bluuebunny/crossref_metadata_2025 huggingface dataset:
Parameters, Calculation of Nuclear Magnetic Resonance
Authors: Jameson Cynthia J.
Published: September 2006

The fundamental parameters that reproduce a nuclear magnetic resonance (NMR) spectrum in gases, liquids, and solids are the four tensor quantities: the NMR chemical shift of the nucleus, the indirect nuclear spin–spin coupling, the nuclear electric quadrupole coupling, and the direct dipolar coupling tensors. The first three are intimately related to the local electronic structure at the nucleus and the chemical bonds connecting the nuclei. On the other hand, the direct nuclear spin dipole–dipole interaction depends directly and entirely on the third power of the inverse of the direct through‐space distance between two nuclei, whether bonded or otherwise. In gases and in liquids where free tumbling of the molecules bearing the nuclear spins leads to isotropic averaging of these quantities, only the isotropic average values, the average of three components along the principal axes of the chemical shift and the indirect spin–spin coupling tensors determine the observed frequencies in the NMR spectrum. In the solid state, restricted motion permits the tensors to manifest all the components, whether the sample is a polycrystalline powder, an amorphous solid, or a single crystal. Theoretical calculations of the NMR chemical shift, indirect spin–spin coupling, and nuclear quadrupole coupling parameters, using quantum mechanical methods, permit the prediction of NMR spectra and provide the physical basis for the relationship between the parameters and molecular electronic structure, which may include local electronic structure (electronic distribution in the immediate vicinity of the nucleus and neighboring bonds), local molecular geometry, bond connectivities, stereochemical structure, as well as subtleeffects of the chemical environment, such as contributions from remote parts of the molecule, tertiary and secondary structure, crystal packing, solvent effects, and isotopic substitution.
Relaxation in Nuclear Magnetic Resonance, General
Authors: Canet Daniel , Mutzenhardt Pierre
Published: September 2006

Relaxation phenomena in nuclear magnetic resonance (NMR) govern the recovery of nuclear magnetization after it has been taken from its equilibrium state (i.e. along the static magnetic field direction). Various interactions, leading to local fluctuating magnetic fields, are responsible for the timescale of these phenomena. By themselves, these interactions yield valuable dynamic and structural information. Conversely, the influence of relaxation on the quantitative aspect of NMR spectra has to be assessed prior to any experiment aiming at using NMR as an analytical tool. This implies an evaluation not only of the relaxation parameters but also of indirect relaxation processes such as the nuclear Overhauser effect (NOE).
Two‐Dimensional Nuclear Magnetic Resonance of Small Molecules
Authors: Pedersen Henrik
Published: September 2006

The article deals with practical aspects in obtaining two‐dimensional (2‐D) nuclear magnetic resonance (NMR) spectra of small molecules [molecular weight (MW) < 1000]. This includes the choice of parameters for acquisition and processing and the required and optional instrument hardware. Examples of 2‐D experiments are given with emphases on robustness and ease of use. The pulse sequences are given in schematic form together with suggestions for basic parameters and the phase cycle. The experiment examples include homonuclear and heteronuclear experiments with direct and indirect detection. Also the use of pulsed field gradients (PFGs) is exemplified. 2‐D NMR on spin‐

$\def\tovr#1#2{{\scriptstyle{#1\over #2}}} \tovr{1}{2}$

nuclei other than
1
H and
13
C is exemplified by
19
F and
15
N experiments. Experiments on gel‐phase samples under magic angle conditions are mentioned briefly.
Charged Particle Activation Analysis
Authors: Strijckmans Karel
Published: September 2006

Charged particle activation analysis (CPAA) is an analytical method for elemental analysis. It is based on charged particle (CP) induced nuclear reactions producing radionuclides, that are identified and quantified by their characteristic decay radiation. CPAA allows trace element determination in the bulk of a solid sample, as well as characterization of a thin surface layer, i.e. determination of mass thickness or composition.

Advantages of CPAA are: (1) good precision, (2) low detection limit, (3) outstanding accuracy, (4) absolute, i.e. reference method, (5) independent, i.e. based on a totally different principle than more common methods for elemental analysis (atomic/optical emission and mass spectrometry), and hence not subject to the same systematic errors, (6) traceability, (7) not subject to surface contamination for trace element determination in the bulk, (8) if instrumental analysis is not possible it is not subject to reagent blank errors, and (9) errors due to nonquantitative yield (and even nonreproducible yield) can be corrected for. Disadvantages of CPAA are its inherent complexity and costs, it is not suitable for liquid samples, and heating occurs during irradiation. Speciation, depth profiling, or scanning is not possible.

