ASTM D5580 for elements in finished gasoline by gas chromatography
ASTM D5580 Standard Test Method for Determination of Benzene, Toluene, Ethylbenzene, p/m-Xylene, o-Xylene, C9 and Heavier Aromatics, and Total Aromatics in Finished Gasoline by Gas Chromatography
6. Apparatus
6.1 Chromatographic System - See Practice E 355 for specific designations and definitions. Refer to Fig. 1 for a diagram of the system.
6.1.1 Gas Chromatograph (GC), capable of operating at the conditions given in Table 1, and having a column switching and backflushing system equivalent to Fig. 1. Carrier gas pressure and flow control devices shall be capable of precise control when column head pressures and flow rates are low.
6.1.2 Sample Introduction System, capable of introducing a representative sample into the gas chromatographic inlet. Microlitre syringes and automatic syringe injectors have been used successfully.
6.1.3 Inlet System, (splitting type) - Split injection is necessary to maintain the actual chromatographed sample size within the limits required for optimum column efficiency and detector linearity.
6.1.3.1 Some gas chromatographs are equipped with on-column injectors and autosamplers which can inject submicrolitre sample sizes. Such systems can be used provided that column efficiency and detector linearity are comparable to systems with split injection.
6.1.4 Detector - A flame ionization detector (Detector A) is employed for quantitation of components eluting from the WCOT column. The flame ionization detector used for Detector A shall have sufficient sensitivity and stability to detect 0.01 volume % of an aromatic compound.
6.1.4.1 It is strongly recommended that a thermal conductivity detector be placed on the vent of the TCEP precolumn (Detector B). This facilitates the determination of valve BACKFLUSH and RESET times (10.5) and is useful for monitoring the separation of the polar TCEP precolumn.
6.1.5 Switching and Backflushing Valve, to be located within a temperature-controlled heated zone and capable of performing the functions in accordance with Section 10, and illustrated in Fig. 1. The valve shall be of low internalvolume design and not contribute significantly to deterioration of chromatographic resolution.
6.1.5.1 A 10-port valve with 1.6-mm (0.06) outside diameter fittings is recommended for this test method. Alternately, and if using columns of 0.32-mm inside diameter or smaller, a valve with 0.8-mm (0.03-in.) outside diameter fittings should be used.
6.1.5.2 Some gas chromatographs are equipped with an auxiliary oven which can be used to contain the valve. In such a configuration, the valve can be kept at a higher temperature than the polar and nonpolar columns to prevent sample condensation and peak broadening. The columns are then located in the main oven and the temperature can be adjusted for optimum aromatic resolution.
6.1.5.3 An automatic valve switching device is strongly recommended to ensure repeatable switching times.
6.2 Data Acquisition System:
6.2.1 Integrator or Computer, capable of providing real-time graphic and digital presentation of the chromatographic data are recommended for use. Peak areas and retention times can be measured by computer or electronic integration.
6.2.1.1 It is recommended that this device be capable of performing multilevel internal-standard-type calibrations and be able to calculate the correlation coefficient (r2) and linear least square fit equation for each calibration data set in accordance with 11.4.
6.3 Chromatographic Columns (two columns are used):
6.3.1 Polar Precolumn, to perform a pre-separation of the aromatics from nonaromatic hydrocarbons in the same boiling point range. Any column with equivalent or better chromatographic efficiency and selectivity in accordance with 6.3.1.1 can be used.
6.3.1.1 TCEP Micro-Packed Column, 560-mm (22-in.) by 1.6-mm (1/16-in.) outside diameter by 0.76-mm (0.030-in.) inside diameter stainless steel tube packed with 0.14 to 0.15 g of 20 % (mass/mass) TCEP on 80/100 mesh Chromosorb P(AW). This column was used in the cooperative study to provide the precision and bias data referred to in Section 15.
6.3.2 Nonpolar (Analytical) Column - Any column with equivalent or better chromatographic efficiency and selectivity in accordance with 6.3.2.1 can be used.
6.3.2.1 WCOT Methyl Silicone Column, 30 m long by 0.53-mm inside diameter fused silica WCOT column with a 5.0-µm film thickness of cross-linked methyl siloxane.
7. Reagents and Materials
7.1 Carrier Gas, appropriate to the type of detector used. Helium has been used successfully. The minimum purity of the carrier gas used must be 99.95 mol %. Additional purification may be necessary to remove trace amounts of oxygen. (Warning - Helium is usually supplied as a compressed gas under high pressure.)
7.2 Methylene Chloride - Used for column preparation. Reagent grade, free of nonvolatile residue. (Warning - Harmful when ingested or inhaled at high concentrations.)
7.3 2,2,4-Trimethylpentane (isooctane) - Used as a solvent in the preparation of the calibration mixture. Reagent grade. (Warning - Isooctane is flammable and can be harmful or fatal when ingested or inhaled.
7.4 Standards for Calibration and Identification, required for all components to be analyzed and the internal standard. Standards are used for establishing identification by retention time as well as calibration for quantitative measurements. These materials shall be of known purity and free of the other components to be analyzed. (Warning - These materials are flammable and may be harmful or fatal when ingested or inhaled.
