Qualitative Identification

A TGA curve has limited ability to identify the sample or determine its composition if its nature is unknown. However, there are still many unique analytical applications based on TGA. Two primary applications are qualitative identification and compositional analysis. 

In known systems, TGA is useful for compositional analysis, especially for additives. TGA often proves effective in dealing with complex systems that are 

A measure of the relative importance of these processes may be obtained by dividing the number of depolymerization steps occurring by the number of chain transfer and termination steps. This parameter is the zip length and gives a measure of the tendency of the polymer to break down into a monomer. If the zip length is low, there will be little monomer in the volatile products with an increased probability for a higher char yield. 

Two factors favor high zip length the absence of easily abstractable hydrogens and the resonance stabilization of the incipient radicals that can participate in monomer production. The following examples illustrate (see Fig. 15.4) the concept [5]. Both poly(methyl methacrylate) I and poly(methyl acrylate) II can generate radicals III and IV, where R represents part of the polymer chain, 

4 Degradation schemes of poly(methyl methacrylate), I and III, and poly(methyl acrylate), II and IV. 

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Qualitative identification. The spectrum is of help in identifying organic compounds. If two compounds are identical, the electronic spectra must be identical the converse is not necessarily true and in this... 

Atomic absorption, along with atomic emission, was first used by Guystav Kirch-hoff and Robert Bunsen in 1859 and 1860, as a means for the qualitative identification of atoms. Although atomic emission continued to develop as an analytical technique, progress in atomic absorption languished for almost a century. Modern atomic absorption spectroscopy was introduced in 1955 as a result of the independent work of A. Walsh and C. T. J. Alkemade. Commercial instruments were in place by the early 1960s, and the importance of atomic absorption as an analytical technique was soon evident. 

The interaction between nuclei splits resonances into multiple peaks, the number and relative intensity of which also assist in qualitative identification of the proton responsible for the absorption. 

Spark Source Mass Spectrometry (SSMS) is a method of trace level analysis—less than 1 part per million atomic (ppma)—in which a solid material, in the form of two conducting electrodes, is vaporized and ionized by a high-voltage radio frequency spark in vacuum. The ions produced from the sample electrodes are accelerated into a mass spectrometer, separated according to their mass-to-charge ratio, and collected for qualitative identification and quantitative analysis. 

In addition to qualitative identification of the elements present, XRF can be used to determine quantitative elemental compositions and layer thicknesses of thin films. In quantitative analysis the observed intensities must be corrected for various factors, including the spectral intensity distribution of the incident X rays, fluorescent yields, matrix enhancements and absorptions, etc. Two general methods used for making these corrections are the empirical parameters method and the fimdamen-tal parameters methods. 

The half-wave potential is also independent of the electrode characteristics, and can, therefore, serve for the qualitative identification of an unknown substance. Owing to the proximity of many different half-wave potentials, its use for qualitative analysis is of limited application unless the number of... 

Normalization, is an adjustment to a data set that equalizes the magnitude of each sample. In other words, normalization removes all information about the distance each data point lies from the origin of the data space but preserves the direction. Normalization has a relatively limited number of special applications. For example, it is frequently used a pre-processing step in preparing reference spectra for a qualitative identification library. The idea is to retain only the information that qualitatively distinguishes one sample from another while removing all information that could separate two samples of identical composition but different concentrations. 

Comparative studies [1028,1052,1053] of the decompositions of Ni, Co and Cu alkanoates from formate to valerate showed that both the cation present and the length of the alkane chain influenced the temperature and enthalpy of decomposition. No such relationship was found [1048], however, between chain length and temperature of reaction of a series of nickel salts between the propionate and the stearate in a study which included some qualitative identifications of the products. Mass... 

Many investigations of the decompositions of coordination compounds have been concerned with the qualitative identification of the steps involved, characterization of any intermediates formed and comparisons of reactivities of related salts containing systematically varied constituents. Observations and conclusions from such work [1113,1114] are outside the scope of this review, though the results can serve to identify systems worthy of more detailed investigation. The content of this section, reflecting the content of the relevant literature, is restricted to accounts of the behaviour of a number of representative substances. Features distinguishing these reactions from those of simple salts are emphasized. 

