Chromium (Cr) is one of the constituents currently undergoing review by the U.S. Environmental Protection Agency (USEPA) as part of the 6
year regulatory review process. California uses total Cr as a screening measurement for
hexavalent chromium (Cr-6) in compliance monitoring. One issue of importance in these areas
is the accuracy of Cr and Cr-6 measurements. Several vendors have developed mixing
chambers 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 sample
into the main quadrupole in ICPMS methods such as 200.8 (e.g. Perkin Elmer Dynamic
Reaction Cell-DRC). Use of the DRC significantly improves accuracy of total Cr
measurements and raises questions about much reported compliance data that was reported
without using this technology.
The study analyzed in excess of 500 samples of groundwater, bottled water, potable water, and
raw surface water for both Cr-6 by USEPA Method 218.6 and total Cr by 200.8 for each of the last
4 years. This
has allowed direct assessments of both the typical percentage of Cr-6 and an assessment of
factors affecting the accuracy of measurements. It has been demonstrated that total Cr by
ICPMS can be subject to false positives due to interference from carbon, even when samples
are held under acidified conditions before analysis to remove CO2. However, false negatives
are not as well understood, although they have been well documented.
In 2001 and 2002, a phenomenon was observed whereby many samples appeared to have more
Cr-6 than total Cr, a physical impossibility. At the time it was hypothesized that total Cr
measurements by ICPMS were subject to a false negative as a result of possible differences in
instrument response to trivalent and hexavalent Cr, because addition of peroxide increased
recovery of hexavalent Cr when measured by ICPMS. Park et al (2004) postulated that false
negatives are due to interaction with iron in samples, selectively removing Cr at low pH. In
2002, Cr measurements by 200.8 were made using a mass correction for
carbon 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. In
2003 the study began using the DRC in lieu of direct carbon correction. In 2002, nearly 50% of
these measurements of total Cr were less than the Cr-6 concentrations, with values as high as
200%. In contrast, in 2003, using DRC mode, less than 20% of samples had apparent "excess
Cr-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 accuracy
due to use of the DRC. While the presence of iron may be a compounding factor in preventing
accurate analysis of Cr, use of the DRC appears to overcome most matrix interferences in
drinking water sources. Includes 4 references, table, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 8 |
| Published : | 11/15/2004 |
