ISO 14596 Petroleum products - Determination of sulfur content - Wavelength-dispersive X-ray fluorescence spectrometry
7 Calibration solutions
7.1 General
Use either certified reference materials (4.3) or primary standards prepared from sulfur compounds (4.2) dissolved in white oil (4.1) as a basis for the preparation of the appropriate range of sulfur stock solutions.

7.2 Preparation of stock solutions
Weigh, to the nearest 0.1 mg, a quantity of sulfur compound (4.2) or certified reference material (4.3) to prepare stock solutions of approximately 2.50 % (m/m) and 0.10 % (m/m) sulfur content, calculated to the nearest 0.001 % (m/m), and dissolve in white oil (4.1) at room temperature. Mix the contents thoroughly using a homogenizer (5.3).

The approximate quantities of sulfur compounds (4.2) for addition to 100 g of white oil (4.1) to prepare the stock solutions are as follows:
DBT (4.2.1): 16.75 g [2.5 % (m/m)] and 0.5 g [0.1 % (m/m)]
DBS (4.2.2): 12.85 g [2.5 % (m/m)] and 0.45 g [0.1 % (m/m)]
TNA (4.2.3): 11.65 g [2.5 % (m/m)] and 0.40 g [0.1 % (m/m)]

It is recommended that a polytetrafluoroethylene or glass-coated magnetic stirrer and stirring device be used to mix the contents of the flask.

Calculate the exact sulfur content, wS,2, as a percentage by mass, to three decimal places, in each case from the amounts of white oil and sulfur compound used as given in Equation (1):
wS,2 = (mC x wS,1)/(mC + mO)
where
mC is the mass of the sulfur compound, expressed in grams;
wS,1 is the sulfur content of the sulfur compound, expressed as a percentage by mass;
mO is the mass of white oil, expressed in grams.

7.3 Preparation of standard solutions
7.3.1 High range [0.1 % (m/m) to 2.5 % (m/m)]
Into 50 ml flasks (5.4), weigh to the nearest 0.1 mg, the appropriate quantity of the 2.5 % (m/m) stock solution (7.2) to produce approximately 25 g of each standard solution of sulfur contents approximately 2.0 % (m/m), 1.5 % (m/m), 1.0 % (m/m), 0.7 % (m/m), and 0.4 % (m/m). Add white oil (4.1) to make up to approximately 25 g and re-weigh to the nearest 0.1 mg. Calculate the sulfur content, wS,3, of the standard solution to the nearest 0.001 % (m/m), using Equation (2):
wS,3 = [(m2 - m1)wS,2]/(m3 - m1)
where
m1 is the mass of the flask, expressed in grams;
m2 is the mass of the flask plus stock solution, expressed in grams;
m3 is the mass of the flask plus stock solution plus white oil, expressed in grams;
wS,2 is the sulfur content of the stock solution, expressed as a percentage by mass.

7.3.2 Low range [0.001 0 % (m/m) to 0.1 % (m/m)]
Prepare standard solutions in the same manner as described in 7.3.1 using the 0.1 % (m/m) stock solution (7.2) to give standard solutions of approximately 0.075 % (m/m), 0.05 % (m/m), 0.025 % (m/m), 0.010 % (m/m), 0.005 % (m/m) and 0.001 % (m/m). Calculate the sulfur content to the nearest 0.000 1 % (m/m) using the Equation (2).

7.4 Preparation of calibration solutions
7.4.1 High range [0.1 % (m/m) to 2.5 % (m/m)]
Weigh 20.00 g +/- 0,01 g of each of the standard solutions (7.3.1) and the two stock solutions (7.2) into separate flasks (5.4), and add 5.00 g +/- 0.01 g of the zirconium solution A (4.4). Mix thoroughly at room temperature (18 °C to 28 °C) using a homogenizer (5.3).

7.4.2 Low range [0.001 0 % (m/m) to 0.1 % (m/m)]
Weigh 20.00 g +/- 0,01 g of the low sulfur stock solution and each of the standard solutions (7.3.2) into separate flasks (5.4) and add 2.00 g +/- 0.01 g of the zirconium solution B (4.5). Mix thoroughly at room temperature (18 °C to 28 °C) using a homogenizer (5.3).

