ASTM D7220 standard test method for sulfur in automotive fuels
ASTM D7220 Standard Test Method for Sulfur in Automotive Fuels by Polarization X-ray Fluorescence Spectrometry
10. Preparation of Apparatus
10.1 Set up the apparatus in accordance with the manufacturer's instructions. Whenever possible, the instrument should remain energized to maintain optimum stability.
11. Calibration and Standardization
11.1 Preparation of Calibration Standards:
11.1.1 Preparation of Stock Calibration Standard - Accurately weigh white oil or matrix-matching blank sample to the nearest 0.1 mg as shown in Table 1 into a suitable narrow-necked container, and then accurately weigh in the nominal quantity of Di-n-butyl sulfide. Mix thoroughly (a PTFE coated stirrer is advisable) at room temperature. Calculate the concentration in the stock standard to 1 mg/kg using the following equation:
SStock = [DBS x SDBS / (DBS + Blank)] x 1000
where:
SStock = mg/kg of sulfur in the stock standard,
DBS = actual mass of Di-n-butyl sulfide, g,
SDBS = the mass % of sulfur in Di-n-butyl sulfide, typically 21.91 %, and
Blank = actual mass of white oil or matrix-matching blank sample.
11.1.2 Preparation of Calibration Standard - Accurately weigh the nominal quantity of white oil or matrix-matching blank to the nearest 0.1 mg as shown in Table 2 into a suitable narrow-necked container, and then accurately weigh in the nominal quantity of stock standard. Mix thoroughly (a PTFE coated stirrer is advisable) at room temperature. Calculate the concentration of sulfur in each standard to 0.1 mg/kg using the following equation:
SStd = STK x SStock / (STK + Blank)
where:
SStd = mg/kg sulfur in the calibration standard, and
STK = actual mass of stock standard, g.
NOTE 3 - Commercially available standards can be used provided their sulfur concentrations are accurately known and they approximate the nominal concentrations listed in Table 2.
NOTE 4 - If desired, additional standards can be analyzed with concentrations between those listed in Table 2.
11.1.3 Storage of Standards and QC Samples - Store all standards and QC samples in glass bottles in a cool, dark place until required. The glass bottles shall either be dark or wrapped in opaque material, and closed with glass stoppers, inert plastic lined screw cups, or impermeable enclosures, As soon as any sediment or change of concentration is observed, discard the sample.
11.2 Establish calibration curve data by carefully measuring the net intensity of the emitted sulfur radiation from each of the standards by the procedure described in Sections 11 and 12.
11.3 Calibration Model - Construct a calibration model by either:
11.3.1 Using the software and algorithms supplied by instrument manufacturer, or
11.3.2 Fitting the data to the following equation:
S = a x R + b
where:
S = sulfur concentration in mg/kg,
R = net intensity for the sulfur radiation,
a = slope, and
b = intercept.
11.4 When using drift correction monitors, determine the intensity of the drift correction monitor sample(s) during the calibration procedure. The value determined corresponds to the factor A in 13.1.
11.5 Immediately after completing the calibration, determine the sulfur concentration of one or more of the calibration check samples (see 8.8). The measured value shall be in the range defined by the certified concentration 6 the repeatability of this test method. When 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. If the calibration intercept b in 11.3.2 shows a negative value, this indicates a significant S concentration in the blank sample.