Natural Matrix Materials Research Articles

Rapid and Quantitative Measurement of Hematite and Goethite in the Chinese Loess-Paleosol Sequence by Diffuse Reflectance Spectroscopy

AbstractThe long, continuous deposition of dust in the Chinese loess plateau offers a unique opportunity to study the nature of Fe oxide formation in a wide range of climatic conditions. A technique to obtain quantitative estimates of the concentration of hematite and goethite in loess and paleosol samples is reported. Experiments using diffuse reflectance spectroscopy on sets of laboratory mixed and natural loess and paleosol samples show that it is possible to obtain rapid and quantitative estimates of the absolute concentration of hematite and goethite in the Chinese loess sediments. Typical loess and paleosol samples were deferrated using the CBD procedure to produce a natural matrix material to which hematite and goethite in known weight percentages were added to produce a set of calibration standards. Spectral violet, blue, green, yellow, orange, red and brightness of standards were calculated from the reflectance data and served as independent variables for a multiple linear regression analysis. The effect of changing matrix from loess to paleosol was overcome by including a variety of different loess and paleosol samples in the regression equations. The resulting calibration equations provide estimates of wt.% hematite and goethite and have correlation coefficients >0.93. The total measured hematite and goethite concentrations exhibited consistent variations with CBD extractable iron. Tests of the equations for buffering changes in matrix composition were run with samples of varying mineralogical composition (calcite, illite, etc.) and demonstrated that the equations are well buffered for changes in matrix composition from loess to paleosol.

Basic considerations for the preparation of performance testing materialsas related to performance evaluation acceptance criteria

The preparation of performance testing (PT) materials for environmental and radiobioassay applications involves the use of natural matrix materials containing the analyte of interest, the addition (spiking) of the analyte to a desired matrix (followed by blending for certain matrices) or a combination of the two. The distribution of the sample analyte concentration in a batch of PT samples will reflect the degree of heterogeneity of the analyte in the PT material and/or the reproducibility of the sample preparation process. Commercial and government implemented radioanalytical performance evaluation programs have a variety of acceptable performance criteria. The performance criteria should take into consideration many parameters related to the preparation of the PT materials including the within and between sample analyte heterogeneity, the accuracy of the quantification of an analyte in the PT material and to what "known" value will a laboratory's result be compared. This manuscript discusses how sample preparation parameters affect the successful participation in performance evaluation (PE) programs having an acceptance criteria established as a percent difference from a "known" value or in PE programs using other acceptance criteria, such as the guidance provided in ANSI N42.22 and N13.30.

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.

Reference materials for small-sample analysis

Many modern analytical techniques use small solid samples and lack proper reference materials for their calibration and quality assurance. A remedy to this deficiency may be in the development of a new genre of highly homogeneous natural matrix materials, their properties being studied with analytical techniques such as PIXE and μ-PIXE, solid sampling AAS, scanning electron microscopy in combination with electron probe X-ray microanalysis, and INAA. Suitable natural materials may be obtained in form of single cell biological materials, finely dispersed suspensions and precipitates such as air particulate matter or sediments, and by appropriate particle size reduction of complex matrices. Initial studies have been carried out on single cell green algae biomass and air particulate matter, as well as several processed materials. Narrow particle size distributions with particles preferably below 10 μm diameter may assure the desired analytical homogeneity. The determination of sampling parameters for individual measurands will ascertain the utility of a material for small-sample analysis.

Microhomogeneity of candidate reference materials: Comparison of solid sampling Zeeman-AAS with INAA

Homogeneity of certified reference materials (CRMs) is a crucial property especially when minimum sample mass of a certain material used for analysis is considered. Particularly in microanalytical techniques CRMs are frequently used to calibrate quantitative analysis, therefore reliable information on the confidence interval of reference values with regard to milligram or sub-milligram sample amounts is urgently needed. Relative element specific homogeneity factors He were determined in order to characterize a single cell algae candidate reference material issued by the IAEA, Vienna, for a number of elements. Solid sampling Zeeman AAS and micro-INAA were applied to determine He values in two batches of monocellular algae with considerably different concentrations of trace elements. It could be shown that the extremely low and narrow particle size distribution of the materials offers the opportunity to use such natural matrix materials for quantitative analysis with microanalytical techniques, such as PIXE, laser ablation MS, or solid sampling AAS, where milligram amounts or less of the material are analyzed. Such homogeneity factors in selected reference materials could be used to attach a precisely determined mass related confidence interval to the certified concentrations, reflecting the properties of the material. If a mass related homogeneity factor reflecting the material’s true heterogeneity could be applied, systematic bias of analytical procedures together with intermethod bias can be traced.

Low-level radioactivity ocean sediment standard reference material

The National Institute of Standards and Technology (NIST), in cooperation with experienced international laboratories, will issue a low-level radioactivity Ocean Sediiment Standard Reference Material (SRM) 4357 in 1996. The Ocean Sediment joins the NIST suite of six other ‘natural matrix’ environmental radioactivity Standard Reference Materials. This family of natural matrix materials have already been used: (1) to develop radiochemical procedures; (2) to test radiochemical procedures already in use for environmental and biokinetic evaluations; (3) to calibrate instruments; (4) to intercompare and evaluate radiochemical methods; (5) to test competency of technicians to do radiochemical assays; and (6) to demonstrate that data output is reliable. The participating laboratoires provided data for the radionuclides are of sufficient quality to establish certified values and reanonable uncertainty limits given the relatively low concentrations of the radionuclides. The uncertainties for a number of certified radionuclides are non-symmetrical and large because of the non-normal distribution of reported values.

