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Are You Being Served? Tailoring Your GC Column for a Lifetime of Analyses

Are You Being Served? Tailoring Your GC Column for a Lifetime of Analyses. Jack Cochran and Michelle Misselwitz Restek Corporation. Determining GC Column Length Why Is It Important?. You must enter the length of the GC column (and the ID) in software for carrier flow control

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Are You Being Served? Tailoring Your GC Column for a Lifetime of Analyses

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  1. Are You Being Served?Tailoring Your GC Column for a Lifetime of Analyses Jack Cochran and Michelle Misselwitz Restek Corporation

  2. Determining GC Column LengthWhy Is It Important? • You must enter the length of the GC column (and the ID) in software for carrier flow control • Translating a method after column trimming maintenance needs an accurate column length

  3. Accurate column length is important for accurate carrier gas flow by electronic pneumatic control...

  4. Determining the Length of a Capillary GC Column • Column length (L) = circumference * turns • Circumference = π * diameter • Determine diameter of column on cage • Count turns (“loops”) of column

  5. S S S S S S S S Determining the Length of a Capillary GC Column • Example for Restek column... • Diameter = 17.5cm or 0.175m • Circumference = 3.1416 (π) * 0.175m = 0.5498m • L = 56 turns * 0.5498m = 30.8m

  6. Another Way to Determine GC Column Length • Use Column Pressure / Flow Calculator • Establish GC column head pressure • Split inject (headspace of) non-retained compound to determine holdup time in sec • Methane for FID • Methylene chloride for ECD • Oven temperature 200°C suggested • Air for MS (monitor N2) • Match column length in Calculator with measured holdup time

  7. “Dead Volume” is synonymous with “Holdup Time” 63.0 sec • Example Dead Volume Determination • 30m x 0.25mm x 0.25µmGC column • Installed in ECD • Entered into GC software • Establish desired flow or linear velocity • Set oven temperature to 200°C • Inject headspace from MeCl2 • Note retention time in sec at peak apex • Average linear velocity is in cm/sec • 30m = 3000cm • 3000cm/63.0 sec = 48 cm/sec

  8. But my holdup time is 65.4 sec Hmmm… 63.0 sec

  9. Adjust holdup time value by increasing (or decreasing) column length... 64.2 sec

  10. Column length 30.6m Adjust holdup time value by increasing (or decreasing) column length... 65.4 sec

  11. Accurate column length is important for accurate carrier gas flow by electronic pneumatic control...

  12. EPA

  13. 30.6m, 20.7°C/min Organochlorine Pesticides 12.55 min

  14. We have to trim a GC column today, Mr. Lucas. Are you free? I’m free!

  15. PAH Standard after “Dirty” Injections Nonvolatile material compromises column Liner was changed BEFORE PAH standard injection Change GC inlet seal, trim GC column

  16. 30.6m, 20.7°C/min Organochlorine Pesticides 12.55 min

  17. GC Method Translation After trimming, column length is 29.7m. Flow rate is maintained at 2 mL/min. New GC oven program rate is 21.6°C/min.

  18. 29.7m, 21.6°C/min Organochlorine Pesticides 12.12 min

  19. 28.6m, 22.7°C/min Organochlorine Pesticides 11.62 min

  20. 26.6m, 25.0°C/min Organochlorine Pesticides 10.65 min

  21. 21.8m, 32.6°C/min Organochlorine Pesticides 8.49 min

  22. 15.7m, 49.9°C/min Organochlorine Pesticides 6.47 min

  23. 30.6m, 20.7°C/min 29.7m, 21.6°C/min 28.6m, 22.7°C/min

  24. 26.6m, 25.0°C/min 21.8m, 32.6°C/min 15.7m, 49.9°C/min 4,4’-DDE Endosulfan I

  25. 30.6m, 20.7°C/min FWHH 0.98 sec 10.57 min 21.8m, 32.6°C/min FWHH 0.80 sec 7.17 min

  26. I trimmed my GC column but I didn’t update the length in my method. I updated the length in my method but I didn’t change the oven program rate?

  27. Coelution of 4,4’-DDE and Endosulfan I 30.6m, 20.7°C/min Actual column length is 21.8m... 11.22 min

  28. 6.75 min 251°C 21.8m, 32.6°C/min 6.78 min 252°C 4,4’-DDE Endosulfan I 30.6m, 20.7°C/min 8.60 min 227°C Actual column length is 21.8m so carrier flow is now 2.75 mL/min... 4,4’-DDE Endosulfan I

  29. 21.8m, 20.7°C/min 4,4’-DDE Endosulfan I 8.94 min 234°C 30.6m, 20.7°C/min 8.60 min 227°C Actual column length is 21.8m so carrier flow is 2.75 mL/min...

  30. Accurate column length is important for accurate carrier gas flow by electronic pneumatic control...

  31. Column length was updated to 21.8m after trimming for maintenance... Flow control will be accurate. Don’t forget to method translate for the new GC oven program.

  32. 14.4m x 0.25mm x 0.10µm Rtx-1614 Original method 71 49 BDEs 20 min 7.9m x 0.25mm x 0.10µm Rtx-1614 Translated method 71 49 8.5 min

  33. Summary Comments • Determining GC column length is important! • Allows EPC to deliver accurate GC carrier flow • With proper method translation, separations are maintained even after substantial column trimming for maintenance • Performing method translation is important! • Allows components to elute at same GC oven temperatures, which maintains separations and avoids elution order changes • Extends column lifetime • GC columns separate longer than most users realize, even after column trimming • Trim 1 turn (~0.5m) of column for maintenance when necessary

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