This page updated July 19, 2007.
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Toxic Performance Audits |
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The Quality Assurance Section (QAS) conducts laboratory and through-the-probe field performance audits which serve as a valuable tool to assure the validity of the collected toxics data. Laboratory performance audits of the analytical methods for toxic volatile organic compounds (VOCs) are conducted annually to assess the accuracy of the methods used by the laboratory to measure the ambient concentrations. Audits are conducted by supplying the laboratory with a cylinder containing a mixture of VOC standards prepared by the National Institute of Standards and Technology (NIST). The laboratory analyzes the contents of the cylinder following their normal standard operating procedures. Toxic Metals laboratory audits are conducted semi-annually by the QAS. Toxic metals laboratory audits are performed using NIST traceable filter standards spiked with a known lead (Pb) concentration. Laboratories undergoing an audit are asked to analyze the filter standards using graphite furnace atomic absorption spectrometry following procedures set forth by Method MLD005. After the results are obtained, a comparison is made with the known concentrations, and percent differences are calculated. Flow audits of the Xontech 920 toxic metals sampler are conducted annually at each site to ensure accuracy of the sampling of toxic metals (total metals and hexavalent chromium) and aldehyde compounds. Flow rates are a determining factor in calculating concentration and are included as part of the quality assurance program. The flow audit is conducted by comparing the indicated flow of each of the three sampling channels to the true flow as measured by a NIST certified flow transfer standard. For comparison purposes, a percent difference is calculated between the sampler flow rate and true flow rate. Summary statistics are available in graphical form for total metals, hexavalent chromium, and aldehydes audit results collected in 2002-2004. Through-the-Probe (TTP) performance audits are conducted annually at each toxics monitoring site to assess the integrity of the sampling equipment and transport system, and the accuracy of the analytical methods used by the laboratory to measure the ambient concentrations of VOCs. In a TTP audit, a gaseous mixture of VOCs prepared by NIST is mixed with purified air under controlled conditions and introduced into the sampling probe of a toxics air sampler. The audit sample is humidified to 50%-75% using a specially designed dilution unit. The audit sample is then shipped to the laboratory along with other regular ambient samples, where it is analyzed following normal standard operating procedures. Whole Air Sampler performance checks are used to compare the analytical methods used by all the laboratories that measure ambient concentrations of toxic compounds. A sampler draws ambient air for three hours, filling up to twelve canisters at a time to an approximate pressure of 14 psig each. A canister is sent to each participating laboratory for analysis. The laboratories follow standard operating procedures in assaying the contents of the canister. The laboratories report the results of their analyses to the QAS, and the results are compared among the other participating laboratories. Summary data for laboratory and TTP audits reflect data collected since 1999. Summary data for toxic metals audits and whole air comparison checks reflect data collected since 1999 and 2001 (respectively). For the laboratory and TTP audits and whole air comparison check, each compound is linked to a page showing the results in a graphical format. Please note that the laboratory analytical methods have undergone changes, either to improve the method's sensitivity and / or precision, or to improve the laboratory's efficiency, and in most cases these results have not been adjusted to reflect any biases that may have been introduced by the new methods. Available Technical Papers:
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Long Liu, Program Contact - lliu@arb.ca.gov |
Toxic Air Contaminants Monitoring