ASTM D7740 Standard Practice for Optimization, Calibration, and Validation of Atomic Absorption Spectrometry for Metal Analysis of Petroleum Products and Lubricants
9. Sampling
9.1 The objective of sampling is to obtain a test specimen that is representative of the entire quantity. Thus, take lab samples in accordance with the instructions in Practice D4057 or D4177. The specific sampling technique can affect the accuracy of analysis.
10. Sample Handling
10.1 Homogenization - It is extremely important to homogenize the used oil in the sample container in order to obtain a representative test specimen.
10.2 Ultrasonic Homogenization - Place the used oil (in the sample container) into the ultrasonic bath. For very viscous oils, first heat the sample to 60°C. Leave the sample in the bath until immediately before dilution.
10.3 Vortex Homogenization - As an alternative to ultrasonic homogenization, vortex mix the used oil in the sample container, if possible. For viscous oils, first heat the sample to 60°C.
11. Preparation of Test Specimens and Standards
11.1 Blank - Prepare a blank by diluting the base oil or white oil tenfold by mass with the dilution solvent. Other dilution factors may be used as necessary.
12. Sample Introduction
12.1 Atomic absorption spectrometry can handle both aqueous as well as non-aqueous samples but AAS being a method for the analysis of liquids, if the sample to be analyzed is a solid or semi-solid, it needs to be brought into solution first. Some of the techniques used for such sample preparation include from simplest to more elaborate. Further discussion of sample preparation techniques used for the elemental analysis can be found in Practice D7455.
(1) Dilution of hydrocarbon liquid samples with organic solvents such as xylene, toluene, MIBK, kerosene, etc., for example, Test Method D3237, D3605, D4628, D5863, and D6732.
(2) Oxidation of organic liquid or solid samples in an oxygen pressurized stainless steel vessel, which converts the elements present to inorganic compounds. The contents are diluted with water or dilute acid for measurement.
(3) Incineration of organic samples with or without sulfuric acid followed by the dissolution of the residue in a mixture of acids or fusing with alkalies and further dissolution in a acid mixture, for example, Test Method D1318, D3340, D5056, D5184, and D5863.
(4) Dissolution in sealed PTFE (polytetrafluoroethlyene) pressure vessels with acids, heated for several hours at ~150°C, and then dilution with water for measurement.
(5) Dissolution in a microwave oven in a mixture of acids in a very short period of time.
(6) Gold amalgamation before cold vapor measurement for mercury determination, for example, D7622 and D7623.
(7) Hydride formation of certain volatile elements - Se, As, Sb, etc. - and direct measurement by AAS.
(8) Incineration of organic samples by low temperature plasma. However, this takes several days to complete the oxidation. The residue is dissolved in a mixture of acids prior to AAS determination.
13. Calibration Standards
13.1 All AAS measurements of samples are preceded by calibration of the instrument with elemental standards. Such calibration need to be undertaken every time the flame is lit because each time the flame conditions cannot be precisely replicated and there will be small differences in the intensity of elemental lines with each flame condition. Such standards could be aqueous or organic solvent based. Aqueous metallic standards are used when samples are converted to aqueous acid forms, and organometallic standards in organic solvents are used in case where samples are simply dissolved or diluted in base oil or organic solvents. Generally the calibration standards are nowadays commercially available prepared in suitable concentrations. Either single element or multi-element standards are available.
13.2 The elements usually determined in petroleum products and lubricants are listed in Table 4 along with their recommended wavelengths, flame conditions, range of analysis and detection limits.
14. Procedure and Calculation
14.1 Analysis - Analyze the test specimen solutions in the same manner as the calibration standards (that is, same integration time, background correction points, flame conditions, etc.). Between test specimens, nebulize dilution solvent for a typical rinse time of 60 s. Calculate elemental concentrations by multiplying the determined concentration in the diluted test specimen solution by the dilution factor. Calculation of concentrations can be performed manually or by computer when such a feature is available.
14.2 Quality Control with Check Standard - Analyze the check standard after every fifth sample, and if any result is not within 5 % of the expected concentration, recalibrate the instrument and reanalyze the test specimens solutions back to the previous acceptable check standard analysis.
NOTE 2 - To verify the accuracy and precision of the instrument calibration, certified standards such as NIST SRM 1085 (wear metals in oil) should be regularly analyzed.
