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
11. Procedure
11.1 Although X-radiation penetrates only a short distance into the sample, scatter from the sample cell and the sample may vary. Consequently, the analyst must ensure that the sample cell is filled with sample above a minimum depth, at which point, further filling causes an insignificant change in the counting rate. Generally speaking, filling the sample cup to at least three-fourths the capacity of the sample cell will be sufficient. Prepare the sample cell by providing adequate head space. Provide a vent hole in the top to prevent bowing of the X-ray film during measurement of volatile samples. (Warning - Avoid spilling flammable liquids inside the analyzer.)
11.2 Instrument Calibration - Calibrate the instrument for the appropriate range, following manufacturer's instructions. Typically, the calibration procedure involves setting up the instrument for recording of net sulfur X-ray intensity, followed by the measurement of known standards. Obtain two readings on the standard using the recommended counting time for the instrument according to Table 3. With minimal delay, repeat the procedure using freshly prepared cells and fresh portions of the standard. Once all the standards have been analyzed, follow the manufacturer's instructions for generating the optimum calibration curve based on the net sulfur counts for each standard that has been analyzed four times. Immediately upon completion of the calibration, determine the sulfur concentration of one or more of the calibration check samples (see 9.3). The measured values should be within 3 % relative of the certified values. If this is not the case, the calibration or calibration standards are suspect and corrective measures should be taken and the calibration rerun. The degree of matrix mismatch between samples and standards should also be considered when evaluating a calibration.
11.3 Analysis of Unknown Samples - Fill the cell with the sample to be measured as described in 11.1. Before filling the cell, it may be necessary to heat viscous samples so that they are easy to pour into the cell. Ensure that no air bubbles are present between the cell window and the liquid sample. Measure each sample (see Table 3 for the recommended counting times for the specific concentration ranges). With minimal delay, repeat the measurement using a freshly prepared cell and a fresh portion of the sample. Obtain the average of the two readings for the sulfur content in the unknown sample. If the average reading is not within the concentration range for that calibration, repeat the sample measurement in duplicate using the range that brackets the sample average determined.
12. Calculation
12.1 The concentration of sulfur in the sample is automatically calculated from the calibration curve.
13. Report
13.1 The preferred concentration unit for reporting the total sulfur content is mass %, although results may also be reported in other SI units, such as mg/kg. Round results to three significant figures using Practice E 29, and state that the results were obtained according to Test Method D4294.