This article covers (1) the principles on the slowing down of CPs in matter, nuclear reactions, and interferences; (2) experimental details concerning irradiation and activity measurement; (3) conceptual and experimental procedures and data handling, from the problem to the final result; (4) perspectives. References to relevant databases are made. A systematic overview of all possible CPAA applications is beyond the scope of this article.
Cyclic Activation Analysis
Authors: Hou Xiaolin
Published: September 2006

Cyclic activation analysis (CAA) is a method of activation analysis for elemental analysis in which a sample is irradiated, decayed, counted, then irradiated again, and this process is repeated for a number of cycles, the spectra from each counting being summed to give a final total spectrum. By this process, the counts of a short‐lived nuclide of interest are considerably increased and the analytical sensitivity of elements is significantly improved. The most commonly used CAA method is cyclic neutron activation analysis (CNAA) by irradiation with thermal, epithermal, and fast neutrons produced from a nuclear reactor, accelerator, or isotopic neutron source. A nuclear reactor can supply a very high neutron flux and is most often used for this purpose. At least 20 elements which produce short‐lived nuclides (half‐life less than 100 s) by thermal neutron bombardment, and also more than 10 elements which produce nuclides with half‐lives of 100–600 s can be determined by thermal and epithermal neutron CAA. This technique has been widely applied in biological, environmental, geological, and industrial studies, and the most often measured elements include Se, F, Pb, Hf, Sc, O, Ag, and Rh.

The advantages of CAA, as compared with conventional activation analysis, include significant improvements of the detection limits, analytical precision, and accuracy for the elements by using short‐lived nuclides, short experimental time, increased number of samples per unit time, a capability for the estimation or confirmation of the half‐lives of short‐lived nuclides and determination of the degree of homogeneity of samples. However, the application of CAA is limited by the number of elements determined, because only some of the elements determined by conventional activation analysis can be determined by this method. In addition, dead time and pile‐up are serious problems in CAA and must be corrected. The principle, selection of optimum experimental conditions, detection limit, and analytical precision of CAA, and also the dead time and pile‐up corrections, are discussed in this article.
Elemental Analysis by Isotope Dilution
Authors: Bahrainwala T.M. , Turel Z.R.
Published: September 2006

The measurement of extremely small traces of materials, which cannot be determined by normal analytical techniques, becomes possible and even relatively rapid and simple if the element can be made radioactive by the action of neutrons or charged particles. Nuclear techniques possess exceptional advantages in having extraordinary sensitivity, selectivity in detection and determination, speed and simplicity.

One of the nuclear techniques applied in trace and ultratrace analysis is isotope dilution analysis (IDA) which was first introduced by Hevesy and Hobbie in 1932. The great advantage of the method lies in the fact that it can be used even when the procedure involves loss of material or does not allow a complete separation of the element or compound. The isotope dilution method has found several applications in the determination of geological ages, fission yields and other problems in nuclear physics and chemistry.

Various techniques involved in IDA such as substoichiometry, liquid chromatography (LC) and radiometric titration are discussed along with the application of this technique to the estimation of elements in complex matrices as reported in the literature.
Instrumental Neutron Activation Analysis
Authors: Alfassi Zeev B.
Published: September 2006

Analysis of the elemental content of a sample by irradiation with neutrons and measurement of the induced radioactivity are described.
Particle‐Induced γ‐Ray Emission
Authors: Mutsaers P.H.A. , de Voigt M.J.A.
Published: September 2006

Particle‐induced γ‐ray emission (PIGE) is commonly applied for quantitative analysis of materials. γ‐Rays, characteristic of the elements, are emitted after nuclear reactions. These reactions are induced by particle beams (e.g. protons) from accelerators. The reaction conditions and probabilities are explained, as well as the γ‐ray properties and probabilities of emission. Knowledge of those parameters enables concentrations of elements to be deduced from the γ‐ray spectra, which under certain conditions are also depth dependent. Therefore the samples must be small and flat. The experimental set‐up and procedures are given together with a table with the most intense proton reactions and the energies of the emitted γ‐rays. Various examples are given to illustrate the method in different applications.
Prompt γ‐Neutron Activation Analysis
Authors: Oliveira Carlos , Salgado José , Carvalho Frederico G.
Published: September 2006

Prompt γ‐neutron activation analysis (PGNAA) is a multielemental analytical technique based on the detection of γ‐rays emitted by excited nuclei following neutron radioactive capture or neutron inelastic scattering. The accumulated count rate relative to the γ‐ray energy characteristic of a certain isotope depends on its concentration. Measurements take place during sample irradiation. The technique can provide accurate information on the elemental composition of materials in a broad range of cases: raw materials, mineral exploration, contaminants, detection of forbidden products, such as drugs and explosives, inspection of containers and applications in health care. Among the main advantages of the PGNAA technique are: faster results; nondestructiveness; flexible sample requirements; bulk sample analysis; in situ application by direct on‐line measurement; detectable unique signatures for most elements; improved accuracy for short‐lived elements; low residual activity. On the other hand, the most significant limitations are: higher detection limits (DLs) as compared with delayed γ‐neutron activation analysis (DGNAA); longer irradiation times; single (one‐cycle) sample irradiation; impracticability of enhancing a specific isotope contribution by radiochemical or physical (half‐life) separation techniques; requirement for relatively larger samples.
Rutherford Backscattering Spectroscopy
Authors: Kimura Kenji
Published: September 2006

