Total Chromium & Hexavalent Chromium Measurement
Chromium (Cr) is the 21th most abundant element in the Earth's crust, and almost all Cr is from chromite (FeCr2O4) ore. Natural weathering (e.g. rock corrosion) and industrial activities (e.g. leather tanning, stainless steel welding, plating solutions, and Cr pigment production) are common ways Cr is transmitted to our air, water, soil, and food.
In its aqueous phase, chromium exists in two different chemical forms:
- Chromium-3, also called chromium (III), Cr(III), or trivalent chromium
- Chromium-6, also called chromium (VI), Cr(VI), or hexavalent chromium
Cr(III) is essential to human beings and involved in different metabolisms in the body, while Cr(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 Cr(VI) are important for drinking water analysis.
In chromium testing, either total chromium or Cr(VI) is tested to meet regulatory compliance.
Chromium as a carcinogen has been regulated in drinking water since 1942. The U.S. EPA currently only regulates total chromium, including Cr (VI). In 1991, the U.S. EPA set the maximum Contaminant Level (MCL) for total chromium at 100µg/L (100 ppb) but different States can have stricter regulations.
After the National Toxicology Program (NTP) published the study results that indicate ingestion of Cr(VI) causes cancer in laboratory animals in 2008, 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 the 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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In addition to the U.S., many other countries, including those of the European Union, also regulate total chromium either at the same or different levels.
Solutions for total chromium analysis
Total chromium in either drinking water or wastewater for regulatory compliance can be measured with inductively coupled plasma optical emission spectrometry (ICP-OES) using EPA method 200.7 or inductively coupled plasma mass spectrometry (ICP-MS) using EPA method 200.8. Only EPA 200.8, not 200.7, is approved for Unregulated Contaminant Monitoring Rule 3 (UCMR 3) for total chromium analysis because the method offers lower detection limit for the occurrence monitoring for the program.
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.
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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 was the only state that implemented a new regulation with the public health goal (PHG) of 0.02µg/L and MCL of 10µg/L for Cr(VI). According to the new regulation, all California public water systems were required to monitor the sources of drinking water by January 2015. However, due to the recent loss of lawsuit, California Water Board removed the MCL of 10µg/L for Cr(VI) in August 2017, a new MCL will be issued in 18 to 24 months. Until then, no Cr(VI) regulation is enforceable.
In addition to California, countries such as Japan and China also regulate Cr(VI).
Solutions to chromium (VI) analysis
Prior to the new California regulation in 2014, EPA method 218.6 was approved for Cr(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 describe important methods developed for Cr(VI) analysis related to EPA Methods 218.6 and 218.7.
EPA method 218.7 uses a Thermo Scientific ion chromatography (IC) 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 0.0054µg/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, column diameter (e.g. 4mm i.d. vs. 2mm i.d.), eluent flow rate, reagent coil volume, and reagent flow rate. It is worth mentioning that 2mm i.d. column can also be used in method 218.7 to get more sensitive results (AU179).
In addition, although not approved for regulatory use, our recently developed method using suppressed conductivity, with detection limit of 1µg/L, eliminates the post-column derivatization step necessary for EPA 218.6 and 218.7 and may be used for screening and evaluation purposes for Cr(VI) contamination.
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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. Because 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 systems since only isocratic elution is needed for separation of chromium species.