ASTM D4420 Determination of Aromatics in Finished Gasoline by Gas Chromatography
7. Sampling
7.1 Gasoline (Warning - Extremely flammable. Vapors harmful if inhaled.) Samples to be analyzed by this test method shall be obtained using the procedures outlined in Practice D4057. After delivery to the laboratory, volatile samples must be cooled before the container is opened.
8. Preparation of Apparatus
8.1 Install the valve and the columns in the column oven in accordance with the flow pattern shown in either Fig. 1 or Fig. 2. Columns can be prepared by any satisfactory method, used in the practice of the art, provided the separation achievable is equivalent to that shown in Figs. 3-5.
NOTE 2 - Typical instructions for preparing columns are given in Practice E 260.
8.1.1 Adjust the operating conditions to those listed in Table 1, but do not turn on the detector circuits. Check the system for carrier gas leaks. After confirming no leaks, turn on the detector and allow the system to equilibrate, as indicated by a stable, recorder baseline.
9. Calibration
9.1 Calibration mixture - Using the procedure described in Practice D4307, prepare a calibration blend of aromatics in isooctane at levels approximately those in the sample to be analyzed. n-Butylbenzene is used to represent the C9 + aromatic composite. Using relative densities, convert the calculated mass (weight) percent to liquid volume percent. (Warning - see 6.4, 6.5, 6.6, and 6.7.)
NOTE 3 - The following aromatics concentrations, in volume percent, are typical: benzene, 3.00; toluene, 10.00; mixed xylenes (C8 aromatics), 15.00, and n-butylbenzene (C9 + aromatics), 15.00.
NOTE 4 - The following relative densities, at 60/60°F, are used: benzene 0.8845, toluene 0.8719, n-butylbenzene 0.8646, and an average of 0.8727 for mixed xylenes.
9.2 Calibration Procedure:
9.2.1 Inject 3 µL of the calibration mixture with the valve in the forward flow position and the detector on polarity B. If a single-filament detector is being used, monitor the signal from Detector B. Determine the time, in seconds, at which benzene begins to elute, and subtract 6 s from this value. Call this time T1.
9.2.2 Chromatograph the calibration mixture with the detector on polarity A. If a single-filament detector is being used, monitor the signal from Detector A. At time T1, switch the valve to the backflush position. When the o-xylene peak returns to baseline, switch the valve to the forward flow position. Call this time T2. An illustrative chromatogram is given in Fig. 3.
9.2.3 Measure the areas of the benzene, toluene, C8 aromatic composite and n-butyl benzene peaks. Calculate the response factor (volume percent per unit of area) for each of these components or groups from the following equation:
where:
F = response factor of the component or group for a 3-µL volume,
M = concentration of the component, volume percent, and
A = peak area for the component
9.2.4 Calculate response factors to at least three significant figures.
