bluuebunny/crossref_metadata_2025 huggingface dataset:
Microbalance, Electrochemical Quartz Crystal
Authors: Ward Michael D.
Published: September 2006

The need for direct measurement of interfacial events at solid surfaces under ambient conditions has provoked surface scientists and electrochemists to develop direct in situ methods. Such processes include electroless or electrochemical metal deposition, insertion of ions into polymer ion‐exchange films, growth of oxide films on metals, or the loss of material from corrosion processes. An increasing interest in molecular films has prompted development of methods for measuring adsorption of molecules from vapor or liquid phases. Many of these processes share a common feature, namely they are accompanied by changes in mass at the solid surface.

During the 1990s a new analytical method for the in situ examination of interfacial electrode processes, the electrochemical quartz crystal microbalance (EQCM), has emerged that has substantially influenced electrochemical science. This method relies on a single crystal of quartz that has been cut into a thin wafer and coated with gold electrodes on both sides. These electrodes are used to provide an alternating electric field so that the crystal is vibrated at a specific resonant frequency, while one of the electrodes is simultaneously used as a working electrode in an electrochemical cell. The resonant frequency shifts upon changes in mass that occur on the working electrode during an electrochemical process, with sensitivity as high as 100 pg cm
−2
of electrode area. Recent developments also have illustrated that the quartz crystal microbalance (QCM) is capable of measuring the viscosity and density of liquids near the QCM surface. The purpose of this article is to provide the reader with a fundamental understanding of the EQCM and several illustrative examples of applications that demonstrate its unique capabilities, as well as the technical details required to build this apparatus.
Neurotransmitters, Electrochemical Detection of
Authors: Colliver Thomas L. , Ewing Andrew G.
Published: September 2006

In this article, various electrochemical methods used in neuronal cell studies are examined. Electrochemical methods useful for investigating neuronal communication directly in the brain and in ex vivo brain slices are discussed. Additionally, we describe how voltammetric methods can be used to make intracellular and extracellular measurements at single cells. Technical aspects of each method are emphasized and selected applications are highlighted.

Although techniques such as electron microscopy and patch clamp can be used to visualize and monitor cellular events, electrochemical methods can provide chemical information about neurotransmitters being released and transported in brain tissue or even at single cells. The main competing technique for in vivo measurements of neurotransmitters is microdialysis. Although microdialysis coupled to separation techniques can be more selective and more sensitive than electrochemical methods, the latter methods are considerably faster and more accurate in determining the in vivo concentration. At single cells, electrochemistry provides extremely small probes compared to microdialysis thus facilitating measurements of single‐cell events. Here, again, the response time associated with electrochemical methods is important for measurements of millisecond exocytosis events.

Electrochemical methods are clearly a powerful means to measure rapid changes in neurotransmitters that are electroactive and found in tissue or cellular microenvironments.
Ultraviolet/Visible Spectroelectrochemistry
Authors: Scherson Daniel A. , Tolmachev Yuriy V. , Stefan Ionel C.
Published: September 2006

This article reviews the field of solution‐phase ultraviolet/visible (UV/VIS) absorption spectroelectrochemistry and its application to problems in thermodynamics, kinetics, and mass transport, with emphasis on both experimental and theoretical aspects. Examples are provided for transmission in thin layer and semi‐infinite media, using optically transparent electrodes, and external and internal reflection in quiescent media. Also illustrated is the coupling of UV/VIS spectroscopy to systems under forced convection, such as rotating disk and channel electrodes, under steady state and transient conditions. Attention is also focused on spatially resolved spectroelectrochemistry for the imaging of diffusion and reaction layers. The last section is devoted to the more complex and particularly powerful modulation techniques involving diffraction, refraction and absorption in stagnant media and in the presence of convective flow. Factors that limit the sensitivity and spatial and temporal resolution of UV/VIS absorption spectroelectrochemistry, as well as its future prospects in analytical chemistry are briefly discussed.
Ultrafast Electrochemical Techniques
Authors: Forster Robert J.
Published: September 2006

Ultrafast electrochemical techniques provide information about the kinetics and thermodynamics of redox processes that occur at submillisecond or even nanosecond timescales. This short timescale is achieved either by making very rapid transient measurements or by using ultrasmall probes to achieve very high rates of diffusion under steady‐state conditions. Microelectrodes (i.e. electrodes with critical dimensions in the micrometer range) play pivotal roles in both approaches. Electrochemistry has several advantages over spectroscopy in that it provides direct information about electron transfer (E) and coupled chemical (C) reactions. Ultrafast electrochemical techniques now allow it to do so at times as short as 10 ns. In transient measurements, decreasing the lower accessible timescale depends critically on fabricating ultramicroelectrodes that continue to respond ideally as their critical dimension (e.g. the radius of a microdisc) decreases. It is now possible to assemble microelectrodes that respond to changes in applied potential within less than a few nanoseconds. In steady‐state approaches, ultrasmall probes are required to make short timescale measurements and various approaches that yield nanodes (i.e. electrodes of nanometer dimension) have been proposed. However, beyond the need for smaller probes and faster instrumentation, the continued development of new theory describing electron transfer is essential, where the dimensions of the zone that is depleted of reactant because of a Faradaic reaction and the electrochemical double layer become comparable.
Phosphorescence Measurements, Applications of
Authors: Díaz García Marta E. , Badía Rosana
Published: September 2006

