Detailed Hydrocarbon Analysis Featuring Rtx-DHA Columns
Accurate DHA Analysis, Including Alcohols, Using Rtx-DHA Columns
- Individually tested to meet DHA method criteria; guaranteed column-to-column reproducibility.
- Improved resolution between oxygenates and hydrocarbons for more accurate reporting.
- Use of hydrogen carrier gas yields 50% faster run times.
Gasolines are complex mixtures of hundreds of compounds. Information about concentrations of the individual components is important for evaluating raw materials and for 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.
To maximize resolution of these complex mixtures, the ASTM method recommends a 100 m x 0.25 mm ID capillary column coated with 0.5 μm of 100% dimethyl polysiloxane stationary phase and sets minimum resolution criteria for several critical pairs of closely eluting compounds. To retain the aromatics and accomplish the separations, a short tuning column, approximately 2–3 m long, coated with 5% diphenyl/95% dimethyl polysiloxane polymer, is connected to the inlet of the 100 m analytical column. Through a series of trial analyses, the length of the tuning column is adjusted to ensure the critical resolutions are achieved. Rtx-DHA-100 columns are ideal for DHA methods and easily meet or exceed performance requirements.
Quality & Consistency
Analytical columns used for DHA applications must exhibit high efficiency and exceptional inertness, especially for polar oxygenates in gasoline. Figure 1 illustrates a column efficiency of 613,600 total theoretical plates, measured on C5, and shows excellent peak symmetry for the oxygenated additives, including ethanol and tert-butanol (tert-butanol skewness = 1.25). We test every Rtx-DHA-100 column for retention (k'), efficiency (n), stationary phase selectivity (α), and bleed, according to ASTM D6730. Data are shown on each test report, assuring that your Rtx-DHA column will meet method specifications.
Figure 1: Rtx-DHA-100 columns meet or exceed ASTM D6730 specifications.
Sharp, symmetric oxygenate peaks!
| Peaks | |
|---|---|
| 1. | Ethanol |
| 2. | C5 |
| 3. | tert-Butanol |
| 4. | 2-Methylbutene-2 |
| Column | Rtx-DHA-100, 100 m, 0.25 mm ID, 0.50 µm (cat.# 10148) |
|---|---|
| using Rtx-5 DHA tuning column, 2.62 m, 0.25 mm ID, 1.0 μm | |
| Standard/Sample | Custom detailed hydrocarbon analysis (DHA) mix |
| Conc.: | Neat |
| Injection | |
| Inj. Vol.: | 0.1 µL split (split ratio 150:1) |
| Liner: | 4 mm cup splitter inlet liner with Siltek deactivation |
| Inj. Temp.: | 200 °C |
| Oven | |
| Oven Temp.: | 35 °C |
| Carrier Gas | He, constant flow |
| Flow Rate: | 2.3 mL/min |
| Linear Velocity: | 28 cm/sec |
| Detector | FID @ 250 °C |
|---|---|
| Notes | C5 efficiency: 613,596 total theoretical plates k' (C5): 0.489 tert-Butanol skewness: 1.25 Resolution (tert-butanol/2-methylbutene-2): 5.60 catalog number 20709-214.1 Cup splitter with Siltek® deactivation obsolete, recommend catalog number 20709 |
Resolution of Critical Pairs
There are a number of critical compound pairs that must be resolved as measured by an oxy set up blend mix. An Rtx-DHA-100 column meets all ASTM D6730-01 requirements for critical pair resolution, as demonstrated in Figure 2. A tuning column was used to achieve the highlighted resolutions based on retention of the aromatics (e.g., resolution for 1-methylcyclopentene/benzene = 1.5). In addition to qualifying for the ASTM D6730-01 analysis, Rtx-DHA-100 columns meet the similarly stringent Canadian General Standards Board CAN/CGSB 3.0 No. 14.3-99 requirements.
Figure 2: Critical pairs of gasoline components resolved per ASTM specifications, using an Rtx-DHA-100 column.
Resolve all critical pairs!
