Because microwave ovens heat unevenly, several short repeated heating steps are less likely to burn samples than a single uninterrupted heating step. When a high degree of accuracy is necessary, there are good practices that apply to all gravimetric methods. The weight of sample containers should be accurately determined before filling. When using paper containers, dry the container first to drive off its moisture. Ideally, after heating and before weighing, samples should be stored in a desiccation chamber until they cool to ambient room temperature.
Warm samples can reabsorb moisture from the air.
If a desiccation chamber is not available, weigh samples as promptly as possible after removing them from the oven. Generally, working with larger samples creates less error than smaller ones. Finally, and most importantly, always get a good representative sample of the material and run multiple analyses. When even a microwave oven is too slow and cumbersome, operators can turn to monitoring instruments with sensors that give a nearly immediate estimate of moisture levels. Sensors correlate moisture content with changes in some quickly-measurable property of the material such as electrical conductivity.