Improved detection limits for trace elements on aerosol filters using Compton suppression counting and epithermal irradiation techniques

Aerosol samples were collected on Whatman 41 filters at two sites near Lake Huron and one site near Lake Ontario. These samples were then analyzed by instrumental neutron activation analysis (NAA) at the University of Illinois. The detection limits for certain trace elements were enhanced by irradiation with both thermal and epithermal neutrons and also by counting with Compton suppression techniques. The sample was divided in half to allow for four irradiations. Short-lived thermal NAA resulted in the determination of Al, Br, Ca, Cl, Cu, K, Mn, Na, Ti, and V. A short epithermal irradiation was used to determine Cu, I, In, Si, Sn and U. A one and one-half hour epithermal irradiation was utilized for the determination of As, Au, Br, Sm, Sb, and W. The elements Cr, Cs, Fe, Hf, Ni, Sc, Se, Th. Zn, and several rare earths were determined with a long thermal irradiation. Utilizing a Compton suppression gamma-ray counting system reduced the background and enhanced the detection of several isotopes which primarily emit only a single gamma-ray upon decay. Counting was simultaneously performed with a normal counting mode so that the detection of isotopes with multiple decay gamma-rays was not impaired.     

The determination of chromium-50 in human blood and its utilization for blood volume measurements

Possible relationships between insufficient blood volume increases during pregnancy and infant mortality could be established with an adequate measurement procedure. An accurate and precise technique for blood volume measurements has been found in the isotope dilution technique using chromium-51 as a label for red blood cells. However, in a study involving pregnant women, only stable isotopes can be used for labeling. Stable chromium-50 can be determined in total blood samples before and after dilution experiments by neutron activation analysis (NAA) or mass spectrometry. However, both techniques may be affected by insufficient sensitivity and contamination problems at the inherently low natural chromium concentrations to be measured in the blood. NAA procedures involving irradiations with highly thermalized neutrons at a fluence rate of 2·10¹³ n·cm^–2·s^–1 and low background gamma spectrometry are applied to the analysis of total blood. Natural levels of chromium-50 in human and animal blood have been found to be <0.1 ng/ml; i.e., total chromium levels of <3 ng/ml. Based on the NAA procedure, a new approach to the blood volume measurement via chromium-50 isotope dilution has been developed which utilizes the ratio of the induced activities of chromium-51 to the iron-59 in three blood samples taken from each individual, namely blank, labeled and diluted labeled blood.     

Determination of58Fe and65Cu enriched stable isotopic tracers in studies of mineral metabolism of babies

Neutron activation analysis is an appropriate method for the determination of the excretion by infants of enriched stable isotopes which have been added to synthetic milks for assessment of elemental bioavailability.⁵⁸Fe enrichments in diet and faeces have been determined by instrumental NAA and a radiochemical procedure established for determination of⁶⁵Cu enrichments. Conditions for the latter facilitate determination of both stable Cu isotopes, thereby providing results for the total concentration of this element. Appearance of added stable isotopes in samples collected between carmine red markers has been confirmed, and the method applied to studies of Fe and Cu absorption.     

Applications of nuclear analytical techniques in environmental research. Plenary lecture.

Among nuclear analytical techniques, neutron activation analysis (NAA) is particularly useful for environmental studies. It affords low detection limits for many elements, high specificity and few sources of systematic error, which means that high accuracy is attainable. Neutron activation analysis is particularly useful for trace and ultra-trace analysis of environmental samples (water, soils, rocks and biological material). In trace element work associated with pollution, instrumental NAA is a powerful technique for multi-element surveys, in particular when combined with other spectroscopic techniques. Nuclear techniques, as with most analytical techniques, cannot be used to distinguish between different physico-chemical forms of an element per se. When used in combination with appropriate separation techniques, however, nuclear techniques can provide valuable information about trace element speciation in environmental and biological systems. From dynamic tracer experiments, i.e., addition of chemically well defined labelled compounds to environmental systems, valuable information can be obtained on the distribution of species and on microchemical processes influencing the physico-chemical forms. In these laboratories, speciation studies on trace elements in natural waters have been carried out by using instrumental NAA in combination with physical separation techniques, such as dialysis and ultrafiltration, in situ and in the laboratory. Dynamic radiotracer experiments have provided important information about processes influencing the speciation of trace elements in aquatic systems. Sequential extraction techniques have proved to be useful in studies on sediments and soils when combined with NAA. Sequential extractions also provide significant information about the physico-chemical behaviour of radionuclides supplied to natural soils from the Chernobyl accident.     

