ASTM D7039 Standard Test Method for Sulfur in Gasoline, Diesel Fuel, Jet Fuel, Kerosine, Biodiesel, Biodiesel Blends, and Gasoline-Ethanol Blends by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry
8. Sampling and Sample Handling
8.1 Sample fuel according to the procedures in Practices D4057 or D4177.

8.2 Use the utmost care in sampling and handling gasoline to prevent evaporation of light ends which could change the concentration of sulfur in the sample. Store gasoline in a leak tight container at 0 °C to 4 °C until ready for analysis. If possible, maintain at this temperature throughout any transfer and handling processes. Allow specimens maintained at 0 °C to 4 °C to reach room temperature before testing, and expose these materials to ambient conditions only as long as necessary to obtain a sample for analysis. Analyze test specimens as soon as possible after sub-sampling from bulk container. Do not allow bulk container to remain uncovered any longer than is needed to obtain desired sub-samples.

8.3 For each sample, an unused piece of X-ray film is required for the sample cell. Avoid touching the inside of the sample cell, any portion of the film exposed to the liquid or the X-ray beam, and also avoid touching the instrument window. (It is highly recommended that clean, disposable rubber or plastic gloves be used when preparing test specimens.) Oil from fingerprints on the film and wrinkles in the film can generate errors in the analysis of sulfur. Therefore, make sure the film is taut and clean to ensure reliable results. Use calibration-check samples (see 7.2) to verify calibration integrity if the type and thickness of the window film is changed. After the sample cell is filled, provide a vent above the sample to prevent bowing of the film by accumulating vapors. When reusable sample cells are used, thoroughly clean and dry cells before each use. Disposable sample cells shall not be reused.

8.4 Because impurities and thickness variations can occur in commercially available transparent films and vary from lot to lot, use calibration-check samples (see 7.2) to verify calibration integrity after starting each new batch of film.

9. Preparation of Apparatus and Specimens for Analysis
9.1 Analyzer Preparation - Ensure that the MWDXRF analyzer has been installed and put into operation according to manufacturer's instructions. Allow sufficient time for instrument electronics to stabilize. Perform any instrument checkout procedures required. When possible, the instrument should be run continuously to maintain optimum stability.
9.1.1 Use the count time (T) recommended by the instrument manufacturer for the lowest sulfur concentration expected. The typical time for each measurement is two to three minutes.

9.1.2 Alternatively, determine T expected for a desired count precision by following the procedure in Appendix X1.

9.2 Specimen Preparation - Prepare a specimen of a test sample or a calibration standard as follows:
9.2.1 Carefully transfer a sufficient portion of the liquid to fill an open-ended sample cell above a minimum depth of 5 mm, beyond which additional liquid does not affect the count rate. Filling the sample cell to three-fourths of the cell's depth is generally adequate.

9.2.2 Fit an unused piece of X-ray-transparent film over the sample-cell opening and attach securely. When available, use the same batch of film for the analysis of test samples and the calibration standards used for constructing the calibration curve; otherwise follow 8.4 to verify the calibration integrity when switching to a new batch of film, and recalibrate using the new batch of film when results obtained on the calibration-check sample(s) fall outside acceptance criteria (see 10). Avoid touching the inside of the sample cell, any portion of the film exposed to the liquid or the X-ray beam, and also avoid touching the instrument window. (It is highly recommended that clean, disposable rubber or plastic gloves be used when preparing test specimens.) Ensure the film is taut, wrinkle-free, and not leaking.

9.2.3 Provide a small vent to prevent bowing of the window film caused by the accumulating vapor. Many commercially available sample cells provide a means to vent the space above the liquid.

9.2.4 Perform the analysis of the specimen promptly after preparing the specimen. Do not let the specimen remain in the sample cell any longer than necessary before collecting the data.

10. Calibration
10.1 Obtain or prepare a set of calibration standards bracketing the expected concentration range (up to 3000 mg/kg sulfur) in the samples by careful mass dilution of di-n-butyl sulfide (DBS) with a suitable base material (BM) (see Section 5). Two suitable base materials include mineral oil (see 7.6) for use with the correction factors in Table 3 and ethanol (see 7.8) for use with the correction factors in Table 4. All standards used in the analysis must be from a reliable and consistent source, which can include commercially available standards. Recommended nominal sulfur concentration standards are listed in Table 5.
10.1.1 Take into account any sulfur in the base materials when calculating the sulfur content (mg/kg) in each of the calibration standards as shown in Eq 1:
S = [(D B S·SDBS) + (B M·SBM)]/(D B S + B M)
where:
S = mass fraction of sulfur in the prepared standards, mg/kg,
DBS = actual mass of di-n-butyl sulfide, g,
SDBS = mass fraction of sulfur in DBS, mg/kg, typically 21.91 %,
BM = actual mass of base material, g, and
SBM = mass fraction of sulfur in the base material, mg/kg.

10.1.2 Alternatively, standards may be prepared by mass serial dilution of polysulfide oils (Note 7) with sulfur-free white oil. A freshly prepared polysulfide oil calibration curve should be verified using CRMs traceable to a national measurement institution that has demonstrated proficiency for measuring sulfur in the matrix of interest.

10.2 Following instrument manufacturer's instructions and the instructions in 11.2, measure the sulfur fluorescence intensity (total sulfur count rate) for each of the calibration standards. Convert total counts (N) to count rate (RS) in counts per second by dividing N by the count time (T) using units of seconds (see 9.1.1, 9.1.2, and Eq 2).
Rs = N/T
where:
Rs = measured total count rate of the sulfur fluorescence from 10.2, counts per second,
N = total counts collected at 0.5373 nm, and
T = seconds required to collect N counts.

10.3 Construct a linear calibration model by either:
10.3.1 Using the software supplied by the instrument manufacturer, or

10.3.2 Perform a linear regression of the calibration measurements. The following linear equation (Eq 3) describes the regression:
RS = Y + (E x S)
where:
RS = measured total count rate of the sulfur fluorescence from 10.2, counts per second,
Y = y-intercept of the calibration curve, counts per second,
E = slope of the calibration curve, counts kg s(-1) mg(-1), and
S = sulfur concentration, mg/kg.

10.4 When using drift correction, measure the total counts of sulfur fluorescence from the drift-monitor sample during the calibration procedure. Determine RS by dividing the total counts by T. The factor, RS, determined on the drift-monitor sample at the time of calibration, is factor A in Eq 4 in 12.1.

10.5 Immediately after analyzing the calibration standards, determine the sulfur concentration of one or more calibration-check samples (see 7.2). The determined value shall be in the range defined by the certified concentration plus or minus the repeatability of this test method. If this criterion is not met, the calibration process and calibration standards are suspect, corrective measures must be taken, and the calibration rerun. The degree of matrix mismatch between calibration check samples and standards should be considered when evaluating a calibration.