ASTM D7876 Standard Practice for Practice for Sample Decomposition Using Microwave Heating (With or Without Prior Ashing) for Atomic Spectroscopic Elemental Determination in Petroleum Products and Lubricants
9. Analysis
9.1 Preparation of Ash or Sulfated Ash:
9.1.1 If petroleum product or lubricant samples need to be ashed or sulfated ashed, use Standard Test Methods D482 or D874, respectively.

9.1.2 A sample may be ashed in a controlled fashion over a Meeker burner using a Vycor or a platinum vessel.

9.1.3 Dry ashing may also be achieved by using microwave assisted dry ashing. Take approximately 1 g of sample in a Vycor crucible, raise the temperature to 525°C over a 2-h period. Hold the temperature for 1 h at that temperature. After the sample cools to room temperature, the ash can be dissolved in suitable acid.

NOTE 2 - During various dry- or wet-ashing techniques there is possibility of volatilization of certain elements or compounds depending upon their volatility, and the ashing temperature used. It would be impossible to categorize all such circumstances here.

9.2 Open-Vessel Microwave Oven Dissolution:
9.2.1 Take approximately 1 g of sample into the digestion vessel and add about 10 mL of nitric acid. Set the microwave oven to 30 W to cause a gentle reflux of the nitric acid. After about one hour or when the sample is nearly decomposed, add perchloric acid drop-wise. (Warning - Perchloric acid is an extremely strong oxidizer that may react violently with organic material. Never let the perchloric acid go to dryness. Experienced personnel only should use it with utmost caution. Note that unattended operation or venting in unmonitored vessel can result in the solution going to dryness.)

NOTE 3 - Optimal conditions for microwave digestion depend on sample weight, sample composition, volume of digestion reagents, and the microwave system used. Usually 0.1 g of the sample is weighed into a pre-cleaned microwave suitable PTFE digestion vessel, and appropriate volume of mineral acids are added to it before sealing the vessel and subjecting it to microwave radiation.

9.2.2 Wash the dissolved and clear content of the vessel into a 50 mL volumetric flask and bring up to volume with water.

9.3 Closed Vessel Microwave Oven Dissolution:
9.3.1 Accurately weigh about 0.1 to 1 g of the sample in a TFE-fluorocarbon digestion vessel with pressure relief mechanism. Add about 4 mL (or at least the minimum acid volume recommended by the instrument manufacturer) of concentrated nitric, hydrochloric, or other appropriate mineral acid. Note that sulfuric and phosphoric acids have boiling points sufficiently high to melt through some fluocopolymer vessels.

9.3.2 Set the microwave oven at 125 W for 15 min. Then ramp the oven up to 190 W for another 15 min. After the cycle is finished, place the vessel in an ice bath for at least one hour to cool. Dissolve the washed sample solution into a 25 mL volumetric flask and bring up to volume with water. An operator initiating a reaction should not cause an overpressure venting event due to disturbing hot vessels. Whether the reaction has gone to completion is dependent on the sample type heating cycle (versus the number of vessels and sample/reagent volumes, and so forth).

NOTE 4 - Care must be taken to keep the internal temperature and pressure within the capability of the vessels. Excessive heat and pressure will cause the pressurized digestion vessels to deform and potentially leak or explode.

NOTE 5 - From a safety viewpoint, when digesting samples containing volatile or easily oxidized organic compounds, initially weigh no more than 0.10 g and observe the reaction before capping the vessel. If a vigorous reaction occurs, allow the reaction to cease before capping the vessel. Ifno appreciable reaction occurs, a sample weight up to 1 g can be used.

NOTE 6 - Some microwave oven models may be capable of simultaneously processing multiple sample digestion vessels.

9.3.3 Temperature control of closed vessel microwave devices provides the main feedback control performance mechanism for the method. Control requires a temperature sensor in one or more vessels during the entire decomposition. The microwave decomposition system should sense the temperature to within +/- 2.5°C and permit adjustment of the microwave output power within 2 s.

NOTE 7 - Ensure that the control sensor is in vessel that is undergoing a similar decomposition reaction as the other vessels in the microwave.

9.3.4 All digestion vessels and volumetric ware must be carefully acid washed and rinsed with reagent water. When switching between high concentration and low concentration samples, all digestion vessels (fluoropolymer liners only) should be cleaned with appropriate acid reagents (for example, by leaching with hot (1:1) hydrochloric acid (greater than 80°C, but less than boiling) for a minimum of two hours followed with hot (1:1) nitric acid (greater than 80°C, but less than boiling) for a minimum of two hours and rinsed with reagent water and dried in a clean environment. It is preferable to use the same reagent mixture that will be used to dissolve the sample as also for cleaning the vessels. This cleaning procedure should also be used whenever the prior use of the digestion vessels is unknown or cross contamination from vessels is suspected. Polymeric or glass volumetric ware and storage containers should be cleaned by leaching with more dilute acids (approximately 10 % V/V) appropriate for the specific plastics used and then rinsed with reagent water and dried in a clean environment.