Activation techniques for the determination of stable isotopes of cerium in blood plasma

Information on the biokinetics of cerium can be obtained directly from humans by using stable isotopes as tracers. Neutron, photon and proton activation analysis have all been tested as analytical techniques able to quantify different isotopes of the same element in biological fluids. The experimental conditions were optimized for Ce analysis in blood plasma samples. The performances of the different techniques have been explored. The simultaneous determination of two Ce isotopes with the required sensitivity in the order of few ppb is difficult to obtain using a single technique, and, therefore, a combination of techniques can be envisaged.     

Neutron Activation Techniques in Environmental Studies

The development over time in applications of nuclear activation techniques in environmental studies is critically reviewed. A vast majority of the work has been based on activation analysis using thermal and sometimes epithermal neutrons from nuclear reactors (NAA). Whereas radiochemical methods were frequently used until about 1975, the work reported more recently has mainly been multi-element studies based on instrumental NAA. The by far most successful application has been the analysis of aerosol samples, but considerable work has also been done in other areas such as precipitation and surface waters, soils, vascular plants, moss and lichen biomonitors, and fossil fuels with by-products. Some interesting examples of speciation analysis, based on pre-irradiation separations, have also been reported. Rapid development in alternative multi-element techniques such as ICP-MS has shown these techniques to be superior in a number of cases where NAA earlier was the technique of choice. Areas where efforts should be concentrated in future NAA work are indicated.     

Precise and accurate isotope ratio measurements by ICP-MS

The precise and accurate determination of isotope ratios by inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) is important for quite different application fields (e.g. for isotope ratio measurements of stable isotopes in nature, especially for the investigation of isotope variation in nature or age dating, for determining isotope ratios of radiogenic elements in the nuclear industry, quality assurance of fuel material, for reprocessing plants, nuclear material accounting and radioactive waste control, for tracer experiments using stable isotopes or long-lived radionuclides in biological or medical studies). Thermal ionization mass spectrometry (TIMS), which used to be the dominant analytical technique for precise isotope ratio measurements, is being increasingly replaced for isotope ratio measurements by ICP-MS due to its excellent sensitivity, precision and good accuracy. Instrumental progress in ICP-MS was achieved by the introduction of the collision cell interface in order to dissociate many disturbing argon-based molecular ions, thermalize the ions and neutralize the disturbing argon ions of plasma gas (Ar+). The application of the collision cell in ICP-QMS results in a higher ion transmission, improved sensitivity and better precision of isotope ratio measurements compared to quadrupole ICP-MS without the collision cell [e.g., for 235U/238U approximately 1 (10 microg x L(-1) uranium) 0.07% relative standard deviation (RSD) vs. 0.2% RSD in short-term measurements (n = 5)]. A significant instrumental improvement for ICP-MS is the multicollector device (MC-ICP-MS) in order to obtain a better precision of isotope ratio measurements (with a precision of up to 0.002%, RSD). CE- and HPLC-ICP-MS are used for the separation of isobaric interferences of long-lived radionuclides and stable isotopes by determination of spallation nuclide abundances in an irradiated tantalum target.     