| Peaks | |
|---|---|
| 15. | Toluene |
| 16. | C8 (n-octane) |
| 17. | Ethylbenzene |
| 18. | p-Xylene |
| 19. | 2,3-Dimethylheptane |
| 20. | C9 (n-nonane) |
| 21. | 5-Methylnonane |
| 22. | 1-Methyl-2-ethylbenzene |
| 23. | C10 (n-decane) |
| 24. | C11 (undecane) |
| 25. | 1,2,3,5-Tetramethylbenzene |
| 26. | Naphthalene |
| 27. | C12 (dodecane) |
| 28. | 1-Methylnaphthalene |
| 29. | C13 (tridecane) |
| Column | Rtx-DHA-100, 100 m, 0.25 mm ID, 0.50 µm (cat.# 10148) |
|---|---|
| using Rtx-5 DHA tuning column 2-5 m, 0.25 mm ID (cat.# 10165) | |
| with universal Press-Tight connectors (cat.# 20429) | |
| Standard/Sample | Oxy setup blend (cat.# 33034) |
| Injection | |
| Inj. Vol.: | 0.1 µL split (split ratio 150:1) |
| Liner: | Premium 4 mm Precision liner w/wool (cat.# 23305) |
| Inj. Temp.: | 250 °C |
| Oven | |
| Oven Temp.: | 5 °C (hold 10 min) to 49 °C at 5 °C/min (hold 50 min) to 200 °C at 1.5 °C/min (hold 10 min) |
| Carrier Gas | He, constant flow |
| Flow Rate: | 2.2 mL/min |
| Detector | FID @ 275 °C |
|---|---|
| Make-up Gas Flow Rate: | 30 mL/min |
| Make-up Gas Type: | N2 |
| Hydrogen flow: | 40 mL/min |
| Air flow: | 368 mL/min |
| Instrument | Agilent 7890B GC |
| Notes | Restek's Rtx-DHA-100 column has the required selectivity to deliver adequate resolution between the critical pairs in ASTM Method D6730. Excellent column inertness provides peak shapes for oxygenated compounds that meet or exceed ASTM method criteria. Chromatogram was obtained using 2.5 m of Rtx-5 DHA tuning column. |
Speed Up Analysis Time Using Hydrogen
Rtx-DHA-100 columns easily meet all ASTM and CGSB method requirements when using helium as the carrier gas. However, analysis times can be improved significantly—with no loss in chromatographic performance—by switching to hydrogen as the carrier gas. Rtx-DHA-100 columns meet or exceed all criteria in these standardized methods in up to 50% less time when using hydrogen (Figure 3).
Figure 3: Achieve up to 50% faster analysis times using hydrogen instead of helium.
|
Optimized D6730 |
Optimized D6730 |
Standard D6730 |
|
|
Approximate analysis time |
72 min. |
98 min. |
146 min. |
|
% Time savings (relative to standard method conditions) |
51% faster |
33% faster |
— |
| * Optimized conditions and chromatographic results for hydrogen shown below; for helium, see Figure 2. | |||
| Peaks | |
|---|---|
| 16. | Toluene |
| 17. | Octane (C8) |
| 18. | Ethylbenzene |
| 19. | p-Xylene |
| 20. | 2,3-Dimethylheptane |
| 21. | Nonane (C9) |
| 22. | 5-Methylnonane |
| 23. | 1,2-Methylethylbenzene |
| 24. | Decane (C10) |
| 25. | Undecane (C11) |
| 26. | 1,2,3,5-Tetramethylbenzene |
| 27. | Naphthalene |
| 28. | n-Dodecane (C12) |
| 29. | 1-Methylnaphthalene |
| 30. | Tridecane (C13) |
| Column | Rtx-DHA-100, 100 m, 0.25 mm ID, 0.50 µm (cat.# 10148) |
|---|---|
| using Rtx-5 DHA tuning column* 2-5 m, 0.25 mm ID, | |
| Standard/Sample | DHA/oxygenates setup blend |
| Injection | |
| Inj. Vol.: | 0.1 µL split (split ratio 150:1) |
| Liner: | 4 mm ID deactivated cup inlet liner |
| Inj. Temp.: | 250 °C |
| Carrier Gas | H2, constant flow |
| Flow Rate: | 3.62 mL/min |
| Linear Velocity: | 55 cm/sec |
| Detector | FID @ 300 °C |
|---|---|
| Notes | GC inlet liner cat.# 20835 was used to produce this chromatogram. It has been replaced by Cup Splitter Inlet Liner (cat.# 20836). *Note that the exact length of the tuning column was determined experimentally based on the resolution of critical pairs as per method D6730-01 (reapproved 2011). 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 setup 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. |
Superior Performance for DHA Analysis
A comparison of DHA columns (also referred to as PIANO, PIONA, or PONA columns by other manufacturers) shows that the Rtx-DHA-100 column is the best overall DHA column for peak shape, theoretical plates, and required resolution. Figure 4 shows that, at 35 °C, as specified in the ASTM methods, the Competitor A and Competitor B PONA columns did not meet ASTM D6730 method specifications. The Competitor C and Competitor D PONA columns performed within specifications, but column efficiency was less than ideal. In contrast, the performance of the Rtx-DHA-100 column at 35 °C was well within ASTM D6730 method specifications, and column efficiency exceeded the specification. The column also performed well at sub-ambient temperature.
Figure 4: Rtx-DHA-100 columns outperform competitor columns for detailed hydrocarbon analysis.
Conclusion
Rtx-DHA-100 columns offer the best overall value and performance for detailed hydrocarbon analysis. These columns meet or exceed all ASTM D6730-01 and Canadian General Standards Board method requirements and outperform other commercially available columns. Make your next DHA column an Rtx-DHA-100 column—individually tested to meet DHA method criteria, assuring outstanding performance and column-to-column reproducibility.