ASTM D4951 standard test method for determination of additive elements in lubricating oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
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 free aspiration or an optional peristaltic pump. By comparing emission intensities of elements in the test specimen with emission intensities measured with the calibration standards and by applying the appropriate internal standard correction, the concentrations of elements in the sample are calculable.
4. Significance and Use
4.1 This test method usually requires several minutes per sample. This test method covers eight elements and thus provides more elemental composition data than Test Method D4628 or Test Methods D4927. In addition, this test method provides more accurate results than Test Method D5185, which is intended for used lubricating oils and base oils.
4.2 Additive packages are blends of individual additives, which can act as detergents, antioxidants, antiwear agents, and so forth. Many additives contain one or more elements covered by this test method. Additive package specifications are based, in part, on elemental composition. 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 additive packages and unused lubricating oils meet specifications with respect to elemental composition.
5. Interferences
5.1 Spectral - There are no known spectral interferences between elements covered by this test method when using the spectral lines listed in Table 1. 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 measurements. However, the biases can be reduced to negligible proportion by using the specified solvent-to-sample dilution and an internal standard.