Low-temperature ashing techniques

Low temperature ashing techniques were employed to reduce the oxidation and decomposition of the minerals which would occur under normal ashing conditions. Mineralogical analyses were performed using X-ray powder diffraction, infra-red spectroscopy, optical petrography, and scanning electron microscopy. Elemental determinations were performed using atomic absorption spectroscopy. 

A variety of instmmental techniques may be used to determine mineral content. Typically the coal sample is prepared by low temperature ashing to remove the organic material. Then one or more of the techniques of x-ray diffraction, infrared spectroscopy, differential thermal analysis, electron microscopy, and petrographic analysis may be employed (7). 

Experience in this laboratory has shown that even with careful attention to detail, determination of coal mineralogy by classical least-squares analysis of FTIR data may have several limitations. Factor analysis and related techniques have the potential to remove or lessen some of these limitations. Calibration models based on partial least-squares or principal component regression may allow prediction of useful properties or empirical behavior directly from FTIR spectra of low-temperature ashes. Wider application of these techniques to coal mineralogical studies is recommended. 

Within the past decade the technique of electronic (radiofrequency) low-temperature ashing has been used to investigate mineral matter in coal. In a low-temperature asher, oxygen is passed through a radiofrequency field, and a discharge takes place. Activated oxygen thus formed passes over the coal sample, and the organic matter is oxidized at relatively low temperatures—usually less than 150°C (14). 

Estep et al. (16) used infrared absorption bands in the region 650-200 cm"1 to analyze quantitatively as well as qualitatively for minerals in low-temperature ash. O Gorman and Walker (2) also applied this technique in their investigations. 

According to Stoeppler et al. [15], severe errors up to a factor of two may result from ETA—AAS analysis of biological materials without some form of sample pretreatment. The approaches that will be discussed here are (a) the use of diluent solutions to minimise matrix and molecular absorption interferences (b) partial decomposition techniques in which metals are extracted from proteins with acids (c) dissolution of tissue samples without complete oxidation (d) complete oxidation procedures such as dry ashing, wet digestion at ambient and elevated pressures, and low temperature ashing with reactive gases at low pressures. 

At the present time there are no ETA—AAS methods that can compete with the cold vapour technique for Hg or with hydride generation methods for Sb and Te. Another attractive method for Sb and Te is low pressure microwave induced plasma (MIP) emission spectroscopy [138]. Using low-temperature ashing and solvent extraction as preparation, physiological concentrations of both elements ([Pg.376]

Modifications of the above techniques could be applied to the analysis of freeze dried and low temperature ashed tissue samples. 

The usual method utilized to identify the clay minerals in coal is X-ray diffraction of the low-temperature ash (LTA), but because of the poor crystallinity of the clays in the coal, the technique cannot be used for quantitative measurements. The Mossbauer effect does not provide much improvement owing to the small iron content of the clays. A coal rich in clays is shown in Figure 7 (about 10% mineral matter). The two peaks at higher... 

In summary, a method for the analysis of molybdenum in biological fluids has been presented. The method requires the destruction of the organic materials in the sample by low-temperature ashing. Detection was accomplished by using a graphite furnace—atomic absorption technique and the standard additions method. The method is sufficiently sensitive to distinguish between molybdenum levels in the blood, serum, and urine from exposed and unexposed individuals. 

A few years ago, an ad hoc group of workers interested in coal minerals. The Mineral Matter in Coal Group, prepared and distributed a round-robin low temperature ash to ten laboratories. Each laboratory was to prepare, mount and quantify the mineral components in the ash by their respective XRD techniques. The data were then compared. Even though a wide variety of techniques was used for each phase of the analysis, with the exception of the clay mineral estimates made by one laboratory (significantly lower than the others) and the pyrite estimate made by another (too high), the data compared reasonably well. The averages of all the submitted estimates are summarized in Table III. 

Another dry technique is that of low-temperature ashing. A radio-frequency discharge is used to produce activated oxygen radicals, which are very reactive and will attack organic matter at low temperatures. Temperatures of less than 100°C can be maintained, and volatility losses are minimized. Introduction of elements from the container and the atmosphere is reduced, and so are retention losses. Radiotracer studies have demonstrated that 17 representative elements are quantitatively recovered after complete oxidation of organic substrate. 

