ASTM D7303 Standard Test Method for Determination of Metals in Lubricating Greases by Inductively Coupled Plasma Atomic Emission Spectrometry
4. Summary of Test Method
4.1 A weighed portion of the grease sample is weighed and subjected to alternate means of sample dissolution which may include sulfated ashing in a muffle furnace or by closed vessel microwave digestion in acid. Ultimately these diluted acid solutions are analyzed using ICP-AES. Aqueous calibration standards are used. The solutions are introduced to the ICP instrument by free aspiration or an optional peristaltic pump. By comparing emission intensities of elements in the test specimen with those measured with the calibration standards, the concentrations of elements in the test specimen can be calculated.
4.2 Additional information on using inductively coupled plasma-atomic emission spectrometry can be found in Practice D7260.
5. Significance and Use
5.1 Lubricating greases are used in almost all bearings used in any machinery. Lubricating grease is composed of ~90 % additized oil and soap or other thickening agent. There are over a dozen metallic elements present in greases, either blended as additives for performance enhancements or as thickeners, or in used greases present as contaminants and wear metals. Determining their concentrations can be an important aspect of grease manufacture. The metal content can also indicate the amount of thickeners in the grease. Additionally, a reliable analysis technique can also assist in the process of trouble shooting problems with new and used grease in the field.
5.2 Although widely used in other sectors of the oil industry for metal analysis, ICP-AES based Test Methods D4951 or D5185 cannot be used for analyzing greases because of their insolubility in organic solvents used in these test methods. Hence, grease samples need to be brought into aqueous solution by acid decomposition before ICP-AES measurements.
5.3 Test Method D3340 has been used to determine lithium and sodium content of lubricating greases using flame photometry. This technique is no longer widely used. This new test method provides a test method for multi-element analysis of grease samples. This is the first DO2 standard available for simultaneous multi-element analysis of lubricating greases.
6. Interferences
6.1 Spectral - Spectral interferences can usually be avoided by judicious choice of analytical wavelengths. There are no known spectral interferences between elements covered by this test method when using the spectral lines listed in Table 2. However, if spectral interferences exist because of other interfering elements or selection of other spectral lines, correct for the interferences using the technique described in Test Method D5185.
6.1.1 Follow the instrument manufacturer's operating guide to develop and apply correction factors to compensate for the interferences.
6.2 Chemical - If the grease sample contains refractory additives such as silicon or molybdenum, it is possible that some of these elements may remain undissolved in the residue, and may result in lower recoveries.
6.2.1 If HF is used for dissolution of grease residues, elements such as silicon may be lost as SiF6. Residual HF can also attack the ICP sample introduction system. HF can be passivated by adding dilute boric acid to the acid solution.
6.2.2 If the dry ashing in sample preparation step is used, elements such as sulfur will be volatilized during combustion.
