Sample preparation microwave dissolution

Sample preparation Fast dissolution, sample degradation unlikely slow dissolution avoid shear, microwave, etc. 

Table 3.4 summarises the main characteristics of a variety of sample preparation modes for in-polymer additive analysis. Table 3.5 is a short literature evaluation of various extraction techniques. Majors [91] has recently reviewed the changing role of extraction in preparation of solid samples. Vandenburg and Clifford [4] and others [6,91-95] have reviewed several sample preparation techniques, including polymer dissolution, LSE and SEE, microwave dissolution, ultra-sonication and accelerated solvent extraction. 

G.J. DeMenna and W.J. Edison, Novel Sample Preparation Techniques for Chemical Analysis - Microwave and Pressure, Dissolution, Chemical Analysis of Metals, ASTM STP 994 (F.T. Coyle, ed.), American Society for Testing and Materials, Philadelphia, PA (1987), p. 45. 

Flow injection analysis is based on the injection of a liquid sample into a continuously flowing liquid carrier stream, where it is usually made to react to give reaction products that may be detected. FIA offers the possibility in an on-line manifold of sample handling including separation, preconcentration, masking and color reaction, and even microwave dissolution, all of which can be readily automated. The most common advantages of FIA include reduced manpower cost of laboratory operations, increased sample throughput, improved precision of results, reduced sample volumes, and the elimination of many interferences. Fully automated flow injection analysers are based on spectrophotometric detection but are readily adapted as sample preparation units for atomic spectrometric techniques. Flow injection as a sample introduction technique has been discussed previously, whereas here its full potential is briefly surveyed. In addition to a few books on FIA [168,169], several critical reviews of FIA methods for FAAS, GF AAS, and ICP-AES methods have been published [170,171]. 

The microwave unit of the sample-preparation system provides heating during sample acidification and oxidation procedures. Heating promotes the dissolution of Al(OH)3 precipitates formed during the acidification step, as well as the removal of nitrites, as gaseous NOx species, which could interfere with subsequent oxidation. 

Methods involve extractions of analytes into organic solvents, as well as treatments with acidic or basic reagents. Solid-phase extraction can be used for removal and pre-concentrations of analytes in aqueous solutions. Applications of low-power focused microwave technology have been investigated as a means of dissolution, and good results have been reported for extractions of organometal-lic compounds of tin and mercury (Schmitt et al., 1996 Szpunar et al., 1996). Analyses of CRMs were used for verification. The time necessary for quantitative isolations of the analytes was greatly reduced, e.g. 24 h to 5 min. In addition, there were reductions in solvent volumes, and improvement in analyte recoveries. Some of the analytical procedures for speciation of particular elements such as mercury, described later in this chapter, include microwave-assisted sample preparation. 

The first step in analysing plastics for metals content in polymers by ICP-AES technique is that they must be prepared in solutions that are suitable for nebulization. There are four general methods applicable for sample preparation for metal analysis by ICP-AES and they are solvent dissolution of some plastics dry ashing using a muffle furnace acid digestion using a microwave oven and oxygen bomb combustion. 

The concentration of metals that are detrimental to catalysts added can vary between 20.0 ppm for Fe to 100 ppm for Ni and lOOOppm for V. The presence of these metals necessitates the need for analysis of these metals to determine their concentrations prior to the cracking process. The best method to analyse these oil samples needs to be rapid and accurate. Careful selection of the method either from experience or by trial and error may be applied depending on the metal and the concentration. Sample dissolution in a solvent or solvent mixture is considered the easiest but may not be suitable for low limits of detection. Destructive sample preparation methods, i.e. oxygen bomb combustion, microwave acid digestion followed by pre-concentrating may be required for trace analysis and/or with the aid of a hyphenated system, e.g. ultrasonic nebuliser. Samples prepared by destmctive methods are dissolved in aqueous solutions that have very low matrix and spectral interferences. 

In many cases, however, the costs arising from sample preparation will become decisive, which favors x-ray spectrometric methods, provided the earlier mentioned limitations are not encountered. Future progress will certainly depend on the avail-abilty of on-line sample treatment using, for example, flow injection and eventually on-line sample dissolution as is possible in some cases with microwave-assisted heating. Also the realization of separations in miniaturized systems and with minute amounts of reagents is very promising. In each instance the question of which method should be selected will have to be discussed for each type of analytical task to be solved. 

Since ET-AAS is a trace element technique, special equipment is required to minimize the risks for contamination or analyte loss at the critical sample preparation stage of the analytical procedure. To minimize blank levels, additions of reagents to the sample should be limited, and for this reason dissolution techniques such as oxygen combustion and microwave-assisted wet digestion in closed cells are to be recommended. Sulfur- and halide-containing reagents may cause interferences in the determination of certain elements by ET-AAS and should thus be avoided. 

See also Atomic Absorption Spectrometry Principles and Instrumentation. Atomic Emission Spectrometry Principles and Instrumentation. Elemental Speciation Overview. Food and Nutritional Analysis Sample Preparation. Ion-Selective Electrodes Overview. Quality Assurance Reference Materials. Sample Dissolution for Elemental Analysis Dry Ashing Oxygen Flask Combustion Wet Digestion Microwave Digestion. Spectrophotometry Inorganic Compounds. Titrimetry ... 

Atomic spectrometry generally requires prior dissolution of the sample, which can be carried out with either acids or organics solvents, but in some cases necessitates destroying the matrix by means of a wet acid treatment or a dry digestion. This can be a serious drawback, but the new strategies for sample preparation, based on the use of microwave-assisted digestion procedures for sample dissolution and... 

As microwave sample preparation has evolved, standard microwave procedures have been developed and approved by numerous standard methods organizations. Table 1 summarizes the different methods approved for either microwave drying or microwave acid dissolution by the Association of Official Analytical Chemistry (AOAC), American Society for Testing and Materials (ASTM), the United States Environmental Protection Agency (US-EPA), Standard Method, and Erench and Chinese national methods. 

Two applications will be described to illustrate automated sample preparation. The first will outline the automation of microwave dissolution of minerals, a solid matrix. The second will describe the use of online LC for the determination of urapadil and its metabolites in human plasma. 

Labreque JM (1988) Manual and robotically controlled microwave pressure dissolution of minerals. In Kingston HM and Jassie LB (eds.) Introduction to Microwave Sample Preparation Theory and Practice, p. 207. Washington, DC American Chemical Society. 

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