ASTM Method D5769: An Easy Solution for Determination of High Concentration Aromatic Compounds in Finished Gasolines

ASTM D5769 is a widely accepted standard method used in the petroleum industry for determination of benzene, toluene, and total aromatics in finished gasolines by GC-MS. A common difficulty encountered with this method is the concern with saturation of the ion source, which leads to nonlinearity in calibration curves, especially for the quantification of the high-concentration aromatics, such as toluene.

2. Experimental

High split, low volume injections into the GC were utilized combined with full mass range acquisition with LECO’s Pegasus BT TOFMS, a benchtop time-of-flight mass spectrometer. LECO’s ChromaTOF® brand software’s Target Analyte Find feature was used to automatically identify analytes of interest and quantify both calibrated and uncalibrated analytes using Quantitation and Semi-Quantitation data processing tools.

3. Results and Discussion

Three fundamental criteria must be met in order to satisfy the requirements for ASTM D5769. They are the following: mass spectrometer sensitivity for 0.01 mass % for 1,4-diethylbenzene, achieving specified ion abundance ratios for key ions of 1,2,3-trimethylbenzene, and calibration linearity for all analytes.

The first requirement stated in Section 6.2.3 of the method stipulates that the signal-to-noise (S/N) ratio of 0.01 mass % 1,4-diethylbenzene at mass 134 must be consistently greater than 5. As can be seen in Figure 2 and Table 2, repeat injections show that the instrument easily surpasses the required S/N of 5, with an average Peak S/N of well over 100.

The second requirement stated in Section 9.2.5 of the method specifies expected ion ratios for 3 masses of 1,2,3-Trimethylbenzene.

This criterion is easily satisfied with values shown in Figure 3 and Table 3.

Linear calibration curves for all analytes are also required, with linear least-squares R2 values greater than 0.99 according to Section 9.3.3. Table 4 shows that the linear least-squares R2 values of each analyte exceed the criteria, with the average value at 0.99987. Figure 4 illustrates the excellent linearity achieved for a known problem compound, toluene, with concentrations ranging from less than 2 mass % to over 20 mass %. The open-style source of the LECO Pegasus BT avoids rollover due to saturation from these high-concentration aromatics, easily yielding linear calibration curves for a wide dynamic range.