D. A. Becker

The role of reactor characterization in NAA quality assurance

As a result of the NBSR upgrade to 20 MW, an extensive program of recharacterization of the irradiation facilities was undertaken. These measurements were made to (1) evaluate the neutron flux density and its variability over the irradiation volume, and (2) help establish safe irradiation conditions for samples in the NBSR. Techniques used to measure the following parameters are described: thermal neutron flux density; flux variations within an irradiation volume and between irradiations; relative fast neutron flux density; maximum temperatures generated by different sample matrices in various irradiation facilities; and pressures generated in sealed containers by radiolytic decomposition.

The use of high accuracy NAA for the certification of nist botanical standard reference materials

Neutron activation analysis is one of many analytical techniques used at the National Institute of Standards and Technology (NIST) for the certification of NIST Standard Reference Materials (SRMs). NAA competes favorably with all other techniques because of its unique capabilities for high accuracy even at very low concentrations for many elements. In this paper, instrumental and radiochemical NAA results are described for 25 elements in two new NIST SRMs, SRM 1515 (Apple Leaves) and SRM 1547 (Peach Leaves), and are compared to the certified values for 19 elements in these two new botanical reference materials.

Characterization of the mineral fraction in botanical reference materials and its influence on homogeneity and analytical results

SummaryThe NIST natural-product leaf Standard Reference Materials have been widely used in developing reliable methods of analysis. A small amount of mineral matter present in these materials was separated by flotation, and characterized qualitatively by microscopy and quantiatively by neutron activation analysis. Several elements are concentrated in the mineral fraction, which can lead to analytical error through incomplete dissolution or sampling statistics. Two new candidate materials prepared by the Office of Standard Reference Materials, SRM 1515 Apple Leaves and SRM 1547 Peach Leaves, have been processed with an air-jet mill, resulting in a very finely ground leaf material, with particle size less than 200 mesh. A cyclone classifier in the process discriminates against coarse grit, so that the content of minerals in the ground material is less than in the first-generation materials. Better homogeneity was in fact observed, even down to 100 mg sample size. One must use caution, however, to assure that any inhomogeneity found in real samples are appropriately considered and dealt with.

Use of INAA, PGAA, and RNAA to determine 30 elements for certification of an SRM: Tomato Leaves, 157a

Analyses for certification have been made for the determination of 30 elements in the National Institute of Standards and Technology (NIST) Tomato Leaves renewal reference material, SRM 1573a. Three of the analytical techniques used were instrumental neutron activation analysis (INAA), radiochemical neutron activation analysis (RNAA), and prompt gamma activation analysis (PGAA). These techniques provided data on 19 elements by INAA, 10 elements by PGAA, and 7 elements by RNAA, with some overlap between techniques. For example, INAA was able to obtain overall analytical uncerainties (at the 95% confidence level) averaging ±2.2% for major and minor constituents (Ca, Mg, K), ±3.3% for constituents from 1 to 1000 μg/g (Na, Fe, Al, Mn, Ba, Zn, Rb, La, Cr), and ±6.4% for elements between 10 and 1000 ng/g (Co, V, Se, Th, Sc, Sb), using sample dry weights of approximately 150 mg. These analyses represent the most extensive use to date of nuclear analytical techniques in the certification of a trace element SRM at NIST.

Unique quality assurance aspects of INAA for reference material homogeneity and certification

SummaryInstrumental neutron activation analysis (INAA) has become one of the primary analytical techniques for certification of elemental content in biological Standard Reference Materials (SRMs) at the National Institute of Standards and Technology (NIST). One important reason why INAA has become so widely used and valuable in the certification of NIST SRMs is that INAA has unique inherent quality assurance (QA) characteristics which provide the capability for accurate analysis and which often allow the analytical values obtained to be internally evaluated and cross checked. While the NAA technique has the general characteristics of most spectroscopic techniques, the specific characteristics include uniform activation, long and well-documented excited states, highly penetrating emitted radiation, and an excited state decay process which is statistically random in nature. These characteristics work together to provide an analytical technique which can provide highly reliable analytical results and is particularly suitable for the certification of major, minor, and trace elements in biological reference materials.

