ASTM D5185 for additive elements, wear metals, contaminants and selected elements
ASTM D5185 Standard Test Method for Determination of Additive Elements, Wear Metals, and Contaminants in Used Lubricating Oils and Determination of Selected Elements in Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
11. Calibration
11.1 The linear range must be established once for the particular instrument being used. This is accomplished by running intermediate standards between the blank and the working standard and by running standards containing higher concentrations than the working standard. Analyses of test specimen solutions must be performed within the linear range of response.
11.2 Working Standard - At the beginning of the analysis of each batch of specimens, perform a two-point calibration consisting of the blank and working standard. Use the check standard to determine if each element is in calibration. When the results obtained with the check standard are within 5 % of the expected concentrations for all elements, proceed with test specimen analyses. Otherwise, make any adjustments to the instrument that are necessary and repeat the calibration. Repeat this procedure with the check standard every five samples.
11.3 Working Standard with Internal Standard - Calibrate the instrument as described in 11.2. Obtain a printed record of the standard's emission intensities and those of the internal standard. Calculate an intensity ratio for each element by the following equation:
I(Re) = (I(e) - I(Be))/I(is)
where:
I(Re) = intensity ratio for element e,
I(e) = intensity for element e,
I(Be) = intensity of the blank for element e, and
I(is) = intensity of internal standard element.
11.3.1 Calculate the calibration factors from the intensity ratios. Alternatively, use the computer programs provided by the instrument's manufacturer to calibrate the instrument using internal standardization.
12. Internal Standardization (Optional)
12.1 The internal standard procedure requires that every test specimen solution have the same concentration (or a known concentration) of an internal standard element that is not present in the original specimen. Specimen to specimen changes in the emission intensity of the internal standard element can be used to correct for variations in the test specimen introduction efficiency, which is dependent on the physical properties of the test specimen.
12.2 Internal Standard Solution - Weigh 20 g of 0.500 mass % cadmium, cobalt, or yttrium (or any other suitable metal) organometallic concentrate into a 1 L volumetric flask and dilute to 1 L with the dilution solvent. Prepare fresh, at least weekly, and transfer this solution into a dispensing vessel. The concentration of the internal standard element is not required to be 100 µg/mL; however, the concentration of the internal standard element in the test specimen solution should be at least 100 times its detection limit.
