An Evaluation of Sources of Error in the Measurement of Chromium and HexavalentChromium in Potable Water Samples

摘要

Chromium (Cr) is one of the constituents currently undergoing review by EPA as part of the 6rnyear regulatory review process. California uses total Cr as a screening measurement forrnhexavalent chromium(Cr-6) in compliance monitoring. One issue of importance in these areasrnis the accuracy of Cr and Cr-6 measurements. Several vendors have developed mixingrnchambers where reaction gasses are introduced to filter out interferences such as Argon-Carbonrn(ArC). The inert gas reacts with the element of interest prior to the introduction of the samplerninto the main quadrupole in ICPMS methods such as 200.8 (e.g. Perkin Elmer DynamicrnReaction Cell-DRC). Use of the DRC significantly improves accuracy of total Crrnmeasurements and raises questions about much reported compliance data that was reportedrnwithout using this technology.rnWe have analyzed in excess of 500 samples of groundwater, bottled water, potable water, andrnraw surface water for both Cr-6 by EPA Method 218.6 and total Cr by 200.8 for each of the lastrn4 years. Many sites have been sampled and analyzed multiple times over this period. Thisrnhas allowed direct assessments of both the typical percentage of Cr-6 and an assessment ofrnfactors affecting the accuracy of measurements. It has been demonstrated that total Cr byrnICPMS can be subject to false positives due to interference from carbon, even when samplesrnare held under acidified conditions before analysis to remove CO2. However false negativesrnare not as well understood, although they have been well documented.rnIn 2001 and 2002, we observed a phenomenon whereby many samples appeared to have morernCr-6 than total Cr, a physical impossibility. At the time it was hypothesized that total Crrnmeasurements by ICPMS were subject to a false negative as a result of possible differences inrninstrument response to trivalent and hexavalent Cr, because addition of peroxide increased