Luminescence is a term used for both fluorescence and phosphorescence. These are emission processes following the absorption of light, and the distinction between them is based upon the process of the deactivation mechanism. Phosphorescence has been identified as the light emitted from the triplet excited state to the ground state, a spin‐forbidden transition. Fluorescence is a radiational transition between electronic states of the same multiplicity.

The first analytical uses of phosphorescence were published in the 1950s, but the technique was not widely accepted due to the practical aspect of measuring the signal at cryogenic temperatures (77 K).

Developments in room temperature phosphorescence (RTP) have given rise to fundamental and practical advances which have stimulated interest in phosphorimetry. Solid‐surface room temperature phosphorescence (SSRTP) has been generally observed by immobilizing the lumiphor on a solid support such as filter paper or polymeric resins. SSRTP has been shown to be a sensitive and selective approach for the analysis of trace components in biological, pharmaceutical, and environmental samples. Techniques such as SSRTP combined with flow injection analysis provide new avenues for optical sensing transduction and improvement of SSRTP techniques. Applications of SSRTP in flowing systems include the sensing of cations, anions, oxygen, glucose, ethanol, antibiotics, and moisture.

A number of developments in RTP have focused on sensitized phosphorescence (SP) and on the use of ordered media (micelles and cyclodextrins (CDs)), allowing the observation of RTP in fluid solution. In SP the triplet energy of a donor (analyte) is transferred to an acceptor that phosphoresces in fluid solution and the emission of the acceptor is measured. Therefore, SP can only provide quantitative information. In ordered media, a micelle (or a CD) protects the long‐lived triplet state from the quenching processes that normally take place in solution, allowing both quantitative and qualitative measurements. SP and ordered media RTP provide the limits of detection for many lumiphors in the nmol
−1
range and calibration graphs have a wide linear range.

As new instrumental and basic insights emerge in SSRTP, Micelle Stabilized Room Temperature Phosphorescence (MSRTP), and solution‐sensitized RTP, these techniques will find much greater use in the trace and ultratrace analysis of dissolved, volatile, and gaseous substances in areas such as pharmaceutical analysis, environmental science, and clinical chemistry.
Column Technology in Gas Chromatography
Authors: Larson Paul
Published: September 2006

Column technology has fostered the development of gas chromatography (GC). As the manufacture of columns, especially capillary columns, has become more reproducible, the instruments have also improved. Chromatographic theory helps to guide the development of both the column and instrument in GC. Aspects of the theory directly related to the construction of columns are summarized. While packed columns can still be made in the local laboratory, the construction of capillary columns has moved to commercial vendors with the introduction of thin‐walled fused‐silica capillary columns. The technology required for the construction of columns is described.
Cavity Ringdown Laser Absorption Spectroscopy
Authors: Miller George P. , Winstead Christopher B.
Published: September 2006

Cavity ringdown laser absorption spectroscopy (CRDS), a distinctly new variant of classical absorption spectroscopy, is based upon the time required for the intensity of a light pulse in an optical cavity to decay. The sensitivity of the technique stems from the large number of passes that the light pulse makes within the cavity. For example, mirrors of 99.99% reflectivity in a 1‐m long cavity yield a ringdown time of approximately 33.4‐µs for an empty cavity. This is equivalent to a 10‐km pathlength traveled during the first time constant! The presence of a sample having an absorbance of 1 × 10−6inside the cavity will yield a readily measurable change in ringdown time of approximately 1%. When compared with traditional single‐pass absorption spectroscopy, which typically detects absorbances of the order of 1 × 10−3, the potential of CRDS becomesclear. As for any absorption technique, the conditions for Beer's law behavior must be fulfilled. One primary advantage of the cavity ringdown method is to allow the use of the extensive wavelength operating range of pulsed lasers for absorption spectroscopy. Because CRDS depends solely on the decay time, it is unaffected by the inherent shot‐to‐shot power fluctuations of pulsed lasers. Although still in the early stages of development, CRDS has been used to study chemical species in environments ranging from molecular beams and gas cells to atmospheric pressure flames and plasmas, in spectral regions ranging from the ultraviolet (UV) to the infrared (IR). It has only been very recently, with first couplings of CRDS and atomization sources (inductively coupled plasma (ICP), graphite furnace), that the technique has broadened to include analytical atomic spectroscopy. The potential of cavity ringdown for atomic spectroscopy has been demonstrated with the determination of mercury detection limits (DLs), in air, of less than 1 part per trillion.

