Total Chromium & Hexavalent Chromium Measurement

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 0.02 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. Only 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.

Total Chromium

The U.S. EPA currently only regulates 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 health effect concerns, the EPA is currently reviewing both the health effects (from NTP and other recent research results) and the 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.

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Solutions for total chromium analysis

EPA method 200.8 can be used for metals analysis, including total chromium, in drinking water, surface water, groundwater, and wastewater. Sample preparation differs depending on:

  • If you are measuring the dissolved analytes or the total recoverable analytes
  • If the sample is drinking water with a turbidity of less than 1 NTU
  • If you are measuring other water samples

Acid digestion is only needed for total recoverable element analysis of water samples, with the exception of drinking water samples with turbidity less than 1 NTU. Nevertheless, all the samples need to be preserved in nitric acid to make sure the pH is less than 2 during storage. For total chromium measurement in UCMR3, all the samples are required to be acid digested regardless of water turbidity.

Instruments for total chromium analysis

The Thermo Scientific™ iCAP™ RQ 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. 

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, EPA method 218.6 was approved for chromium (VI) analysis. To comply with the new regulation, EPA method 218.7 must be used to achieve the low limit of detection. Our application updates  AU144 and  AU179 are important methods developed for chromium (VI) analysis. In addition, although not approved for regulatory use, our recently developed method below meets the low detection limit requirement for the new California regulation and eliminates the post-column derivatization step necessary for EPA 218.6 and 218.7.

Solutions to chromium (VI) analysis

EPA method 218.7 uses a Thermo Scientific™ ion chromatography system to measure chromium (VI), as described in Table 1 in the method. The IC instrument combined with a guard column, to remove hydrophobic organics, and the analytical column can achieve the detection limit of 0.0044 or 0.0054 ug/L, 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 (e.g. 4 mm i.d. vs. 2 mm i.d.), eluent flow rate, reagent coil volume and reagent flow rate. It is worth mentiong that 2 mm i.d. column can also be used in method 218.7 to get more sensitive results (AU179).

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Chromium speciation

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 with 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.

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