Quantitative chemical techniques

The main techniques employed in quantitative analysis are based upon (a) the quantitative performance of suitable chemical reactions and either measuring the amount of reagent needed to complete the reaction, or ascertaining the amount of reaction product obtained (b) appropriate electrical measurements (e.g. potentiometry) (c) the measurement of certain optical properties (e.g. absorption spectra). In some cases, a combination of optical or electrical measurements and quantitative chemical reaction (e.g. amperometric titration) may be used. 

Recent developments in microsystems technology have led to the widespread application of microfabrication techniques for the production of sensor platforms. These techniques have had a major impact on the development of so-called Lab-on-a-Chip devices. The major application areas for theses devices are biomedical diagnostics, industrial process monitoring, environmental monitoring, drug discovery, and defence. In the context of biomedical diagnostic applications, for example, such devices are intended to provide quantitative chemical or biochemical information on samples such as blood, sweat and saliva while using minimal sample volume. 

During the latter part of the nineteenth century and the early years of the twentieth century, there was considerable controversy over the composition of chemical compounds—were compounds strictly stoichiometric, with an immutable composition, or could the composition vary. Indeed, at the turn of the twentieth century, even the existence of atoms was a subject of debate. The principal techniques involved at this epoch were accurate quantitative chemical analysis and metallo-graphic studies of phase equilibria. The advent of X-ray diffraction studies effectively resolved the problem, and the experimental evidence for composition ranges of many solids became incontestable. 

Gonzalez et al. 2008). Laser ablation is a direct sampling technique by which a high energy laser is focused on the surface of a material and atoms, ions, and particles are ejected. The particles, which are chemically representative of the bulk sample, are then transported into an ICPMS for analysis. In LIBS, a luminous, short-lived plasma is created on the sample surface by the focused laser beam and its emission spectra are analyzed to provide both qualitative and quantitative chemical compositional analysis (Cremers... 

Heterocycles with conjugated jr-systems have a propensity to react by substitution, similarly to saturated hydrocarbons, rather than by addition, which is characteristic of most unsaturated hydrocarbons. This reflects the strong tendency to return to the initial electronic structure after a reaction. Electrophilic substitutions of heteroaromatic systems are the most common qualitative expression of their aromaticity. However, the presence of one or more electronegative heteroatoms disturbs the symmetry of aromatic rings pyridine-like heteroatoms (=N—, =N+R—, =0+—, and =S+—) decrease the availability of jr-electrons and the tendency toward electrophilic substitution, allowing for addition and/or nucleophilic substitution in yr-deficient heteroatoms , as classified by Albert.63 By contrast, pyrrole-like heteroatoms (—NR—, —O—, and — S—) in the jr-excessive heteroatoms induce the tendency toward electrophilic substitution (see Scheme 19). The quantitative expression of aromaticity in terms of chemical reactivity is difficult and is especially complicated by the interplay of thermodynamic and kinetic factors. Nevertheless, a number of chemical techniques have been applied which are discussed elsewhere.66... 

Difficulties are encountered in the qualitative and quantitative analysis of carbohydrate mixtures because of the structural and chemical similarity of many of these compounds, particularly with respect to the stereoisomers of a particular carbohydrate. As a consequence, many chemical methods of analysis are unable to differentiate between different carbohydrates. Analytical specificity may be improved by the preliminary separation of the components of the mixture using a chromatographic technique prior to quantitation and techniques such as gas-liquid and liquid chromatography are particularly useful. However, the availability of purified preparations of many enzymes primarily involved in carbohydrate metabolism has resulted in the development of many relatively simple methods of analysis which have the required specificity and high sensitivity and use less toxic reagents. 

Recently, introductory books about chemometrics have been published by R. G. Brereton, Chemometrics—Data Analysis for the Laboratory and Chemical Plant (Brereton 2006) and Applied Chemometrics for Scientists (Brereton 2007), and by M. Otto, Chemometrics—Statistics and Computer Application in Analytical Chemistry (Otto 2007). Dedicated to quantitative chemical analysis, especially using infrared spectroscopy data, are A User-Friendly Guide to Multivariate Calibration and Classification (Naes et al. 2004), Chemometric Techniques for Quantitative Analysis (Kramer 1998), Chemometrics A Practical Guide (Beebe et al. 1998), and Statistics and Chemometrics for Analytical Chemistry (Miller and Miller 2000). 

The confluence of improved experimental, dynamical and quantum chemical techniques are making possible the quantitative testing of dynamical rate theories. The ketene molecule (CH2CO) is a superb example. First, the dissociation of singlet ketene... 

Quantitative Analytical Techniques (chemical/physical measurements)... 

In many manufacturing processes there exists the potential for aldehyde formation. Often these aldehydes occur in low concentrations in the presence of much higher levels of aliphatics, olefinics and aromatic hydrocarbons. Gas chromatography or combined gc/ms methods are often ineffective in determining aldehydes in such a matrix. Several wet chemical techniques have been devised for estimating the total aldehyde concentration in these streams, but quantitation of the individual aldehydes has remained a difficult task. 

An analytical chemical technique that utilizes radioactive (or stable) isotopes for the quantitative analysis of the amount of substance. In the absence of a kinetic isotope effect, isotopic isomers react identically with respect to their unlabeled counterparts. The method offers the advantage that specific activity (or gram-atom excess in the case of stable isotopes) is an intensive variable. Therefore, one only needs to recover sufficient labeled metabolite to determine amount of substance and disintegrations per minute (or, gram-atom excess) to reach an accurate determination of specific activity. The technique is feasible so long as one can accurately determine the initial and final specific activities. 

Dramatic improvements in instrumentation (lasers, detectors, optics, computers, and so on) have during recent years raised the Raman spectroscopy technique to a level where it can be used for species specific quantitative chemical analysis. Although not as sensitive as, for example IR absorption, the Raman technique has the advantage that it can directly measure samples inside ampoules and other kinds of closed vials because of the transparency of many window materials. Furthermore, with the use of polarization techniques, one can derive molecular information that cannot be obtained from IR spectra. Good starting references dealing with Raman spectroscopy instruments and lasers are perhaps [34-38]. 

The modern investigations of trace elements in coals were pioneered by Goldschmidt, who developed the technique of quantitative chemical analysis by optical emission spectroscopy and applied it to coal ash. In these earliest works, Goldschmidt (31) was concerned with the chemical combinations of the trace elements in coals. In addition to identifying trace elements in inorganic combinations with the minerals in coal, he postulated the presence of metal organic complexes and attributed the observed concentrations of vanadium, molybdenum, and nickel to the presence of such complexes in coal. 

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

Abstract Blood and urine are frequently analyzed for their chemical content. Raman spectroscopy, with its high specificity, can provide chemical information in a non-contact and potentially non-invasive manner. In this chapter, key experimental and analytical techniques for converting Raman spectra into quantitative chemical concentration measurements are presented, along with a survey of the current status of the field. 

M. Nambayah and T.I. Quickenden, A quantitative assessment of chemical techniques for detecting traces of explosives at counter-terrorist portals, Talanta 63 (2004) 461. 

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