At the time of writing (1999), CRDS is very much still in its infancy, especially with regard to analytical applications. For a possible glimpse of the future, it should be noted that as the wavelength coverage of diode lasers continues to grow, the possibilities for constructing small, ultrasensitive CRDS spectrometers for analytical atomic and molecular spectrometry continue to increase. However, any inherent advantage of CRDS over other analytical techniques will be determined by the quality of mirrors used for cavity construction, the stability of the baseline ringdown time measurement, the atomization source, and the fluorescence characteristics of the element under consideration. Although the technology associated with the ringdown technique continues to advance rapidly, CRDS is as yet a novel approach for many analytical measurements, and is sure to continue to find new areas of application. The next few years should prove very exciting as new instrumentation becomes available and the technique's full potential is explored.
Interpretation of Infrared Spectra, A Practical Approach
Authors: Coates John
Published: September 2006

The vibrational spectrum of a molecule is considered to be a unique physical property and is characteristic of the molecule. As such, the infrared spectrum can be used as a fingerprint for identification by the comparison of the spectrum from an “unknown” with previously recorded reference spectra. This is the basis of computer‐based spectral searching. In the absence of a suitable reference database, it is possible to effect a basic interpretation of the spectrum from first principles, leading to characterization, and possibly even identification of an unknown sample. This first principles approach is based on the fact that structural features of the molecule, whether they are the backbone of the molecule or the functional groups attached to the molecule, produce characteristic and reproducible absorptions in the spectrum. This information can indicate whether there is backbone to the structure and, if so, whether the backbone consists of linear or branched chains. Next it is possible to determine if there is unsaturation and/or aromatic rings in the structure. Finally, it is possible to deduce whether specific functional groups are present. If detected, one is also able to determine local orientation of the group and its local environment and/or location in the structure. The origins of the sample, its prehistory, and the manner in which the sample is handled all have impact on the final result. Basic rules of interpretation exist and, if followed, a simple, first‐pass interpretation leading to material characterization is possible. This article addresses these issues in a simple, logical fashion. Practical examples are included to help guide the reader through the basic concepts of infrared spectral interpretation.
Liquid Chromatography/Infrared Spectroscopy
Authors: Somsen Govert W. , Visser Tom
Published: September 2006

Analytical techniques that combine liquid chromatography (LC) and infrared (IR) spectroscopy have been developed primarily to permit specific detection and/or identification of sample constituents. LC is an important and extensively used method for the separation of mixtures into their individual components. IR spectroscopy is very useful for the characterization of functional groups and has strong compound‐identification capabilities which are especially suited for the differentiation of structural isomers. Over the past years the coupling of LC and IR spectroscopy (LC/IR) has been accomplished by two different approaches. The first and simplest approach is to use a flow cell through which the effluent from the LC column is passed while the IR spectra are continuously measured. The merits of flow‐cell IR detection include ease of operation, real‐time detection and low maintenance, but its main disadvantage lies in the significant IR‐absorption of the solvents commonly used in LC. These absorptions seriously limit both the detection sensitivity and the obtainable spectral information. The second approach involves elimination of the LC solvent prior to IR detection. In this approach an interface is used to evaporate the eluent and deposit the separated compounds onto a substrate suitable for IR detection. The primary advantages of solvent‐elimination LC/IR are the possibility to obtain full spectra of the analytes and the considerably enhanced sensitivity when compared to flow‐cell detection. Unfortunately, common LC solvents, and particularly aqueous eluents, are not easily removed and therefore the evaporation interfaces are often rather complex. This article reviews the developments, practical aspects, applications and current status of LC/IR, covering both coupling approaches. It follows that despite the unfavorable detection limits, flow‐cell LC/IR can be useful for the specific and quantitative detection of major components of mixtures. However, solvent‐elimination‐based IR‐detection should be used when small amounts of sample constituents have to be characterized with a high level of confidence. In general, the practical use of IR detection in LC is still limited, but the advent of various (commercial) flow‐cell and interface designs shows that LC/IR is more and more being recognized as a feasible and rewarding technique.
Electrocatalysis‐Based Kinetics Determinations
Authors: Bǎnicǎ Florinel Gabriel , Ion Ana
Published: September 2006

The term “electrocatalysis” denotes the catalysis of an electrochemical reaction. In accord with the general definition of catalysis1 the effect of an electrocatalyst is to increase the rate of an electrochemical reaction without modifying the overall standard Gibbs energy change in the reaction. Taking the view of Faraday's law, the rate of an electrochemical reaction is expressed by the electrolytic current. Consequently, an increase in the electrolytic current is the main result of electrocatalysis. The benefits of electrocatalysis include making possible the quantitation of the catalyst itself or of some substrate compounds that are not electrochemically active or are not generally amenable to direct electrochemical determination. An appreciable improvement in selectivity may be brought about by electrocatalysis. Additionally, appropriate engineering of catalytic layers on electrode surfaces has led to the preparation of a huge variety of chemical and biochemical sensors. However, close control of the experimental conditions (temperature, electrolyte composition) is often required when dealing with electrocatalytic processes. Usually, such requirements do not give rise to any particular difficulties.
Capillary Electrophoresis
Authors: Siles Barbara A.
Published: September 2006