Results of the intercalibration study of laboratories involved in assessing the environmental consequences of the Chernobyl accident

Within the framework of the International Chernobyl Project, the IAEA's Seibersdorf Laboratories organized an intercalibration exercise among some of the laboratories which were involved in assessing the environmental contamination in the USSR due to the accident. The objective was to assess the reliability of the radioanalytical data for food and environmental samples, which wre used to assess the doses. In the initial study reference materials from the stocks of the IAEA's Analytical Quality Control Services (AQCS) were re-labelled and submitted to 71 laboratories as blind samples. These natural matrix materials included samples of milk (containing 2 different levels of radioactivity), soil, air filters and clover. The concentrations of radionuclides in these samples were known from previous intercalibration exercises. The overall range in performance was broad, which is similar to what has been observed in previous international intercomparisons. The results obtained by gamma-ray spectrometry tended to be somewhat underestimated, on average. On the other hand, the laboratories showed an overall tendency to overestimate 90Sr and possibly 239Pu, which were analysed radiochemically. The intercalibration exercise is continuing with nine materials, including: soil, grass, hay and milk powder contaminated with fallout from the Chernobyl accident. These materials, which were prepared by laboratories in the USSR, are now being tested by AQCS prior to future intercomparison exercises. Work with these materials is expected to continue for several years.

A reliable and accurate procedure for preparing low-activity efficiency calibration standards for germanium gamma-ray spectrometers

An empirical procedure is described which is readily capable of producing reliable and accurate efficiency calibration sources of low activity for high-resolution γ-ray spectrometers. Known activities of a multiisotopic solution are adsorbed onto a mixture of cation and anion-exchange resins and chromatographic-grade cellulose. The dried powder that results can be mixed with any desired solid natural matrix material to produce a homogeneous, extended geometry, efficiency calibration source. Results are presented which enable the method to be critically evaluated.

Toughening mechanisms in abalone shell

Abalone shell (Haliotis Rufescens) is a naturally ocurring ceramic/polymer composite material. The system displays a unique laminated structure of calcium carbonate (aragonite) crystals in a matrix of biological macromolecules. The CaCO3 crystals and the organic matrix are arranged in a miniature “brick and mortar” structure referred to as nacre. Figure 1 is a TEM bright field micrograph illustrating the high degree of order observed in this microstructure.Although the nacre region of the shell is more than 95% CaCO3 by volume, the natural matrix material and the arrangement of the microstructure lead to a substantial increase in the observed mechanical properties. Mechanical tests performed on the nacre region show a fifty-fold increase over that of pure bulk CaCO3 (Fig. 2), which also compares with other ceramic and cermet systems.Vickers microhardness testing was performed on samples polished for optical microscopy. Crack propagation features were observed by standard SEM techniques and analyzed in an attempt to identify the possible toughening mechanisms that are operating in the nacre structure. The cracks generally travel by a tortuous path, often displaying microcracks and crack branching. However, these mechanisms alone are not sufficient to account for the observed mechanical properties.

Spiked natural matrix materials as quality assessment samples

The Environmental Measurements Laboratory has conducted the Quality Assessment Program (QAP) since 1976 to evaluate the quality of the environmental radioactivity data, which is reported to the Department of Energy by as many as 42 commercial contractors involved in nuclear work. In this program, matrix materials of known radionuclide concentrations are distributed routinely to the contractors and the reported results are compared. The five matrices used are: soil, vegetation, animal tissue, water and filter paper. Environmental soil, vegetation and animal tissue are used, but the water and filter paper samples are prepared by spiking with known amounts of standard solutions traceable to the National Bureau of Standards. A summary of results is given to illustrate the successful opertion of the program. Because of the difficulty and high cost of collecting large samples of natural matrix material and to increase the versatility of QAP, an attempt was recently made to prepare the soil, vegetation and animal tissue samples with spiked solutions. A description of the preparation of these reference samples and the results of analyses are presented along with a discussion of the pitfalls and advantages of this approach.

Standard reference materials for the determination of polycyclic aromatic hydrocarbons

Since 1980 a number of Standard Reference Materials (SRM's) have been issued by the National Bureau of Standards (NBS) to assist in validating measurements for the determination of polycyclic aromatic hydrocarbons (PAH) and other polycyclic aromatic compounds (PAC). These SRM's are certified for selected PAC and range in analytical difficulty from calibration solutions to complex natural matrix materials, such as air and diesel particulate matter, shale oil, and crude oil. In the past year three new SRM's have been introduced: (1) SRM 1647a “Priority Pollutant PAH in Acetonitrile”, (2) SRM 1491 “Aromatic Hydrocarbons in Hexane/Toluene”, and SRM 1597 “Complex Mixture of PAH from Coal Tar”. The SRM's available from NBS for use in the determination of PAC are described and the concentrations of PAC determined in the natural matrix SRM's are summarized and compared. The primary analytical techniques used for the measurement of PAC in these SRM's were gas chromatography, liquid chromatography, and gas chromatography/mass spectrometry.