Temperature microwave digestion

Levine, K.E., Batchelor, J.D., Rhoades, C.B., Jones, B.T. Evaluation of a high-pressure, hlgh-temperature microwave digestion system. J. Anal. At. Spectrom. 14, 49-59 (1999)... 

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],... 

An analytical procedure for the determination of about 20 trace elements in coal samples after microwave digestion is described by Wang et al.15 It was found that high temperature digestion with HN03 alone (250 °C, 7.5 MPa) led to an extensive decomposition of the organic matrix and clay in coal, whereby solid carbon remained in the final solution after evaporation. Most of the trace elements were quantitatively dissolved in three standard coals. A lost of Hg was observed during the evaporation step. 

Florian, D. and G. Knapp. 2001. High-temperature, microwave-assisted UV digestion A promising sample preparation technique for trace element analysis. Anal. Chem. 73 1515-1520. 

Total nitrogen was determined in water using high-temperature oxidation and chemiluminescence,119 as well as microwave digestion-UV spectrophotometry.120 Total nitrogen in solid waste was determined by modified Kjeldahl-Nessler reagent colorimetrically and modified Kjeldahl titration with hydrochloric acid.121 No significant differences were found between the two methods in terms of accuracy and precision. 

Modern microwave digestion systems monitor both pressure and temperature in the containers. As the temperature or pressure reaches the set point, power to the oven is cut. The oven power as well as the maximum pressure and temperatures can be set. Both digestion time and oven power can be programmed so that each sample is treated in a reproducible manner. The initial digestion is done slowly at low temperature, and the temperature is increased after the majority of the readily digested matrix is dissolved. 

Very recently, a novel microwave-assisted high-temperature UV digestion system for accelerated decomposition of dissolved organic compounds or slurries was developed [95, 96]. The technique is based on a closed, pressurized, microwave decomposition device wherein UV irradiation is generated by immersed electrodeless Cd discharge lamps (228 nm) operated by the microwave field in the oven cavity. The immersion system enables maximum reaction temperatures of up to 250-280 °C, resulting in a tremendous increase in mineralization efficiency. 

Modern microwave digestion systems monitor both pressure and temperature in the containers with automatic shut-down safety valves. The oven temperature and pressure can be set and when this is achieved, the power is switched off. This continues for several stages of the set programs until completion of mn and it will turn off automatically. The microwave vessels are available commercially and toughened to handle most samples. 

Microwave digestion systems have become very popular for decomposing samples. The photo shown is a closed-vessel microwave digestion system for high-pressure digestions. A microwave oven with a built-in fume exhaust system is shown along with sample trays that contain up to 1 2 samples. Teflon sample vessels can be operated at temperatures up to 2300 C and 625 psi. 

The use of microwave ovens for the decomposition of both inorganic and organic samples, first proposed in the mid-1970s, is now an important method of sample preparation. Microwave digestions can be carried out in either closed or open vessels, but closed vessels are more popular because of the higher pressures and higher temperatures that can be achieved. 

Microwave digestion Method of digesting an organic matrix to liberate metal content using acid at elevated temperatures (and pressures) based on microwave radiation. Can be carried out in either open or sealed vessels. 

---