Characterization of neutron transmuted zinc traces in pure copper materials by isotope dilution mass spectrometry

The neutron transmutation doping (NTD) of highly pure copper with zinc was investigated as a promising means of achieving controlled gradation of the zinc content in the range 1-20 microg g(-1). The doping process leads to the enrichment of two stable isotopes 64Zn and 66Zn in a ratio which differs from that of natural isotopic distribution. Mass spectrometric investigations by thermal ionization mass spectrometry (TIMS) were performed to validate the results obtained by gamma spectrometry. The investigations included both determination of the isotopic ratios of the doped zinc isotopes and the analysis of the accumulated zinc contents by isotope dilution (ID) analysis. Thereby a sample-specific correction of the blank could be performed because the isotope 68Zn was not influenced, because of the transmutation process. The results obtained by TIMS prove the strict proportionality of the doped zinc content, in the range 5 to 20 microg g(-1), to the neutron fluence. Comparison with gamma spectrometric results showed a very good agreement within the uncertainties.     

Importance of double neutron-capture as a second-order reaction interference in NAA

In order to estimate gut absorption by determining tracer concentration in plasma, a technique based on the administration of two stable isotopes of the same element was combined with proton activation analysis. The optimization for the determination of Zr isotopes in biological samples is presented together with the results of a preliminary study on Zr biokinetics in animals. (p,n) reactions on⁹⁰Zr and⁹⁶Zr resulted the most convenient. The obtained minimum detectable quantities are 3 and 2 ng/ml plasma, respectively, for⁹⁰Zr and⁹⁶Zr. Zr plasma clearance and Zr response to a simple oral test were studied separately in different subjects by using the natural Zr solution. The data analysis was performed measuring the concentration of⁹⁰Zr to obtain indication on the time behavior and fractional level of Zr appearance in plasma depending on the administration routes. Two rabbits were intravenously injected 50 g⁹⁰Zr and a third rabbit was orally given 2.5 mg of⁹⁰Zr. Concentration in plasma samples of intravenously and orally given Zr isotopes are reported, as a function of time after administration. The injected tracer concentration relative to the first two rabbits were fitted simultaneously to obtain clearance parameters. Zr intestinal absorption is evaluated to be less than 0.2%. The work confirms that proton activation is a powerful tool for biokinetic studies with stable isotopes.     

Stable isotopes and the international system of units

It is now over 60 years since Nier built the first isotope ratio mass spectrometer. The introduction of continuous-flow techniques heralded a huge expansion in the use of stable isotopes in biomedical and environmental sciences, yet there is no consensus on the appropriate units, especially in the biomedical field. Most isotope ratio mass spectrometry (IRMS) instruments calculate isotopic abundance in terms of delta notation (delta, per thousand, per mil), which is a convention determined by geochemistry, because most of the original IRMS instruments were developed in isotope geochemistry laboratories to measure natural abundance variations. Delta units are not SI units. This paper considers the appropriate units for studies using stable isotopes based on the International System of Units (SI). The SI base unit for concentration is the mol, from which atom fraction and mol fraction are derived. The units of stable isotope abundance, atom % and mol %, are the atom and mol fractions expressed as percentages. Atom % excess and mol % excess are the SI units of enrichment and are to be recommended for use in tracer studies.     

Variation in the isotopic composition of zinc in the natural environment and the use of zinc isotopes in biogeosciences: a review

Zinc (Zn) is a trace element that is, as a building block in various enzymes, of vital importance for all living organisms. Zn concentrations are widely determined in dietary, biological and environmental studies. Recent papers report on the first efforts to use stable Zn isotopes in environmental studies, and initial results point to significant Zn isotope fractionation during various biological and chemical processes, and thus highlight their potential as valuable biogeochemical tracers. In this article, we discuss the state-of-the-art analytical methods for isotopic analysis of Zn and the procedures used to obtain accurate Zn isotope ratio results. We then review recent applications of Zn isotope measurements in environmental and life sciences, emphasizing the mechanisms and causes responsible for observed natural variation in the isotopic composition of Zn. We first discuss the Zn isotope variability in extraterrestrial and geological samples. We then focus on biological processes inducing Zn isotope fractionation in plants, animals and humans, and we assess the potential of Zn isotope ratio determination for elucidating sources of atmospheric particles and contamination. Finally, we discuss possible impediments and limitations of the application of Zn isotopes in (geo-) environmental studies and provide an outlook regarding future directions of Zn isotope research.     

