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
6. Apparatus
6.1 Analytical Balance, capable of weighing to 0.001 g or 0.0001 g, capacity of 150 g.
6.2 Atomic Absorption Spectrometer, with appropriate hollow cathode lamps for analytes of interest, readout, and so forth.
6.3 Inductively Coupled Plasma Atomic Emission Spectrometer - Either a sequential or simultaneous spectrometer is suitable, if equipped with a quartz ICP torch and RF generator to form and sustain the plasma. Suggested wavelengths for the determinations of elements in dissolved acid solutions are given in Table 2. These wavelengths are only suggested and do not represent all possible choices. Wavelengths for boron, phosphorus, and sulfur below 190 nm require that a vacuum or inert gas purge optical path be used in ICP-AES instrument.
6.4 Microwave Oven, commercially available laboratory microwave digestion oven of sufficient power is suitable. The units should be capable of 1 % power adjustment and 1 s time adjustment. The oven cavity should be fluorocarbon- coated or coated with a material that has equivalent acid resistance and microwave properties and be equipped with exhaust ventilation at 2.8 m3/min (100 ft3/min) for acid vapor protection of the unit and operator. The unit must have a rotating or alternating turntable, capable of holding multiple digestion vessels, to ensure even sample heating. Safety interlocks, to shut off magnetron power output, must be contained in the oven door opening mechanism. The unit may contain a temperature control device capable of controlling vessel pressure to a minimum of 100 psig.
6.4.1 The unit must comply with U. S. Health and Human Services Standards under CFR Part 1030.10, sub parts (C) (1), (C) (2), and (C) (3), for microwave leakage. The unit should have FCC-type approval for operations under FCC Rule Part 18.
6.5 Sample Digestion system with Closed Vessel, quartz or polytetrafluoroethylene (PTFE) digestion vessels capable of holding 100 mL of solution. The vessel must be transparent to microwave energy and capable of withstanding internal pressures of 100 psig and temperatures of 200°C generated from the digestion of 0.2 g of sample. (Pressure achieved with a 100 mL vessel and 0.2 g of sample could be in excess of 100 psi).The vessel must contain a safety pressure relief valve, rupture disc, pressure venting system or be connected to an external safety relief valve that will prevent possible vessel rupture or ejection of the vessel cap. Microwave digestion systems with temperature and pressure monitoring devices are recommended for safety of personnel and accuracy of sample preparation.
6.5.1 The closed vessel should be operated in accordance with the manufacturer's recommended operating and safety instructions.
NOTE 1 - Follow the manufacturer's suggested vessel cleaning instructions to avoid possible sample contamination.
6.5.2 The microwave digestion dishes are also commercially available. (Warning - Take all necessary precautions to prevent exposure to radio frequency (RF) radiation. See Sections 6 and 9 for specifics.)
6.6 For cleaning the microwave vessels follow the manufacturer's recommended cleaning procedure. Commonly, soak the fluoropolymer vessel parts in cleaning solution (1:1 mixture of nitric acid and water) at 60°C for 10 min. Other suitable cleaning reagents may be used if appropriate. Remove the vessel parts from the cleaning solution and thoroughly rinse the parts with tap water and then with reagent water. Allow the vessel parts to air-dry or wipe dry using a clean, soft cloth.
6.7 The sample digestion system requires essentially microwave transparent and reagent resistant suitably inert polymeric materials (examples are PFA or TFM) to contain acids and samples. For higher pressure capabilities, the vessel may be contained within layers of different microwave transparent materials for strength, durability, and safety. The vessels internal volume should be at least 45 mL, capable of withstanding pressures of at least 30 atm (30 bar or 435 psi), and capable of controlled pressure relief. These specifications are given to provide an appropriate, safe, and durable reaction vessel of which there are many adequate designs by many commercial suppliers.
6.8 Rotating Turntable, to ensure homogenous distribution of microwave radiation within most systems. The speed of the turntable should be a minimum of 3 r/min. (Warning - It is not recommended to place a microwave unit in a fume hood, where it is surrounded by acid fumes, which can cause corrosion of the equipment. Acid fumes generated inside the oven cavity should be air swept away from the oven cavity to a hood.)
6.9 Combustion Dishes, Vycor, quartz, or platinum evaporation dishes of 250 mL size.
6.10 Volumetric Flasks, polypropylene or similar materials of 25, 50, or 100 mL sizes.
6.11 Electric Muffle Furnace, capable of maintaining 525 +/- 25°C and sufficiently large to accommodate several 250 mL Vycor beakers. The capacity of an air bleed is advantageous and optional. (Warning - Take all necessary precautions to prevent exposure to very hot surfaces.)
6.12 Heating Lamp, commercial infrared heating lamp.
6.13 Specimen Solution Containers, of appropriate size, glass or polyolefin vials or bottles, with screw caps without metal liners, to prevent trace element contamination.
7. Reagents and Materials
7.1 Purity of Reagents - At a minimum, reagent grade chemicals shall be used in all tests. When ICP-MS analysis is desired, the reagents should be of ultra-pure quality. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
7.2 Concentrated Sulfuric, Nitric, Hydrochloric, Perchloric, and/or Hydrofluoric Acids, as needed. (Warning - Causes severe burns. Corrosive.) (Warning - Hydrofluoric acid is poisonous and must be handled with care as it causes painful sores on the skin usually noticed on the next day only. Avoid inhaling the fumes. Liquid or vapor causes severe irritation of eyes. Skin contact causes severe burns. Wash the skin with copious amounts of water. HF should not be used in the laboratory unless there is antidote material such as calcium gluconate or other remedies are on hand.)
7.3 Aqueous Standard Solutions, individual aqueous elemental solution calibration standards in appropriate concentration ranges of elements of interest. These can be prepared by dissolving pure metal compounds in water or dilute acids, or may be purchased from commercial sources.
7.3.1 Multi-element aqueous standards may be advantageous to use.
7.4 Water, distilled or deionized, unless otherwise indicated, references to water shall be understood to mean Type II reagent grade water as defined in Specification D1193.
7.5 Quality Control (QC) Samples, preferably are portions of one or more sample type materials that are stable and representative of the samples of interest. These QC samples can be used to check the validity of the testing process.
8. Sampling
8.1 The objective of sampling is to obtain a test specimen that is representative of the entire quantity. Thus, take laboratory samples in accordance with the instructions in Practice D4057 and D4177. The specific sampling technique can affect the accuracy of eventual final measurement of elements in samples prepared per this Practice.