Rutherford backscattering spectroscopy (RBS) is one of the most powerful techniques for measuring elemental depth profiles. It allows quantitative and nondestructive analysis with a reasonable depth resolution. In conventional RBS (combination of 1–4 MeV He ions with a silicon surface‐barrier detector), the typical depth resolution is about 10 nm and the typical mass resolution is about 1 u for light elements (mass numberM < 20) and about 20 u for heavy elements (Mca. 200). Sensitivity of 100 ppm can be easily obtained for heavy elements in light hosts although analysis of light elements in heavy hosts is difficult. In the present article, physical concepts which RBS relies on are briefly introduced. Some examples of analyses are demonstrated and several advanced techniques which have been developed to improve capabilities of the conventional RBS are discussed.
Scattering and Absorption of γ‐Rays and Thermalization and Disappearance of Neutrons
Authors: Borsaru M.
Published: September 2006

Nuclear techniques are widely used in geological and mineral applications. Owing to the high penetration of γ‐radiation and neutrons, nuclear techniques have found many applications, including well logging, bulk analysis and on‐stream analysis. The first part of this article presents the physics of the interaction of γ‐rays and neutrons with matter. Applications based on these interactions to well logging for oil, uranium, coal and iron ore are presented in the second part of the article. Applications based on the natural radioactivity of rocks are also covered.
β‐Particle Emitters, Determination of
Authors: Caprette David R.
Published: September 2006

Radioisotopes have unstable nuclei that decay in a statistically predictable manner with the release of radiation in the form of particles or electromagnetic waves. In β‐decay “fast electrons” called β‐particles are released, which have a spectrum of kinetic energies ranging from near zero to a characteristic maximum (E

max

). E

max

can vary by orders of magnitude among different types of emitters. The low mass of β‐particles causes them to lose energy quickly through molecular collisions, producing excitation of valence electrons, ionization of atoms, and in some cases specialized radiation (bremsstrahlung and

$\widehat{\hbox{C}}\hbox{erenkov}$

radiation, and Compton scattering). Detection of β‐particle emissions is complicated by the wide range of possible energies and the weakness of some emissions, so that a single type of detector is inadequate for the analysis of radiation from all β‐particle emitters.

Detectors are characterized by a sensitive volume in which particles interact with matter to produce a signal, an example of which is the emission of photons. The signal can produce a continuous current or a pulse. In pulse mode it is desirable to choose a detector such that the pulse is proportional to the energy of the particle, giving the instrument the property of energy discrimination. This property permits spectral analysis and methods of quantitation that are based in part on energy spectra. Liquid scintillation counting (LSC) is the most accurate and widely used method for detecting low‐energy β‐particles. Usually the information obtained by LSC must be corrected for signal loss due to absorption of incident particles or emitted photons. This signal loss is called quenching. For quantitation of low‐energy β‐radiation, quench corrections are usually made using the sample channels ratio (SCR) or an external standard method. All such calculations are subject to error and derived values are subject to the compounding of precision error during mathematical manipulation of the data.

Scintillation counting with organic crystal detectors is the method of choice for high‐energy β‐particle emitters. Gas‐filled detectors are of limited value for β‐particle detection, although windowless proportional counters are appropriate for detection of low‐energy particles, especially in gaseous form. When neither energy discrimination nor precise quantitation is required, Geiger–Müller counters are acceptable for high‐energy β‐particle detection. They can detect all but the least energetic of low‐energy emitters, although Geiger–Müller counters are not practical for the analysis of activity from such emitters because the signal is very weak, and the energies from incident particles are frequently absorbed before reaching the sensitive volume. Silicon‐barrier semiconductors support energy discrimination and are of value for some applications. Visual imaging with photographic film is used for localization of β‐particle emitter activity, as in labeling of biological tissues or macromolecules.
γ‐Spectrometry, High‐Resolution, for Radionuclide Determination
Authors: Toivonen Harri I.K. , Nikkinen Mika T.
Published: September 2006

