Application Note Energy and Chemical A High Performance and Cost-Effective GC/MS Solution for Measuring Aromatics in Gasoline Using ASTM Method D5769 Author Abstract James D. McCurry Agilent Technologies, Inc. This application note describes an Agilent 8860/5977 gas chromatography/mass spectrometry (GC/MS) system for the analysis of benzene, toluene, and total aromatics in motor gasoline using ASTM method D5769. A modified stainless-steel detector source was employed to lower costs while providing the high performance required by the ASTM method. Before running samples, the 8860/5977 system was verified by passing all performance and QC tests required by the D5769 method. Gasoline samples run with this system showed excellent precision and agreement with other laboratory results. Introduction detector linearity requirements and Experimental method performance tests were met, ASTM method D5769 uses GC/MS to the 8860/5977 GC/MS system was A 8860/5977 GC/MS system was measure the total aromatic content used to measure the benzene, toluene, configured to meet the D5769 in gasoline.1 The U.S. Environmental and total aromatic contents in two method requirements (Table 1). The Protection Agency (EPA) has designated Tier III‑reformulated gasoline samples stainless-steel source on the mass D5769 as the official gasoline total from an inter-laboratory cross-check spectrometer (MSD) was modified by aromatics method for regulatory program (ILCP). The results obtained replacing the standard 3 mm draw-out compliance.2 The EPA also allows ASTM from these samples were evaluated plate with a 9 mm draw-out plate. Larger D1319 as an alternative method for the for precision and agreement with the aperture draw-out plates have previously same total aromatics measurement. reported ILCP results. been shown to improve linear response D1319 is a simple technique using a for higher concentration components in fluorescence-indicating dye on a silica gasoline samples.3,4 The D5769-specified gel column to separate and detect operating parameters are listed in the total aromatics in gasoline. This Tables 2 and 3, and were used to acquire method is commonly used for routine all data. aromatics analysis, but the results must be correlated to the primary D5769 Table 1. GC/MS hardware configuration. Table 3. D5769 MS operating conditions. method. Recently, supplies of the dye have been disrupted with no alternative 5977B MSD bundle with 8860 GC GC/MS Interface replacement. This event prompted many G7082BW Stainless steel MSD source Type Direct laboratories to transition their gasoline Split/splitless inlet Temperature 280 °C analysis operations from D1319 to G3440-20022 9-mm draw-out plate for MSD source Agilent 5977 MSD D5769 and begin investment into GC/MS G4567A 7650A auto injector (ALS) Ionization Voltage 70 eV systems. To ease the higher cost of Source Temperature 230 °C transition to the more complex GC/MS Table 2. D5769 GC operating conditions. Quad Temperature 150 °C analysis, this application note presents Auto Injector (ALS) Scan Range 45 to 300 amu a cost-effective GC/MS instrument Syringe 5 µL (p/n G4513-80206) Scan Rate 3 scans/s platform that delivers the necessary Injection Volume 0.1 µL Tuning Algorithm Atune performance to meet D5769 analysis Wash Solvent isooctane requirements. Split/Splitless Inlet An operational challenge often Carrier Gas Helium encountered with the D5769 method is Mode Split, 350:1 achieving calibration linearity for each Temperature 250 °C target compound as measured by their Liner Split (p/n 5190-2295) coefficients of determination. GC/MS Column systems are designed to measure DB-1, 60 m × 0.25 mm, 1.00 µm Type compounds at low concentrations, (p/n 122-1063) 1.8 mL/min constant flow typically in the parts-per-million (ppm) to Flow Rate (35 cm/s) parts-per-billion (ppb) range. Individual Initial Temperature 60 °C aromatic compounds in gasoline are present at single- or double-digit percent Initial Hold Time 0 min levels; greater than 105 times higher than Ramp Rate 1 3 °C/min levels usually measured using GC/MS. Temperature 1 120 °C This application note demonstrates Hold Time 1 0 min an effective way to improve detection Ramp Rate 2 10 °C/min linearity for higher concentrations by Temperature 2 250 °C making a simple modification to the Hold Time 2 2 min mass spectrometer source. Once the Total Run Time 35 min 2 Calibration standards and performance Results and discussion The method requires a correlation check standards were obtained from of determination (R2) of 0.99 or Accustandard, Inc. The quality control The highest-level calibration mix was run greater for each compound’s linear check sample and reformulated to establish identity and elution order regression calibration. This can be gasoline samples were obtained from for the target compounds