Environmental and occupational exposure to toxic metals is assessed through whole blood analysis. Due to advances in instrument technology, clinical laboratories that perform whole blood analyses are moving away from single-element methods towards methods capable of measuring panels of toxic elements and species.
To support method development and provide quality assurance of clinical measurements of toxic metals in blood, NIST has provided a multi-level standard reference material (SRM) 955c Toxic Metals in Caprine Blood consisting of frozen caprine blood value-assigned for mass fraction of: lead (Pb), arsenic (As), cadmium (Cd), total mercury (THg), inorganic mercury (IHg), methylmercury (MeHg) and ethylmercury (ETHg) .
Over the last three decades, NIST, with support from the Centers for Disease Control (CDC), and in conjunction with the New York State Department of Health’s (NYSDOH) Wadsworth Center, has produced a number of frozen whole blood certified reference materials containing endogenous lead (Pb). These are used by clinical laboratories to evaluate the accuracy of blood lead concentration measurements and for use in validating working or secondary reference materials for lead in blood analysis. Starting with the issue of SRM 955c, additional toxic elements and species were spiked into the blood in order to serve both the Pb testing and the toxic metals measurement communities. Mercury toxicity is dependent on chemical form, necessitating measurement of the specific mercury species. Ethyl mercury is a concern, because it is a metabolite of thimersol, which has been used as a preservative in vaccine and is being studied in the proposed link between autism and mercury exposure. Arsenic, Cd, and THg are typically analyzed in concert as a “heavy metal screen” for diagnosis of acute poisoning, utilizing the multi-element analysis method, inductively coupled plasma mass spectrometry (ICP-MS). Half of all blood analyses in the U.S. are performed using ICP-MS. SRM 955c was produced at NYSDOH’s Wadsworth Center from Pb-dosed goats and is comprised of four vials of frozen caprine blood at four concentration levels: a base level and three progressively elevated levels. Each vial contains 2 mL of whole blood. SRM 955c serves multiple measurement communities and is the first available whole blood CRM with ETHg and As values.
To provide a single reference material with expanded analyte coverage that serves the measurement needs of both the childhood/adult Pb testing community and the wider, heavy metal toxicological measurement community.
New ICP-MS-based technologies were utilized to measure As and Hg species in SRM 955c. Gas chromatography (GC-) coupled with ICP-MS was used to measure IHg, MeHg, and ETHg. A GC-ICP-MS chromatogram obtained for SRM 955c L3 is shown in Figure 1. A new quantification scheme based on triple spike speciated isotope dilution (TS-SID) GC-ICP-MS was developed to simultaneously and accurately quantify the three mercury species . Collision cell technology (CCT-) ICP-MS coupled with standard additions was used to mitigate the effects of matrix-based interference that make it difficult to measure As. Results for the CCT-ICP-MS method were combined with results obtained using a newly developed radiochemical neutron activation analysis (RNAA) method . A highly sensitive and accurate method based on IDA cold vapor (IDA CV-) ICP-MS developed at NIST was used to measure THg . The determination of Cd by IDA ICP-MS is complicated by matrix-based interference that cannot be resolved instrumentally and matrix separation is often required . However, separation procedures can lead to greater uncertainty from contamination. An improved separation scheme was developed to reduce contamination levels enabling a more accurate measurement of Cd in SRM 955c than had been achieved for prior blood CRMs. Isotope dilution analysis coupled with ICP-MS was used to accurately quantify Pb in SRM 955c. IDA ICP-MS is a well-established, but time consuming method of analysis primarily used for the certification of reference materials . In order to evaluate the efficacy of routine methods of the analysis for Pb in blood, including graphite furnace atomic absorption spectrometry and a rapid multi-element method utilizing ICP-MS, a comparison with the IDA ICP-MS results was performed .