The development and certification of Standard Reference Materials (SRMs) to assess and ensure accurate measurement of Pb in the environment

The National Institute of Standards and Technology (NIST) has had a major quality-assurance role in the federal effort to reduce lead poisoning of children in the United States through its mission of ensuring the accuracy of chemical measurements. NIST certifies reference materials (standard reference materials--SRMs) that are used to benchmark measurements by secondary and field methods of analysis--to ensure that decisions of great health and economic impact are soundly based on good measurement science. Over the past 10 years, in cooperation with the US Environmental Protection Agency (EPA), US Department of Housing and Urban Development (HUD), and the United States Geological Survey (USGS), NIST has prepared and certified SRMs for lead content in soil, indoor dust, and paint. The role of these materials in meeting regulatory and abatement needs is described and their certified values are summarized.

Development of isotope dilution cold vapor inductively coupled plasma mass spectrometry and its application to the certification of mercury in NIST standard reference materials

An isotope dilution cold vapor inductively coupled plasma mass spectrometry (ID-CV-ICPMS) method featuring gaseous introduction of mercury via tin chloride reduction has been developed and applied to the quantification and certification of mercury in various NIST standard reference materials: SRM 966 Toxic Metals in Bovine Blood (30 ng x mL(-1)); SRM 1641d Mercury in Water (1.6 microg x mL(-1)); and SRM 1946 Lake Superior Fish Tissue (436 ng x g(-1)). Complementary mercury data were generated for SRMs and NIST quality control standards using cold vapor atomic absorption spectroscopy (CVAAS). Certification results for the determination of mercury in SRM 1641d using two independent methods (ID-CV-ICPMS and CVAAS) showed a degree of agreement of 0.3% between the methods. Gaseous introduction of mercury into the ICPMS resulted in a single isotope sensitivity of 2 x 10(6) counts x s(-1)/ng x g(-1) for 201Hg and significantly reduced the memory and washout effects traditionally encountered in solution nebulization ICPMS. Figures of merit for isotope ratio accuracy and precision were evaluated at dwell times of 10, 20, 40, 80, and 160 ms using SRM 3133 Mercury Spectrometric Solution. The optimum dwell time of 80 ms yielded a measured 201Hg/202Hg isotope ratio within 0.13% of the theoretical natural value and a measurement precision of 0.34%, on the basis of three replicate injections of SRM 3133.

Isotope dilution mass spectrometry and the National Reference System

The clinical laboratory community of the United States, which is well represented by the NRSCL/NCCLS, has endorsed the IDMS/DMs developed at NIST. These DMs provide the accuracy (true value) base for the U.S. National Reference System for a number of specific analytes in human serum. Fortunately, the U.S. government through (a) actions of NIST administrators and scientists, (b) financial support from NIH (NIGMS) and FDA, and (c) interagency agreements with CDC has accepted the responsibility for developing and maintaining IDMS/DMs for clinically important analytes as an essential part of this national measurement system infrastructure. Furthermore, it is important to note that several professional organizations, particularly, The American Association for Clinical Chemistry (AACC) and The College of American Pathologists (CAP), have interacted heavily with NIST in full support of these national standardization activities. CAP supports three full-time Research Associates at NIST so that target values on serum samples used in its Interlaboratory Comparison Survey Programs may be traced to DMs. This remarkable cooperation and teamwork between government agencies and private sector organizations, as well as numerous individual scientists and physicians, which promotes greater accuracy of patient results, depends heavily upon the continued timely availability of IDMS/DM measurements. In short, NIST's value assignments on human serum samples (e.g., SRMs and materials for CLIA '88 proficiency testing programs) by this critical IDMS/DM metrology provide the pragmatic base for assuring accurate test results in medicine. The resources required to support IDMS/DM technology at NIST over many decades are not trivial and from time to time require renewed R&D efforts to upgrade methodology and recapitalization in mass spectrometry instrumentation.

Determination of iodine in oyster tissue by isotope dilution laser resonance ionization mass spectrometry

The technique of laser resonance ionization mass spectrometry has been combined with isotope dilution analysis to determine iodine in oyster tissue. The long-lived radioisotope, 129I, was used to spike the samples. Samples were equilibrated with the 129I, wet ashed under controlled conditions, and iodine separated by coprecipitation with silver chloride. The analyte was dried as silver ammonium iodide upon a tantalum filament from which iodine was thermally desorbed in the resonance ionization mass spectrometry instrument. A single-color, two-photon resonant plus one-photon ionization scheme was used to form positive iodine ions. Long-lived iodine signals were achieved from 100 ng of iodine. The precision of 127I/129I measurement has been evaluated by replicate determinations of the spike, the spike calibration samples, and the oyster tissue samples and was 1.0%. Measurement precision among samples was 1.9% for the spike calibration and 1.4% for the oyster tissue. The concentration of iodine determined in SRM 1566a, Oyster Tissue, was 4.44 micrograms/g with an estimate of the overall uncertainty for the analysis of +/- 0.12 microgram/g.

Detection of Uranium from Cosmos-1402 in the Stratosphere

The nuclear reactor from the Soviet radar reconnaissance satellite, Cosmos-1402, reentered the earth's atmosphere on 7 February 1983 and disintegrated over the South Atlantic Ocean. The reactor was powered by approximately 50 kilograms of uranium-235 ((235)U). In an effort to determine the fate of the reactor core, a series of aerosol samples were collected at altitudes between 27 and 36 kilometers in the Northern Hemisphere approximately 1.1 years later by high-altitude balloons. At an altitude of 36 kilometers a 53 +/- 20 percent excess in the (235)U concentration was measured. The total excess of (235)U in the stratosphere was calculated to be 44 +/- 15 kilograms.

Laser Resonance Ionization Mass Spectrometry

The analysis of inorganic atomic species is greatly facilitated by the coupling of lasers with mass spectrometers. A tunable dye laser, alone or in combination with a pump laser, ionizes atoms by resonant excitation processes; the ions are then analyzed in the mass spectrometer. The laser-mass spectrometer system promises to overcome traditional limits of sensitivity and selectivity and to have diverse applications in analytical chemistry.