Ashing method disadvantages

Widening interest in the quaHty of the environment has led to increased demand for information on a wide range of trace-metal contents of foodstuffs. Trace metals in foodstuffs are normally determined by spectroscopic techniques after complete destruction of the organic matrix. Destruction is achieved either by wet oxidation or by dry ashing additional treatment is normally required in order to obtain the metals of interest in a form suitable for analysis. Both methods of destruction are time consuming and tedious this is particularly true of the wet-oxidation procedure, which has the additional disadvantage of being potentially hazardous the methods require considerable analytical skill and experience. Both methods are prone to produce erroneous results either by the loss of an element of interest or by adventitious contamination from the component parts... 

Disadvantages Emissions, especially particulate, less efficient than other methods New technology High NO, fly ash, pulverizing expensive... 

A representative elemental analysis, however, may be obtained by subjecting ashed paper to SEM X-ray analysis this method, which is presently under study, avoids the disadvantages noted above and will be reported upon shortly (32). 

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. 

Dry ashing is rarely applied now and has largely been replaced by wet decomposition (ashing) because it has several disadvantages, such as losses caused by volatilization, very low ashing of some materials, difficult dissolution of ashed materials, and contamination. Advantages of this method are that no reagents are used and little operator attention is required. 

Most commercial soy protein products have been developed from the acid-precipitated fraction [85]. However, local extremes in pH can cause irreversible denaturation of the proteins [85]. The disadvantages of this method include denaturation of protein on exposure to alkali and acid treatment, high-ash content, and alteration of protein solubility after rehydration [86]. 

A critical review is presented of the methods commonly used to isolate and concentrate aquatic humic substances, important steps in any extraction scheme are filtration, concentration, isolation, and preservation. Various filtration options are outlined and compared filtration is important for obtaining low-ash humic substances. Advantages and disadvantages of commonly used concentration and isolation techniques are discussed, and sorption on the synthetic macroporous resins XAD-8, a nonionic resin, and Duolite A-7, a weak anion-exchange resin, are recommended as the most efficient methods. Two established extraction schemes utilizing these resins are discussed. 

Advantages and disadvantages of the commonly used methods to isolate and concentrate aquatic humic substances have been presented in this chapter. For most waters, the process of producing low-ash humic material involves filtration, concentration, isolation of humic substances from inorganic and other organic solutes, and lyophilization. Development of... 

The diethyldithiocarbamate copper complex is unstable in acid medium and is light-sensitive. Dibenzyldithiocarbamate (avaUable as zinc salt) lacks these disadvantages moreover, it is more sensitive (Table 1) and more specific (M13, S3). Maytum et al. (M17) and most recently Giorgio et al. (G5) have described methods for urinary copper determination with this reagent. The latter method does not require preliminary ashing. The only disadvantage of these methods is the use of carbon tetrachloride, which is potentially toxic. 

The homogenous acid catalyzed hydrolysis of sucrose uses food approved mineral acids (sulfuric acid, hydrochloric acid) at elevated temperature. The degree of inversion could be adjusted by the point of neutralization (sodium or potassium hydroxide). This step leads inevitably to the formation of the respective salts, thereby causing high ash contents in the product. A further disadvantage of this method is the applied elevated temperature in combination with the low pH, thus causing by-product formation. 

The emission of sulphur oxides may be controlled by adding substances into the combustion chamber, which are able to bind sulphur dioxide (additive method). Finely group limestone, calcium hydroxide, calcium oxide or sometimes dolomite may be used for this purpose. Limestone is calcinated at high temperature, yielding calcium oxide, which binds the sulphur dioxide. Sulphate, sulphite, non-reacted adsorbent and fly ash are separated in a separating system. The disadvantage of this process is its low efficiency (only about 20 to 40% SO2 are retained). 

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