ASTM D7041 Standard Test Method for Determination of Total Sulfur in Liquid Hydrocarbons and Hydrocarbon-Oxygenate Blends by Gas Chromatography with Flame Photometric Detection
5. Apparatus
5.1 Gas Chromatograph, equipped with automatically controlled valves, capable of automatic calibration with an external standard and having a flame photometric detector with an overall sensitivity to detect at least 0.5 mg/kg of SO2. It must be able to automatically control all valve switching times. Although originally developed with online analytical measurement equipment in an offline mode of operation, suitable online or laboratory gas chromatographs may apply this test method as described. Typical instrument parameters are listed in Table 1.
5.1.1 Carrier and Detector Gas Control - The chromatograph must be equipped with flow controllers or pressure controllers capable of maintaining a constant supply of carrier gas and detector supply gases. Electronic pressure or flow control is highly recommended.

5.1.2 Sample Injection System - An automatic sample injection device is required. The injector must allow the introduction of small sample sizes (0.1 µL to 1 µL). The sample must be accurately and repeatably injected into the gas chromatograph. Rotary or stem type liquid injection valves or auto injectors are recommended. The valve or injector must be equipped with a heated vaporizer section capable of being heated to at least 285 °C.

5.2 Pyrolysis Furnace - A furnace capable of maintaining a sufficient temperature (>900 °C) to pyrolyze the entire sample and oxidize the sulfur compounds to SO2.

5.3 Quartz Combustion Tube - Quartz tube capable of withstanding temperatures up to 1200 °C. The oxidation section shall be large enough to ensure complete oxidation of the sample.

5.4 Column - A column that can provide complete separation of SO2 from the CO2 quench and the other oxidized components such as H2O.

5.5 Detector - Any flame photometric detector (FPD) can be used, provided it can detect a minimum peak height twice that of the baseline noise for a 1 µL injection of a 0.5 mg S/kg standard. Detector linearity shall be at least equal to or greater than 10(3). The user is referred to Practice E840 for assistance in optimizing the operation and performance of the FPD.

5.6 Data Acquisition System - Use any integrator or computerized data acquisition system for peak area integration, as well as for recording the chromatographic trace. The device and software must have the following capabilities:
5.6.1 Identification of peak by retention time.

5.6.2 Calculation and use of response factors.

5.6.3 External standard calibration calculation.

5.6.4 Graphic presentation of the chromatogram.

5.7 Analytical Balance - Any balance capable of accurately weighing materials to the nearest 0.01 mg.

6. Reagents and Materials
6.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.

6.2 Carrier-Gas - Zero grade air is recommended. (Warning - Compressed air is a gas under high pressure that supports combustion.)

6.3 Hydrogen - Chromatographic grade recommended, minimum purity 99.995 %. (Warning - Hydrogen is an extremely flammable gas under high pressure.)

6.4 Solvent (Reagent Grade) - the solvent chosen should be capable of dissolving the sulfur-containing compound used to prepare the standard. The solvent of choice should have a density similar to the samples being analyzed and it should have sulfur concentrations less than the instrument detection limit. Mixed solvents such as an isooctane/toluene mixture can be used to reach the desired density. (Warning - Solvents used as reagents such as toluene and isooctane are flammable and may be harmful or fatal if ingested or inhaled.)

6.5 Standards for Calibration and Peak Identification - Standards are used for peak identification and retention time determination. Also standards of known concentrations are required for external standard calibration of the gas chromatograph.
6.5.1 Preparation of Stock Solution (mass/volume), 100 µg S/mL (see Notes 3 and 4). Accurately weigh to the nearest 0.1 mg, 0.0456 g of butyl sulfide into a suitable container such as a 100 mL volumetric flask. Dilute to volume with the selected solvent. This stock solution can be further diluted to the desired sulfur concentration. Other sulfur containing compounds such as thiophene or thianaphthene can be substituted for n-butyl sulfide if desired. The concentration of the stock solution can be calculated as follows:
µg S/mL = (M x 32.06) x (1 x 10(6))(µg/g)/(100 mL x FW)
where:
M = exact mass of sulfur reference compound (g), and
FW = formula weight of sulfur reference compound.

NOTE 3 - Commercial standards can be used provided they are checked for accuracy.

NOTE 4 - Stock solutions will have a shelf life of approximately 2 to 3 months and should be remixed accordingly.

6.5.2 Preparation of Stock Solution: (mass/mass), 100 µg S/g (see Notes 3 and 4). Accurately weigh to the nearest 0.1 mg, 0.0456 g of butyl sulfide into a suitable container. Add 100 g (accurately weighed to the nearest 0.1 g) of the selected solvent. This stock solution can be further diluted to the desired sulfur concentration. Other sulfur containing compounds such as thiophene or thianaphthene can be substituted for butyl sulfide if desired. The concentration of the stock solution can be calculated as follows:
mgS/kg = (M x 32.06) x (1 x 10(6))(mg/kg)/(100 g x FW)

6.6 Butyl Sulfide - FW 146.29, 21.92 % (m/m) S.

6.7 Thiophene - FW 84.14, 38.1 % (m/m) S.

6.8 Thianaphthene - FW 134.20, 23.89 % (m/m) S.

7. Hazards
7.1 Consult current Occupational Safety Health Administration (OSHA) regulations, supplier Material Safety Data Sheets, and local regulations for all materials used in this test method.

7.2 High temperatures are used in this method; extra precaution should be exercised when working with flammable materials near the pyrolysis furnace.