High‐resolution γ‐spectrometry is a nondestructive assay method that gives low detection limits for many radioactive nuclides. γ‐Spectrometers use an electric field to collect the free charge carriers, electrons and holes, in a semiconductor detector. Modern detectors are based on high‐purity germanium (HPGe). The material properties of germanium containing extremely low concentrations of impurity atoms are excellent at low temperature (77 K) for creating and collecting charges in the sensitive region of the detector. The major components of the spectrometer are the detector, cooling system, shielding system, signal processing unit, and analysis software. The charge created at the detector is directly proportional to the energy deposited in the detector by the radiation. The weak charge pulse is transformed, shaped and amplified to a voltage pulse, which is then converted to digital form. A multichannel analyzer (MCA) collects and stores the spectrum, which comprises counts partitioned among different channels according to energy dissipated in the detector. The spectrum contains peaks that can be unequivocally associated with certain radionuclides. Peak areas are proportional to the activity of the nuclide. Meticulous calibrations are required to operate the instrument properly. Software plays a major role in the analysis of the spectra, and interactive human work and artificial intelligence, i.e. an expert system, are needed to handle complex cases. Stable elements can be analyzed through neutron activation and subsequent γ‐spectrometric determination. γ‐Spectrometry is not a sensitive method for radionuclides that have a long half‐life (≫30 years).
Neutron Activation Analysis in the Determination of Very Long‐Lived Radionuclides
Authors: Rosenberg Rolf J.
Published: September 2006

This article describes the use of neutron activation analysis (NAA) for the quantitative determination of very long‐lived radionuclides in the environment and nuclear waste. Because of its sensitivity and isotope specificity NAA is used for the determination of a few radionuclides with half‐lives of the order of 10
4
years or longer. The detection limits for some nuclides are lower than those obtainable with most kinds of mass spectrometry. The method has a substantial freedom from systematic errors.

The benefit of NAA, compared with direct radiometric measurement of the parent nuclide, is based on the fact that neutron activation may produce radionuclides with shorter half‐lives and higher specific activities than those of the parent nuclides. An especially significant improvement is obtained when a γ‐emitting nuclide is produced from a nuclide which only emits β‐particles.

There are a few long‐lived radionuclides which need to be determined because they are produced in nuclear fuel, and may pose a hazard in the final disposal of nuclear waste. Of these possible applications, the determination of
99
Tc,
129
I,
135
Cs and
237
Np is described here. NAA has proven to be the most sensitive method to determine
129
I and a useful method to determine
237
Np in environmental samples. Therefore, NAA can be recommended as an analytical method for these nuclides, when a research reactor is available.
Speciation of Radionuclides in the Environment
Authors: Salbu Brit
Published: September 2006

To obtain information on physicochemical forms of radionuclides in the environment, speciation techniques should be applied in situ, at‐site or in laboratories shortly after sample collection. Among speciation techniques for radionuclides in waters, combined in situ ultrafiltration and exchange chromatography are most promising as radionuclides species are fractionated with respect to nominal molecular mass and change properties simultaneously. However, the fractionation and characterization of chemically well‐defined low molecular mass (LMM) species in waters is still a challenge. Surface analytical techniques (e.g. electron microscopy) are useful for characterizing colloids and particles with respect to structure and radionuclide distribution. Recent development within synchrotron‐based X‐ray microbeam techniques allows detailed information of crystallographic structure (micro‐X‐ray diffraction (μ‐XRD)), oxidation states (micro‐X‐ray absorption near‐edge spectroscopy (μ‐XANES)) andvolumedistribution (μ‐XANES tomography) of particle‐associated radionuclides on the micrometer scale. Information on radionuclides reversibly or irreversibly associated with solid surfaces (binding mechanisms) is attained by different extraction techniques, while dynamic tracer experiments can provide information on the kinetics involved.
Inorganic Systems, Thermal Analysis Applications to
Authors: Gallagher Patrick K.
Published: September 2006

Thermoanalytical techniques can be used to characterize a wide range of inorganic materials and processes concerning them. The topics covered involve the measurement of thermal properties, phase equilibria, chemical analysis, and the study of solid‐state reactivity. Examples are selected to include applications to metals, ceramics, minerals and a variety of inorganic compounds. Applications to processes such as catalysis, crystal growth, and synthesis are described.

The principle techniques chosen for these purposes are associated with the measurement of different properties as a function of temperature and time. Thermogravimetry (TG), thermomagnetometry (TM), and evolved gas analysis (EGA) are based on changes in weight and analysis of the materials evolved or consumed. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are concerned with changes in temperature and heat capacity. Thermodilatometry (TD) measures changes in length or volume. Examples illustrating the use of each of these techniques are described.
X‐Ray Techniques: Overview
Authors: Jenkins Ron
Published: September 2006

The use of X‐ray methods in the field of materials analysis is now entering its seventh decade. While the broad definition of X‐ray methods covers many techniques based on the scatter, emission and absorption properties of X‐radiation, the two most common are X‐ray fluorescence (XRF) spectrometry and X‐ray powder diffractometry (XRD). When a sample of material is bombarded with energetic radiation (X‐rays, γ‐rays, electrons, protons, etc.) vacancies may arise from the removal of inner orbital electrons. One of the processes by which the atom regains stability is by transference of electrons from outer to inner electron shells. Each of these transitions is accompanied by the emission of an X‐ray photon having an energy equal to the energy difference between the two states. The X‐ray emission wavelengths are characteristic of the atom in question and there is a simple relationship (Moseley's law) between the wavelength of the emission line and the atomic number of the atom. Thus when a sample is made up of many different types of atoms, each atom will produce a series of wavelengths, and all of the contributions add up to become the total X‐ray emission from the sample. XRF is a technique which utilizes the diffracting power of a single crystal, or the proportional characteristics of a photon detector, to separate the polychromatic beam of radiation from the sample into separate wavelengths, thus allowing qualitative and quantitative elemental measurements to be made.