酸及其浓度对多接收器等离子体质谱法测定Cu和Zn同位素的影响

运用多接收器等离子体质谱法 (MC-ICP-MS)测定同位素时,必须进行仪器质量歧视校正,而酸浓度对仪器质量歧视的影响是过去被忽视的问题.本实验以Cu和Zn同位素分析为例,研究了在HNO3和HCl介质进样条件下酸浓度对仪器质量歧视的影响.结果表明:HNO3 的浓度对Cu和Zn同位素的仪器质量歧视有显著影响,但二者并不同步;HCl的浓度在0.05 mol/L ～ 0.3 mol/L范围内对Cu和Zn同位素的仪器质量歧视没有影响.这说明与HNO3相比,HCl是一种更好的进样介质.在0.1 mol/L HCl 介质条件下,65Cu/63Cu 和66Zn/64Zn比值测定的长期重现性 (外部误差)均为0.00008 (2sd),比在HNO3介质下的重现性有明显改善.     

Study of Chromium Speciation in Normal and Diabetic Rats by Activable Enriched Stable Isotope Technique

Chromium speciation was investigated in the liver cytosol, serum and urine of normal and diabetic rats after a single intravenous injection of enriched stable isotope ⁵⁰Cr tracer solution. Sephadex G-25 gel chromatography combined with instrumental neutron activation analysis was used to isolate and characterize protein-bound chromium in the above materials. The results indicate that Cr is mainly combined with a high-molecular-weight protein either in liver cytosol or serum. A low-molecular-weight, Cr-containing compound (LMWCr) was found in all the observed liver, serum and urine samples of both normal and diabetic rats. Chromium is excreted chiefly as LMWCr in urine.     

Performance of NAA Methods in an International Interlaboratory Reference Material Characterization Campaign

An extensive database of analytical results from a recent biological matrix Reference Material Characterization Campaign permitted an intercomparison of the performances of various methods among each other and with "true" best estimate concentration values established for these materials. Six different variants of neutron activation analysis (NAA) methods were employed including: instrumental neutron activation analysis, instrumental neutron activation analysis with acid digestion, neutron activation analysis with radiochemical separation, neutron capture prompt gamma activation analysis, epithermal instrumental neutron activation analysis, and neutron activation analysis with preconcentration. The precision and accuracy performance of NAA-based analytical methods are compared with three other major techniques, atomic absorption spectrometry (AAS), atomic emission spectrometry (AES) and mass spectrometry (MS) for 28 elements in 10 natural matrix materials.     

Precocentration method using an activatable yield tracer for neutron activation analysis

Previously, the use of an enriched stable isotope as an activatable yield tracer in preconcentration steps has been tested by use of commercially available enriched¹¹⁶Cd and¹⁵⁶Dy for biological standard reference materials.^1,2 In the present work, this method has been further applied to the determination of lanthanoid contents in various kinds of samples: one coal fly-ash, three Japanese standard rocks, and eight standard soils. Samples were decomposed by alkali fusion in the preconcentration step. Thirteen elements were determined for coal fly-ash and soil samples, and 14 elements for rocks. The data obtained for coal fly-ash and standard rocks are compared with the data reported in literature. The data for soil samples have been newly determined in the present work. The ordinary instrumental neutron activation analysis and radiochemical neutron activation analysis were also performed to confirm the accuracy and usefulness of the present method.     

Microanalytical isotope ratio measurements and elemental mapping using laser ablation ICP-MS for tissue thin sections: zinc tracer studies in rats

