Wet and dry ashing

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. 

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Adrian, W.J. (1973). A comparison of wet pressure digestion method with other commonly used wet and dry ashing methods. Analyst. 98, 213-216 Alcock, N.W. (1987). A hydrogen peroxide digestion system for tissue trace metal analysis, Biol.Trace Bern. Res. 13, 363-370... 

Hill, A.D., Patterson, K.Y., Veillon, C. and Morris, E.R. (1986). Digestion of biological materials for mineral analysis using a combination of wet and dry ashing. Anal. Chem. 58, 2340-2342... 

Projects in the CCT program demonstrated innovative applications for both wet and dry or seniidry FGD systems. The wet FGD systems, which use limestone as an absorber, have met or exceeded the 90 percent SO, removal efficiency required to meet air quality standards when burning high-sulfur coal. The di"y or semidry systems use lime and recycled fly ash as a sorbent to achieve the required removal. 

Resist stripping- Both wet and dry removal processes are used at this step in processing. Dry ashing removes the bulk of the photoresist and wet stripping removes remsuning residues. 

The description of the workings of the control technologies is beyond the scope of this article. However, it is worth noting that many of these technologies create substantial amounts of solid or liquid waste that needs to be disposed of properly. For example, in the USA the total amount of fly ash produced from coal combustion alone is about 57 Mt/y (Kalyoncu 2000). About one-third is utilized as secondary raw material (e.g., for aggregate and asphalt), but the rest is usually disposed of in landfills. The wet and dry scrubbers for S02 control produce a sludge or dry waste that finds little secondary use, and a large amount is disposed in landfills. 

Calcium and magnesium may be determined by titration with EDTA or by atomic absorption spectroscopy on TCA filtrates or on wet- or dry-ashed samples. 

In a pulverized boiler, the coal is ground to the consistency of talcum powder in a mill, and then entrained in an air stream that is fed through the burners to the boiler combustion chamber.6 Firing, therefore, occurs in suspension. Pulverized boilers can be wet-bottom, which means that coals with low ash fusion temperatures are used, and molten ash is drained from the bottom of the furnace, or can be dry bottom, which means that coals with high ash fusion temperatures are used, and dry ash removal techniques can occur.6... 

The total amount of iodine present in the sample in all forms was thus determined. If bromide—bromate reagent with 2 N acetic acid and 0.05 N sulphuric acid was applied to the liberation of bound iodine, the result did not include tightly bound iodine, which can be released only by wet or dry ashing. With the omission of this step from the procedure, a result was obtained which represented iodide and free iodine only without the addition of iodate, free iodine only was determined. 

The validity of the method of standard additions depends on the forms of the analyte element in the sample and in the added standard responding in the same way during the atomisation step. This may not always be so in practice. For example, in determining lead in whole blood by this method it would have to be proved (or assumed) that lead added as a lead nitrate solution is atomised to the same extent as lead bound organically in the sample. This is most easily checked by running a certified standard material through the procedure. If no such reference standard exists, as with the above blood example, the sample should also be run after pre-treatment for removal of the matrix, e.g. wet or dry ashing, and the results compared. 

Prior to quantitation of analyte by atomic spectrometry it is usually necessary to destroy the organic matrix and bring the element into solution. Most of the multitude of decomposition procedures reported fall into one of two classes, wet digestion and dry ashing. Often many variants of each procedure provide adequate results with a variety of analytes and matrices. Several commonly used procedures of general applicability are described below specific details are found in the sections dealing with the determination of individual elements. The reader is referred to several good sources of information on sample decomposition for fuller details and discussion [8h, 34,138—140]. The procedures described below result in essentially... 

Wet digestion and dry ashing of samples for Fe estimation have been discussed in a number of publications [139, 175,176, 178, 179,191, 202, 209]. Although some difficulties have been reported with both approaches, wet digestion may be preferred. 

Paper and pulp, being based on plant materials represent particularly favourable matrices for atomic absorption analysis by flame or by furnace. Such applications have been reviewed in the specialist literature [165—168], Differences of opinion exist as to whether wet or dry ashing is to be preferred for flame analysis, but increasingly, paper samples are being added to graphite furnaces without any sample pre-treatment. 

Many of the remarks made in the previous section concerning fibres can be applied to the analysis of plastics. Some polymers are soluble in organic solvents and samples may be prepared for direct aspiration into a flame in this way, e.g. MIBK is a suitable solvent for polyesters, polystyrene, polysiloxanes, cellulose acetate and butyrate dimethyl formamide for polyacrylonitrile, dimethyl acetamide for polycarbonates and polyvinyl chloride cyclohexanone for polyvinyl chloride and polyvinyl acetate formic acid for polyamides and methanol for polyethers. These organic solutions may alternatively be injected into a graphite furnace. Otherwise, polymers may be wet or dry ashed and the resultant ash dissolved in acid. An approach which is attracting increasing interest is the direct insertion of solid samples into a graphite furnace. 

Dabeka RW and Lacroix GMA (1987) Total arsenic in foods after sequential wet digestion, dry ashing, coprecipitation with ammonium pyrrolidene dithio-carbamate, and graphite-furnace atomic absorption spectrometry. J Assoc Off Anal Chem 70 866-870. 

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