A beam of monochromatic radiation may also be scattered when X‐ray photons collide with atomic electrons. Where the scattered wavelengths interfere with one another diffraction of X‐rays occurs. All substances are built up of individual atoms and nearly all substances have some degree of order of periodicity in the arrangement of these atoms. It is the scattering from these periodic arrays that leads to the diffraction effect, and there is a simple relationship (Bragg's law) between the scattering angle, the wavelength of the radiation and the spacings between the planes of atoms. Since the distances between the atomic planes are dependent on the size and distribution of atoms – i.e. the structure of the material, XRD can be used for qualitative and quantitative phase identification.
Ultrafast Diffraction Techniques
Authors: Rousse A.
Published: September 2006

The need for femtosecond time resolution in the analysis of processes governed by atomic motion is well established. In biology, chemistry, and physics different times are of interest, but the shortest relate to the vibration of matter, being typically about 10
−13
 s (100 femtoseconds). Time‐resolved experiments are used to monitor time‐dependent processes. Depending on the spectral range of observation, different properties can be studied. Usually, visible or near‐visible radiation provides information on the electronic properties, but atomic motion can be directly monitored only through very short wavelength radiation, such as X‐rays. The electronic changes associated with vibrations in single molecules, liquids or crystal lattices, isomerization and the breaking or formation of chemical bonds, can be monitored by ultrafast optical spectroscopy techniques, but the accompanying ultrafast structural rearrangements cannot be directly observed. In the detailed characterization of structures at the atomic level, X‐ray and electron diffraction is an unparalleled tool, allowing reaction intermediates to be followed.

Synchrotrons have become essential to the study of structures. Nevertheless, the shortest durations attainable with the latest generation of synchrotrons, hold the time resolution of experiments around 50 ps. This article shows how subpicosecond time resolution can be achieved in the analysis of very short‐lived transient structures. The two emerging techniques in subpicosecond time‐resolved X‐ray (electron) diffraction are (1) ultrashort X‐ray (electron) probe pulses, produced through a femtosecond laser system, and (2) fast X‐ray detection during quasicontinuous synchrotron‐produced X‐ray probe illumination.
Wavelength‐Dispersive X‐Ray Fluorescence Analysis
Authors: Jenkins Ron
Published: September 2006

When an element is bombarded by high‐energy particles, orbital electrons may be ejected creating inner orbital atomic vacancies. These vacancies may be filled by transition of outer level electrons giving rise to characteristic X‐radiation. X‐ray fluorescence spectrometry provides the means of the identification of an element by measurement of its characteristic X‐ray emission wavelength of energy. The method allows the quantization of a given element by first measuring the emitted characteristic line intensity and then relating this intensity of element concentration. While the roots of the method go back to the early part of this century, where electron excitation systems were employed, it is only during the last 30 years or so that the technique has gained major significance as a routine means of elemental analysis. Wavelength‐dispersive spectrometers employ diffraction by a single crystal to separate characteristic wavelengths emitted by the sample. Today, nearly all commercially available X‐ray spectrometers use the fluorescence excitation method and employ a sealed X‐ray tube as the primary excitation source. The first commercial X‐ray spectrometer became available in the early 1950s and although these earlier spectrometers operated only with an air path, they were able to provide qualitative and quantitative information on all elements above atomic number 22 (titanium). Later versions allowed use of helium or vacuum paths that extended the lower atomic number cut‐off to around atomic number 9 (fluorine). X‐ray detectors used include the flow counter, the scintillation counter and the Si(Li) detector.

The X‐ray method has good overall performance characteristics. In particular, the speed, accuracy and versatility of X‐ray fluorescence are the most important features among the many that have made it the method of choice in over 30 000 laboratories all over the world. Most wavelength‐dispersive spectrometers fall into two broad categories – single channel and multichannel. Single channel spectrometers are typically employed for both routine and nonroutine analysis of a wide range of products, including ferrous and nonferrous alloys, oils, slags and sinters, ores and minerals, thin films, and so on. These systems are very flexible but relative to multichannel spectrometers are somewhat slow. The multichannel wavelength‐dispersive instruments are used almost exclusively for routine, high‐throughout analysis where the great need is for fast accurate analysis, but where flexibility is of no importance.

