Total Chromium & Hexavalent Chromium Measurement

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.

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.

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.

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.

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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