Faster DHA Analyses Using Helium or Hydrogen

Accurate information about the concentrations of individual components in gasolines is critical for evaluating raw materials and controlling refinery processes. A high-resolution GC method for detailed hydrocarbon analysis (DHA) of gasolines is outlined in American Society of Testing and Materials (ASTM) Method D6730-01. ASTM D6730-01 is specific for the analysis of hydrocarbon components, plus oxygenated additives such as methanol, ethanol, tert-butanol, methyl tert-butyl ether (MTBE), and tert-amyl methyl ether (TAME) in spark-ignition engine fuels.

Historically, columns for DHA analysis have had several challenges, primarily centering on inertness and selectivity. Highly inert columns are required for the accurate analysis of polar compounds and inadequately deactivated columns result in poor peak symmetry and unpredictable retention times (Figure 1). Similarly, developing columns with adequate selectivity for resolving key aromatics such as benzene, toluene, and p-xylene has also been difficult. Both of these issues have been critical to overcome as they prevent accurate quantitative analysis and reduce the precision with which refineries can control blending.

Figure 1 Rtx®-DHA-100 columns are highly inert, resulting in sharp oxygenate peaks eluting at predictable retention times.

Peaks 1. Ethanol 2. C5 3. tert-Butanol 4. 2-methylbutene-2 Column Rtx®-1 PONA, 100 m, 0.25 mm ID, 0.50 µm (cat.# 10195) Sample oxy set-up blend, neat Injection Inj. Vol.: 0.01 µL split (split ratio 150:1) Liner: Cup Splitter (cat.# 20709) Inj. Temp.: 250 °C Oven Oven Temp: 5 °C Carrier Gas H2, constant linear velocity Linear Velocity: 40 cm/sec. Detector FID @ 275 °C Notes chromatograms aligned on pentane (C5)

Restek has worked in a long-term collaboration with Neil Johansen, one of the original DHA method developers, to engineer a selective and highly inert DHA column that meets or exceeds all ASTM D6730-01 and Canadian General Standards Board (CGSB) requirements. Originally, this column was sold as the Rtx^®-1PONA column, but it has recently been renamed as the Rtx^®-DHA column line, for better alignment with method nomenclature. No changes were made to manufacturing, so the columns and chromatographic performance are exactly the same. In addition to meeting or exceeding all method parameters, Rtx^®-DHA columns are exceptionally robust. They can be used with either helium under an accelerated temperature program, or with hydrogen carrier gas, providing two options for significantly increasing sample throughput without compromising chromatographic performance.

33% Faster Run Times Using Helium under Optimized Conditions

The Rtx^®-DHA-100 column is a 100 m x 0.25 mm ID capillary column coated with 0.5 µm of 100% dimethyl polysiloxane stationary phase, as recommended in D6730-01. The method sets criteria for efficiency and capacity (both based on C5), t-butanol peak symmetry, and t-butanol/2-methylbutene-2 resolution. As shown in Figure 2, Rtx^®-DHA-100 columns meet or exceed all method criteria and show exceptional peak symmetry for polar oxygenates. Sharp, symmetric peaks mean maximum response and stable, predictable retention time positions for the eluting alcohols. Since Rtx^®-DHA-100 columns are individually tested for peak symmetry and retention (as well as efficiency, selectivity, resolution, and bleed), consistent performance is assured. The reliable inertness of these columns results in near symmetric oxygenate peaks, which in turn give more accurate quantitative results and allow refiners to make better decisions in the blending process.

Figure 2 Critical pairs of gasoline components are reliably resolved per ASTM specifications 33% faster using helium under optimized conditions.

Peaks Peaks 1. Ethanol 16. Toluene 2. C5 17. C8 3. tert-Butanol 18. Ethylbenzene 4. 2-Methylbutene-2 19. p-Xylene 5. 2,3-Dimethylbutane 20. 2,3-Dimethylheptane 6. Methyl tert-butyl ether (MTBE) 21. C9 7. C6 22. 5-Methylnonane 8. 1-Methylcyclopentene 23. 1,2-methylethylbenzene 9. Benzene 24. C10 10. Cyclohexane 25. C11 (undecane) 11. 3-Ethylpentane 26. 1,2,3,5-Tetramethylbenzene 12. 1- tert-2-dimethylcyclopentane 27. Naphthalene 13. C7 28. C12 (dodecane) 14. 2,2,3-Trimethylpentane 29. 1-Methylnaphthalene 15. 2,3,3-Trimethylpentane 30. C13 (tridecane) GC_PC00743 Column Rtx®-DHA-100, 100 m, 0.25 mm ID, 0.50 µm (cat.# 10148) using Rtx®-5 DHA Tuning Column, 2.62m, 0.25mm ID, 1.0?m (cat.# 10165) with Universal Angled Press-Tight Connectors (cat.# 20446) Sample custom detailed hydrocarbons analysis (DHA) mix Conc.: neat Injection Inj. Vol.: 0.01 µL split (split ratio 150:1) Liner: Cup Splitter (cat.# 20709) Inj. Temp.: 200 °C Oven Oven Temp: 5 °C (hold 15 min.) to 50 °C at 5 °C/min. (hold 50 min.) to 200 °C at 8 °C/min. (hold 10 min.) Carrier Gas He, constant flow Flow Rate: 2.3 mL/min. Linear Velocity: 28 cm/sec. Detector FID @ 250 °C

