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Understanding of gchs parameters


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Brief about the parameters used in headspace GC.

Published in: Science

Understanding of gchs parameters

  1. 1. Understanding of Gas chromatography Headspace parameters Ankur Gupta (Ph.D. Chemistry)
  2. 2. Terminology used for HSGC Headspace sampler Loop-pressure system Pressure Balanced system (Agilent) (Perkin Elmer) Temperature Oven Oven Temperature Loop Needle Temperature Transfer line Transfer line Time GC Cycle GC Cycle Time Sample Eq. Thermostating Time Pressurize Pressurize Time Loop fill Not applicable Time Loop Equilibrium Not applicable Time Injection Injection Vial pressure Psi Psi Shaking Low/High On/off loop volume 1ml/3ml Not applicable
  3. 3. Pressure Balanced system (Perkin Elmer)
  4. 4. Loop-pressure system (Agilent)
  5. 5. Oven (Vial) temperature: The temperature at which sample is equilibrate in HS vial. It is set so that the maximum amount of analyte is moved into the headspace in the minimum time without (or minimum) disturbing the sample thermal stability.
  6. 6. Needle/Loop temperature:  The needle/loop is heated so that the sample vapors does not condense in the needle during injection.  For best reproducibility, set the needle/loop temperatures to a value 5-10 ºC higher than the oven temperature.
  7. 7. Transfer Line Temperature:  The transfer line is heated so that the sample vapors does not condense in the transfer line during injection.  For best reproducibility, set the transfer line temperatures to a value 5-10 ºC higher than the needle/loop temperature.
  8. 8. Thermostatting/vial equilibration time:  The thermostatting (vial) equilibration time is the time required to thermostat the vial in HS oven.
  9. 9. Pressurization time:  After equilibrium has been reached, the vial is pressurized by the carrier gas to a pressure equal to the sampling head pressure (P2).
  10. 10. Injection time:  After pressurizing vial, the carrier gas supply is interrupted by closing a valve in the carrier gas supply line and the pressurized gas in the vial expands onto the column, resulting in a flow of the headspace gas from the vial to the column.
  11. 11. Withdrawal Time:  The length of time after the injection, before the sample needle is withdrawn from the sample vial or lowered into the vent position. During this time the needle remains in the vial.
  12. 12. GC Cycle Time:  The GC cycle time is the minimum time between a sample injection and the time at which the GC is ready for the next injection. This time will be determined from the GC analysis time and the time required to return the GC to its ready state.
  13. 13. Vial Venting:  This will be the length of time during which the vial is vented to atmosphere.
  14. 14. Column Pressure:  This is the carrier gas pressure for the HS system. The carrier gas is used to pressurize the vials and then carry the sample through the transfer line to the GC injector or column.
  15. 15. Loop Fill Time:  Time in which the headspace/vial pressurization gas mixture passes through the sample loop to vent. This fills the sample loop with a precise amount of headspace gas.
  16. 16. loop equilibration time:  Time after the vent valve closes to allow the analytes in the sample loop to equilibrate to the higher loop temperature and to allow pressure and flow in the loop to stabilize.
  17. 17. Inject Time:  Time in which the gases in the sample loop are injected into the GC. The actual time needed depends on the carrier flow rate and the sample loop size.
  18. 18. Multiple Headspace Extraction (MHE)  MHE is used to sample the same vial repeatedly. The vial is raised on to the sampling probe, sampled, and lowered off of the probe for each extraction.
  19. 19. Pressure Balanced system Loop-pressure system Standby Standby
  20. 20. Pressure Balanced system Loop-pressure system Pressurization Pressurization
  21. 21. Pressure Balanced system Loop-pressure system Loop filling Not applicable
  22. 22. Pressure Balanced system Loop-pressure system Not applicable Loop equilibrium
  23. 23. Pressure Balanced system Loop-pressure system Sampling
  24. 24. THANK YOU