ASTM D2622 for sulfur in petroleum products
ASTM D2622 standard test method for sulfur in petroleum products by wavelength dispersive X-ray fluorescence spectrometry
10. Procedure
10.1 Instrument Setup - Before using any WDXRF spectrometer, it is essential that the instrument is performing to the manufacturer's specifications. Consult with the manufacturer on how to perform spectrometer quality control checks. Practice D7343, Section 7 also provides more detailed information in this area.
10.2 Place particular attention on goniometer settings for sequential instruments, that is, ensuring goniometer positions are set correctly. Before performing a calibration of the goniometer angles carry out pulse height discriminator settings (PHD's) for each element and background to be used. The angle should be checked first, then the PHD, then the angle re-checked if the PHD settings are changed significantly. A very poor angle calibration may lead to bad PHD. The only reasonable alternate line is the sulfur Kb with significantly less than 10 % of the sensitivity of the Ka; this will only be practical for samples with high sulfur concentrations.
10.3 Account for observations of known instrument interferences. These include crystal fluorescence, tube line overlaps and any element spectral contamination from the materials within the instrument construction. Lead is a particularly bad interference for sulfur measurement. A number of these interferences can be avoided by careful selection of window settings during PHD set-up and for element interference the selection of an alternative line or minimizing overlap using higher resolution collimators and crystal selection.
10.4 When the factor F8 is used in Eq 8, regularly analyze a blank sample to determine the factor F8. On a sulfur free sample, such as the base material, determine the counting rate at the appropriate sulfur peak and background angles.
10.5 Place the sample in an appropriate cell using techniques consistent with good practice for the particular instrument being used. Although sulfur radiation will penetrate only a small distance into the sample, it will escape from only a small distance into the sample, and scatter from the sample cell and the sample can vary. Ensure that the sample cell is filled above a minimum depth, beyond which additional sample does not significantly affect the counting rate. Generally, fill the sample cell to a minimum of two-thirds of the cell's capacity. Provide a small vent hole in the sample cell unless using a sealed cell.
10.6 Place the sample in the X-ray beam and allow the X-ray optical path to come to equilibrium.
10.7 Determine the intensity of the sulfur Ka radiation at 0.5373 nm by making counting rate measurements at the precise angular settings for this wavelength.
NOTE 12 - It is suggested that a sufficient number of counts be taken to satisfy an expected coefficient of variation (% rsd) of 1.0 % or less when practical. When sensitivity or concentration, or both, make it impractical to collect a sufficient number of counts to achieve a 1.0 % coefficient of variation, accepted techniques, which will allow the greatest statistical precision in the time allotted for each analysis, should be used.
10.8 The coefficient of variation, CV, is calculated as follows:
CV = (100(Np + Nb)1/2) / (Np – Nb)
where:
CV = Coefficient of Variation, %,
Np = number of counts collected at sulfur line peak, 0.5373 nm, and
Nb = number of counts collected at background wavelength in the same time interval taken to collect Np counts.
10.9 Measure background counting rate at a previously-selected fixed angular setting, adjacent to the sulfur Ka peak.
NOTE 13 - Suitability of any background setting will depend on the X-ray tube anode employed. A wavelength of 0.5190 nm is recommended where chromium or scandium is used whereas 0.5437 nm has been found suitable for rhodium, 2u, peak and background, angles for various crystals are listed in Table 3.
10.10 Determine the corrected counting rate and calculate the concentration of the sample as described in Section 11.
10.11 When, from the measurements made in accordance with 10.5-10.10, the counting rate is higher than that of the highest point of the calibration curve, dilute the sample with the base material used to prepare the calibration standards until the sulfur counting rate is within the limits of the calibration curve and repeat the procedure described in 10.5-10.10.
10.12 For samples containing 100 mg/kg total sulfur or less, duplicate determinations are required. Each determination must be made on a new portion of sample material and analyzed in accordance with 10.5-10.10. The difference between the duplicate analyses should be equal to or less than the repeatability values indicated in 14.1.1. If the difference is larger, investigate sample preparation to identify any possible sources of sample contamination, and repeat the analysis. The reason for duplicate measurements is to identify problems associated with sample contamination, leading to improved results precision at the lower sulfur levels.
10.13 When the sample is known or believed to contain concentrations of interfering substances higher than those listed in Table 1, dilute the sample by mass with base material to concentrations below those listed.
10.13.1 The data collected (see Note 2) showed reasonable X-ray results when the calculated sum of mass absorption coefficients times mass fractions for samples was not greater than 4 to 5 % above the sum of mass absorption coefficients times mass fractions for the calibration standards. Absorption interferences are additive, and they are only minimized by dilution, not completely eliminated. Table 1 is therefore to be used as a guide to concentrations that can be tolerated without significant error, not as an absolute quantity.
NOTE 14 - The effect of matrix interferences can also be corrected on an empirical or theoretical basis.
NOTE 15 - The concentrations of ethanol and methanol were calculated assuming a theoretical mixture of hydrocarbons and di-butyl sulfide to which ethanol (or methanol) was added until the sum of the mass coefficients times mass fractions increased by 5 %. In other words, the amount of ethanol (or methanol) that caused a negative 5 % error in the sulfur measurement was calculated. This information is included in Table 1 to inform those who wish to use Test Method D2622 for determining sulfur in gasohol (or M-85 and M-100) of the nature of the error involved.
10.13.2 Thoroughly mix the blend to ensure homogeneity (mixing method will depend on the matrix type), and transfer it to the instrument for measurement.
10.13.3 Determine the sulfur content of the blend in the normal manner as described in 10.5-10.9, and calculate the sulfur content of the original sample as described in Section 11.
