ASTM D4927 Method for Elemental Analysis of Lubricant and Additive Components
ASTM D4927 Standard Test Methods for Elemental Analysis of Lubricant and Additive Components - Barium, Calcium, Phosphorus, Sulfur, and Zinc by Wavelength-Dispersive X-Ray Fluorescence Spectroscopy
TEST METHOD B (MATHEMATICAL CORRECTION PROCEDURE)
14. Reagents and Materials
14.1 Helium, for optical path of spectrometer.
14.2 P-10 Ionization Gas, 90 volume % argon and 10 volume % methane for the flow proportional counter.
14.3 Diluent Solvent, a suitable solvent free of metals, sulfur, and phosphorus (for example, kerosine, white oil, or xylenes).
14.4 Calibration Standard Materials:
14.4.1 Barium 2-Ethylhexoide, with concentrations not less than 5 mass % barium and certified to better than more or less 0.1 % relative, so that calibration standards can be prepared as stated in 15.1. Other barium containing organic matrices (free of other metals, sulfur, and phosphorus) may be used, provided the barium is stable in solution and the concentration is not less than 5 mass % barium and certified to better than more or less 0.1 % relative.
14.4.2 Calcium Octoate, with concentrations not less than 4 mass % calcium and certified to better than more or less 0.1 % relative, so that calibration standards can be prepared as stated in 15.1. Other calcium containing organic matrices (free of other metals, sulfur, and phosphorus) may be used, provided the calcium is stable in solution and the concentration is not less than 4 mass % calcium and certified to better than more or less 0.1 % relative.
14.4.3 Bis(2-Ethylhexyl)Hydrogen Phosphate, 97 % purity (9.62 mass % phosphorus). Other phosphorus containing organic matrices (free of other metals and sulfur) may be substituted, provided the phosphorus is stable in solution and the concentration is not less than 2.5 mass % phosphorus and certified to better than more or less 0.1 % relative, so that calibration standards can be prepared as stated in 15.1.
14.4.4 Zinc Octoate, with concentrations not less than 2.5 mass % zinc and certified to better than more or less 0.1 % relative, so that calibration standards can be prepared as stated in 15.1. Other zinc containing organic matrices (free of other metals, sulfur, and phosphorus) may be used, provided the zinc is stable in solution, and the concentration is not less than 2.5 mass % zinc and certified to better than more or less 0.1 % relative.
14.4.5 Di-n-Butyl Sulfide, 97 % purity (21.9 mass % sulfur). Other sulfur containing organic matrices (free of metals and phosphorus) may be substituted, provided the sulfur is stable in solution and the concentration is not less than 7.5 mass % sulfur and certified to better than more or less 0.1 % relative, so that calibration standards can be prepared as stated in 15.1.
14.5 Quality Control (QC) Samples, preferably are portions of one or more lubricating oils or additives that are stable and representative of the samples of interest. These QC samples can be used to check the validity of the testing process and performance of the instrument as described in Section 18.
15. Preparation of Calibration Standards
15.1 Prepare calibration standards by precise dilution of each of the elements that meet the requirements of 14.4.1 to 14.4.5, with the diluent solvent for the recommended concentrations prescribed in Table 3.
15.2 Although Table 3 is an abbreviated listing of all the possible combinations of elements and concentration range interactions that can be tested to determine mathematical correction factors for the various elements, the number of standards and the varying degree of element concentrations in the matrix are sufficient.
16. Calibration
16.1 Fill respective X-ray cups at least half full with the calibration standard solutions. Make sure that no wrinkles or bulges are present in the film. The film must be flat.
16.2 Place the sample cups in the X-ray beam in order to measure and record the net intensity (peak intensity - background intensity) for each element and in each calibration standard according to the wavelengths and conditions suggested in Table 4. Up to 60-s counting periods may be used at each wavelength position.
NOTE 7 - The parameters indicated in Table 4 are presented for guidance only and they will vary according to the instrument used.
16.3 Interelement correction factors and the slope and intercept of the calibration line are obtained by the regression analysis using the program supplied with the particular instrument used (if available) or a model similar to the following form:
Ci = (Di + EiIi) (1 + ∑j αij Cj)
where:
Ci = concentration of the analyte Element i,
Di = intercept of the calibration curve for Element i,
Ei = slope of the calibration curve for Element i,
Ii = measured net intensity for Element i,
αij = interelement correction factor for effect of Element j on analyte Element i, and
Cj = concentration of interfering Element j.
A slope, intercept, and a set of interelement correction factors are calculated for each analyte.
16.4 The initial calibration to obtain the slope, intercept, and interelement correction factors is performed initially when the test method is set up, after any major maintenance is performed on the instrument that can affect the calibration (for example, new X-ray tube installed, new crystal added, and so forth), and as deemed necessary by the operator (for example, triggered by quality control sample results). Subsequent re-calibration is performed with a minimum of three standards containing each of the calibration elements at nominal concentrations across the respective calibration ranges in order to check the values of the slope and intercept. An optional stable pellet can also be prepared which can be measured on a periodic basis for the purpose of monitoring instrumental drift.
17. Report
17.1 Report the mass % element content to three significant digits (x.xx, 0.xxx, 0.0xxx).
17.2 State which test procedure was used.
18. Analysis of Quality Control Samples
18.1 A QC sample shall be analyzed each day samples are analyzed to verify the testing procedure and instrument performance. Additional QC samples may be analyzed. The QC samples shall be treated as outlined in Section 19.
19. Procedure
19.1 Fill X-ray cups at least half full with the sample specimens to be analyzed. Make sure that no wrinkles or bulges exist in the film. The film must be flat.
19.2 Obtain intensities for all of the elements for all of the samples in the manner prescribed for the standards (16.2).
19.3 The elemental concentrations for each sample specimen are calculated using the measured intensities combined with the correction factors obtained from the calibration procedure (16.3).
19.4 Procedures 19.1 to 19.3 are repeated on diluted sample specimens in those cases where elemental concentrations exceed 1 mass % for barium, calcium, phosphorus, or zinc, or 2 mass % for sulfur.
