Chemical identities, compositional analysis

Samples of potential chemical interferences were also prepared by placing samples of selected materials inside clean cardboard boxes. The potential inter-ferent items were materials commonly encountered in demining operations such as petrol. These materials were sampled in an identical fashion as the positive samples. Blank samples were also prepared by sampling empty cardboard boxes. All samples were marked by sampling personnel in a manner that made it impossible for analysts to determine the composition of the sample during analysis. Nomadics personnel and dog handlers were not given any information on sample identity until analysis of samples was completed and results were submitted for scoring (i.e., the tests were conducted in a blind fashion). 

Furthermore, as an extract of a natural product is concentrated, the number of odorants detected increases indefinitely. Clearly, most of the odorants in a natural product are below their odor threshold, and it is only the most potent compounds that are involved in generating the flavor response. An odorant can be very potent at extremely low concentrations if it has an extremely low odor threshold, (unit go). In practice, early GC/O analysts attempted to concentrate the sample as far as possible to identify as many potential odorants as possible. Compositional studies combined with threshold studies were then used to sort out the important odorants from the ones that did not contribute to the flavor experience. Rothe s odor units (OU = concentration in sample/threshold in sample) were an early attempt to rank odorants by potency. The process of determining OU values for a food required a lot of chemical and psychophysical analysis. Dilution analysis was developed to produce an OU-like value directly from GC/O without the need to know the identity of the odorant. In fact, the real value of dilution analysis is that it can tell the analyst which compounds to identify. 

Compositional analysis involves the determination of three quantities. The most fundamental of these is the elemental identity of surface species, i.e., the atomic number of each species. It also is desirable to know, however, the chemical identities of these species. For example, is CO adsorbed as a molecule or is it dissociated into separate C and 0 complexes with the substrate. Finally, it is necessary to determine the approximate spatial location of the various chemical species. Are they "on top" an otherwise undisturbed substrate Do they reconstruct the substrate or diffuse into it, e.g., along grain boundaries Or perhaps they form localized islands or even macroscopic segregated phases at various positions across the surface. An important trend in modern compositional analysis is the increasing demand for spatial resolution laterally across the surface on a scale (d 0.1 u m = 10 A) comparable to the dimensions of modern integrated circuits (10-12). Compositional analysis is by far the most extensively used form of surface analysis and is the subject of most of the papers in this symposium as well as of numerous reviews in the literature (5-9., 13, 14). 

That solids with different compositions can adopt identical crystal shapes was documented in 1819 by Mitscherlich, who called the phenomenon isomorphism (Mitscherlich 1819, Melhado 1980). Isomorphism can describe phases with similar atomic architectures but unlike constituents, such as NaCl and PbS, and it also can refer to members of a continuous solid solution series, such as the olivine group with formula (Mg,Fe)2Si04. Three years later, Mitscherlich documented the complementary property of polymorphism, whereby phases with identical compositions occur as different structures (Mitscherlich 1822). Although mineralogists of the nineteenth century recognized the important inter-relationship between crystal structure and composition, the crystallographic probes available for structure determination did not keep pace with advances in wet chemical analysis. Consequently, understanding the effects that chemical modifications exert on crystal structures could be revealed only by careful measurements of subtle variations in habit. 

Analytical chemistry deals with methods for determining the chemical composition of samples. A compound can often be measured by several methods. The choice of analytical methodology is based on many considerations, such as chemical properties of the analyte and its concentration, sample matrix, the speed and cost of the analysis, type of measurements i.e., quantitative or qualitative and the number of samples. A qualitative method yields information of the chemical identity of the species in the sample. A quantitative method provides numerical information regarding the relative amounts of one or more of the species (the analytes) in the sample. Qualitative information is required before a quantitative analysis can be performed. A separation step is usually a necessary part of both a qualitative and a quantitative analysis. 

While the two first conclusions could be obtained from the measurement of the total H/C ratio of coke deposits or from their in situ investigation by spectroscopic methods (IR, CP MAS NMR, UV VIS, EPR) this is not the case for the other conclusions. Indeed these methods can lead only to information concerning the coke content and the chemical identity of its components, which is not sufficient for specifying the mode of coke formation, the location of coke molecules and the cause of their retention in or on the zeolite. All this can be obtained from the coke composition. To determine this composition requires the recovery of coke from the zeolite followed by its analysis through GC, HNMR, SM... However great care must be taken not to modify coke components during their recovery. It has been shown that this is the case when coke is recovered from the coked samples through dissolution of the zeolite in a hydrofluoric acid solution. 

Compositional analysis is based on the fact that an x-ray diffraction pattern is unique for each crystalline material. Thus, if an exact match can be found between the pattern of the unknown material and an authentic sample, chemical identity can be assumed. The International Center for Diffraction Data (ICDD-JCPDS) publishes a database containing powder diffraction patterns for several thousands of materials. Commonly, it is possible to identify an unknown material by searching the ICDD-JSPDS database for a matching pattern. 

The nature of the material to be studied, which means its degree of crystallinity and perfectness of crystal structure, may have a significant effect on the thermoanalytical behavior. In spite of identical chemical composition of a certain material the variations with respect to structure, imperfections, grain boundaries, etc. are almost infinite. Of course many of these will not show in normal thermogravimetric analysis, with very sensitive apparatus characteristically different TG curves18, 19 may be obtained however. As an example Fig. 26 shows the thermal decomposition of hydrozincite, Zn5(OH)6(003)2, whereby equal amounts of samples from natural origin and synthetic preparations are compared. 

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