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Chromium (Cr) is one of the constituents currently undergoing review by the U.S. Environmental Protection Agency (USEPA) as part of the 6year regulatory review process. California uses total Cr as a screening measurement forhexavalent chromium (Cr-6) in compliance monitoring. One issue of importance in these areasis the accuracy of Cr and Cr-6 measurements. Several vendors have developed mixingchambers where reaction gasses are introduced to filter out interferences such as Argon-Carbon(ArC). The inert gas reacts with the element of interest prior to the introduction of the sampleinto the main quadrupole in ICPMS methods such as 200.8 (e.g. Perkin Elmer DynamicReaction Cell-DRC). Use of the DRC significantly improves accuracy of total Crmeasurements and raises questions about much reported compliance data that was reportedwithout using this technology.The study analyzed in excess of 500 samples of groundwater, bottled water, potable water, andraw surface water for both Cr-6 by USEPA Method 218.6 and total Cr by 200.8 for each of the last4 years. Thishas allowed direct assessments of both the typical percentage of Cr-6 and an assessment offactors affecting the accuracy of measurements. It has been demonstrated that total Cr byICPMS can be subject to false positives due to interference from carbon, even when samplesare held under acidified conditions before analysis to remove CO2. However, false negativesare not as well understood, although they have been well documented.In 2001 and 2002, a phenomenon was observed whereby many samples appeared to have moreCr-6 than total Cr, a physical impossibility. At the time it was hypothesized that total Crmeasurements by ICPMS were subject to a false negative as a result of possible differences ininstrument response to trivalent and hexavalent Cr, because addition of peroxide increasedrecovery of hexavalent Cr when measured by ICPMS. Park et al (2004) postulated that falsenegatives are due to interaction with iron in samples, selectively removing Cr at low pH. In2002, Cr measurements by 200.8 were made using a mass correction forcarbon following acidification, in order to eliminate false positives from ArC. However,corrections for carbon are empirical by nature and not accurate over all ranges of carbon. In2003 the study began using the DRC in lieu of direct carbon correction. In 2002, nearly 50% ofthese measurements of total Cr were less than the Cr-6 concentrations, with values as high as200%. In contrast, in 2003, using DRC mode, less than 20% of samples had apparent "excessCr-6" and almost all of these were within the analytical error of ICPMS and IC measurements.Studies on several specific sample matrices have also supported the improvements in accuracydue to use of the DRC. While the presence of iron may be a compounding factor in preventingaccurate analysis of Cr, use of the DRC appears to overcome most matrix interferences indrinking water sources. Includes 4 references, table, figures.

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Edition: Vol. - No. Published: 11/15/2004 Number of Pages: 8File Size: 1 file , 800 KB