Over the past two decades, capillary electrophoresis (CE) has emerged as a family of powerful and versatile separation techniques. The electrically driven flow profiles generated in CE provide, routinely, separation efficiencies of 10
5
theoretical plates. Although theory predicts that the achievable separation efficiencies will be greatest for macromolecules with small diffusion coefficients, CE has also been successful in the analysis of small molecules, such as inorganic ions. Strategies to increase separation efficiencies on‐line have involved sample stacking and isotachophoretic techniques. The former promotes higher separation efficiencies based on differences in conductivity between the background electrolyte (BGE) and the sample buffer; the latter utilizes a discontinuous buffer system to create discrete solute zones. The instrumentation employed in CE is simple in design and is compatible with a variety of detection modes, including ultraviolet/visible (UV/VIS) absorbance, fluorescence, electrochemistry, mass spectrometry (MS), and chemiluminescence (CL). CE methodologies can be miniaturized on to a silicon wafer and integrated with sample preparation and chemical reactions. In addition to free‐solution capillary electrophoresis (FSCE) utilizing a BGE and potentially a variety of BGE additives, several alternative separation modes have been developed and implemented. These include micellar electrokinetic chromatography (MEKC), capillary isoelectric focusing (cIEF), size‐sieving capillary electrophoresis (SSCE), and capillary electrochromatography (CEC). Some of these separation modes have been adapted from the slab gel electrophoresis (SGE) format, or are reminiscent of separation mechanisms in liquid chromatography (LC). Currently, one of the greatest challenges in CE involves the maintenance of the capillary surface to provide reproducible electro‐osmotic flow (EOF) and significantly reduce capillary wall interactions. According to the mode of separation employed and the characteristics of the solutes to be analyzed, both covalent and noncovalent (dynamic) capillary wall modifications have been demonstrated.
Microscale High‐Performance Liquid Chromatography and the Evolution of Capillary Electrochromatography
Authors: Vallano Patrick T. , Chirica Gabriela S. , Remcho Vincent T.
Published: September 2006

Miniaturization is a general trend in analytical chemistry. In the field of separation science, this trend has led to the development of microscale techniques based on high‐performance liquid chromatography (HPLC) and electrophoresis. Two such techniques, microscale high‐performance liquid chromatography (or micro LC) and capillary electrochromatography (CEC) are the subjects of this article. Micro LC is a general term that encompasses HPLC separations in which the column diameter is less than 1.0 mm. CEC is a hybrid of micro LC and capillary electrophoresis (CE). Relative to the conventional scale technique, micro LC possesses several advantages, namely higher efficiency, often shorter analysis times, reduced solvent consumption and higher mass sensitivity. Micro LC is uniquely suited for analysis of very small sample volumes (e.g. biological cells). It is this type of analysis, a formidable challenge, which has propelled the technique's continued development. CEC most commonly employs conventional HPLC particles packed into fused silica capillary columns. A potential gradient is applied along the capillary; the resulting electro‐osmotic flow (EOF) then propels the mobile phase (and analytes) through the column. In CEC, the electric field (generated by a power supply) takes the place of pressure (generated by a pump) in an HPLC separation. One of the most important advantages of CEC relative to micro LC or CE is enhanced resolution. High resolution in CEC arises from the combination of high separation efficiency and selectivity inherent in the technique. CEC, however, is a relatively new technique and its ability to become a “workhorse” in the laboratory has not yet been demonstrated.
Supercritical Fluid Chromatography
Authors: Yuan Huimin , Olesik Susan V.
Published: September 2006

Supercritical fluid chromatography (SFC) employs fluids that are raised above their critical pressure (Pc) and critical temperature (Tc) as mobile phases. Carbon dioxide, CO2, is the supercritical fluid most commonly used as a mobile phase for SFC. It is capable of solvating mostly nonpolar analytes. Cosolvents, such as methanol and acetonitrile, are added toincrease the polarity of the analytes that can be analyzed by SFC. SFC is most commonly practiced using a normal‐phase retention mechanism. In other words, the stationary phase is typically more polar than the mobile phase and the retention order is in increasing order of polarity. The advantages of SFC over liquid chromatography (LC) include higher efficiency separations and faster speed of analysis. The advantage that SFC has over gas chromatography (GC) is that SFC can efficiently separate thermally labile compounds. The disadvantage of SFC is that the polarity of the mobile phase is limited. Highly polar solutes are not soluble in supercritical fluids, even when modifiers are added. SFC is commonly used to separate petroleum distillates and by‐products, moderately polar pesticides and herbicides, and more recently for the separation of chiral compounds, such as pharmaceutical drugs.
Time‐of‐Flight Mass Spectrometry
Authors: Weinberger Scot R. , Davis Stephen , Makarov Alexander , Thompson Steve , Purves Randy , Whittal Randy M.
Published: September 2006

This article reviews the developmental history, fundamental technology, and general applications of time‐of‐flight mass spectrometry (TOFMS). The fundamentals of instrument operation and components are discussed. Applications in terms of gas chromatography (GC), liquid chromatography (LC), capillary electrophoresis (CE), laser desorption, multiphoton desorption, plasma desorption (PD), matrix‐assisted laser desorption, surface‐enhanced laser desorption, inductively coupled plasma (ICP), and secondary ion mass spectrometry (SIMS) are presented. A review of tandem time‐of‐flight (TOF) techniques with a focus on ion trap TOF instrumentation is provided.

