Microwave oven digestions

Abstract

Microwave energy for use in sample dissolution during acid digestions was first demonstrated in 1975. Since the first experiments in this type of digestion, the technique has advanced and become the most favored means of sample digestion. The method grew from the use of home microwave ovens with open vessels to the more sturdy laboratory microwave system with closed vessels. Microwave digestions have become a widely used laboratory practice for heating liquid above their normal boiling points with the use of pressurized vessels. The method is a more efficient means of digestion than conventional methods. Two distinct advantages in the use of microwave digestions are those of decreased time and good reproducibility. In addition, the use of microwave energy approaches ideal sample preparation for analysis by Inductively Coupled Plasma Atomic Emission Spectroscopy and Atomic Absorption Spectroscopy. These sample preparation ideals would result in the following goals being attained: 1. Conversion of all solid and liquid sample components to a homogeneous aqueous solution. This would help to prevent any obstructions from being caught in the nebulizers of the instruments and permit complete atomization. 2. Elimination of all organic material to prevent combustion problems and help to eliminate background complications. 3. The retention of all elements of interest, especially those that are most volatile, such as lead, mercury, arsenic, and selenium. 4. No addition of any interfering elements of compounds particularly from the environment or digestion vessels. 5. Adjustment of viscosity and percent dissolved solids within the solution to help with introduction into the instrument. In this study, efforts were made to facilitate microwave oven digestion method development. Difficulty in digesting several samples acquired through investigations conducted at Youngstown State University were encountered using the manufacturer's suggested method development schemes. Action to enhance digestion completeness and temperature control were undertaken. Electronic devices, including infrared-temperature-to-millivolt, digital-to-analog and analog-to-digital converters, and computational software needed to simplify the methodology were employed. A significant improvement in digestion efficiency and ease of oven operation was effected. The major manufacturer of chemical microwave oven digestion equipment will incorporate the design developed at the Department of Chemistry of Youngstown State University into its next oven model.