ISO 20846 Petroleum products - Determination of sulfur content of automotive fuels - Ultraviolet fuorescence method
9 Procedure
9.1 Determine the density of the sample, in accordance with the hydrometer method specifed in ISO 3675 or the oscillating U-tube method specifed in ISO 12185.

If the temperature of sample injection is more than 3 °C from the temperature of determination of density, or if the density used is at a reference temperature, ISO 91-1 may be used to calculate the density at the injection temperature.

9.2 Estimate the sulfur content of the sample, and prepare a test portion that contains a quantity of sulfur within the range of the calibration curve selected, if the multi-point calibration is chosen, or close to the sulfur content of the calibration standard (±50 % maximum), if one-point calibration is chosen. If the expected sulfur content is above the content of the calibration standards, prepare either a volumetric or mass-diluted solution with a selected solvent (4.3).

When the sulfur content of the sample is above 500 mg/kg, dilute to the most convenient level.

9.3 Using the procedure described in 8.1.2 to 8.1.4, analyse an appropriate sample size three times, as specifed by the manufacturer.

9.4 Inspect the combustion tube and other fow path components to verify complete oxidation of the test portion. If deposits are noted, follow the procedures given in 9.4.1 and 9.4.2.
9.4.1 If coke or soot is observed, clean any parts of the combustion tube in accordance with the manufacturer's instructions. After cleaning and/or adjustment, assemble the apparatus and check for leaks. Carry out a check on the calibration quality prior to reanalysis.

9.4.2 Use a reduced sample size or a reduction of injection rate, or both.

10 Calculation
10.1 Using multi-point calibration
For analysers calibrated using a standard curve, calculate the sulfur content, wS, of the sample, expressed in milligrams per kilogram, using Equation (7) or (8):
wS = (A - Y)/(m s Ss x Fg)
wS = (A - Y)/(V x Ss x Fv)
where
A is the integrated detector response for the sample, expressed in number of counts;
Y is the y-intercept of the standard curve, expressed in number of counts;
Ss is the slope of the standard curve, expressed in counts per nanogram of sulfur;
Fg is the gravimetric dilution factor, mass of test portion/mass of test portion and solvent, expressed in grams per gram;
V is the volume of the test portion solution injected, expressed in microlitres;
Fv is the volumetric dilution factor, mass of test portion/volume of test portion and solvent, expressed in grams per millilitre;
m is the mass of the test portion solution injected, expressed in milligrams, either measured directly or calculated from the measured volume injected and density using Equation (9):
m = V x D
where
D is the density of the test portion solution at measurement temperature, expressed in grams per millilitre.

10.2 Using one-point calibration
Calculate the sulfur content, wS, of the sample, expressed in milligrams per kilogram, using Equation (10) or (11):
wS = A/(m x K x Fg)
wS = A/(V x K x Fv)
where
K is the calibration factor, expressed in counts per nanogram of sulfur;
V is the volume of the test portion solution injected, expressed in microlitres;
A is the integrated detector response for the sample, expressed in number of counts;
Fg is the gravimetric dilution factor, mass of test portion/mass of test portion and solvent, expressed in grams per gram;
Fv is the volumetric dilution factor, mass of test portion/volume of test portion and solvent, expressed in grams per millilitre;
m is the mass of the test portion solution injected, expressed in milligrams, either measured directly or calculated from the measured volume injected and density using Equation (12):
m = V x D
where
D is the density of the test portion solution at measurement temperature, expressed in grams per millilitre.

If there is a dilution in low sulfur content, the calculation shall take into account the correction with the sulfur content of the solvent.

10.3 Calculation
Calculate the average sulfur content of the sample from three determinations.