The purpose of this article is to provide the reader with a fundamental understanding of TOFMS principles and applications. Information is presented in a simple, straightforward, didactic fashion.
Carbon‐13 Nuclear Magnetic Resonance Spectroscopy
Authors: Neiss Thomas G.
Published: September 2006

Carbon‐13 (13C) nuclear magnetic resonance spectroscopy (NMR) is the measurement of the precession or resonance frequencies of the net magnetization for13C nuclei whose individual magnetic moments have been oriented in a strong magnetic field. Nuclei differing in their electronic shielding precess about the magnetic field at different Larmor or resonance frequencies. A high‐power radiofrequency (rf) pulse is used to perturb the magnetization vectors from their equilibrium distribution, generating an observable transverse magnetization. Precession of this net magnetization vector about the static magnetic field induces a voltage into the NMR probe coil. Relaxation pathways promote the repartitioning of the individual magnetic moments to their equilibrium Boltzmann distributions and a dephasing of the individual magnetization vectors in the transverse plane. This signal is detected as a function of time through a phase‐sensitive receiver, digitized and Fourier transformed (FT) into a frequency domain spectrum. The NMRs, referenced relative to the resonance frequency of a standard are shifted in a manner characteristic of hybridization of the atom, electronegativity of the substituents attached and the steric environment of the nucleus. These shifts typically follow a standard set of rules and thus spectra can be simulated either empirically through measurement of the additive effects of substituents or through ab initio and semi‐empirical computational methods. Scalar couplings to directly attached hydrogen‐1 (1H) nuclei split resonance lines into (nH + 1) lines and require the use of broadband1H decoupling to remove this splitting for sensitivity enhancement. The presence of attached or nearest neighbor1H atoms also provides a means of enhancing the signal intensity of13C spectra and for selective observation of13C signals through polarization transfer. Multidimensional NMR experiments are available which permit the13C13C or13C1H correlations as well as a means of measuringnJCHcouplings.

Analysis of samples can be done without the need for internal standards or calibration standards since NMR signals are directly proportional to the moles of analyte present and there are no response factors or absorptivities to be determined for quantitative analysis. Although13C NMR spectroscopy is widely used and is a very powerful structural technique in organic and polymer chemistry, the technique suffers from an inherent lack of sensitivity due to a natural abundance of only 1.1 percent and a small magnetogyric ratio (γC). Long relaxation times present in small molecules undermine the quantitative accuracy of this method or require lengthy amounts of instrument time to acquire an NMR spectrum. Quantitative results can be obtained with the selection of appropriate experimental parameters often combined with the use of relaxation agents.
Electron Spin Resonance Spectroscopy
Authors: Chiesa Mario , Giamello Elio
Published: September 2006

Electron spin resonance (ESR) or Electron paramagnetic resonance (EPR) spectroscopy is an experimental technique for detecting and characterizing chemical systems bearing one or more unpaired electrons. The application of the technique, the background of which is similar to the more diffused nuclear magnetic resonance (NMR) spectroscopy, therefore concerns (i) organic and inorganic free radicals, (ii) trapped radicals in various matrices, including irradiated solids (radiation chemistry), frozen inert matrices and solid surfaces, (iii) transition metal ion compounds in classical inorganic systems or in biological systems, (iv) excited paramagnetic states (triplets) and several other systems of relevant scientific interest. The aim of this article is to provide an overall survey on the basic principles and on the various applications of the technique. This is done by describing, first, the physical basis of the electron resonance phenomenon (microwave absorption when the system is in the line of force of a strong external magnetic field) and, subsequently, the main types of interactions that the unpaired electron undergoes in the chemical system to which it belongs and which determines the features of the experimental spectra. These are essentially (i) the magnetic interaction of the electron with nuclei of nonzero nuclear spin (hyperfine interaction) which determines the multiline structure of the spectrum (hyperfine structure) and (ii) the interaction of the electron spin with the electron orbital angular momentum, occurring through the so‐called spin–orbit coupling. This latter interaction causes the dependence of the resonance on the orientation of the radical in the external magnetic field. Particular emphasis will be given to the different types of spectra observed according to the physical state of the investigated sample (liquid solutions, single solid crystals or microcrystalline powdered solids). In the last case (powders), the essential methodology for understanding the often complex profiles of the experimental spectra is described. Advanced electron resonance techniques, essentially ENDOR (electron–nuclear double resonance) and time‐resolved EPR will also be briefly mentioned.

