ASTM D5623 for sulfur compounds in light petroleum liquids
ASTM D5623 standard test method for sulfur compounds in light petroleum liquids by gas chromatography and sulfur selective detection
5. Apparatus
5.1 Chromatograph - Use a gas chromatograph (GC) that has the following performance characteristics:
5.1.1 Column Temperature Programmer - The chromatograph must be capable of linear programmed temperature operation over a range sufficient for separation of the components of interest. The programming rate must be sufficiently reproducible to obtain retention time repeatability of 0.05 min (3 s) throughout the scope of this analysis.

5.1.2 Sample Inlet System - The sample inlet system must have variable temperature control capable of operating continuously at a temperature up to the maximum column temperature employed. The sample inlet system must allow a constant volume of liquid sample to be injected by means of a syringe or liquid sampling valve.

5.1.3 Carrier and Detector Gas Control - Constant flow control of carrier and detector gases is critical to optimum and consistent analytical performance. Control is best provided by the use of pressure regulators and fixed flow restrictors or mass flow controllers capable of maintaining gas flow constant to more or less 1 % at the required flow rates. The gas flow rate is measured by any appropriate means. The supply pressure of the gas delivered to the gas chromatograph must be at least 70 kPa (10 psig) greater than the regulated gas at the instrument to compensate for the system back pressure of the flow controllers. In general, a supply pressure of 550 kPa (80 psig) is satisfactory.

5.1.4 Cryogenic Column Cooling - An initial column starting temperature below ambient temperature may be required to provide complete separation of all of the sulfur gases when present in the sample. This is typically provided by adding a source of either liquid carbon dioxide or liquid nitrogen, controlled through the oven temperature circuitry.

5.1.5 Detector - A sulfur selective detector is used and shall meet or exceed the following specifications: (1) linearity of 10(4), (2) 5 pg sulfur/s minimum detectability, (3) approximate equimolar response on a sulfur basis, (4) no interference or quenching from co-eluting hydrocarbons at the GC sampling volumes used.

5.2 Column - Any column providing adequate resolution of the components of interest may be used. Using the column and typical operating conditions as specified in 5.2.1, the retention times of some sulfur compounds will be those shown in Table 1. The column must demonstrate a sufficiently low liquid phase bleed at high temperature, such that loss of the detector response is not encountered while operating at the highest temperature required for the analysis.

5.2.1 Typical Operating Conditions:
5.2.1.1 Column - 30 m by 0.32 mm inside diameter fused silica wall coated open tube (WCOT) column, 4-µm thick film of methylsilicone.

5.2.1.2 Sample size - 0.1 to 2.0-µL.

5.2.1.3 Injector - Temperature 275°C; Split ratio: 10:1 (10 % to column).

5.2.1.4 Column Oven - 10°C for 3 min, 10°C/min to 250°C, hold as required.

5.2.1.5 Carrier Gas - Helium, Head pressure: 70 to 86 kPa (10 to 13 psig).

5.2.1.6 Detector - Sulfur chemiluminescence detector.

5.3 Data Acquisition:
5.3.1 Recorder - The use of a 0 to 1 mV recording potentiometer, or equivalent, with a full-scale response time of 2 s, or less, is suitable to monitor detector signal.

5.3.2 Integrator - The use of an electronic integrating device or computer is recommended for determining the detector response. The device and software must have the following capabilities: (1) graphic presentation of the chromatogram, (2) digital display of chromatographic peak areas, (3) identification of peaks by retention time or relative retention time, or both, (4) calculation and use of response factors, (5) internal standardization, external standardization, and data presentation.

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 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.

6.1.1 Alkane Solvent - Such as, iso-octane (2,2,4-trimethylpentane), Reagent grade, for use as solvent (diluent) in preparation of system test mixtures and for preparation of internal standard stock solution. (Warning - Iso-octane is flammable and can be harmful when ingested or inhaled.)

6.1.2 Aromatic Solvent - Such as, toluene, Reagent grade, for use as solvent (diluent) in preparation of system test mixtures. (Warning - Reagent grade toluene is flammable and is toxic by inhalation, ingestion, and absorption through skin.)

6.1.3 Carrier Gas - Helium or nitrogen of high purity. (Warning - Helium and nitrogen are compressed gases under high pressure.) Additional purification is recommended by the use of molecular sieves or other suitable agents to remove water, oxygen, and hydrocarbons. Available pressure must be sufficient to ensure a constant carrier gas flow rate (see 5.1.3).

6.1.4 Detector Gases - Hydrogen, nitrogen, air, and oxygen may be required as detector gases. These gases must be free of interfering contaminants, especially sulfur compounds. (Warning - Hydrogen is an extremely flammable gas under high pressure. Warning - Compressed air and oxygen are gases under high pressure and they support combustion.)

6.1.5 External Standards - The sulfur compounds and matrices of external standards should be representative of the sulfur compounds and sample matrices being analyzed. Test Methods D2622 and D3120 can be used to analyze materials for calibration of this test method. The internal standardization procedure can also be used for generating external standards. Alternatively, primary standards prepared as described in 6.1.4 can be used for method calibration when it is demonstrated that the matrix does not affect calibration. Only one external standard is necessary for calibration, provided that the system performance specification (8.3) is met. An external standard must contain at least one sulfur compound at a concentration level similar, for example, within an order of magnitude to those in samples to be analyzed.

6.1.6 InternalStandards - Diphenylsulfide, 3-chlorothiophene, and 2-bromothiophene are examples of sulfur compounds that have been used successfully as internal standards for samples within the scope of this test method (Warning - Sulfur compounds can be flammable and harmful or fatal when ingested or inhaled.). Any sulfur compound is suitable for use as an internal standard provided that it is not originally present in the sample, and is resolved from other sulfur compounds in the sample. Use the highest purity available (99 + % when possible). When purity is unknown or questionable, analyze the material by any appropriate means and use the result to provide accurate internal standard quantities.

6.1.6.1 An internal standard stock solution should be made up in the range of 0.1 to 1 g of the internal standard on a sulfur basis to 1 kg of solvent.

6.1.7 Sulfur Compound Standards - 99 + % purity (if available). Obtain pure standard material of all sulfur compounds of interest (Warning - Sulfur compounds can be flammable and harmful or fatal when ingested or inhaled.). If purity is unknown or questionable, analyze the individual standard material by any appropriate means and use the result to provide accurate standard quantities.

6.1.8 System Test Mixture - Gravimetrically prepare a stock solution of sulfur compounds in accordance with Practice D4307. This solution should cover the volatility range encountered in samples of interest; for example, dimethyl sulfide (~0.1 g/kg), 2-propanethiol (~0.1 g/kg), dimethyl disulfide (~10 g/kg), 3-methylthiophene (~100 g/kg), and (~10 g/kg) benzothiophene. Prepare a working test mix solution by making a 1000:1 dilution of the stock solution in a mixture of 10 % toluene in iso-octane. Although 2-propanethiol is not stable in the long term, peak asymmetry of a thiol (mercaptan) is an indicator of GC system activity.