5.1 Often it is necessary to dissolve the sample, particularly if it is a solid, before atomic spectroscopic measurements. It is advantageous to use a microwave oven for dissolution of such samples since it is a far more rapid way of dissolving the samples instead of using the traditional procedures of dissolving the samples in acid solutions using a pressure decomposition vessel, or other means.

5.2 The advantage of microwave dissolution includes faster digestion that results from the high temperature and pressure attained inside the sealed containers. The use of closed vessels also makes it possible to eliminate uncontrolled trace element losses of volatile species that are present in a sample or that are formed during sample dissolution. Volatile elements arsenic, boron, chromium, mercury, antimony, selenium, and tin may be lost with some open vessel acid dissolution procedures. Another advantage of microwave aided dissolution is to have better control of potential contamination in blank as compared to open vessel procedures. This is due to less contamination from laboratory environment, unclean containers, and smaller quantity of reagents used (9).

5.3 Because of the differences among various makes and models of satisfactory devices, no detailed operating instructions can be provided. Instead, the analyst should follow the instructions provided by the manufacturer of the particular device.

5.4 Mechanism of Microwave Heating—Microwaves have the capability to heat one material much more rapidly than another since materials vary greatly in their ability to absorb microwaves depending upon their polarities. Microwave oven is acting as a source of intense energy to rapidly heat the sample. However, a chemical reaction is still necessary to complete the dissolution of the sample into acid mixtures. Microwave heating is internal as well as external as opposed to the conventional heating which is only external. Better contact between the sample particles and the acids is the key to rapid dissolution. Thus, heavy nonporous materials such as fuel oils or coke are not as efficiently dissolved by microwave heating. Local internal heating taking place on individual particles can result in the rupture of the particles, thus exposing a fresh surface to the reagent contact. Heated dielectric liquids (water/acid) in contact with the dielectric particles generate heat orders of magnitude above the surface of a particle. This can create large thermal convection currents which can agitate and sweep away the stagnant surface layers of dissolved solution and thus, expose fresh surface to fresh solution. Simple microwave heating alone, however, will not break the chemical bonds, since the proton energy is less than the strength of the chemical bond (5).

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 CO₂ are formed. These gases combine with the vapor pressure from the reagents, at any temperature, to produce the total pressure inside the vessel. Since the heat flow from a microwave digestion vessel is reversed from that of resistive devices, the total pressures generated for microwave dissolutions are significantly lower at the same temperature than other comparably heated devices or systems. This means larger samples can be digested at higher temperatures and lower pressures than would normally be expected from such pressurized vessels. Sample size should be controlled to prevent rapid exotherm rupture, exacerbated by excess CO₂ generation. However, the pressure limitations of the vessel still restrict both the sample size that can be used and the maximum temperature that can be achieved due to the vapor pressure resulting from the reagents (17).

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 CO₂ and NO_x. As a result larger sample sizes will produce higher pressures inside the digestion vessel. Generally, no more than 1 g of these sample types can be digested in a closed vessel (18).

5.4.3.1 While in open digestion vessel systems the operating temperatures are limited by the acid solutions’ boiling points, temperatures in the 200 °C to 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 (19).

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 (20).

1.1 This practice covers the procedure for use of microwave radiation for sample decomposition prior to elemental determination by atomic spectroscopy.

1.1.1 Although this practice is based on the use of inductively coupled plasma atomic emission spectrometry (ICP-AES) and atomic absorption spectrometry (AAS) as the primary measurement techniques, other atomic spectrometric techniques may be used if lower detection limits are required and the analytical performance criteria are achieved.

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.

1.3.1 Some examples of actual use of microwave heating for elemental analysis of fossil fuel products and other materials are given in Table 1.

1.3.2 Some additional examples of ASTM methods for microwave assisted analysis in the non-fossil fuels area are included in Appendix X1.

1.4 During the sample dissolution, the samples may be decomposed with a variety of acid mixture(s). It is beyond the scope of this practice to specify appropriate acid mixtures for all possible combinations of elements present in all types of samples. But if the dissolution results in any visible insoluble material, this practice may not be applicable for the type of sample being analyzed, assuming the insoluble material contains some of the analytes of interest.

1.5 It is possible that this microwave-assisted decomposition procedure may lead to a loss of “volatile” elements such as arsenic, boron, chromium, mercury, antimony, selenium, and/or tin from the samples. Chemical species of the elements is also a concern in such dissolutions since some species may not be digested and have a different sample introduction efficiency.

1.6 A reference material or suitable NIST Standard Reference Material should be used to confirm the recovery of analytes. If these are not available, the sample should be spiked with a known concentration of analyte prior to microwave digestion.

1.7 Additional information on sample preparation procedures for elemental analysis of petroleum products and lubricants can be found in Practice D7455.

1.8 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. Specific warning statements are given in Sections 6 and 7.

1.10 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.