Purity Substances

Joint Analytical Laboratory Nizhny Novgorod State University - SHIMADZU, Gagarin Avenue 23, 603950, Russia Institute of Chemistry of High-Purity Substances of HAS,... 

Efremov A.A., Grinberg EE., Tartakovskii V.E High Purity Substances Consultant Bureau, New York, London, 2,2, 231, 1988. 

This is demonstrated by the success story of Merck s liquid crystals, high-tech chemicals with unique properties. Discovered over a century ago by German scientists, they caused controversy in the scientific community at first, but - with the help of Merck s highest-purity substances - liquid crystals were eventually accepted as a fourth state of aggregation. 

In the field of chemical measurements, the question as to which are the national measurement standards is far from being completely answered. There is no doubt, however, that primary reference materials, e.g. high-purity substances which are ultimately necessary as reference points, will play a role as national (or even better international) measurement standards. But these alone will not be sufficient. As the task of chemical analysis is usually the determination of chemical composition, national reference points closer to this task, namely standard reference mixtures, are also required, and if the preparation of these is not feasible, e.g. due to instability problems, devices and procedures furnishing well-known compositions with small uncertainties must also be included as national measurement standards. All these kinds of national references or standards are currently in use or under development. 

Another possible approach, which is broadly used, is to use high-purity substances (indirectly assayed) as standards. For use at the highest level, this approach requires the determination of all important impurities in the sample. This means not only metallic impurities, commonly stated in the manufacturers certificates, but also non-metals as oxygen, carbon, etc. The content of impurities is not always known in advance. If the total content of impurities is very low, the uncertainty of their determination does not affect the required uncertainty of the sample assay. Some other problems are discussed in Ref. [5], The need of determining the molar weight may equally apply here. 

This analytical figure of merit is of special importance especially in the control of high-purity substances and materials for microelectronics, and also in food analysis and other disciplines. 

In the analysis of high-purity substances, general matrix removal is often very important so as to pre-concentrate the elements to be determined. To this aim all separation techniques such as ion exchange, liquid-liquid extraction of a metal complex with organic solvents, fractionated crystallization, precipitation and coprecipitation as well as electrochemical methods may be used (for a systematic treatment, see Ref. [300]). These principles can also be applied in on-line systems, as is now possible with solid phase extraction. Here matrix elements or the analytes can be adsorbed as complexes onto the column and eluted for direct determination by AAS. 

Both for high-purity substances [617] and semiconductor-grade materials [618] up to 16 elements can be determined in 5N and 6N type samples. At high resolu-... 

Accuracy in the purity value for high-purity substances io- % + 5 x 10 2%... 

A very common and useful technique to identify elements in a spectrum is by comparison with known spectra. It is useful for the spectroscopist to prepare spectra of known elements on his own spectrograph for comparison purposes. Figure 7-2 is a typical spectral plate, prepared with high-purity substances. The more useful spectral lines for each element are identified in each spectrum. The spectral plate also includes an iron spectrum, which is convenient for orienting the known and unknown spectra. When an unknown spectrum is obtained, an iron spectrum also should be placed on the spectral plate to aid in lining up the known and unknown spectra. 

High-purity materials are available as pure solids for chemical uses such as metals used as reference substances for metallurgical analysis, and compounds used as primary standards for many types of titrimetry, and as standards for elemental microanalysis. Other available high-purity substances intended for diverse physical properties include ion activity standards for the calibration of pH and ion-selective electrodes standards for various thermodynamic uses including melting point determinations, differential scanning calorimetry and bomb calorimetry and standards for the calibration of spectrophotometers. 

In this sense it has found wide use for analysis of dry solution residues, e.g. in the case of the rare earth elements, or subsequent to matrix separation in the analysis of high-purity substances (see Refs, in [468]). The Doppler widths of the lines are low and accordingly this source has even been used for isotopic analysis (determination of To date it is still employed for the determination of volatile... 

In the analysis of high-purity substances, matrix removal is often very important for preconcentration. For this, separation techniques such as ion exchange, liquid-liquid extraction of metal complexes with organic solvents, fractional crystallization, precipitation, coprecipitation, and electrochemical methods may be used [194],... 

O A simpiified representation ot a side view ot a "perfect" crystai ot a poiar substance at 0 K. Note the pertect aiignment of the dipoies in aii moiecuies in a perfect crystai. This causes its entropy to be zero at 0 K. There are no perfect crystais, however, because even the purest substances that scientists have prepared are contaminated by traces ot impurities that occupy a few of the positions in the crystai structure. Additionaiiy, there are some vacancies in the crystai structures of even very highiy puritied substances such as those used in semiconductors (Section 13-17). 

Table 42 shows that the saturation vapor pressure and orthobaric density of vapor and liquid have been measured repeatedly. We adopted experimental data of MEI [4.3] as reference values of p. These data were obtained statistically for a high-purity substance and with an error estimated by the authors at 0.1-0.2%. The results of comparison with the data of other researchers are shown in Fig. 34. 

Standards in use for quantitation are essentially employed in three ways. With the internal standard technique, known quantities of a carefully selected (usually high purity) substance, the internal standard, are added to both samples and standards. The internal standard (preferably a non-commercial product) should have similar chemical and physical properties to the analyte, in particular, volatility and functional groups, in order to react in the same way to changes in the chemical environment (e.g. dinonyl adipate may serve as an internal standard for the determination of di(2-ethylhexyl) adipate). Solutions of pure additives used as standards may be unsatisfactory due to the difference in the evaporation profile between pure additives and those blended in the polymer samples. For example, a pure Permanax WSP sample evaporates in the ion source from about 30 to 150°C, whereas Permanax WSP blended in PE evaporates from about 120 C (m.p. of PE) to 350°C [17]. If one opts for polymer-based calibration standards the homogeneity of the samples is of crucial importance. Using internal standards in quantitative analysis is advantageous, for instance, in cases where the sample thickness cannot be determined exactly, or in gaseous samples with unknown total pressure. 

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