7.5 Storage of standards
Store certified reference standards in accordance with the instructions of the certifying organization, and use within the timeframe specified.

Store standards prepared from white oil and sulfur compounds in dark glass-stoppered bottles in a cool dark place.

NOTE The stability of prepared standards, under the above storage conditions, is at least six months.

8 Calibration
8.1 General
After the spectrometer has been set up and checked (5.1), purge the optical path thoroughly with helium.

8.2 High-range calibration curve [0.1 % (m/m) to 2.5 % (m/m)]
Transfer each of the calibration solutions (7.4.1) to a sample cell, and in a sequence of increasing sulfur content, place them in the spectrometer for exposure to the primary radiation.

Measure the count rates, Is and Izr , of the excited S-Kα (0.537 3 nm) and Zr-Lα1 (0.607 0 nm) fluorescence radiation.

Calculate the gross count ratio, R, using Equation (3). To determine the calibration curve, the relevant gross count ratio, R, is plotted against the corresponding sulfur content of the calibration solution; the calibration curve shall then be calculated and stored using the linear model according to Equation (4). The regression calculation may be carried out either separately or using the spectrometer calculator.
R = Is/Izr
R(x) = a + bx
where
R is the gross count ratio for the relevant determination;
Is is the count rate of the S-Kα X-ray fluorescence radiation at 0.537 3 nm;
Izr is the count rate of the Zr-Lα1 X-ray fluorescence radiation at 0.607 0 nm;
x is the sulfur content of the relevant calibration solution, expressed as a percentage by mass;
a,b are the parameters from the regression calculation;
R(x) is the gross count ratio computed from the regression calculation for the content, x.

8.3 Low-range calibration curve [0.001 0 % (m/m) to 0.1 % (m/m)]
Proceed as in 8.2 using the low range calibration solutions (7.4.2), and also measure the background radiation, IB, at 0.545 nm. As the fluorescence radiation is diminished by the sample cell window (see Table 1), a thin film of approximately 2 µm thickness (polyester) or approximately 4 µm thickness (polypropylene) shall be used for the calibration solutions and test portions (9.2) of low sulfur content. The setting of the spectrometer shall be such that at least 50000 counts are counted for the Zr-Lα1 line over the total measurement period.

Calculate the net count ratio, RO, using Equation (5). To determine the calibration curve, the relevant net count ratio, RO, is plotted against the corresponding sulfur content of the calibration solution; the calibration curve shall then be calculated and stored using the linear model according to Equation (6). The regression calculation may be carried out either separately or using the spectrometer calculator.
RO = (Is - IB)/(IZr - IB)
RO(x) = a +bx
where
RO is the net count ratio for the relevant determination;
IS is the count rate of the S-Kα X-ray fluorescence radiation at 0.537 3 nm;
IZr is the count rate of the Zr-Lα1 X-ray fluorescence radiation at 0.607 0 nm;
IB is the count rate of the background radiation at 0.545 nm;
x is the sulfur content of the relevant calibration solution, expressed as a percentage by mass;
a,b are the parameters from the regression calculation;
RO(x) is the net count ratio computed from the regression calculation for the content, x.

8.4 Checking
Prior to a measurement series (calibration and/or measurement), but in any case at least once daily, the manufacturer's specifications shall be used to check that the spectrometer is operating correctly, so that an optimum performance and a consistent standard of highest possible quality is ensured.

NOTE 1 Many modern WDXRF systems use drift monitoring and correction to ensure that systems produce accurate data over long periods of time without the requirement to recalibrate.

Performance checks should be performed on a regular basis, since such checks give valuable information about the status and stability of the spectrometer.

Check the calibration curves regularly. In regular use, check at least two points on each calibration curve by using quality control samples with known sulfur contents not less frequently than every three months. Check immediately when using a new batch of films. If the check result differs by more than 0.71 of the reproducibility given in this International Standard (see 12.2), execute a recalibration.

It is recommended that a control chart analysis be carried out.