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
9. Calibration and Standardization
9.1 Preparation of Calibration Standards:
9.1.1 Although it is possible to make a single calibration to measure sulfur in a variety of matrices, it is strongly recommended that, whenever possible, the calibration is matrix specific, that is, a diesel calibration should be based on diesel standards. This is especially true for the analysis of sulfur at low levels. Hence, the matrix diluent should be as close to the form of the matrix being analyzed as possible. White mineral oil (see 7.3) is acceptable as an alternative matrix diluent.
9.1.2 Make primary standards independently at normal concentrations of 0.1 and 5 mass % sulfur and not by serial dilution from a single concentrate. The exact sulfur content in each standard is to be calculated to four decimal places.
9.1.3 Accurately weigh the nominal quantity of matrix diluent 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 magnetic stirrer is advisable) at room temperature.
9.1.4 Prepare calibration standards that will bracket the expected sulfur concentrations in samples, using the matrix diluent (9.1.1) and the primary standards (9.1.2) to prepare the blank and known standard concentrations, respectively, for each calibration range that is established. Consult the manufacturer's instructions for determining the number of standards and ranges to prepare. Table 2 provides an example of how two concentration ranges can be prepared by diluting each primary standard with the applicable matrix diluent.
9.1.4.1 Standard 1 on Range 2 in Table 2 may not be required if the instrument is also calibrated on Range 1. Consult the manufacturer's calibration instructions for specific guidance.
9.1.5 Alternatively, National Institute of Standards and Technology (NIST) traceable certified standards, prepared as described above or composed of the matrix to be analyzed, can be used.
9.1.6 If the matrix diluent being used for the preparation of standards contains sulfur, add this value to the calculated sulfur content of the prepared standards (consult your supplier for a certified sulfur concentration or test the mineral oil using Test Method D3120 or any other equivalent low level sulfur analyzing method with an MDL no higher than 1 ppm).
9.1.7 Weigh the DBS and matrix diluent to the recommended mass to the nearest 0.1 mg. It is important that the actual mass is known; thus, the actual concentration of the prepared standards is calculated and entered into the instrument for calibration purposes. The concentration of sulfur can be calculated using the following equation:
S = [DBS x S(DBS) + (MO x S(MO)]/(DBS + MO)
where:
S = mass % sulfur of the prepared standards,
DBS = actual mass of DBS, g,
S(DBS) = the mass % sulfur in DBS, typically 21.91 %,
MO = actual mass of mineral oil, g,
S(MO) = mass % sulfur in the mineral oil.
9.2 Certified Calibration Standards - Calibration standards, which are certified by a national standards organization, may be used in place of some or all of the standards prescribed in 9.1 when of similar matrix to the sample of interest. Such standards include Standard Reference Materials (SRM) prepared and certified by the National Institute of Standards and Technology (NIST), that is, SRM 2724 for sulfur in diesel. The standards used must cover the nominal concentrations ranges identified in Table 2.
9.3 Calibration Check Standards - Several additional standards (calibration check standards) that were not used in generating the calibration curve can be used to check the validity of the calibration. Calibration check standards may be independently prepared according to 9.1, or certified standards according to 9.2. The concentration of the calibration check standards shall be near the expected concentration of the samples being analyzed.
9.4 Quality Control Samples - Stable petroleum or product samples (that is, quality control samples) representative of the samples of interest may be run on a regular basis to verify that the system is in statistical control (see Section 14).
9.5 Storage of Standards and Quality Control Samples - Store all standards in glass bottles, either dark or wrapped in opaque material, closed with glass stoppers, inert plastic lined screw caps, or other equally inert, impermeable closures, in a cool, dark place until required. As soon as any sediment or change of concentration is observed, discard the standard.
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.
