ASTM D3120 test method for Trace Quantities of Sulfur in Light Liquid Petroleum Hydrocarbons by Oxidative Microcoulometry
14. Calculation
14.1 Calculate the sulfur concentration in the test sample in mg/kg (ppm) as follows:
where:
C = sulfur found in test sample (either directly from the instrument or calculated manually from the calibration curve), µg.
K = gravimetric dilution factor, mass of test sample/mass of test sample and solvent, g/g.
M = mass of test sample solution injected, either directly measured or calculated from measured volume injected and density, V x D, mg.
V = volume of test sample solution injected, either measured directly or calculated from measured mass injected and density, M/D, µL.
D = density of sample, g/mL.
1000 = Factor to convert µg/mg to mg/kg (ppm).
15. Precision and Bias
15.1 The precision of this test method was determined by statistical examination of test results obtained in two separate interlaboratory studies. Details of the first interlaboratory study (Case I) can be found in RR:D02-1036. The second interlaboratory study (Case II) involved 16 samples each of low level sulfur gasoline and diesel sample types analyzed by eight laboratories. A practical limit of quantitation (PLOQ) of ≈3 µg/g S, was determined for the gasoline sample types and ≈5 µg/g S for diesel sample types.
15.1.1 Repeatability:
15.1.1.1 Case I - The difference between two test results obtained by the same operator with the same apparatus under constant operating conditions on identical test material would, in the long run, in the normal and correct operation of the test method, exceed the following values in only in 1 case in 20, where x = the average of the of two test results.
15.1.1.2 Case II - Gasoline samples between approximately 3 and 100 µg/g and diesel samples between approximately 5 and 40 µg/g were analyzed. The repeatability values determined were as follows:
15.1.2 Reproducibility:
15.1.2.1 Case I - 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, in the normal and correct operation of the test method, exceed the following values in 1 case in 20, where x = the average of two test results.
15.1.2.2 Case II - Gasoline samples between approximately 3 and 100 mg/g and diesel samples between approximately 5 and 40 µg/g were analyzed. The reproducibility values determined were as follows:
15.2 Bias - Three National Institute of Standards and Technology (NIST) Standard Reference Materials (SRM) were analyzed to determine the bias. These samples were gasoline SRMs 2298 (4.6 µg/g S) and 2299 (13.6 µg/g S), and diesel SRM 2723a (10 µg/g S). There was no significant bias for SRMs 2299 and 2723a. However, there was a bias of -1.25 for SRM 2298.
16. Keywords
16.1 light hydrocarbons; microcoulometry; sulfur