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
6. Apparatus
6.1 Inductively-Coupled Plasma Atomic Emission Spectrometer - Either a sequential or simultaneous spectrometer is suitable, if equipped with a quartz ICP torch and r-f generator to form and sustain the plasma.
6.2 Analytical Balance, capable of weighing to 0.001 g or 0.0001 g, capacity of 150 g.
6.3 Peristaltic Pump (Required) - A peristaltic pump is required to provide a constant flow of solution. The pumping speed shall be in the range 0.5 mL/min to 3 mL/min. The pump tubing shall be able to withstand at least a 6 h exposure to the dilution solvent. Fluoroelastomer copolymer tubing is recommended.
6.4 Solvent Dispenser (Optional) - A solvent dispenser calibrated to deliver the required weight of diluent can be advantageous. Ensure that solvent drip does not affect accuracy.
6.5 Specimen Solution Containers, ofappropriate size, glass or polyolefin vials, or bottles with screw caps.
6.6 Vortexer (Optional) - Vortex the sample plus diluent mixture until the sample is completely dissolved.
6.7 Ultrasonic Homogenizer (Optional) - A bath-type or probe-type ultrasonic homogenizer can be used to homogenize the test specimen.
7. Reagents and Materials
7.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available.
7.2 Base Oil, U.S.P. white oil, or a lubricating base oil that is free of analytes, having a viscosity at room temperature as close as possible to that of the samples to be analyzed.
7.3 Internal Standard (Required) - An oil-soluble internal standard element is required. The following internal standards were successfully used in the interlaboratory study on precision: Co (most common), Sc, and Y. Other appropriate internal standards may also be used.
7.4 Organometallic Standards - Multi-element standards, containing known concentrations (approximately 0.1 mass %) of each element, can be prepared from the individual metal concentrates. Refer to Practice D4307 for a procedure for preparation of multi-component liquid blends. When preparing multi-element standards, be certain that proper mixing is achieved. Commercially available multi-element blends (with known concentrations of each element at approximately 0.1 mass %) are also satisfactory.
7.4.1 It can be advantageous to select concentrations that are typical of unused oils. However, it is imperative that concentrations are selected such that the emission intensities measured with the working standards can be measured precisely (that is, the emission intensities are significantly greater than background) and that these standards represent the linear region of the calibration curve. Frequently, the instrument manufacturer publishes guidelines for determining linear range.
7.4.2 Some commercially available organometallic standards are prepared from metal sulfonates and, therefore, contain sulfur.
7.4.3 Petroleum additives can also be used as organometallic standards if their use does not adversely affect precision nor introduce significant bias.
7.5 Dilution Solvent - Mixed xylenes, o-xylene, and kerosine were successfully used in the interlaboratory study on precision.
8. Internal Standardization (Required)
8.1 The internal standard procedure requires that every test solution (sample and standard) have the same concentration (or a known concentration) of an internal standard element that is not present in the original sample. The internal standard is usually combined with the dilution solvent. Internal standard compensation is typically handled in one oftwo different ways, summarized as follows:
8.1.1 Calibration curves are based on the measured intensity of each analyte divided (that is, scaled) by the measured intensity of the internal standard per unit internal standard element concentration. Concentrations for each analyte in the test specimen solution are read directly from these calibration curves.
8.1.2 For each analyte and the internal standard element, calibration curves are based on measured (unscaled) intensities. Uncorrected concentrations for each analyte in the test specimen solution are read from these calibration curves. Corrected analyte concentrations are calculated by multiplying the uncorrected concentrations by a factor equal to the actual internal standard concentration divided by the uncorrected internal standard concentration determined by analysis.
8.2 Dissolve the organometallic compound representing the internal standard in dilution solvent and transfer to a dispensing vessel. The stability of this solution shall be monitored and prepared fresh (typically weekly) when the concentration ofthe internal standard element changes significantly. The concentration of internal standard element shall be at least 100 times its detection limit. A concentration in the range of 10 mg/kg to 20 mg/kg is typical.
NOTE 1 - This test method specifies that the internal standard is combined with the dilution solvent because this technique is common and efficient when preparing many samples. However, the internal standard can be added separately from the dilution solvent as long as the internal standard concentration is constant or accurately known.