NOTE 8 - The cleanliness of the laboratory is even more important as the sample size decreases. Trace contamination becomes magnified when dilution factors are large.

9.3.4.1 Alternative cleaning procedures may be utilized if they are shown to be satisfactory.

9.3.5 The analyst should be aware of the potential for a vigorous reaction (exotherms due to rapid exothermic reaction between reagents and samples). This will have effect on maximum temperature/pressure attained, inability to stop an exotherm with suspending input energy. Pure compounds offer greater potential for this problem. If a vigorous reaction occurs upon the initial addition of reagent or the sample is suspected of containing easily oxidizable materials, allow the sample to predigest in the uncapped digestion vessel. Heat may be added in this step for safety considerations (for example the rapid release of carbon dioxide from carbonates, easily oxidized organic matter, and so forth). Once the initial reaction has ceased, the sample may continue through the digestion procedure.
9.3.5.1 The maximum sample weight used on closed vessel microwave digestion systems for total organics should not be more than 1 g; less with pure organic compounds.

9.3.5.2 Sample types make significant difference to their reaction with reagents in a microwave digestion system. This may span from instant, to more moderate reaction (for example, with crude oil and nitric acid) to slow or no reaction with coke samples.

9.3.6 Seal the vessel according to the manufacturer's directions. Properly place the vessel in the microwave system according to the manufacturer's recommended specifications and connect appropriate temperature and pressure sensors to vessels according to manufacturer's specifications.

9.3.7 Pressure control for a specific matrix is applicable if instrument conditions are established using temperature control. Because each matrix will have a different reaction profile, performance using temperature control must be developed for every specific matrix type prior to use of the pressure control system. At the end of the microwave program, allow the vessels to cool for a minimum of 5 min before removing them from the microwave system.
9.3.7.1 The acids used in the dissolution procedure may attack the pressure transducer and become contaminated.

9.3.8 Different microwave systems operate at different heating and ramping rate cycles.
9.3.8.1 For some microwave ovens, program the microwave oven to heat at 125 W for 15 min, then ramp up to 190 W for another 15 min.

9.3.8.2 For devices delivering a measured power of 575 to 635 W, program the instrument time for 50 min and the power to 100 %. For devices with a measured power of 635 to 700 W, program the instrument time for 30 min and the power for 100 %. These heating parameters will allow the samples to reach a maximum temperature of 164 +/- 4°C. For some sample types it may be necessary to reach 180°C or higher for proper dissolution and satisfactorily recover many of the elements of interest. Instruments delivering greater than 700 W must be operated at reduced powers such that the sample heating rates match the heating rate suggested by the manufacturer. Depress the start key and allow the sample mixtures to heat for the programmed time.

NOTE 9 - Different microwave oven models may require different temperature ramping and holding profiles based on the exact requirements of decomposition.

9.3.8.3 Every vessel being monitored for temperature and pressure, still will not stop an overpressure due to an exotherm.

9.3.9 Ifa turntable is used in the microwave oven device for uniform distribution of heat, the sample dissolution containers should be placed on the turntable in a balanced fashion. If an odd number of samples is to be digested, use an extra vessel to mass balance the carousel or a reagent blank to balance the power. Manufacturer's instructions should be followed.

9.3.10 Rotate the turntable through several 360° rotations of the carousel, Check to make sure that the lines are not tangled and to preserve the integrity of the safety features.

9.3.11 Detailed safety recommendations specific to the model and manufacturer of the microwave digestion system is beyond the scope of this practice. The user of this practice is advised to consult the equipment manual, the manufacturer and other literature sources for proper safe operation of the digestion system. The user should be advised that digestion of samples within the scope of this method could rapidly generate high pressure beyond the mechanical capacity of the vessel, which may cause a rupture of the vessel and damage to the digestion system. The user of this method should exercise caution when handling vessels after they have been heated since they may posses high internal pressures. (Warning - The outer layers of vessels are frequently not as acid or reagent resistant as the liner material and must not be chemically degraded or physically damaged to retain the performance and safety required. Routine examination of the vessel materials may be required to ensure their safe use.) (Warning - Another safety concern relates to the use of sealed containers without pressure relief devices. Temperature is the important variable controlling the reaction. Pressure is needed to attain elevated temperatures, but must be safely contained. However, many digestion vessels constructed from certain suitably inert polymerics may crack, burst, or explode in the unit under certain pressures. Only suitably inert polymeric (such as PFA or TFM and others) containers with pressure relief mechanisms or containers with suitably inert polymeric liners and pressure relief mechanisms are considered acceptable. Users are therefore advised not to use domestic (kitchen) type microwave ovens or to use inappropriate sealed containers without pressure relief for microwave acid digestions by this method. Use of laboratory-grade microwave equipment is required to minimize safety hazards.) (Warning - Laboratories should not use domestic (kitchen) type microwave ovens for this method. There are several significant safety issues. First, when an acid such as nitric is used to effect sample digestion in microwave units in sealed vessels equipment, there is the potential for the acid gas vapor released to corrode the safety devices that prevent the microwave magnetron from shutting off when the door is opened. This can result in operator exposure to microwave energy. Use of a system with isolated and corrosion resistant safety devices prevents this from occurring.) (Warning - Toxic nitrogen oxide(s), hydrogen fluoride, and toxic chlorine (from the addition of hydrochloric acid) fumes are usually produced during digestion. Therefore, all steps involving open or the opening of microwave vessels must be performed in a properly operating fume ventilation system.) (Warning - The analyst should wear protective gloves and face protection and must not at any time permit a solution containing acid to come in contact with skin or lungs.)

