ASTM D4951 for additive elements in lubricating oils by ICP-AES
ASTM D4951 standard test method for determination of additive elements in lubricating oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
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 of two different ways, which can be 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 must be monitored and prepared fresh (typically weekly) when the concentration of the 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 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.
9. Sampling
9.1 The objective of sampling is to obtain a test specimen that is representative of the entire quantity. Thus, take lab samples in accordance with the instructions in Practice D4057. The specific sampling technique can affect the accuracy of this test method.
