ASTM D6550 Test Method for Determination of Olefin Content of Gasolines
ASTM D6550 Standard Test Method for Determination of Olefin Content of Gasolines by Supercritical-Fluid Chromatography
9. Procedure
9.1 Determination of Density of Samples and Standards - Determine density of samples and standards using Test Method D4052 or equivalent.
9.2 Switching Times:
9.2.1 Establish the loading time period, tL, between injection (t = 0) and the onset of the aromatic peak, as described in 8.2.2.1.
9.2.1.1 Only the silica column need be in the flow path to the detector (forward-flush mode; see 6.1.7.4) for this step. This will protect the silver-loaded column from aromatics (see 8.2.2.2) when using a quality control sample to determine the loading time period.
9.2.1.2 If both columns are positioned in the flow path, use the loading-time mixture (see 7.5) to protect the silver-loaded column.
9.2.2 Determine the time period, tAR, required to back-flush all aromatic and polar compounds from the silica column after the loading-time period, tL, using a quality control sample. Columns are initially connected in series (see 6.1.7.1). Valves are switched at the end of time period, tL, such that only the silica column is in the flow path (back-flush mode; see 6.1.7.2). Measurement of tAR can then be made while protecting the silver-loaded column from aromatics.
9.2.3 Determine the time period, tC, required to clean all saturated and cyclic compounds from the silver-loaded column in the forward-flush mode, using a quality control sample. Columns are initially connected in series (see 6.1.7.1). Valves are switched at time, tL, such that only the silica column is in the flow path (see 6.1.7.2). Valves are then switched at the end of time period, tAR, (total time = tL + tAR). The silica and silver-loaded columns are connected in series (see 6.1.7.1), allowing the elution of the saturated and cyclic compounds. The time period, tC, shall be short enough to avoid the onset of the elution of olefins from the silver-loaded column.
9.2.4 Determine the time period, tBF, required to elute all olefins from the silver-loaded column in the backflush mode. Columns are initially connected in series (see 6.1.7.1). Valves are switched at the end of time period, tL, such that only the silica column is in the flow path (see 6.1.7.2). Valves are then switched at the end of time period, tAR, such that the two columns are again connected in series (see 6.1.7.1). Valves are switched at the end of time period, tC, (total time tL+ tAR + tC) such that only silver-loaded column is in the flow path in the backflush mode (see 6.1.7.3), allowing the elution of the olefins. The resulting switching table, with typical values corresponding to the conditions listed in Table 1 and Table 2, is shown as Table 3.
9.3 Application of this Test Method:
9.3.1 Calibration Curve - Record the calibration curve, as described in 8.2.4.
9.3.2 Analyzing Samples - Inject undiluted aliquots of all gasoline samples. An example chromatogram is shown as Fig. 2 (see Fig. 3 for the details of Fig. 2).
10. Calculation
10.1 Determine the total area of the olefins peak. Determine the total concentration of olefins directly from the calibration curve. An example of a calibration curve is shown in Fig. 4.
10.2 When the density of the standards used for calibration is different from the samples being analyzed, a density correction shall be applied. This can be accomplished using the following calculation:
Mcor = Mcal [(ρstd)/(ρsam)]
where:
Mcor = corrected mass % olefin,
Mcal = mass % olefins as interpolated from calibration curve,
ρstd = density of the standard solution used to make the calibration curve, and
ρsam = density of gasoline sample.