The final part of the article is devoted to some analytical applications of the technique, including spin trapping (an essential tool for the detection and quantitative evaluation of reactive short‐lived radicals), radiation dosimetry (with applications to medical problems and irradiated foodstuff analysis) and dating of geologically relevant systems.
High‐Performance Liquid Chromatography Nuclear Magnetic Resonance
Authors: Dorn Harry C.
Published: September 2006

The direct coupling of high‐performance liquid chromatography (HPLC) to nuclear magnetic resonance (NMR) has evolved as a powerful new tool (HPLC/NMR) for the analysis of mixtures. The technique was originally reported in the late 1970s, but was generally limited by NMR detector sensitivity constraints and the availability of routine high magnetic field dispersion instruments (11.8–17.6 T, 500–750 MHz1H‐NMR). During the early 1980s, the HPLC/NMR technique was applied to problems involving the analysis of mixtures that were typically not sample limited, such as petroleum fuels. In the early 1990s, the advent of commercial HPLC/NMR equipment provided the impetus for a worldwide growth spurt of the HPLC/NMR technique. As outlined in this review, the HPLC/NMR technique has evolved as a powerful tool for applications involving polymers, pharmaceuticals, and biological mixtures.

An inherent advantage of HPLC/NMR is the ability to monitor the NMR chemical shift dimension. This dimension is particularly relevant for those applications involving structural isomer identification and has distinct advantages in comparison with the more commonly employed HPLC/MS (mass spectrometry) technique. Although sensitivity limitations of NMR as a detector are still a concern, the development of more complex hyphenated combinations (such as HPLC/NMR/MS) will undoubtedly continue, especially in the areas of pharmaceutical and drug development. Other technical and phenomenological considerations of the HPLC/NMR technique are presented in this review. Finally, a number of related variant techniques including capillary electrophoresis nuclear magnetic resonance (CE/NMR), capillary zone electrophoresis (CZE), capillary electrochromatography (CEC), supercritical fluid chromatography (SFC), and dynamic nuclear polarization (DNP) are also discussed.
Two‐, Three‐ and Four‐ Dimensional Nuclear Magnetic Resonance of Biomolecules
Authors: Craik David J. , Scanlon Martin J.
Published: September 2006

Nuclear magnetic resonance (NMR) is associated with transitions, induced by radiofrequency (RF) irradiation, between energy levels of nuclei with nonzero spin quantum numbers in a magnetic field. Traditional “one‐dimensional” (1D) NMR spectra are presented as a plot of signal intensity versus applied frequency and provide information about the chemical environment of magnetically active nuclei. This is used to deduce information about the chemical structure and dynamics of molecules containing the magnetically active nuclei (most often protons). The frequencies of individual nuclei are referred to as their chemical shifts. Multidimensional NMR encompasses a range of related techniques, based on application of a variety of precisely timed RF pulses to the sample, which extends traditional 1D NMR into two, three, or four frequency dimensions. These additional frequency dimensions may be the same as the first frequency dimension, referred to as homonuclear multidimensional NMR, or different, referred to as heteronuclear NMR. In multidimensional NMR of biomolecules the most common frequencies correspond to
1
H in the directly detected dimension and one or more of
1
H,
13
C or
15
N in the additional dimensions. Peaks in such spectra are defined by their intensity and by their frequency coordinates in two, three, or four dimensions obtained by extrapolation back to the respective frequency axes, which are in turn determined by the nature of the applied RF pulse sequences. The intensity and chemical shifts of such peaks provide information about chemical environment, molecular connectivity, or spatial proximity of the participating nuclei. Multidimensional NMR has major advantages over 1D NMR: reduction of peak overlap and provision of information on connectivity (either through bonds or through space). Multidimensional NMR of biomolecules is usually done for samples in solution at millimolar concentration. Where experiments require the use of
13
C or
15
N frequencies, it is usually necessary to enrich isotopically the macromolecule with these nuclei to >90%. This is readily achieved with modern molecular biology techniques and does not present a major limitation. The biggest limitation relates to molecular size, with studies generally limited to macromolecules of less than approximately 35 kDa.
X‐Ray Photoelectron andAuger Electron Spectroscopy
Authors: Turner Noel H.
Published: September 2006

The use of X‐ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) to analyze the surface composition and chemical environment of solids has been well established since about the early 1970s. These approaches find many applications in both basic and applied scientific investigations. The basic principles, i.e. surface sensitivity, the experimental observations employing these procedures, and the working methods, are described. The instrumentation utilized with the commonly available equipment for these approaches is summarized also.
Portable Systems for Energy‐DispersiveX‐Ray Fluorescence
Authors: Cesareo Roberto , Gigante Giovanni E. , Castellano Alfredo , Iwanczyk Jan S.
Published: September 2006

Portable energy‐dispersive X‐ray fluorescence (EDXRF) spectrometers are becoming very popular in many fields for the on‐site analysis of elements. This is mainly because EDXRF is a nondestructive, multielemental technique that is extremely well suited for the analysis of any material.

