Ash Analysis

Ashing is an important first step in proximate or specific mineral analysis. Ash refers to the inorganic (mineral) residue remaining after complete combustion or complete acid-facilitated oxidation of organic matter in food. This chapter covers the instrumentation, principles, procedures, advantages, and disadvantages of various ashing procedures. The two major types of ashing analysis, dry ashing and wet ashing, can be accomplished by conventional means or by the use of microwave systems. The choice of procedure depends upon the use of the ash following its determination and limitations based on cost, time, and sample numbers. Conventional dry ashing is based upon sample incineration at high temperatures (500–600 °C) in a muffle furnace. Wet ashing, which combines lower (100–120 °C) temperatures and acid-facilitated oxidation, is often used as a preparation for specific elemental analysis by atomic absorption, inductively coupled plasma, and/or mass spectrometry. Wet ashing is especially useful for metals that may partially volatilize at high temperatures if open vessels are used. Microwave ashing (dry or wet) is faster than conventional methods, especially when closed, pressurized vessels are used. Microwave ashing requires little additional equipment or space, but sample throughput may be a limiting factor.