Interelement (matrix) effects often complicate quantitative analysis by X‐ray fluorescence. However, a wide selection of methods is now available for minimizing these effects, allowing excellent accuracy to be obtained in many cases. Detection limits are achievable down to the low parts per million (ppm) range and it is possible to obtain reasonable responses from as little as a few milligrams of material.
Gravimetry
Authors: Widmer M.
Published: September 2006

Gravimetry is one of the oldest techniques of quantitative analysis. In gravimetry, the analyte is precipitated as a stoichiometrically defined compound. After collecting and drying, the precipitate is weighed on an analytical balance and the analyte is determined from the mass and the known stoichiometry of the analytes in the precipitated compound. Although many instrumental methods have superseded gravimetry it is a very important method for standardization processes. Since calibration is inherent in the relative molecular masses and the stoichiometry of the chemical reaction, it represents one of the few primary methods.

As a closely related method, electrogravimetry is also considered. In electrogravimetry the analyte is deposited on a metal, for example a platinum electrode, by electrolysis with a controlled current and its mass is evaluated from the weight difference of the electrode before and after electrolysis. Electroseparation of two analytes is feasible by judicious choice of the electrode potential.
Quality Assurance in Analytical Chemistry
Authors: Fischbacher Christoph
Published: September 2006

Analytical methods have to meet certain quality requirements, so that the results are credible and can be used with confidence for regulatory, economic and research decisions. Analytical quality assurance (QA) contains preventive action and comprises planning steps, quality control (QC), quality assessment, documentation and quality improvement. All factors leading to the analytical result have to be controlled, clearly described and documented. Furthermore, the analytical result should be linked to the sample and related to reference materials (RMs) or measurement standards by an unbroken chain of traceability. Two elements are needed in order to achieve this goal: internal QA measures comprise elements performed by the laboratory, such as method validation and documentation and the use of RMs, external QA relates to performance evaluation by persons outside the laboratory, such as proficiency tests and accreditation.
Titrimetry
Authors: Widmer (Deceased) M.
Published: September 2006

In a titration, the analyte reacts with a reagent added as a solution of known concentration. The standard solution added from a buret is called a titrant. The volume of the titrant required to react completely with the analyte is measured as the volume at the equivalence point. The notions titrimetry and volumetry have the same meaning. Only well‐defined stoichiometric reactions are applied. No additional calibration is necessary, i.e. titrimetric methods are primary methods. Indication of the equivalence point can be carried out by visual indicators or instrumental methods.

Acidimetric and alkalimetric titrations can be described by Brønsted's theory of strong and weak acids and bases. Theoretical titration curves for reactions of strong or weak acids (bases) with strong bases (acids) are demonstrated. Extremely weak electrolytes can be titrated in the presence of precipitating, oxidizing or complexing agents.

Determinations of metal ions are feasible by means of complexometric titrations. The theory of complexometry, including coupled chemical equilibria, can be derived from the fundamentals of complex‐formation reactions in aqueous solutions. Applications are important for analyzing individual metal ions, mainly for their standardization, or for determining the hardness of water.

With regard to the instrumental indications of a titration, the potentiometric, amperometric and conductometric principles are outlined. Amperometric titrations of metal ions are performed with one polarizable electrode. The use of two polarizable electrodes (biamperometry) is especially important to determine the water content of organic solvents.
Traceability in Analytical Chemistry
Authors: Adams Freddy
Published: September 2006

This article describes the concepts of traceability for chemical measurements (in chemical analysis) as they are defined at present, and their relation to analytical chemistry. Traceability concerns the direct or indirect linkage of analytical results to the international metric system of units [Système International (SI)], thereby showing a way to achieving comparability in chemical analysis. The metrological concepts to do this, as they have been developed up to the present, are critically evaluated. They concern the units in which analytical results should be expressed and the tools, such as primary reference methods and primary standards, certified reference materials (CRMs) and other reference materials (RMs), that can be used to increase the accuracy of results of analysis.
Absorption and Ingestion Toxicology
Authors: Sen Pranab K.
Published: September 2006

Absorption and ingestion toxicology relates to the prevalence of ambient toxins, their intake process by human being through skin (epidermal) absorption or ingestion as foods, drinks, or drugs (narcotic or not) either taken orally or by injection, biological/neurological/behavioral reactions, related molecular and carcinogenic developments, and resulting health hazards.
Abundance: Population Size and Density Estimation
Authors: Udevitz Mark S.
Published: September 2006

Estimates of population size (total number of individuals) or density (number of individuals per unit area) are some of the most basic requirements for wildlife research and management. This article provides a brief overview of approaches for wildlife population estimation.
Agricultural Runoff
Authors: Jarrell Wesley M.
Published: September 2006