The application of instrumental neutron activation analysis for the certification of the new NIST Fly Ash SRM

The National Institute of Standards and Technology (NIST) recently released the second renewal of its “Trace Elements in Coal Fly Ash” Standard Reference Material (SRM 1633b). This new material is currently certified for 23 major, minor and trace elements, and concentrations of an additional 24 elements are provided for “information only” purposes. Current plans are to certify the concentrations of a number of rare earths upon completion of additional analytical work now in progress. Instrumental neutron activation analysis (INAA) has played a major role in the certification of this new material in view of its potential for accuracy, multielemental capability, ability to assess homogeneity, high sensitivity for many elements, and essentially blank-free nature. For an element to be certified in a NIST SRM its concentration is usually determined by at least two independent analytical techniques. INAA has provided analytical information for 15 of the 23 elements certified, as well as for 22 of the 24 elements listed for “information only”. In addition, INAA has provided much of the homogeneity information for this SRM. This paper will describe these analytical procedures, and highlight those designed to optimize and assess the accuracy of the INAA measurements.

Determination of Cr in certified reference material HISS-1, marine sediment, by cold plasma isotope dilution ICP-MS and INAA: comparison of microwave versus closed (Carius) tube digestion

The recovery of Cr from National Research Council Canada (NRCC) certified reference material HISS-1 sediment was investigated by non-destructive instrumental neutron activation analysis (INAA) and isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) after two different chemical treatments: (1) microwave dissolution using HNO3, HF, and HClO4, followed by reflux with concentrated HClO4, and (2) a closed system method using sealed Carius tube digestion with HNO3 and HCl at high temperature (240 °C) and high pressure (1 × 107 Pa, ∼100 atm). The first chemical treatment gave recoveries of only ∼50% in agreement with observations of Yang et al. of NRCC. Samples digested in Carius tubes gave recoveries that ranged from 79 to 110% which may reflect heterogeneity in the samples rather than differing recoveries. This is supported by the excellent agreement of the mean value determined by INAA of 31.4 ± 4.5 µg g−1, and the mean value of 28.1 ± 3.9 µg g−1 by Carius tube digestion and the observation that these standard deviations are essentially equal and much larger than the measurement precisions. The results from this study are also in excellent agreement with the certified value of 30.0 ± 6.8 µg g−1. This preliminary study demonstrates that chemical treatment of this sediment with HNO3 and HCl acids in closed system Carius tubes is an efficient and capable preparation method for the determination of Cr in silicate materials which typically contain refractory Cr-bearing minerals. This study also demonstrates the unique advantage of using two independent methods for the determination of Cr in a complex matrix which has a high potential for low chemical recovery.

Certifying the chemical composition of a biological material — a case study

A worldwide laboratory intercomparison was organized by the International Atomic Energy Agencys Analytical Quality Control Services (AQCS) involving the determination of trace elements in plant materials used for human consumption. The National Institute of Standard and Technology (NIST) Standard Reference Materials Program donated 5 kg of spinach designated for the production of the future Standard Reference Material SRM 1570a to this intercomparison; the AQCS provided a similar amount of cabbage. For the study, 150 units of each material were distributed and 114 laboratories reported results on both materials to AQCS. The results for the spinach, encompassing more than forty elements, have been compiled and evaluated; estimates of the elemental concentrations were made based on statistical evaluations, principles of analytical procedures and the laboratory performance indicated by the results on the cabbage material. Satisfactory estimates were obtained for 27 elements. Comparison with IAEA laboratory and NIST reference data did not reveal any significant bias that might have been introduced by the intercomparison approach or its evaluation.

Primary standards in activation analysis

The availability of natural matrix reference materials evaluated for trace element content has resulted in their widespread use as standards (i.e., calibration materials; comparators) for instrumental neutron activation analysis (INAA). Due to the uncertainties associated with their certified values, the limited number available, and the relative matrix independence of INAA, these reference materials are more properly utilized as quality assessment materials, after calibration of the INAA analytical system with true primary standards. Terminology is defined, the use of matrix reference materials to evaluate the analytical system is discussed, techniques for the accurate preparation of primary standards for trace element analyses are reviewed, and necessary precautions in the accurate comparison of samples to standards are presented.

TRACE ANALYSIS FOR PLATINUM IN GLASSES BY NEUTRON ACTIVATION.

Abstract Trace amounts of platinum were determined in eight glasses. These glasses had been prepared under special conditions, and information on both the surface and interior platinum was desired. Previous microscopic examination of the glass samples had identified platinum on the surface of at least four of the samples. The platinum was analyzed by means of the nuclear reaction The 199Au product has a 3.15-day T 1 2 and Eγ of 158 and 208 keV. Samples were irradiated in the NBSR for 2 h at 1.3·1013 n cm-2 sec-1. After irradiation, the glasses were etched with aqua regia for the determination of surface platinum, and then dissolved in hydrofluoric and perchloric acids for the determination of interior platinum. The radioactive 199Au was separated from the matrix by spontaneous deposition on metallic zinc. Experimental sensitivities of 0.04 μg of platinum were obtained.