ASTM D6730 Test Method for Individual Components in Spark Ignition Engine Fuels
ASTM D6730 Standard Test Method for Determination of Individual Components in Spark Ignition Engine Fuels by 100-Metre Capillary (with Precolumn) High-Resolution Gas Chromatography
9. Preparation of Apparatus
9.1 Install the 100-m column and, if required, a precolumn according to the manufacturer's or supplier's instructions and Annex A1. See Practice E 1510 for recommended installation procedures.
9.2 Determine the required length of the precolumn in accordance with Annex A1. Adjust the operating conditions of the gas chromatograph to those listed in Table 2 or as determined by Section 12 and Annex A1.
9.3 During setup and, when not performing analyses, it is advisable to turn off the cryogenic operation and set the column oven temperature at 35°C. Attach the column outlet to the flame ionization detector inlet and check for leaks throughout the system. If leaks are found, tighten or replace fittings before proceeding.
9.4 Confirm or adjust, or both, the column carrier gas flow rate by making injections of methane or natural gas. The methane retention time shall be 7.00 more or less 0.02 min with the column oven temperature at 35°C, which results in an average linear velocity of 24 cm/s, as determined using Eq 1. This will result in a methane retention time of 6.53 min at 5°C. Raising or lowering the carrier gas pressure to the injector makes flow rate adjustment. A starting point of 277-kPa (40-psig) helium pressure is recommended, although columns requiring as high as 332-kPa (48-psig) helium have been encountered.
average linear gas velocity: uave(cm/s) = column length (cm)/tM(s)
9.5 After final adjustment of the carrier gas flow rate, note the carrier gas inlet pressure. Measure and, if necessary, readjust the injector split flow rate to give the specified or desired split ratio. Calculate the column outlet flow rate using 9.5.1 and the split ratio using 9.5.2.
9.5.1 Column Carrier Gas Flow Rate (at outlet):
9.5.1.1 P = (head pressure (psig) + ambient pressure)/ambient pressure.
9.5.1.2 j = compressibility factor = 3/2((P2-1)/(P3-1)).
9.5.1.3 uo = uave/j = column outlet velocity.
9.5.1.4 Ac = pi(r)2 = column cross-sectional area (cm2).
where r = column internal radius (cm).
9.5.1.5 Flow rate (cm3/min) = u0 x Ac x 60.
9.5.2 Injection Split Ratio - (Split flow rate + column flow rate)/column flow rate.
9.5.3 Example - Using a 100-m x 0.25-mm capillary column:
9.5.3.1 Uave = 100 x 100/6.98 x 60 = 23.88 cm/s.
9.5.3.2 P = 40 psig + 12.0/12.0 = 4.33.
9.5.3.3 j = 3/2((18.778-1)/(81.370-1)) = 0.33
9.5.3.4 uo = 23.88/0.33 = 71.96 cm/s.
9.5.3.5 Ac = pi(0.025/2)2 = 4.9 x 10(-4) cm2.
9.5.3.6 Flow rate = 71.96 3 4.9 x 10(-4) x 60 = 2.12 cm3/min.
9.5.3.7 Split Ratio = (192 + 2.12)/2.12 = 91.6:1.
9.6 Make a blank analysis (no sample injection) run to ensure proper instrument operation and further condition the column and instrumentation. If stray peaks or a rising baseline signal is observed, the column oven shall be kept at the upper temperature until the baseline becomes steady and returns to within approximately 5 % of the starting temperature detector signal.
9.7 After any extended conditioning period, or if the instrument has been shut down, it is advisable to repeat 9.4, 9.5, and 9.6 to ensure proper carrier gas flows are being used and the column is clean.
