Pressure microwave digestion

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]. 

Kojima, I., Uchida, T., lida, C. Pressurized microwave digestion of biological samples for metal determination. Anal. Sci. 4, 211-214 (1988)... 

Heltai, G., Percsich, K. Moderated pressure microwave digestion system for preparation of biological samples. Talanta 41, 1067-1072 (1994)... 

Schramel, P., Hasse, S. Destruction of organic materials by pressurized microwave digestion. Fresenius J. Anal. Chem. 346, 794—799 (1993)... 

Campbell MB, Kanert GA. 1992. High-pressure microwave digestion for the determination of arsenic, antimony, selenium and mercury in oily wastes. Analyst (London) 117(2) 121-124. 

What are the major limitations of closed-vessel, high-pressure microwave digestions ... 

Figure 5.7 Schematic of a pressurized microwave digestion system.

The closed vessel, high-pressure microwave digestion gave good results for calcium, magnesium, phosphorous, sulfur, and zinc. It has been proven that closed-vessel microwave digestion can be used to accurately prepare samples containing volatile boron and silicon... 

The direct dilution approach works well for lubricant additives and fresh oils. However, this approach caimot handle used oils containing particulate or volatile compounds. Accurate analysis for sulfiir by ICP can be problematic if the samples contain volatile sulfur such as in sulfide form. There is a vapor enrichment phase separation that occurs during the pneumatic nebulization process in ICP. Hence, the use of closed vessel, high-pressure microwave digestion can be used to accurately prepare samples containing volatile sulfides. ICP can be used to determine the accmate sulfur result. 

Maher, W. et al.. Determination of total phosphorus and nitrogen in turbid waters by oxidation with alkaline potassium peroxodisulfate and low pressure microwave digestion, autoclave heating or the use of closed vessels in a hot water bath Comparison with Kjeldahl digestion, Analytica Chimica Acta 463, 283, 2002. 

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. 

While additive analysis of polyamides is usually carried out by dissolution in HFIP and hydrolysis in 6N HC1, polyphthalamides (PPAs) are quite insoluble in many solvents and very resistant to hydrolysis. The highly thermally stable PPAs can be adequately hydrolysed by means of high pressure microwave acid digestion (at 140-180 °C) in 10 mL Teflon vessels. This procedure allows simultaneous analysis of polymer composition and additives [643]. Also the polymer, oligomer and additive composition of polycarbonates can be examined after hydrolysis. However, it is necessary to optimise the reaction conditions in order to avoid degradation of bisphenol A. In the procedures for the analysis of dialkyltin stabilisers in PVC, described by Udris [644], in some instances the methods can be put on a quantitative basis, e.g. the GC determination of alcohols produced by hydrolysis of ester groups. 

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. 

A microwave-assisted, high-temperature, and high-pressure UV digestion reactor has been developed by Florian and Knapp [44] for analytical purposes. The apparatus consists of the immersed electrodeless discharge lamp operating as a result of the MW field in the oven cavity (Fig. 14.8). An antenna fixed to the top of EDL enhanced the EDL excitation efficiency. Another interesting MW-UV reactor has... 

Microwave digestion Sample heated with acids using microwaves under controlled temperature and pressure Metals in organic/inorganic solid samples (e.g. metals in sediments)... 

Closed vessel microwave digestion for bones, teeth, hair, and soil Specially designed closed pressurized Teflon vessels may be used for microwave digestion. Teflon is transparent to microwaves, which enhances the effect of the acids by raising the temperature and pressure within the vessel. In addition the closed vessels will retain any volatile components (e.g., Si) in solution. It must, however, be emphasized that any sealed vessels must only be heated if they have been designed for the purpose. Examples of applications include Kingston and Walter (1992), Baldwin et al. (1994), Sheppard et al. (1994), and Tamba et al. (1994). 

More recently, microwave ovens have been used for sample dissolution. The sample is sealed in a Teflon bottle or a specially designed microwave digestion vessel with a mixture of suitable acids. The high-frequency microwave, temperature (ca. 100-250°C) and increased pressure have a role to play in the success of this technique. An added advantage is the significant reduction in sample dissolution time [25, 26],... 

Decomposition in closed Teflon vessels at medium pressure (up to 8 bar) with microwave heating [43-44]. Microwave Digestion System MDS-81D (CEM Corporation, North Carofina, USA Floyd Inc., South Carolina, USA). 

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