In addition to reliable inertness, Rtx^®-DHA-100 columns have the required selectivity to adequately resolve all critical pairs as measured using an oxy set-up blend. Here, it is important to note that the high thermal stability of these columns has a significant benefit: it allows a more aggressive temperature program to be used with minimal bleed, resulting in separation of all critical pairs in 33% faster run times, compared to the standard method conditions (Table I). This allows refinery labs to significantly increase DHA sample throughput, while assuring separation of all critical pairs.

51% Faster Run Times with Proposed Hydrogen Method

While D6730-01 specifies helium as the carrier gas, hydrogen is a better alternative because it can be used at higher linear velocities without compromising critical separations. For DHA analysis, using hydrogen as the carrier gas offers three distinct advantages over helium: (1) it can be used at twice the linear velocity, (2) it increases column longevity by eluting high molecular weight compounds at lower temperatures, and (3) it is less expensive and more readily available. Despite these advantages, some labs have reservations regarding the safety of using hydrogen. In fact, with basic precautions, hydrogen can be used safely and reliably, particularly when hydrogen generators are used instead of free-standing cylinders.

Restek has developed a new DHA method to take advantage of the benefits of hydrogen. As shown in Figure 3, all critical components are resolved using this method, but in the greatly reduced run time of 72 minutes, versus 146 minutes or 98 minutes using helium. This 51% savings in analysis time has the potential to virtually double sample throughput. Restek has submitted this proposed DHA method based on hydrogen to ASTM where it is currently under review.

Figure 3 Increase sample throughput&151;cut analysis times in half without compromising resolution by using hydrogen instead of helium.

Peaks 1. Ethanol 16. Toluene 2. C5 17. C8 3. tert-Butanol 18. Ethylbenzene 4. 2-Methylbutene-2 19. p-Xylene 5. 2,3-Dimethylbutane 20. 2,3-Dimethylheptane 6. Methyl tert-butyl ether (MTBE) 21. C9 7. C6 22. 5-Methylnonane 8. 1-Methylcyclopentene 23. 1,2-Methylethylbenzene 9. Benzene 24. C10 10. Cyclohexane 25. C11 11. 3-Ethylpentane 26. 1,2,3,5-Tetramethylbenzene 12. 1,2-Dimethylcyclopentane 27. Naphthalene 13. C7 28. C12 14. 2,2,3-Trimethylpentane 29. 1-Methylnaphthalene 15. 2,3,3-Trimethylpentane 30. C13 GC_PC00774 Column Rtx®-1 PONA, 100 m, 0.25 mm ID, 0.50 µm (cat.# 10195) using Rtx®-5 PONA tuning column 5 m, 0.25 mm ID (cat.# 10196) with Universal Angled Press-Tight Connectors (cat.# 20446) Sample DHA/oxygenates setup blend (see notes) Injection Inj. Vol.: 0.01 µL split (split ratio 150:1) Liner: Cup Splitter (cat.# 20709) Inj. Temp.: 250 °C Oven Carrier Gas H2, constant flow Flow Rate: 3.62 mL/min. Linear Velocity: 55 cm/sec. Detector FID @ 300 °C Notes Oven Temp: A: 35°C; B: 5°C (hold 8.32 min.) (elute C5) to 48°C at 22°C/min. (hold 26.32 min.) (elute ethylbenzene) to 141°C at 3.20°C/min. (elute C12) to 300°C at 1°C/min. A: Front end of DHA/oxygenates set-up blend C5 Efficiency: 586,825 plates C5 k': 0.476 tert-Butanol skew: 2.10 Resolution: tert-butanol/2-methylbutene: 2:5.39 Acknowledgement Chromatogram courtesy of Neil Johansen, Inc., Aztec, New Mexico, in association with Envantage Analytical Software, Inc., Cleveland, Ohio.

Conclusion

Restek Rtx^®-DHA-100 columns are individually tested and stringently controlled for retention, efficiency, peak symmetry, selectivity, resolution, and bleed. These columns meet or exceed all ASTM D6730-01 and CGSB method requirements and display excellent peak symmetry for polar oxygenates and reliable separation of all critical pairs.

This allows users to obtain more correct and consistent data, especially for oxygenate content. Additionally, the robustness of these columns allows them to be used with either helium under accelerated temperature conditions or with hydrogen, resulting in 33% or 51% faster analysis times respectively. These columns offer significant performance gains, allowing refinery labs to process samples faster and make more profitable decisions during product blending.

References

(1) J.V. Hinshaw, LC-GC November (2008) 1100.
(2) P. Froehlich, Lab Manager Magazine February (2007) 1