The simplest solid—solid reactions are those involving two solid reactants and a single barrier product phase. The principles used in interpreting the results of kinetic studies on such systems, and which have been described above, can be modified for application to more complex systems. Many of these complex systems have been resolved into a series of interconnected binary reactions and some of the more fully characterized examples have already been mentioned. While certain of these rate processes are of considerable technological importance, e.g. to the cement industry [1], the difficulties of investigation are such that few quantitative kinetic studies have been attempted. Attention has more frequently been restricted to the qualitative identifications of intermediate and product phases, or, at best, empirical rate measurements for technological purposes. 

Many pitfalls await the unwary. Here is a short list, compiled from more detailed considerations by Bunnett.8 One should properly identify the reactants. In particular, does each retain its integrity in the reaction medium A spectroscopic measurement may answer this. The identities of the products cannot be assumed, and both a qualitative identification and a quantitative assay are in order. Pure materials are a must—reagents, salts, buffers, and solvent must be of top quality. Careful purification is always worth one s time, since much more is lost if all the work needs repeating. The avoidance of trace impurities is not always easy. If data are irreproducible, this possibility must be considered. Reactions run in the absence of oxygen (air) may be in order, even if the reactants and products are air-stable. Doing a duplicate experiment, using a spent reaction solution from the first run as the reaction medium, may tell whether the products have an effect or if some trace impurity that altered the rate has been expended. 

Qualitative (identification) applications depend upon the comparison of the retention characteristics of the unknown with those of reference materials. In the case of gas chromatography, this characteristic is known as the retention index and, although collections of data on popular stationary phases exist, it is unlikely that any compound has a unique retention index and unequivocal identification can be effected. In liquid chromatography, the situation is more complex because there is a much larger number of combinations of stationary and mobile phases in use, and large collections of retention characteristics on any single system do not exist. In addition, HPLC is a less efficient separation... 

Human milk Purged warm trapped in Tenax(I)-GC thermally desorbed HRGC/MS Qualitative identification NR Pellizzari et al. 1982... 

Euphane triterpenes Schinus molle SiOj T - AcOEt - AcOH Anisaldehyde reagent Qualitative identification 52 n 3"... 

J.M. Andrews and S.H. Lieberman, Neural network approach to qualitative identifications of fuels from laser induced fluorescence spectra. Anal. Chim. Acta, 285 (1994) 237-246. 

Visualization Techniques for Thin-Layer Chromatography.892 Qualitative Identification and Nicroreaction Techniques it ... 

Table 5.9 summarises the main features of FTIR spectroscopy as applied to extracts (separated or not). Since many additives have quite different absorbance profiles FTIR is an excellent tool for recognition. Qualitative identification is relatively straightforward for the different classes of additives. Library searching entails a sequential, point-by-point, statistical correlation analysis of the unknown spectrum with each of the spectra in the library. Fully automated analysis of... 

Principles and Characteristics The fastest growing area in elemental analysis is in the use of hyphenated techniques for speciation measurement. Elemental spe-ciation analysis, defined as the qualitative identification and quantitative determination of the individual chemical forms that comprise the total concentration of an element in a sample, has become an important field of research in analytical chemistry. Speciation or the process yielding evidence of the molecular form of an analyte, has relevance in the fields of food, the environment, and occupational health analysis, and involves analytical chemists as well as legislators. The environmental and toxicological effects of a metal often depend on its forms. The determination of the total metal content... 

Although often used as a qualitative (identification) tool, MS may act as a quantitative inorganic mass detector. Quantification of organic analytes often takes place in combination with chromatography or in tandem MS mode. It should be realised that mass spectrometry is certainly not a panacea for all polymer/additive problems, although it is developing into a major tool for this purpose. 

Krzek et al. [35] reported the qualitative identification and quantitative analysis of the mixtures of OTC, tiamulin, lincomycin, and spectinomycin in the veterinary preparations by using TLC/densitometry. As stationary phase, they used precoated TLC aluminum sheets, and the mobile phases were mixtures of 10% citric acid solution, hexane, ethanol (80 1 1, v/v), and n-butanol, ethanol, chloroform, 25% ammonia (4 5 2 5, v/v). The other application of TLC or HPTLC for analyzing OTC in the various samples is summarized in Table 2 [36]. 

A universal system for qualitative identification was devised by Kovats some years ago. Based on the linear relation between log t R and the number of carbon atoms for a homologous series, Kovats selected //-alkanes as standards for the following reasons ... 

Methods similar to those used in GC are applicable to HPLC. Thus, comparison of retention data is the most useful means of qualitative identification, the retention factor, , generally being used in preference to... 

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