ASTM D4294 for sulfur in petroleum and petroleum products
ASTM D4294 standard test method for sulfur in petroleum and petroleum products by energy-dispersive X-ray fluorescence spectrometry
14. Quality Control
14.1 The use of quality control programs, such as the one described in 14.1.1, can assist in maintaining statistical control of this test method.
NOTE 4 - Verification of system control through the use of QC samples and control is highly recommended. It is recognized that QC procedures are the province of the individual laboratory.
14.1.1 For the purpose of establishing the statistical control status of the testing process since the last valid calibration, quality control samples prepared from material(s) selected and stored according to 9.3 and 9.4 are to be regularly tested as if they were unknown production samples. Results are recorded and immediately analyzed by control charts or other statistically equivalent techniques to ascertain the statistical control status of the total testing process. Any out of control data shall trigger investigation for root cause(s). The outcome of the investigation may result in instrument recalibration. Depending on the criticality of the quality being measured and the demonstrated stability of the testing process, the frequency of quality control sample testing can range from once each day the test apparatus is in use to twice per week. It is recommended that at least one type of quality control sample be analyzed that is representative of samples routinely analyzed (as in 9.4).
15. Precision and Bias
15.1 Precision - The precision of this test method as obtained by statistical analysis of interlaboratory test results is as follows:
15.1.1 Repeatability - The difference between successive test results obtained by the same operator with the same apparatus under constant operating conditions on identical test materials would, in the long run, in the normal and correct operation of the test method, exceed the following values only in one case in twenty:
0.02894 (X + 0.1691)
where X is the sulfur concentration in mass %.
15.1.2 Reproducibility - The difference between two single and independent results obtained by different operators working in different laboratories on identical test material would, in the long run, exceed the following values only in one case in twenty:
0.1215 (X + 0.05555)
where X is the sulfur concentration in mass %.
15.2 Bias - The interlaboratory study included eight NIST reference materials. The certified values and bias are given in Table 4.
16. Keywords
16.1 analysis; energy dispersive; petroleum; spectrometry; sulfur, X-ray
