). The EU limit for total chromium, in parallel with the World Health Organization (WHO) and the German Drinking Water Regulation, has been set at 50 μg/L. Switzerland accepts 20 μg/L as the limit for hexavalent chromium.

Equipment and Software Used

Working in close cooperation with the U.S. EPA, Metrohm has developed a highly sensitive ion chromatographic method for Cr (VI) analysis within the framework of the current EPA Method 218.7. The detection limit obtainable with this analytical method can fall below the strict California limit of 0.02 μg/L Cr (VI).

Table 1: Equipment and software used

Figure 1: Ion Chromatography system components

Sampling

Samples subject to method development and application were obtained by collecting tap water samples from various cities and contamination areas located in Oklahoma, Florida, Texas and California. High levels of total hardness (in CaCO3 form) were observed in the samples in question, as well as the presence of high levels of chloride, sulfate, free chlorine (disinfectant) and other anionic species. All samples and standards were prepared and stored in accordance with U.S. EPA Method 218.7 Section 8 guidelines.

Analysis

Following separation on an anion-exchange column, the Cr (VI) ion is derivatized by post-column reaction with 1,5-diphenylcarbazide. The resulting purple-colored chromium-diphenylcarbazone complex is detected at 530 nm wavelength using a UV/Vis detector.

Figure 2: IC analysis system connection diagram

Table 2: Method analysis parameters

Instrument settings and method parameters are optimized within the framework of the separation column, column oven temperature, wavelength and flow rates.

 

Figure 3: Typical Cr (VI) chromatogram, drinking water

Calibration and Linearity

The calibration curve used within the method was created using 8 different low-level Cr (VI) standard solutions. Table 3: Calibration standards Cr (VI) in water (μg/L) The resulting curve is linear across all concentration ranges, with a correlation coefficient of 0.999994 and a standard deviation of 0.595%.

Effect of Chloride and Sulfate

The effects of standard ions such as chloride and sulfate on chromate were examined in detail. Studies were conducted by adding chloride and sulfate in the range of 0–200 mg/L to a tap water sample spiked with 0.2 μg/L chromate standard. The optimized chromatographic method enables recovery in the range of 90–110% up to a standard anion concentration of 150 mg/L.

Eluent - PCR Reagent Relationship

Precise maintenance of the eluent/PCR reagent ratio in the PCR reservoir is of vital importance for achieving correct and precise color formation for the Cr (VI)-DPC complex. Thanks to the Metrohm Dosino™ unit integrated into the system, precise and accurate PCR dosing can be performed and method robustness is assured.

Method Detection Limit (MDL)

The method detection limit (MDL) was calculated from results obtained from standard deviations of seven replicate water samples spiked with two different Cr (VI) concentration levels at ppt levels. MDL values of 0.007 μg/L and 0.016 μg/L were calculated for Cr (VI) spikes of 0.025 μg/L and 0.05 μg/L, respectively. These values meet EPA requirements and demonstrate that the method is suitable for routine analysis.

Lowest Concentration Minimum Reporting Level (LCMRL)

The EPA has developed user-friendly and freely downloadable software to determine single-laboratory lowest concentration minimum reporting levels (LCMRL). LCMRL is defined as the lowest spike concentration at which spike recovery in the 50–150% range can be achieved with at least 99% probability. Unlike the MDL, which only considers standard deviations based on repeated low-level spike measurements, LCMRL also includes measurement accuracy.

Figure 4: LCMRL chart for Cr (VI).

Within the scope of LCMRL calculations, any laboratory or analyst is expected to be able to quantitatively detect at least half of the LCMRL value calculated by the software. Within the scope of the study conducted, the LCMRL value calculated by the software was found to be 0.032 μg/L. In addition, one of the calibration standards is already at the 0.025 μg/L level.

Conclusion

The release of easily water-soluble hexavalent chromium into the environment is a global problem affecting millions of people and is of vital importance, particularly for drinking water sources. In this context, there is a need to use measurement methods with high sensitivity for the detection of trace-level Cr (VI) content. Within the scope of the application detailed above, a precise, accurate and reliable analysis platform is defined for trace Cr (VI) analysis in full compliance with U.S. EPA Method 218.7. Metrohm Ion Chromatography Systems provide a wide range of options ranging from entry-level routine instruments to highly automated modular platforms for trace analysis, research and method development, combining ease of use and excellent reliability with maximum flexibility to provide superior solutions that can meet the needs of all quality control and research laboratories, particularly in the water and environmental sectors. Sources 1- U.S. EPA Method 218.7, Determination of Hexavalent Chromium by IC-UV/PCR 2- Metrohm Application Work AW IC US6-0152-012011 Determination of Hexavalent Chromium (Cr+6) in drinking water, ground water and surface water by Ion Chromatography followed by Post Column Reaction (PCR) and UV/Vis detection (conforming to U.S. EPA method 218.7) 3- Metrohm Information 1195794_MI_2012_EN Determine hexavalent chromium according to EPA Method 218.7     Furkan Özçil Ion Chromatography Product Group Manager Metrohm Turkey