ASTM D4951 standard test method for determination of additive elements in lubricating oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
13. Calibration
13.1 The linear range of all calibration curves must 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 must be performed within the linear range of the calibration curve.
13.2 At the beginning of the analysis of each set of test specimen solutions, perform a two-point calibration using the blank and working standard.
13.3 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.
13.4 Calibration curves can be constructed differently, depending on the implementation of internal standard compensation.
13.4.1 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.
13.4.2 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.
14. Analysis
14.1 Analyze the test specimen solutions in the same manner as the calibration standards (that is, same integration time, background correction points (optional), plasma conditions, and so forth). Between test specimens nebulize dilution solvent for a minimum of 60 s.
14.2 When the concentration of any analyte exceeds the linear range of the calibration, prepare another test specimen by mixing the sample with base oil before adding diluent (refer to 11.2.2.1, for example). Then, reanalyze.
14.3 Analyze the check standard after every fifth test specimen solution. If any result is not within 5 % of the expected concentration, recalibrate the instrument and reanalyze the test specimen solutions back to the previous acceptable check standard analysis.
15. Quality Assurance/Quality Control (required)
15.1 Confirm the performance of the instrument and the test procedure by analyzing a control (QC) sample.
15.1.1 When QA/QC protocols are already established in the testing facility, these may be used to confirm the reliability of the test result.
15.1.2 When there is no QA/QC protocol established in the testing facility, Appendix X1 can be used as the QA/QC protocol.
15.2 Users of this test method are advised that in contractual agreements, one or more of the contracting parties can and may make Appendix X1 a mandatory practice.
16. Calculation and Report
16.1 Calculate concentrations, based on sample, using (Eq 1). Generally, the ICP software performs this calculation automatically.
where:
C = analyte concentration in the sample, mass %,
S= analyte concentration in the test specimen, mass % (refer to Section 14),
W1 = sample mass, g,
W2 = diluent mass, g, and
W3 = base oil mass (if any), g.
16.2 For each analyte, report mass % to three significant figures.