An EDXRF spectrometer mainly consists of an X‐ or γ‐ray excitation source, an X‐ray detector with electronics, and a pulse height analyzer. Recent technological developments have resulted in small, low‐power, dedicated X‐ray tubes, thermoelectrically cooled semiconductor detectors, and small pulse height analyzers. Therefore, completely portable EDXRF spectrometers are available that can be assembled on‐site, having the size of a book and a weight ranging from as light as 500 g (using a radioactive source) to a few kilograms (using an X‐ray tube). These spectrometers can be employed for on‐site analysis in various fields, such as works of art, alloys, soil, environmental samples, forensic medicine, paper, waster materials, mineral ores and their products, or anywhere a portable apparatus would be required.

This paper reviews the present status of the development and application of EDXRF portable systems. The various components of a portable system are described: the radiation source, i.e. small, low‐power, dedicated X‐ray tubes or alternatively radioactive sources that emit X‐rays or low‐energy γ‐rays; and X‐ray detectors, i.e. scintillators, proportional gas counters and semiconductor detectors, with special emphasis on the recent thermoelectrically cooled X‐ray detectors: Si‐PIN (silicon positive–intrinsic– negative), Si‐strip and CZT (cadmium–zinc–telluride).

Commercial systems are considered, and finally the most common and significant applications are described.
K
arl
F
ischer Moisture Determination
Authors: Schöffski Katrin , Strohm Dieter
Published: September 2006

The Karl Fischer (KF) titration is a method of determining the water content of solid, liquid and gaseous samples. It is the technique preferred for use in industrial quality control. In principle it involves the oxidation of sulfur dioxide by iodine, in the presence of water, in a buffered solution. An alcohol is used as the preferred solvent. The water is converted stoichiometrically and therefore its quantity is determined indirectly. The end‐point (EP) is reached when there is an excess of iodine. It can be indicated visually, photometrically or electrochemically. According to state‐of‐the‐art technology a double platinum electrode determines a voltammetric indication. The KF titration can be carried out either volumetrically or by coulometry. For a volumetric titration the iodine is added by volume to the titration cell containing the sample. In a coulometric titration iodide is oxidized at a platinum electrode and the iodine formed reacts with the water. The amount of current necessary in the generation of the iodine is directly related to the quantity of iodine generated, according to Faraday's first law. In practice, special titrators for the KF titration are available. Preformulated reagents are similarly available ready for use. Water in an extensive number of materials can be determined by KF titration. Chemicals, pharmaceuticals, oils, plastics and foodstuffs are examples of typical sample types. The measurement range spans a few ppm to 100% water. The KF titration is described in ISO 760 and as part of many other international standard procedures. Other methods for the determination of water content include loss‐on‐drying, IR spectroscopy or azeotropic distillation.
Quantitative Spectroscopic Calibration
Authors: Mark Howard
Published: September 2006

Spectroscopic calibration is the process of using mathematical calculations to relate the measured spectroscopic properties of a set of samples to the chemical composition (and sometimes other properties) of those samples. Any of several algorithms are employed to perform these calculations. Regardless of the algorithms used, however, successful execution of this process depends on properly performing several tasks before and after the main calibration algorithm is applied: selecting a suitable set of calibration samples to “train” the system, using the correct algorithm to match the characteristics of the measured data, and properly evaluating and validating the calibration model developed. In this article, these various aspects of calibrating spectroscopic instruments to perform quantitative analysis are discussed. The article ends with an overview of the various algorithms that are available to perform the calibration process.
Acid Rain
Authors: Butler Thomas J.
Published: September 2006

Acid rain is a popular term that can include all forms of precipitation, as well as fog and cloudwater, that is more acidic than expected from natural causes. Natural background levels of precipitation acidity tend to be around pH of 5.1 to 5.2. This compares with present annual average values of pH 4.3 to 4.4 for parts of eastern North America, and pH values less than 4.5 for large areas of Europe, and eastern Asia. As pH decreases, H
+
concentration or acidity increases exponentially. Individual storms in affected areas, especially in the summer, can often produce pH values below 3.5, and cloudwater in some areas can be below pH 3.0. A more appropriate term than acid rain is ‘acidic deposition’, which includes both wet and dry deposition (gases and particles) of acidic compounds to the earth's surface from the atmosphere. Accurate measurements of dry deposition are unfortunately much more difficult to measure than wet deposition. However, dry deposition can account for 10%–70% of the total amount of acidic deposition.
Age–Growth Modeling
Authors: Nokoe K. Sagary
Published: September 2006