and internal difficult to achieve using the standard Spectrum Quality Standards. The standard compounds. Figure 1 shows 3 mm draw-out plate in the MS source, Tier III-reformulated gasolines were the resulting total ion chromatogram especially for toluene where the upper commercial samples obtained from an (TIC). There are two pairs of target calibration standard has a concentration interlaboratory cross-check program compounds not completely resolved of 19% (v/v). In Figure 2, the profile for (ILCP) used to help laboratories meet the on this column: m-xylene/p-xylene and toluene’s extracted ion chromatogram EPA Tier III §80.47 Performance-based 1,4-dimethylbenzene/n-butylbenzene. (EIC) shows evidence of detector Analytical Test Method Approach. Ten The ASTM D5769 method allows saturation, with a peak height ion replicates of each gasoline sample were each pair to be reported as a count exceeding 8.0 × 106. Ideally the prepared with each replicate run once on single compound. maximum peak height should be less the 8860/5977 system. than 5 × 106 ion counts. The resulting ×106 3 7 1. Benzene-D6 (ISTD1) 11. 1-Methyl-3-ethylbenzene 20. 1,2-Diethylbenzene 2. Benzene 12. 1-Methyl-4-ethylbenzene 21. 1,2,4,5-Tetramethylbenzene 3. Toluene 13. 1,3,5-Trimethylbenzene 22. 1,2,3,5-Tetramethylbenzene 6 4. Ethylbenzene-D10 (ISTD2) 14. 1-Methyl-2-ethylbenzene 23. Naphthalene-D8 (ISTD3) 5. Ethylbenzene 15. 1,2,4-Trimethylbenzene 24. Naphthalene 5 6,7. m+p-Xylene 16. 1,2,3-Trimethylbenzene 25. Pentamethylbenzene 8. o-Xylene 17. Indan 26. 2-Methylnaphthalene e 9. 1-Methylethylbenzene 18. 1,4-Diethylbenzene 27. 1-Methylnaphthalene 4 10. n-Propylbenzene 19. n-Butylbenzene Abundanc 3 26 6,7 8 12 18,19 5 15 2 25 27 11 17 20 24 2 16 21 9 10 13 14 1 22 1 4 23 810121416182022242628 Time (min) Figure 1. Total ion chromatogram (TIC) showing the elution order of aromatic compounds in the D5769 calibration mix. The three deuterated internal standards are shown in red text. 3 ×106 A 8 Toluene EIC 6 B e Toluene 4 o 10 Abundanc 2 Response rati 5 R2 = 0.926 0 10.05 10.15 10.25 10.35 02468 Time (min) Concentration ratio Figure 2. Detector saturation was observed for toluene at the 19 % (v/v) calibration level. The resulting linear calibration curve failed the ASTM required coefficient of determination (R2) of 0.99 or greater. calibration curve exhibits a nonlinear The linear calibration curve for toluene performed by running a 0.01% (m/m) response over the five-point calibration now has an R2 of 0.99999. Table 4 of 1,4-dimethylbenzene in isooctane. range, with a failed R2 of 0.93. shows the R2 values for all calibrated The ASTM D5769 method requires a The detector linearity problem was compounds greatly exceeding the ASTM minimum S/N of five for this test. The solved by replacing the standard 3 mm id requirement for acceptable calibration. 8860/5977 GC/MS system delivered draw‑out plate with a 9 mm id draw‑out After successful calibration, several a S/N of 16.2, as shown in Figure 4, plate. In Figure 3, the toluene EIC does other required performance measures showing no detrimental effect on method not show any detector saturation and the were met before running the samples. sensitivity with the 9 mm draw-out plate height is well below 5 × 106 ion counts. First, a signal-to-noise (S/N) test was installed in the MS source. ×106 3.4 A Toluene EIC 2.6 B Toluene e 20 1.8 o Abundanc 1.0 10 Response rati R2 = 0.99999 0 10.12 10.16 10.20 10.24 10.28 10.32 0 2468 Time (min) Concentration ratio Figure 3. Improved peak shape and response for toluene was obtained using a 9 mm draw-out plate in the MS source (A). The resulting linear calibration now has an ideal R2 of 0.99999 (B). 4 Table 4. Coefficients of determination. 0.01% 1,4-Diethylbenzene (EIC) Peak Compound R2 2 Benzene 0.99992 3 Toluene 0.99999 5 Ethylbenzene 0.99990 6,7 m,p-Xylene 0.99996 Column 1 8 o-Xylene 0.99999 S/N = 16.2 9 1-Methylethylbenzene 0.99986 10 n-Propylbenzene 0.99997 11 1-Methyl-3-ethylbenzene 0.99996 12 1-Methyl-4-ethylbenzene 0.99993 13 1,3,5-Trimethylbenzene 0.99999 14 1-Methyl-2-ethylbenzene 0.99993 15 1,2,4-Trimethylbenzene 0.99998 22.7 22.9 23.1 23.3 23.5 23.7 23.9 16 1,2,3-Trimethylbenzene 0.99994 Time (min) 17 Indan 0.99999 Figure 4. A signal-to-noise ratio of 16.2 exceeds the D5769 minimum required value of 5. 18 1,4-Diethylbenzene 0.99996 19 n-Butylbenzene 0.99940 1-Methyl-2-ethylbenzene 20 1,2-Diethylbenzene 0.99999 1,3,5-Trimethylbenzene 21 1,2,4,5-Tetramethylbenzene 0.99993 22 1,2,3,5-Tetramethylbenzene 0.99997 24 Naphthalene 0.99991 25 Pentamethylbenzene 0.99954 26 2-Methylnaphthalene 0.99988 27 1-Methylnaphthalene 0.99996 Resolution (R) = 3.3 Two other performance tests, one for chromatographic resolution and another for mass spectral response, were also run using procedures described in the D5769 method.