Microwave dissolution: Development and application of a new sample preparation technique

Results presented here establish and quantify the theoretical basis for understanding the technique of microwave sample dissolution. Obstacles associated with temperature and pressure measurement in closed-vessel microwave digestions were overcome and studied in a systematic fashion. By measuring these parameters fundamental principles have been investigated. A complete microwave digestion system using 2450 MHz is described and mechanisms of microwave heating are discussed. Since microwave energy couples directly with the solvent medium, the behavior of mineral acids used for dissolution has been investigated and characterized. Equations for estimating the power absorptions of nitric, hydrochloric, hydrofluoric and sulfuric acids from 25-1000g are given. Procedures for determining the maximum power output of a microwave unit and a calibration scheme for determining the most useful linear region are documented. This information permits the transfer of analytical procedures from one microwave unit to another. Continuous, real-time monitoring of temperature provides new insights into matrix component decompositions. This capability allows the mechanism of decomposition to be observed and the procedures to be generalized to a variety of samples. Analysis of the decomposition behavior of the three principal classes of biological materials in nitric acid ranks carbohydrates, proteins, and lipids according to their decomposition temperatures. Optimal conditions for digestions now can be tailored specifically to the matrix. Reproducibility of thermal conditions of similar samples prepared by microwave dissolution is achieved by monitoring the temperature. The concept of modeling dissolution on the behavior of the acid is presented as an alternative to real-time measurements. Studies of the microwave interaction with mineral acids include (1) a comparison of acid absorption efficiencies and their relationship with mass and proportional power; (2) the prediction of the power absorption of mixtures by treating them additively; (3) the nature of heat loss from the vessels, power rejection, and their influence on measurement and prediction. Results for the analysis of selenium and mercury in sediment suggest that volatile analytes are retained in solution; these elements may now be determined directly and simultaneously with other trace metals. Practical applications for the microwave dissolution of soils, sediments, inorganic and biological matrices are presented along with a recommended procedure for high-particulate water.