Dry ashing of dental samples can be carried out in a muffle furnace at temperatures not exceeding 500°C when the elements having moderate volatility (e.g. Zn, Pb) are quantitatively retained. However, it cannot be recommended when volatile elements (Cd, Se, Hg) are to be determined. The use of low temperature ashing in an oxygen plasma (LTA) has not been reported for dental samples but the preliminary studies carried out in the author s laboratory showed that the technique is slow and is only effective if combined with initial wet dissolution using nitric acid. 

Low temperature ashing in an oxygen plasma can be carried out in the same way as described for bone after finely powdering the soft tissue using the brittle fracture technique. 

Mercury was included in a thorough review of XRF techniques in clinical studies (Leyden and Noddy, 1977). For normal XRF analysis of 1 g of dried soft tissue, the lower limit of detection was found to be a few mg/kg. For PIXE, the relative detection limits in biological material are in the order of a few tenths of mg/g. For soft tissue analysis, this is normally orders of magnitude too high, compared with "normal" levels of mercury. Thus, the X-ray techniques must be combined with a preconcentration stage. Preconcentration techniques prior to mercury determination means serious risks of losses due to the volatility. To minimize such losses, lenient concentration procedures, e.g. low temperature ashing (Pallon and Malmqvist, 1981), are required. 

A variety of advanced techniques exists for determining the species composition of the fuel, which is vital to improved predictive methods for coal, and especially ash behavior, in boilers. X-ray diffraction, performed on low-temperature ash, is the most widely used technique for qirahtatively identifying the presence of minerals in their crystalline form in concentrations of a few weight percent or greater. Thermal analytical techniques such as differential thermal analysis (DTA) and thermogravi-metric analysis (TGA) have been used as a signature analysis based on changes in physical properties with temperature. 

Multielement analytical techniques - atomic absorption spectrometry, inductively coupled plasma mass spectrometry. X-ray fluorescence, neutron activation analysis, etc. - are used. The experimentation can be done directly on the mineral matter of the coal sample after the removal of the organic matter by a prolonged treatment of activation with oxygen plasma (low-temperature ashing). Neutron activation is also applied to online analyses of coal and fly-ashes on feeding-belts in order to provide information on a continuous basis. 

Low-temperature ashing (LTA) can be used on oxidizable blasting debris — for example, plastic abrasive — to achieve a high degree of volume reduction in the waste. Trials performed with this technique on plastic abrasive resulted in a 95% reduction in the volume of solid waste. The ash remaining after oxidation must be disposed of as hazardous waste, but the volume is dramatically reduced [9],... 

Simultaneously with the efforts to determine the origin of mineral matter in coal, systematic efforts were underway to estimate the quantitative distribution of trace and minor elements in American coals. The early analyses were performed on high-temperature ashes, and as a consequence, the investigators had to be content with determining the nonvolatile metallic oxides. However, with the advent of the low temperature asher and improvisations and advances in wet chemical, radiochemical, and instrumental analytical techniques, we not only can analyze nondecomposed mineral matter but also can study the composition of whole coal. 

Identification of Minerals in Coal. Once the low-temperature mineral matter residue has been obtained by radiofrequency ashing, the minerals can be identified, and their concentrations can be determined by a variety of instrumental techniques. The best developed, most inclusive, and probably most reliable method used thus far in distinguishing minerals in coal is x-ray diffraction analysis. It has been used extensively by Gluskoter (15), Wolfe (17), O Gorman and Walker (2), and Rao and Gluskoter (1) and has been somewhat successful in quantifying mineral analyses. 

The disadvantages of both wet and dry ashing which have only been outlined here, have led to the development of alternative methods for sample oxidation based on these two techniques. These are wet ashing using vapour phase attack or at elevated pressures, and dry ashing at low temperatures and pressures with reactive 02. 

Decomposition of biological tissues in a low pressure (<1 torr) stream of radio-frequency excited 02 gas takes place at relatively low temperatures (70°C) with no volatilisation losses and 99—102% recovery of Sb, As, Cs, Co, Cr, Fe, Pb, Mn, Mo, Se, Na and Zn [43]. Appreciable losses of Hg, I, Ag, Au and Pt do occur, the latter three probably as a result of catalytic reaction with the excited 02 [44], The main disadvantage of this technique is the very long ashing time, which can be up to 32 h for 1 g samples. The use of more reactive gas mixtures can reduce the ashing time. Lopez-Escobar and Hume [45] described a mixed-gas technique in which 1.4 nmol min-1 of 03 in 02 effected the release of 98.5% of Hg from organic matrices in only... 

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