ASTM D7040 Standard Test Method for Determination of Low Levels of Phosphorus in ILSAC GF 4 and Similar Grade Engine Oils by Inductively Coupled Plasma Atomic Emission Spectrometry
3. Summary of Test Method
3.1 A sample portion is weighed and diluted by mass with mixed xylenes or other solvent. An internal standard, which is required, is either weighed separately into the test solution or is previously combined with the dilution solvent. Calibration standards are prepared similarly. The solutions are introduced to the ICP instrument by a peristaltic pump (required). By comparing emission intensity of phosphorus in the test specimen with emission intensities measured with the calibration standards and by applying the appropriate internal standard and background corrections, the concentrations of phosphorus in the sample is calculated.
4. Significance and Use
4.1 This test method usually requires several minutes per sample. Other test methods which can be used for the determination of phosphorus in lubricating oils include WDXRF Test Method D4927 and ICPAES Test Methods D4951 and D5185. However, this test method provides more precise results than Test Methods D4951 or D5185.
4.2 Lubricating oils are typically blends of additive packages, and their specifications are also determined, in part, by elemental composition. This test method can be used to determine if unused lubricating oils meet specifications with respect to elemental composition.
4.3 It is expected that GF 4 grade engine oils marketed in the years 2004 to 2005 will have a maximum phosphorus concentration level of 500 mg/kg to 800 mg/kg. These limits are required to minimize poisoning of automotive emission control catalysts by volatile phosphorus species. It is anticipated that the later grades of oils may have even lower phosphorus levels.
5. Interferences
5.1 Spectral - There are no known spectral interferences between phosphorus and other elements covered by this test method when using the spectral lines 177.51 nm, 178.29 nm, 185.94 nm, 213.62 nm, 214.91 nm, or 253.40 nm for phosphorus. These wavelengths are only suggested and do not represent all possible choices. Wavelengths below 190 nm require a vacuum or inert gas purged optical path be used. However, if spectral interferences exist because of other interfering elements or selection of other spectral lines, correct for the interference using the technique described in Test Method D5185.
5.2 Viscosity Index Improver Effect - Viscosity index improvers, which can be present in multi-grade lubricating oils, can bias the measurements. However, the biases can be reduced to negligible proportion by using the specified solvent-to-sample dilution and an internal standard.
