ASTM D6839 for Hydrocarbon Types, Oxygenated Compounds and Benzene
ASTM D6839 Standard Test Method for Hydrocarbon Types, Oxygenated Compounds and Benzene in Spark Ignition Engine Fuels by Gas Chromatography
9. Preparation of Apparatus
9.1 Assemble the analyzer system (gas chromatograph with independent temperature controlled components) as shown in Fig. 1 or with an equivalent flow system. If using a commercial system, install and place the system in service in accordance with the manufacturer's instructions.
9.2 Impurities in the helium carrier gas, hydrogen, or air will have a detrimental effect on the performance of the columns and traps. Therefore, it is important to install efficient gas purifiers in the gas lines as close to the system as possible and to use good quality gases. The helium and hydrogen gas connection lines shall be made of metal. Check that all gas connections, both exterior and interior to the system, are leak tight.
9.3 The gas flow rates on commercial instruments are normally set prior to shipment and normally require little adjustment. Optimize flow rates on other systems to achieve the required separations. Typical flow rates for the commercial instrument used in the precision study are given in Table 3; however, the flows can differ somewhat from system to system.
9.3.1 Set air flow rates for column/trap cooling and for operation of air actuated valves, if required.
9.4 System Conditioning - When gas connections have been disconnected or the flow turned off, as on initial start up, condition the system by permitting carrier gas to flow through the system for at least 30 min while the system is at ambient temperature. After the system has been conditioned, analyze the system validation test mixture, as described in Section 11, discarding the results.
10. Standardization
10.1 The elution of components from the columns and traps depends on the applied temperatures. The switching valves also need to be actuated at exact times to make separations of compounds into groups, for example, to retain specific compounds in a column or trap while permitting other compounds to elute. Therefore, the separation temperatures of the columns/traps and the valve timing are critical for correct operation of the system. These parameters need to be verified on the start up of a new system (see Note 6) for correctness. They also require evaluation and adjustment as necessary on a regular basis to correct for changes to columns and traps as a result of aging. To do this, the analyst shall analyze several test mixtures and make changes, as required, based on an evaluation of the resulting chromatograms and test reports.
10.2 Using the procedure outlined in Section 11, analyze the system validation test mixture. Carefully examine the chromatogram obtained to verify that all the individual components of the test mixture are correctly identified as compared to the reference chromatogram (Figs. 2 and 3). Test results for group totals shall agree with the known composition (see Table 2) within more or less 0.5 mass %. Test results for groups by hydrocarbon number shall agree with the known composition (see Table 2) within 0.2 mass %. If these specifications are met, proceed to the analysis of the quality control samples (see 10.3).
10.2.1 If the specifications in 10.2 are not met, adjust the temperature of specific columns and traps or valve timing according to the manufacturer's guidelines and reanalyze the system validation test mixture until they are met.
10.3 Analyze quality control samples; see 8.6. Verify that results are consistent with those previously obtained and that the separation of olefins and saturates is correct.
10.3.1 Breakthrough of olefins to the saturate fraction is indicated by a rising baseline under the C5 to C6 saturates region or additional peaks between the C4 and C6 peaks. If breakthrough is observed, optimize the olefin trap temperature or, if necessary, replace the trap.
10.3.2 If the fraction containing C4 to C6 olefins and C7 to C10 saturates shows peaks in the C7 region, optimize the olefin trap temperatures or, if necessary, replace the trap.
10.3.3 Loadability limits for olefins are listed in 1.2. These limits depend on the condition of the olefin trap, and an aged trap may not have this capacity. Use the quality control sample (see 8.6) to verify olefin capacity.
10.3.4 Check that the correct qualitative and quantitative analysis of oxygenates are present in the quality control sample. If qualitative or quantitative specifications are not met, optimize the alcohol trap temperature and ether-alcohol-aromatic trap temperature or replace the columns as necessary.
10.4 Reanalyze the system validation test mixture whenever the quality control sample does not conform to expected results (see 10.3) and make adjustments as necessary (see 10.2).
