ISO 20846 Petroleum products - Determination of sulfur content of automotive fuels - Ultraviolet fuorescence method
4 Reagents and materials
4.1 Inert gas, argon or helium, high purity grade with a minimum purity of 99.998 % (V/V).

4.2 Oxygen, high purity grade with a minimum purity of 99.75 % (V/V).

CAUTION - Vigorously accelerates combustion.

4.3 Solvent
4.3.1 General
Use either that specifed in 4.3.2 or 4.3.3, or a solvent similar to that occurring in the sample under analysis. Correction for sulfur contribution from solvents used in standard preparation and sample dilution is required. Alternatively, use of a solvent with non-detectable sulfur contamination relative to the unknown sample makes the blank correction unnecessary.

4.3.2 Toluene, reagent grade.

4.3.3 Isooctane, reagent grade.

CAUTION - Flammable solvents.

4.4 Sulfur compounds
4.4.1 General
Compounds with a minimum purity of 99 % (m/m). Examples are given in 4.4.2 to 4.4.4. Where the purity of these compounds is less than 99 % (m/m), the concentrations and nature of all impurities shall be established.

NOTE A correction for chemical impurity can be applied when the sulfur content is known with accuracy.

Certifed reference materials (CRM) from accredited suppliers are suitable alternatives to the compounds listed in 4.4.2 to 4.4.4.

4.4.2 Dibenzothiophene (DBT), of molecular mass 184.26, with a nominal sulfur content of 17.399 % (m/m).

4.4.3 Dibutyl sulfde (DBS), of molecular mass 146.29, with a nominal sulfur content of 21.915 % (m/m).

4.4.4 Thionaphthene (Benzothiophene) (TNA), of molecular mass 134.20, with a nominal sulfur content of 23.890 % (m/m).

4.5 Sulfur stock solution
Prepare a stock solution of approximately 1000 mg/l sulfur content by accurately weighing the appropriate quantity of sulfur compound (4.4) in a volumetric fask (5.9). Ensure complete dissolution with solvent (4.3). Calculate the exact sulfur concentration of the stock solution to the nearest 1 mg/l. This stock solution is used for the preparation of calibration standards. As an alternative procedure, a sulfur stock solution of approximately 1000 mg/kg can be prepared by accurately weighing the appropriate quantity of sulfur compound (4.4) in a volumetric fask (5.9) and reweighing the volumetric fask once it has been flled to the mark with the solvent (4.3). Take precautions to ensure that evaporation of the solvent and/or sulfur compounds is not causing weighing errors.

The appropriate mass of sulfur compound described in 4.4.2 to 4.4.4 to add to the 100 ml fask is 0.5748 g (DBT), 0.4563 g (DBS) and 0.4186 g (TNA).

NOTE The shelf life of the stock solution is approximately three months when stored at low temperature, typically in a refrigerator.

4.6 Calibration standards
Prepare the calibration standards by dilution of the stock solution (4.5) with the selected solvent (4.3).

Calculate the exact sulfur content of each calibration standard.

Calibration standards with a known sulfur concentration, in milligrams per litre, (or content, in milligrams per kilogram) can be obtained with a volume/volume dilution (or mass/mass dilution, respectively) of the stock solution at 1000 mg/l (or 1000 mg/kg respectively). Other practices are possible, but those mentioned above avoid any density correction.

New calibration standards should be prepared on a regular basis, depending upon the frequency of use and age. When stored at low temperature, typically in a refrigerator, the calibration standards with a sulfur content above 30 mg/kg (or mg/l) have a shelf life of at least one month. Below this sulfur content (30 mg/kg), the shelf life should be reduced.

4.7 Quality control samples
Quality control samples are stable samples representative of the materials being analysed, which have a sulfur content that is known by this test method over a substantial period of time. Alternatively, there are standard materials with a certifed value commercially available. Prior to use, ensure that the material is within its shelf life.

4.8 Quartz wool
Follow the manufacturer's recommendations.

5 Apparatus
Figure 1 illustrates the basic pieces of the UVF.

5.1 Furnace, comprising an electric device, capable of maintaining a temperature suffcient to pyrolyse all of the sample and oxidize all sulfur to sulfur dioxide (SO2).

It can be set either in a horizontal or vertical position.

5.2 Combustion tube, of quartz, constructed to allow the direct injection of the sample into the heated oxidation zone of the furnace (5.1).

The combustion tube shall have side arms for the introduction of oxygen and carrier gas. The oxidation section shall be large enough to ensure complete combustion of the sample. It can be set either in a horizontal or vertical position.

5.3 Flow controllers, capable of maintaining a constant supply of oxygen and carrier gas.

5.4 Vapour drier, capable of removing water vapour formed during combustion prior to measurement by the detector (5.5).

5.5 UV fuorescence detector, selective and quantitative, capable of measuring light emitted from the fuorescence of sulfur dioxide by UV light.

WARNING - Exposure to excessive quantities of UV light is injurious to health. The operator must avoid exposing any part of his/her person, especially his/her eyes, not only to direct UV light, but also to secondary or scattered radiation that may be present.

5.6 Microlitre syringe, capable of accurately delivering between 5 µl to 50 µl quantities.

Follow the manufacturer's instructions for determining the length of the needle required. For vertical injection, syringes with a polytetrafuoroethylene (PTFE) plunger are recommended.

5.7 Sample inlet system, either positioned vertically or horizontally.

It shall consist of a direct injection inlet system capable of allowing the quantitative delivery of the material to be analysed into an inlet carrier stream which directs the sample into the oxidation zone at a controlled and repeatable rate. A syringe drive mechanism, which discharges the sample from the microlitre syringe at a constant rate of approximately 1 µl/s maximum, is required.

NOTE Boat injection systems can be used if they meet the performance requirements of Clause 12.

5.8 Balance, capable of weighing with an accuracy of at least 0.1 mg.

5.9 Volumetric fasks, Class A one-mark volumetric fasks, conforming to ISO 1042, of appropriate capacities, including 100 ml, for the preparation of sulfur stock solution (4.5) and calibration standards (4.6).