ASTM D6443 Copper, Phosphorus and Sulfur in Unused Lubricating Oils and Additives
ASTM D6443 Standard Test Method for Determination of Calcium, Chlorine, Copper, Magnesium, Phosphorus, Sulfur, and Zinc in Unused Lubricating Oils and Additives by Wavelength Dispersive X-ray Fluorescence Spectrometry (Mathematical Correction Procedure)
9. Preparation of Calibration Standards
9.1 Prepare calibration standard blends by accurate dilution of the oil-soluble standard solutions with the dilution solvent. These blends (Practice D4307), with accurately known analyte concentrations, shall approximate the nominal values listed in Table 1.
9.1.1 When empirical alphas are determined by regression, prepare and measure all standard blends listed in Table 1.
9.1.2 When theoretical alphas are used, a subset of the standard blends (for example, standards 2, 6, 8, and 10) can be satisfactory.
9.2 Drift Correction Monitors (Optional) - The use of drift correction monitors for determination and correction of instrument drift can be advantageous. Monitors are stable, solid disks or pellets containing all elements covered by this test method. Two disks are preferred to correct for both sensitivity and base line drifts. The high-concentration drift monitor provides high-count rates, so that for each analyte, counting error is less than 0.25 % relative. The low-concentration drift monitor provides low-count rates, so that for each element, count rate is similar to that obtained with the calibration blank, or zero mass % standard.
10. Calibration
10.1 For the Ka-spectral line for each analyte, assemble a channel per operating instructions of the X-ray instrument. Suggested, approximate instrument settings are listed in Table 3. Actual settings can be instrument dependent; hence, the information in Table 3 is for guidance only.
10.2 For correct operation of the X-ray instrument, assemble the required measurement program, calculation program, and monitor program (when drift correction monitors are implemented), as appropriate.
10.3 When drift correction monitors are implemented, measure monitor intensities for each analyte.
10.4 Fill, to at least three-fourths full, X-ray sample cells with calibration standard blends, and ensure that the film is flat with no wrinkles or bulges. Punch a vent hole in the top of the cell. Introduce the calibration standard blends into the X-ray instrument in random order, and for each analyte, measure X-ray intensities. In general, a total count time of about 4 to 5 min per sample is typical. Common background measurement for two or more analytes can be used. If a standard must be remeasured, use a fresh aliquot, a fresh cup, and a new piece of film.
10.4.1 Measure magnesium and chlorine intensities first because they will be most sensitive to changes in the sample cell configuration.
10.5 Following measurement of the calibration standard blends, for each analyte, regress the concentration data with measured intensity data. Typically, the model describing the concentration-intensity relationship is:
Ci = (Di + EiIi)(1 + ∑jαijCj)
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 = net measured intensity for element i,
αij = influence coefficient for the effect of each absorbing element j on analyte element i, and
Cj = concentration of interfering element j.
10.5.1 When empirical alphas are used exclusively, the complete set of calibration standards (see Table 1) is measured. Then, for each analyte in turn, regression software is used to determine the D value, the E value, and also the relevant alphas.
10.5.1.1 Experimental results indicate that the calibration for magnesium does not require alphas because interelement effects on magnesium are not significant. Because of limited magnesium sensitivity on many X-ray instruments, empirically determined alphas can often be unrealistic and problematic.
10.5.1.2 Experimental results indicate that for each analyte, an alpha for the effect of copper can be ignored because the maximum copper concentration covered by this test method is only 0.05 mass %. When copper alphas are determined empirically, they can often be unrealistic and problematic.
10.5.2 When theoretical alphas are used exclusively, a subset of the calibration standard blends (see Table 1, typically standards 2, 6, 8, and 10) can be measured. Then, for each element in turn, the theoretical alphas are edited into the calibration parameter list, and the D value and E value are determined by regression.
10.5.3 When a mix of empirical and theoretical alphas is used, the complete set of calibration standards (see Table 1) is measured. Then, for each element in turn, the relevant theoretical alphas are edited into the calibration parameter list and the D value, the E value, and remaining alphas (if any) are determined by regression.
10.6 Typically, the initial calibration to obtain the slope, intercept, and alphas is performed only once. Subsequent recalibration is performed with two standards (typically, the drift correction monitors) in order to correct for changes in X-ray sensitivity and blank. The two standards are chosen such that they span the range of expected concentrations for the unknown samples.