Agricultural runoff describes the water that moves across the land surface before it enters a permanent surface water body. The water originates as natural precipitation – rainfall or snow melt – or irrigation water. As the water moves across the landscape, it frequently picks up impurities and moves them into surface waters, especially where soils have been disturbed.
Agroforestry
Authors: Lund H. Gyde
Published: September 2006

Agroforestry is a collective name for land‐use systems and practices for the deliberate growing of trees and crops on the same piece of land. In its broadest interpretation, agroforestry can include the planting of trees and shrubs around a farmstead for beauty and shade, or the management of public forests for timber and livestock grazing, mushroom and berry production, or fisheries. Key in all definitions of agroforestry is that the trees are grown deliberately for a purpose, along with the plants and/or animals, and that they are not merely remnants left over from land clearing or conversion.
A
merican
S
tatistical
A
ssociation, Section on Statistics and the Environment
Authors: Nussbaum Barry D.
Published: September 2006

The objective of the Section on Statistics & the Environment of the American Statistical Association is to serve as the focal point for Association members with special interests in the following areas:

Application and interpretation of statistics in disciplines related to the environment.

Development of improved statistical theory, concepts, and procedures for application to problems of the environment.

Improvement of training in environmental statistics.
Atmospheric Dispersion: Chemistry
Authors: Builtjes P.J.H.
Published: September 2006

Most trace gases and particles that are brought into the atmosphere are not inert, i.e. are passive. They are chemically, or physically, transformed in the atmosphere, and are removed from the atmosphere by dry deposition – reaction with the surface – and wet deposition – taken up in water‐droplets (rain‐out) or removed by falling droplets (wash‐out). The environmental impacts of the effects of chemistry relative to atmospheric dispersion are presented in this article by considering different spatial scales, namely local, regional and global.
Automatic Differentiation
Authors: Skaug Hans Julius
Published: September 2006

Automatic differentiation (AD) is a technique for numerical differentiation of computer algorithms.
Hierarchical
B
ayesian Space–Time Analysis
Authors: Mark Berliner L.
Published: September 2006

The hierarchical Bayesian (HB) approach to stochastic modeling and statistical analysis offers a framework for dealing with spatio‐temporal structures as well as the uncertainties prevalent in both our observations and scientific understanding of environmental processes. This article gives an introduction to the basics of HB space–time analysis with emphasis on modeling and prediction. Realistic space–time HB models lead to analyses requiring intensive computations. Until the advent of Markov Chain Monte Carlo (MCMC) as a device for Bayesian calculation, such computations were typically infeasible.
B
ayesian Methods and Modeling
Authors: Gelfand Alan E.
Published: September 2006

A recent article in
Science
magazine (286, 19 November 1999, pp. 1460–1464) noted the rejuvenation of the Bayesian inference paradigm and its increasingly warmer reception in the scientific community. Pertinent here, is its recent application to a wide range of environmetrics problems, allowing inferential capability that may not be available using classical methodology. This article provides a brief overview of Bayesian methodology and modeling beginning with Bayes' theorem. It then elaborates the Bayesian inference paradigm and the Bayesian modeling perspective. Historically, computation was the major impediment to the use of Bayesian analysis for complex models. Now, the wide availability of inexpensive, fast computing combined with breakthroughs in simulation‐based model fitting have removed this impediment. Bayesian models, which were formerly infeasible to examine, can now be fitted. More realistic models can be investigated.
Bioindicators
Authors: McGeoch Melodie A.
Published: September 2006

A biological indicator is a species (or group of species) that readily reflects some aspect of the state of the environment within which it is found. Bioindicators are used to detect or demonstrate the existence of a particular environmental state, and to monitor changes in that state. In this way they are used as tools to extract abiotic or biotic information from the environment so that this information can be used for making scientifically‐based management decisions.
Biomarkers
Authors: Thurston Sally W.
Published: September 2006

Recent advances in molecular biology have enabled investigators to measure a variety of biological indicators that can potentially help elucidate the causal pathway between environmental exposure and disease. Often called ‘biomarkers’, these can be defined as cellular, biochemical or molecular changes in a biological system such as human cells, tissues or fluids. Recent years have seen increasing interest in incorporating biomarkers into epidemiological research in general, but particularly in relation to cancer research. Such work generally falls under the rubric of molecular epidemiology. In the context of cancer, the contributions of molecular epidemiology include: (a) providing evidence that certain environmental agents act as carcinogens; (b) establishing evidence of a causal role between these agents and cancer; (c) identifying whether certain subpopulations are at an increased cancer risk when exposed to the agent; and (d) providing a basis for suggesting and developing strategies which can potentially reduce cancer risk.
Biomass
Authors: Parresol Bernard R.
Published: September 2006

