EXPLANATION
GF 4 grade engine oils marketed in the year 2004 - 2005 and beyond will have a maximum phosphorus content of 500 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 levels of phosphorus. This test method provides more precise results than other similar elemental analysis methods ASTM D4927 WDXRF, and ASTM D4951 or ASTM D5185 ICP-AES methods. The precision statements given in this method are valid for dilutions in which the mass % of sample in solvent is held constant in the range of 1 to 5 mass % oil. Although the precision tables in this test method define the concentration range of 500 to 800 mg/kg phosphorus, both higher and lower concentrations of phosphorus can be determined by this method.
TEST SUMMARY
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 similarly prepared. 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.
INTERFERENCES - There are no known spectral interferences between phosphorus and other elements covered by this method when using the spectral lines 177.51, 178.29, 185.94, 213.62, 214.91, or 253.40 nm for phosphorus. 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 the Test Method ASTM D5185. Viscosity index improvers, which may be present in the multi-grade lubricating oils, can bias the measurements. However, the biases can be reduced to negligible proportions by using the specified solvent-to-sample dilution and an internal standard.
TEST PRECISION
Where X is the mean concentration of phosphorus in mg/kg.
The bias of this method cannot be determined since there are no accepted standard reference materials with certified values for phosphorus in these engine oils.