The kinetics of zinc absorption, metabolism and excretion is extensively studied by nutritionists. Stable isotopes of zinc can be used to identify body zinc compartments that have different turnover kinetics. Since the compartments might belong to physiological subsections of different organs, there is a need for microsampling analysis to determine isotope ratios of the trace element zinc in tissue samples. Here, we study the feasibility to use laser ablation coupled to quadrupole ICP-MS for the determination of zinc tracers given to rats at different time points with the aim to generate isotope ratio bioimages of heart tissue. A double tracer (⁷⁰Zn and ⁶⁷Zn) experiment with rats was designed to label the exchangeable zinc pool as well as the stable zinc pool. The isotope ratios determined by laser ablation ICP-MS were evaluated by additional measurements of tissue digests. Accumulated tracers which made up more than 0.1% of total zinc could be identified in the tissues of the treated rats. It was established that at least 50 measurements from the microsampling were necessary to distinguish between controls and a tracer treated rat resulting in reduced resolution of the bioimage. With the parameters used, features in the tissue thin sections of at least 250 μm² in size are necessary to detect the incorporation of a tracer. When different time points have to be measured, higher precisions are required and therefore a larger area needs to be ablated (1 mm²). Using the bioimages and pool measurements from one physiological feature, it was possible to show that the aorta cell walls incorporate the zinc tracer at the different time points.     

Proton, Photon and Neutron Activation Analysis for the Determination of Stable Isotopes of Gadolinium in Human Blood Plasma

The biokinetics of radioactive substances can be studied using stable tracers. For the highly radiotoxic actinides, for which no stable isotopes are available as tracers, the use of stable isotopes of lanthanides as chemically related surrogates has been suggested. In this work, the possibility of using activation analysis with protons, photons, or thermal neutrons for the determination of single stable isotopes of gadolinium in biological samples has been tested. All the techniques show very good linearity response, and may be considered as complementary. Whereas activation analysis with protons is recommendable for the simultaneous determination of two different isotopes, neutron and photon (gamma) activation analysis should be chosen whenever a better sensitivity or simplicity of the analysis is required.     

Application of two-tracer isotope dilution mass spectrometry in the determination of uranium in geological samples

The determination of the uranium concentration in mineral samples using two tracer (²³³U and ²³⁵U) mass spectrometric isotope dilution techniques is described. The precision and accuracy are discussed and results are compared with those obtained by x-ray fluorescence and instrumental neutron activation methods. Based on the two independent values obtained for the same dilution, parameters such as the chemical procedures adopted, effect of mass fractionation and uranium distribution in minerals are evaluated. The ability of the method to distinguish between the analytical errors and heterogeneous distribution of uranium is discussed.     

Analysis of undigested honey samples by isotope dilution inductively coupled plasma mass spectrometry with direct injection nebulization (ID-ICP-MS)

Undigested honey samples were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) using direct injection nebulization (DIN). Samples were diluted 25-fold using dilute HNO₃ to which Triton X-100 was added. Isotope dilution (ID) was used to quantify Ba, Cu, Pb and Zn in these natural matrix samples. DIN parameters were optimized to obtain a stable transient signal enough to determine multiple isotope ratios. The Ba, Cu and Zn isotope ratios were measured in a single sample injection of 50 μl diluted honey and the Pb isotopes in a separate injection. The analysis was characterized by precisions given by R.S.D. values from 0.20 to 1.24% for Ba, 0.49–1.23% for Cu, 0.61–2.28% for Pb and 0.50–1.51% for Zn. The results were in agreement, at the 95% confidence level, with those obtained by analyzing digested samples using external calibration. Using this direct method, 30 real honey samples per hour could be analyzed.     

Nuclear activation methods for the characterization of fossil fuels

A review is presented on the use of neutron activation analysis (NAA) for the analyses of coal, oil shale, tar sands and petroleum. Fast NAA has been widely used for the determination of oxygen, and to a limited extent, of other elements such as nitrogen and silicon. Reactor NAA followed by instrumental counting, and in specific cases, after radiochemical separations is discussed. Thermal and epithermal neutrons are both used. Limited use of the²⁵²Cf source has been made in fuel analysis. A complementary technique to NAA is the photon-activation analysis with linear accelerator. It can determine over thirty elements, many of them not possible to do by NAA. Round-robin analyses of standard coal, fly ash, or oil shale samples indicate nuclear activation methods are comparable in accuracy and precision to X-ray fluorescence or atomic spectrometric methods for most elements.