9.3.12 Ifrupture disc fails in the pressure sensing vessel, the microwave will automatically stop, but if it fails in one of the other vessels, the microwave may or may not stop. Press the stop button and wait at least one hour before removing the vessels. Do not open the microwave oven door for at least 15 min after stopping because another disc may fail, spewing hot acid.

9.3.13 After the heating cycle is complete, (as an option if necessary) place the heating pressure vessels in an ice bath for about an hour to cool. Samples may also be cooled in a refrigerator for a sufficient length of time. Shake the vessels with caution (to prevent any rapid out-gassing of vapor or liquid causing acid burns of the exposed skin of the operator) to mix the sample solutions and vent to atmosphere any gas pressure that may be present in the vessel. There could be a potential for contamination from the melted ice on the vessel exterior (assuming that the ice is not from deionized water).
9.3.13.1 The analyst may choose to monitor the temperature and pressure, and not remove the vessels from the microwave unit until they are suitable to handle.

9.3.13.2 Some microwave pressure vessels have vent holes that prohibit their placement in ice bath without sample contamination. Some microwave heat manufacturers include a cool-down step in the digestion process to prevent users from accidently venting a highly pressurized vessel.

9.3.14 Quantitatively transfer the dissolved sample into a 25 or 50 mL volumetric flask and bring it up to volume with deionized water. Dilution may be carried out on a weight or volume basis.

9.3.15 Detach the vent tubing and remove the vessel assembly from the turntable. Weigh the cooled vessel system. If there is a weight loss of greater than 0.5 g, open the vessel and add an amount of reagent water equal to the weight loss. Liquid loss of less than 10 % has not been found to result in any analyte loss. Recap the vessel and then shake the vessel to mix the sample solution.

9.3.16 If any pressure remains in the vessels when they are opened, then hot acidic fumes or liquids may spew forth endangering the operator.

NOTE 10 - Care must be exercised when opening the pressure vessel so that the contents do not spill out.

NOTE 11 - There may be some insoluble residue at the end of dissolution steps above if the sample contains refractory elements such as silicon, molybdenum, barium, and so forth. In such cases use of hydrofluoric acid to dissolve the residue may be required. If HF is used, HF-resistant labware must be used during the dissolution steps. Also, an HF-resistant ICP sample introduction system should be used if silicon is to be quantified and to minimize potential damage to the "solution wetted" glass components in the ICP. If HF is used, volatile elements such as silicon may be lost as silicon hexafluoride. HF can be passivated by adding dilute boric acid to the acid solution Some post-digestion organic residue may also remain in the microwave digestion vessels and seals. They can be identified as floating immiscible organic residue or "ring" around the liquid level of the microwave digestion vessel, most likely leached of any analyte and not containing any metals.

NOTE 12 - Samples containing large amounts of organics may experience excessive loss of liquid (greater than 10 %); therefore, a study may be required to determine if any analyte loss has occurred. CO2 loss is a potential but there could be losses of other volatile analytes.

9.4 Spectrochemical Measurements:
9.4.1 Once the sample solution is prepared by the above procedure, the elements of interest in it can be analyzed using AAS or ICP-AES instrumentation using aqueous metal calibration standards.

9.4.2 Suggested wavelengths for AAS and ICP-AES measurements are given in Table 2. These are the most commonly used lines. However, other suitable interference-free lines may also be used.

NOTE 13 - Examples of spectroscopic measurement procedures can be found in various ASTM standards such as Test Method D4628 for AAS, or Test Methods D4951 and D5185 for ICP-AES. Standard operating protocols for AAS and ICP-AES can be found in Practices D7740 and D7260, respectively.

10. Calculations
10.1 Calculate the concentration of metals in the original sample by comparing the intensity of sample solution signal multiplied by the dilution factor, and divided by the intensity of the calibrant solution.

11. Keywords
11.1 atomic absorption spectrometry; inductively coupled plasma atomic emission spectrometry; lubricants; metals determination; microwave heating; petroleum products