Microwave ashing

As microwave sample preparation has evolved, standard microwave procedures have been developed and approved by numerous standard methods organisations (ASTM, AOAC International, EPA, etc.), see ref. [64]. Examples are standard test methods for carbon black/ash content (ASTM Method D 1506-97), lead analysis in direct paint samples (ASTM Method E 1645-94), etc. Table 8.15 shows some microwave ashing references (detection weight). A French AFNOR method utilises the atmospheric pressure single-mode microwave method as an alternative sample preparation procedure for Kjeldahl nitrogen determination [84], The performance of a microwave-assisted decomposition for rapid determination of glass fibre content in plastics for QC has been described [85]. 

Analytical instmments have been developed recently to dry ash samples using thermal and microwave heating examples are the APION A dry mode mineralizer (Tessek, Czech Republic), MLS-1200 PYRO microwave ashing furnace (Milestone, USA), and MAS 7000 microwave ashing system (CEM, USA). 

Table 6.2 Program for microwave ashing using ramping and holding stages...

Human and animal blood and plasma. Human Blood Plasma RM (Versieck) As Br Cd Ca Cl Co Cu 1 Fe Pb Mn Mo Ni Se Sn Zn Microwave ashing System pj DM] Analyze with energy dispersive X- [WDM-EDXRF] ray fluorescence spectrometer with total reflection optics [EDTXRF] Savage and Haswell 1998... 

Ash Carbon black Microwave ashing Balance ASTM D1506-94b carbon black - ash content... 

The first published report on microwave ashing was that of Koirtyohann et al., which appeared in 1975 [24]. Initially it attracted little attention because of the lack of appropriate expertise and apparatus. 

Raw foods were freeze-dried and analyzed for carbon isotopes using mass spectrometry. Cooked foods were prepared following historic recipes, then were freeze-dried prior to analysis. For the trace element analysis, foods (both raw and cooked) were wet ashed using nitric acid in Teflon lined pressure vessels and digested in a CEM Microwave oven. Analysis of Sr, Zn, Fe, Ca and Mg was performed using Atomic Absorption Spectrometry in the Department of Geology, University of Calgary. 

The apphed pretreatment techniques were digestion with a combination of acids in the pressurized or atmospheric mode, programmed dry ashing, microwave digestion and irradiation with thermal neutrons. The analytical methods of final determination, at least four different for each element, covered all modern plasma techniques, various AAS modes, voltammetry, instrumental and radiochemical neutron activation analysis and isotope dilution MS. Each participating laboratory was requested to make a minimum of five independent rephcate determinations of each element on at least two different bottles on different days. Moreover, a series of different steps was undertaken in order to ensure that no substantial systematic errors were left undetected. 

Method development for high-pressure ashing and closed microwave digestion was reported for wet oxidation and extraction of Pb, Cd, Cr and Hg from various food packaging materials [57]. Use of HPA resulted in the highest median recoveries of the spiked elements (Pb and Cd, 92% Cr, 97% Hg, 83%). The use of In as an internal standard improved the accuracy... 

High-temperature/low-pressure inorganic digestions are an area of application that has benefited from recent advances in vessel and sensor design. The inert properties of Teflon and its resistance to acid attack make it the material of choice for microwave pressure-vessel construction. Improved commercial systems offer additional safety precautions and improved facilities for pressure and/or temperature control. Also, the distribution of microwave radiation inside the oven cavity is fairly homogeneous. Low-pressure systems allow decomposition temperatures of about 180 °C. However, for many matrices, such temperatures are not sufficient to guarantee the complete ashing of thermoresistant sample components. 

The use of microwave digestion-wet oxidation overcomes sample preparation problems for many polymer-based materials. However, this will result in a reduction in sensitivity compared with an ashing procedure, because of dilution. Use of an aqueous phase is not... 

Numerous microwave applications have been published on decomposition, fusion, dry and wet mineralisation, ashing and extraction. Knapp et al. [67] have reported decomposition efficiencies of over 96% for PE, PVC, PS and PB, using PMD. Boron in polyolefins was determined after high-pressure microwave digestion followed by ICP-MS [80]. 

Table 8.22 shows some rubber analyses by FAAS after dry ashing. The concentration of Rh in polymers was measured by FAAS [128], The accuracy of 10-20% was in agreement with a dissolution procedure the precision obtained for direct solid analysis was between 10 and 20 %. Due to the relatively high analyte content of lead in paint, the determination is mostly performed by FAAS. Typical digestion procedures include dry ashing, wet and microwave digestion. 

Activated Fly ash , an industrial waste pollutant, is found to be a good catalyst in Beckmann rearrangement promoted by microwaves under solvent-free conditions. The amides are obtained from the corresponding ketoximes in high yields (75-94%). 

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