Biomass, the contraction for biological mass, is the amount of living material provided by a given area or volume of the earth's surface, whether terrestrial or aquatic. Biomass is important for commercial uses (e.g. fuel and fiber) and for national development planning, as well as for scientific studies of ecosystem productivity, energy and nutrient flows, and for assessing the contribution of changes in forestlands (especially tropical) to the global carbon cycle.
Biomonitoring
Authors: Conquest Loveday L.
Published: September 2006

According to
Webster's Dictionary
, the word
monitor
is related to the verb to warn, and includes the idea of observing a signal over time to see whether change is occurring. When the prefix
bio
is added to form the word biomonitoring, this encompasses the idea of tracking possible changes in a particular biological response, sometimes in correspondence to changes in human activity. In its common usage, the term biomonitoring refers to nonhuman biological systems, such as wildlife (e.g. birds, mammals), fish, and other aquatic organisms. For many of these organisms, the observations are recorded in the field (e.g. population counts, observations on breeding success), rather than in controlled laboratory experiments. The emphasis is that some living biological population is being observed (e.g. toxicants in bird populations, as opposed to toxicants from soil samples).
Bivariate Distributions
Authors: Christman Mary C.
Published: September 2006

Bivariate distributions play a special role in environmental science since they are the underpinnings of analyses of relationships between two random variables. These relationships include such analyses as biserial correlation between continuous and discrete random variables; measures of association in an
r
×
c
contingency table; Pearson's product–moment correlation; Kendall's tau; Spearman's rank correlation; and many others. In addition to studies of the dependency between two variables, interest may center on other analyses; for example, multivariate general (or generalized) linear modeling when two or more response variables are involved. Inferences involving two random variables rely on knowledge of their bivariate distribution. Probably the best known example is estimation and testing of the correlation coefficient in the bivariate normal distribution for two continuous random variables. There are many other distributions of importance as well, especially those relating two discrete random variables or a discrete to a continuous variable. This article describes a few of the more common distributions in environmental practice and, where appropriate, mentions the multivariate distribution of which the bivariate is a special case.
Bootstrap Resampling
Authors: Dixon Philip M.
Published: September 2006

The bootstrap is a resampling method for statistical inference. It is commonly used to estimate confidence intervals, but it can also be used to estimate bias and variance of an estimator or calibrate hypothesis tests. This article will illustrate bootstrap concepts using a simple example, describe different types of bootstraps and some of their theoretical and practical properties, discuss computation and other details, and indicate extensions that are especially appropriate for environmetric data. The methods will be illustrated using data on heavy metal concentrations in groundwater and magnesium concentration in blood.
Branch‐and‐Bound Algorithm
Authors: Whitaker David
Published: September 2006

The branch and bound algorithm is mostly used to solve global optimization problems with integer variables (unknowns), linear objective and linear constraints (integer linear programming (LP) problems). The recent increase in the speed of computation on desktop computers has meant that the algorithm can be applied to large problems with several hundred integer variables, which only a decade ago were intractable to any true optimization algorithms. Although a number of algorithms have been proposed for the integer LP problem, the branch and bound technique is used in almost all of the available computer software. This technique has proven to be reasonably efficient for practical problems because it has the embedded advantage of solving continuous linear programmes as subproblems. Furthermore, in some situations the embedded subproblem is a special case of a LP problem (such as assignment and transportation problems), which have more efficient algorithms than the general LP algorithm. Branch and bound methods are also easy to implement on parallel processors. An increasing number of applications are currently forthcoming in this area.
Capture–Recapture Sampling Designs
Authors: Lindberg Mark , Rexstad Eric
Published: September 2006

Reliable results from capture–recapture (CR) studies can only be ensured if diligent care is exercised prior to the acquisition of the first datum. We describe the considerations necessary for competent completion of CR studies. Rather than focusing on analytical issues, we discuss the stochastic processes that give rise to the data derived from such studies. Through this focus, we provide investigators with concepts they can influence to most closely adhere to the assumptions of analyses associated with CR studies. We hope to expose investigators to data collection issues associated with classic CR designs, and also to point out novel applications of these designs.
Carcinogenicity Model, Three‐State
Authors: Dinse Gregg E.
Published: September 2006

Early methods for analyzing tumor response data did not adjust for any of the following factors: (i) tumors generally are not observable in live animals, in which case all onset times are censored; (ii) much of this censoring is informative in the presence of tumor lethality and (iii) differential mortality exacerbates any biases that result from not properly adjusting for survival, censoring, and lethality. As experience was gained and the techniques for analyzing carcinogenicity data became more sophisticated, attention gradually focused on the tumor incidence function as the most appropriate endpoint. A multistate stochastic model provides a sensible statistical framework within which to make inferences about tumor incidence. At a minimum, the model must have two states, as incidence refers to the transition between a tumor‐free state and a tumor‐bearing state. Most models also include a third state to represent death. This article discusses the analysis of tumor incidence within the context of a three‐state model, concentrating on the first occurrence of occult, irreversible tumors at a single organ site.