Chromium is a carcinogen and has been regulated in drinking water since 1942. In 1991, the U.S. EPA set strict regulation levels for total chromium contamination at 100 µg/L (100 ppb).
In its aqueous phase, chromium exists in two different chemical forms: chromium-3, chromium (III), or trivalent chromium and chromium-6, chromium (VI), or hexvalent chromium. Cr(III) is essential to human beings and involved in different metabolisms in the body, while chromium (VI) is carcinogenic. Cr(VI) can be released from cooling towers into drinking water and can also come from oxidation of Cr(III) by Mn(III/IV) in drinking water. Therefore, chromium speciation and determination of chromium (VI) are important for drinking water analysis.
California’s Office of Environmental Hazard and Heath Assessment (OEHHA) set the Public Health Goal (PHG) for Cr (VI) at 2.5 µg/L in 2001 and modified to 0.2 µg/L in 2011. Effective July 1, 2014, Department of Public Health in California established a Cr(VI) Maximum Contaminant Level (MCL) in drinking water at 10 µg/L (10 ppb), and the U.S. EPA and European Food Safety Authority (EFSA) is evaluating new legislation for Cr(VI). The solution leans toward adopting speciation as the ideal method for determining chromium toxicity in drinking water.
This trend would enable laboratories to measure total chromium with inductively coupled plasma optical emisssion spectrometry (ICP-OES) or inductively coupled plasma mass spectrometry (ICP-MS) using EPA method 200.7 or EPA method 200.8, respectively. But EPA 200.8, not 200.7, is approved for Unregulated Contaminant Monitoring Rule 3 (UCMR 3) for total chromium analysis. Cr(VI) can be measured using ion chromatography as discussed in EPA method 218.6 and EPA method 218.7. But only EPA 218.7 can be used to measure Cr (VI) for the new California regulation.
The U.S. EPA currently only regulates the total chromium, including Cr (VI). The National Toxicology Program (NTP) study published in 2008 indicates that ingestion of chromium (VI) causes cancer in laboratory animals. Due to the health effects concerns, the EPA is currently reviewing both the health effects (from NTP and other recent research results) and levels and frequency of occurrence data. These data were collected through the second-six year review of regulated contaminants (for total chromium) and through Unregulated Contaminant Monitoring Rule 3 (UCMR3) program (for both total chromium and Cr(VI)). These data will be used to decide if a new regulation on Cr(VI) will be proposed.
Solutions to total chromium analysis
The EPA method 200.8 can be used for metals analysis, including the total chromium, in drinking water, surface water, groundwater, and wastewater. Sample preparation differs depending on if the dissolved analytes or the total recoverable analytes are measured, and if the sample is drinking water with turbility of less than 1 NTU or other watersamples. Acid digestion is only needed for total recoverable element analysis of water samples (except the drinking water samples with turbility less than 1 NTU). Nevethless, all the samples need to be preserved in nitric acid to make sure the pH is less than 2 during the storage. For total Chromium measurement in UCMR3, all the samples are required to be acid digested regardless of the water turbility.
Instruments for total chromium analysis
iCAP Q ICP-MS provides high sensitivity/low detection limit (for ultratrace detection) and wide dynamic range (for analytes with order of magnitude difference in concentration), in addition to low maintenance and easy to clean-up benefits.
For water samples with high matrices (high total dissolved solids, TDS), such as wastewater samples, it is always a challenge to use ICP-MS to get accurate and precise results due to signal drift caused by the high matrix. Typically, ICP-MS can handle <0.2% TDS. To improve high matrix tolerance, argon gas dilution or PrepFAST for sample autodilution can be used.
For more information
- View the on-demand webinar : Prepping your laboratory for advanced environmental analysis to view how iCAP Q ICP-MS are used for environmental applications
More on argon gas dilution and PrepFAST for ICP-MS:
Hexavalent chromium [Cr(VI)] regulations
Based on the NTP study results, in July 2014, California implemented a new regulation with the public health goal (PHG) of 0.02 µg/L and MCL of 10 µg/L for Cr(VI). According to the new regulation, all public water systems in California are required to monitor the sources of drinking water before January 2015.
Prior to the new regulation, the EPA method 218.6 was approved for chromium (VI) analysis. To comply with the new regulation, the EPA 218.7 method has to be used to achieve the low limit of detection. Our application updates AU144 and AU179 are important method development for chromium (VI) analysis. In addition, although not approved for the regulatory use, our recently developed method meets the requirement of the low detection limit required for the new Califonia regulation and eliminates the post-column derivatization step necessary for the EPA 218.6 and 218.7.
Solutions to chromium (VI) analysis
The the EPA 218.7 method uses Thermo Scientific™ ion chromatography system to measure chromium (VI), as described in Table 1 in the method. The IC instrument combined with the guard column (to remove hydrophobic organics) and the analytical column can achieve the detection limit of 0.0044 or 0.0054 ug/L in the method 218.7, depending on the solid or liquid preservation reagent used.
As shown in AU144 and AU179 , multiple parameters can influence the detection limit: injection volume, coulmn diameter (4 mm i.d. vs. 2 mm i.d., e.g.), eluent flow rate, reagent coil volume and reagent flow rate. It is worh mentiong that 2 mm i.d. column can also be used in the method 218.7, and get more sensitive results (AU179).
Speciation is used to separate and quantify different chemical forms (species) of a particular element. Since Cr(III) is not toxic and Cr(VI) is highly toxic, chromium speciation analysis can provide data for the percentage of Cr(VI) in a given water sample. One of the most powerful hyphenated techniques of Cr speciation is to use IC-ICP-MS. Since the IC system is completely metal-free, it proves especially valuable for metal speciation as traces of metal contamination can change the result and conclusion. For analytical labs that are cost concerns, the IC systems do not have to be the high-end sytems since only isocratic elution is needed for separation of chromium species.