ASTM D7876-13

5.4.1 In the electromagnetic irradiation zone, the combination of the acid solution and the electromagnetic radiation results in near complete dissolution of the inorganic constituents in the carbonaceous solids. Evidently, the electromagnetic energy promotes the reaction of the acid with the inorganic constituents thereby facilitating the dissolution of these constituents without destroying any of the carbonaceous material. It is believed that the electromagnetic radiation serves as a source of intense energy which rapidly heats the acid solution and the internal as well as the external portions of the individual particles in the slurry. This rapid and intense internal heating either facilitates the diffusion processes of the inorganic constituents in solution or ruptures the individual particles thereby exposing additional inorganic constituents to the reactive acid. The heat generated in the aqueous liquid itself will vary at different points around the liquid-solid interface and this may create large thermal convection currents which can agitate and sweep away the spent acid solution containing dissolved inorganic constituents from the surface layers of the carbonaceous particles thus exposing the particle surfaces to fresh acid (16).

5.4.2 Unlike other heating mechanisms, true control of microwave heating is possible because stopping of the application of energy instantly halts the heating (except the exotherms which can be rapid when pure compounds are digested). The direction of heat flow is reversed from conventional heating, as microwave energy is absorbed by the contents of the container, energy is converted to heat, and the bulk temperature of the contents rises. Heat is transferred from the reagent and sample mixture to the container and dissipated through conduction to the surrounding atmosphere. Newer synthesized containers made up of light yet strong polymers can withstand over 240°C temperatures and over 800 psi pressure. During the digestion process of samples containing organic compounds, largely insoluble gases such as CO_{5.4.3 Organic and polymer samples can be especially problematic because they are highly volatile and produce large amounts of gaseous by-products such as CO5.4.3.1 While in open digestion vessel systems the operating temperatures are limited by the acid solutions’ boiling points, temperatures in the 200–260°C range can be typically achieved in sealed digestion vessels. This results in a dramatic acceleration of the reaction kinetics, allowing the digestion reactions to be carried out in a shorter time period. The higher temperatures, however, result in a pressure increase in the vessel and thus in a potential safety hazard. Rapid heating of the sample solution can induce exothermic reactions during the digestion process. Therefore in modern microwave digestion systems, sensors and interlocks for temperature and pressure control are introduced. Since different types of sample behave differently in microwave field, heating control is necessary in this operation 5.4.4 Microwave heating occurs because microwave reactors generate an electromagnetic field that interacts with polarizable molecules or ions in the materials. As the polarized species compete to align their dipoles with the oscillating field, they rotate, migrate, and rub against each other, causing them to heat up. This microwave effect differs from indirect heating by conduction achieved by using a hot plate 1.1 This practice covers the procedure for use of microwave radiation for sample decomposition prior to elemental determination by atomic spectroscopy.}

1.2 This practice is applicable to both petroleum products and lubricants such as greases, additives, lubricating oils, gasolines, and diesels.

1.3 Although not a part of Committee D02’s jurisdiction, this practice is also applicable to other fossil fuel products such as coal, fly ash, coal ash, coke, and oil shale.