ASTM D7212 Standard Test Method for Low Sulfur in Automotive Fuels by Energy-Dispersive X-ray Fluorescence Spectrometry Using a Low-Background Proportional Counter
12. Procedure
12.1 Prepare and fill the sample cup with the test portion as described in Section 9.
12.1.1 When using an autosampler, follow the manufacturer's advice on the number of samples than can be measured in one batch.
12.2 Take measurements for fluorescent sulfur K series intensity, argon K series intensity and two intensities to determine spectral overlap from chlorine (see 11.3) using the same counting time as used for calibration. With minimal delay, repeat the measurement on a freshly prepared sample cell and a fresh portion of the sample. Calculate the average of the two results.
12.3 When there is significant chlorine present in a sample (see 6.1), then identify it at this point and apply the necessary correction.
12.3.1 Compare the three sulfur results from the three calibration lines (see 11.3). Normally, the three measurements agree and no correction is necessary, but when chlorine is present, they differ. When the result from the wider region exceeds that from the narrower wider region by greater than 1.5 mg/kg then a correction is applied by using the following equation:
Scoor = Smeas - 1.47 x (Swide - Snarrow)
where:
Scoor = result corrected for spectral overlap by chlorine,
Smeas = result calculated from the calibration line using the normal region of interest,
Swide = result calculated from the calibration line using the wide region of interest, and
Snarrow = result calculated from the calibration line using the narrow region of interest.
12.4 Use the difference between the results on the two portions of sample to indicate possible problems. If the figures differ by more than 3 mg/kg then there may be an error in the measurement. Possible causes are contaminated cell window, leaking cell window, wrinkled or slack cell window, and use of the wrong window material.
12.5 After running a tray of samples, always remove them to ensure no leakage of sample into the spectrometer.
13. Calculation of Results
13.1 The concentration of sulfur in the sample is automatically calculated from the calibration line (see 11.3).
14. Quality Control
14.1 Confirm the performance of the instrument or test procedure by analyzing a quality control (QC) sample (see 8.6) after each calibration and at least each day of use thereafter (see 12.2).
14.1.1 When QC/quality assurance (QA) protocols are already established in the testing facility, these may be used when they confirm the reliability of the test result.
14.1.2 When there is no QC/QA protocol established in the testing facility, Appendix X2 can be used as the QC/QA system. Practice D6792 should be consulted for protocols used in laboratory data analysis regarding quality control and quality assurance.