Growth of an individual or group of organisms is defined as the change in dimensions over time. Growth rates vary between and within plant and animal species. They are generally influenced by both genetic and phenotypic factors, and may be negative or positive. Positive growth relates to increases in dimensions (weight, height, and number of component parts) over time, while negative or decay growth refers to the decline over time. Human interference, either intentionally or accidentally, environmental influences and management practices can substantially influence the growth forms and patterns of individuals and population. Age/growth functions constitute important analytical tools in the management of natural resources. These models are explicit expressions of both theory and empirical knowledge, and are useful for growth and yield prediction, computation of sustainable crop yields, and evaluation of alternative management strategies, among others. The approach to age/growth modeling may be based on either primary (single) unit or stand (aggregate of units). Irrespective of modeling philosophy, the goal is identification of the most parsimonious and biologically‐reasonable model that best describes the relationship between the response variable (which is a measure of growth) and time (age). Biologically reasonable models involve parameters to which meaningful interpretation may be assigned.
Asthma
Authors: Bellamy Scarlett
Published: September 2006

Statistics plays an important role in discovering the link between environmental exposures and the expression of asthma among urbanized, poor children who seem to be most vulnerable.
Bayesian Graphical Models
Authors: Jensen Finn Verner
Published: September 2006

Mathematically, a Bayesian graphical model is a representation of the joint probability distribution for a set of variables. The most frequently used type of Bayesian graphical models are Bayesian networks, but other types exist and Bayesian networks can be seen as a special type of Chain Graphs. The structural part of a Bayesian graphical model is a graph consisting of nodes and edges. The nodes represent variables. The set of all variables, called the universe, is denotedU. A variable may be discrete or continuous. For the sake of simplicity this article focuses on models with only discrete variables, and refers to the variable values as states. An edge between two nodesAandBindicates a direct influence between the state ofAand the state ofB.
Binary Data
Authors: Piegorsch Walter W.
Published: September 2006

Environmental data often occur in discrete form, such as the number of individuals living near a waste site, or whether or not a subject develops cancer after exposure to an environmental agent. This latter example is one of the simplest forms of discrete data: a dichotomous indicator of whether or not an event has occurred, i.e. a ‘success’ or a ‘failure’. Such an outcome can be coded simply as either 1 or 0 – corresponding to success or failure – and we refer to this as a case of
binary data
. If the data on each subject or unit in the study are coded in this binary fashion, and if each subject contributes one and only one such binary observation to the experimental data, we call this a case of
ungrouped binary data
. Alternatively, when the subjects have some underlying or coordinating feature that relates them, it is common to group the like subjects and record the proportion of subjects responding; i.e. take the proportion Y/N, where Y is the number of observed successes out of the N observed subjects in the common group. This is
grouped binary data
.
Bioaccumulation
Authors: Voit Eberhard O.
Published: September 2006

The accumulation of contaminants in organisms is crucial to environmental risk assessments. It depends on the chemical and physical properties of the contaminant and the ecosystem, the type of exposure (e.g. acute or chronic), and the efficiency with which organisms assimilate, metabolize, and eliminate the compound. To judge the severity of exposure and the efficacy of remediation, one must understand the complex dynamics of chemicals as they affect ecosystems. The sheer number of chemical components and the multitude, nonlinearity, and temporal dependence of the governing biological, chemical, physical, and geological processes and cycles make this a daunting task. Most research has focused on specific scenarios, emphasizing aquatic (freshwater and marine) ecosystems and either heavy metals or large organics. While much has been learned, generalizations of the results often fail, and a true understanding of the dynamics of contaminant accumulation has not yet been achieved.
Biodiversity
Authors: Ver Hoef Jay M.
Published: September 2006

Biodiversity is closely related to diversity. However, biodiversity has come to be concerned with the conservation of variation of different levels of biological organization, from genes to populations, communities, ecosystems, and landscapes. Biodiversity ranges across spatial and temporal scales. For the elements of concern (genes to landscapes), biodiversity can be measured as the number of different types of elements, the relative frequencies of the different elements, or the number of different processes such as gene flow or nutrient flow. This article emphasizes methods for conserving biodiversity. Biodiversity is rarely used in a sentence without the word ‘conservation’. When discussing biodiversity, it is assumed that high biodiversity is desirable and should be preserved. The arguments for conserving biodiversity are many and varied, from aesthetic to the stability of our environment to future cures for cancer and other uses of nature's raw biological material that cannot yet be imagined.
Biogeography
Authors: Ver Hoef Jay M.
Published: September 2006

Biogeography, as defined by Brown and Lomolino, is ‘the science that attempts to document and understand spatial patterns of biodiversity. It is the study of distributions of organisms, both past and present, and of related patterns of variation over the earth in the numbers and kinds of living things.’ Biogeography can be split taxonomically, with phytogeographers studying plants, and zoogeographers studying animals. Biogeography can also be split by time, with paleoecology studying relationships between organisms and past environments, and ecological biogeography studying present distributions of organisms and how they relate to the physical and biotic environment. Biogeography is at the crossroads of multiple disciplines: sytematics, ecology, evolution, geology, and climatology, to name a few.