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
9. Sampling
9.1 The objective of sampling is to obtain a test specimen that is representative of the entire quantity. Thus, take laboratory samples in accordance with the instructions in Practice D4057. The specific sampling technique can affect the accuracy of this test method.
10. Preparation of Apparatus
10.1 Instrument - Design differences between instruments, ICP excitation sources, and different selected analytical wavelengths for individual spectrometers make it impractical to detail the operating conditions. Consult the manufacturer's instructions for operating the instrument with organic solvents. Set up the instrument for use with the particular dilution solvent chosen.
10.2 Peristaltic Pump - Inspect the pump tubing and replace it, if necessary, before starting each day. Verify the solution uptake rate and adjust it to the desired rate.
10.3 ICP Excitation Source - Initiate the plasma source at least 30 min before performing an analysis. During this warm-up period, nebulize the dilution solvent. Inspect the torch for carbon buildup during the warm-up period. If carbon buildup occurs, replace the torch immediately and consult the manufacturer's operating guide to take proper steps to remedy the situation.
NOTE 2 - Carbon that accumulates on the tip of the torch injector tube can be removed by using nebulizer gas that consists of approximately 1 % oxygen in argon.
10.3.1 Generally, carbon buildup can be minimized by increasing the intermediate argon flow rate or lowering the torch, or both, relative to the load coil.
NOTE 3 - Some manufacturers recommend even longer warm-up periods to minimize changes in the slopes of the calibration curves.
10.4 Wavelength Profiling - Perform any wavelength profiling that is specified in the normal operation of the instrument.
10.5 Operating Parameters - Assign the appropriate operating parameters to the instrument task file so that the desired elements can be determined. Parameters to be included are element, wavelength, background correction points (required), interelement correction factors (refer to 5.1), integration time, and internal standard compensation (required). Multiple integrations (typically three) are required for each measurement. A typical integration time is 10 s.
11. Preparation of Test Specimens
11.1 Diluent - Diluent refers to the dilution solvent containing the internal standard (refer to 8.2).
11.2 Test specimen solutions are prepared in the same way that calibration standards are prepared (refer to 12.2). The mass % oil in diluent shall be the same for calibration standards and test specimen solutions.
11.2.1 Lubricating Oil Specimens - Weigh appropriate amount of the test specimen to the nearest 0.001 g. The weight of the test specimen taken will vary depending upon the metal concentration ofthe specimen. Dilute by mass with the diluent. Mix well.
11.3 Record all weights and calculate dilution factors by dividing the sum ofthe weights ofthe diluent, sample, and base oil (if any) by the weight of the sample.
11.4 The user of this test method has the option of selecting the dilution factor, that is, the relative amounts of sample and diluent. However, the mass % sample in diluent (for calibration standards and test specimens) shall be constant throughout this test method, and the mass % sample in diluent shall be in the range of 1 mass % to 5 mass %.
11.4.1 All references to dilute and diluting in this test method refer to the user-selected dilution.
11.5 Blank - Prepare a blank by diluting the base oil or white oil with the diluent.
11.6 Working Standards - Weigh to the nearest 0.001 g, approximately 1 g to 3 g of each multi-element standard (refer to 7.4) into separate bottles. Dilute by mass with the diluent.
11.7 Check Standard - Prepare instrument check standards in the same manner as the working standards such that the concentrations ofelements in the check standards are similar to the concentrations ofelements in the test specimen solutions. It is advisable to prepare the check standard from alternative sources of certified organometallic standards.
12. Calibration
12.1 The linear range of all calibration curves shall be determined for the instrument being used. This is accomplished by running intermediate standards between the blank and the working standards and by running standards containing higher concentrations than the working standards. Analyses of test specimen solutions shall be performed within the linear range of the calibration curve.
12.2 At the beginning of the analysis of each set of test specimen solutions, perform a two-point calibration using the blank and working standard.
12.3 Use the check standard to determine if each element is in calibration. When the results obtained with the check standard are within 5 % (relative) of the expected concentrations for all elements, proceed with the analysis. Otherwise, make any adjustments to the instrument that are necessary and repeat the calibration.
12.4 Calibration curves can be constructed differently, depending on the implementation of internal standard compensation.
12.4.1 When analyte intensities are ratioed to internal standard intensities, the calibration curve is, in effect, a plot of I(Re) versus analyte concentration and:
I(Re) = (I(e) - I(Be))/I(is)
where:
I(Re) = intensity ratio for analyte e,
I(e) = intensity for analyte e,
I(Be) = intensity of the blank for analyte e, and
I(is) = intensity of internal standard element.
12.4.2 When internal standard compensation is handled by multiplying all results for a certain test specimen by the ratio of the actual internal standard concentration to the determined internal standard concentration, the calibration curve is, in effect, a plot of (I(e) - I(Be)) versus analyte concentration.
