GC Technical Tip: Retention Times Prolonged or Shortened

In this technical tip we will examine the possible causes of prolonged or shortened retention times and suggest some remedies. We will consider first retention time shifts on the same column. After that we will examine cases where retention times shifted after replacing the column.

Troubleshooting Column Performance in Gas Chromatography

When the current column (previously performing well) shows problems:

Possible Causes and Remedies

• Column temperature different from previous analyses
  • Adjust the oven temperature to match previous analytical conditions.
• Carrier gas flow rate lower or higher than before
  • Adjust the flow rate to the correct setting.
• Carrier gas leak at septum or column connection
  • Check for leaks and repair them. Replace the septum or tighten fittings if needed.
• Column contaminated or deteriorated
  • For bonded-phase columns, rinse to remove contaminants or trim 1–2 coils from the inlet end. If performance does not improve, replace the column.
• Phase damage in nonbonded columns
  • Rapid temperature changes can cause the stationary phase to “puddle” (form droplets inside the column), leading to major efficiency loss. Replace the column.
    Important: Always heat and cool columns gradually to prevent thermal rearrangement. Use bonded-phase columns whenever possible.
• Incorrect recorder chart speed or defective recorder
  • Verify chart speed and adjust as needed.
• Column overloaded
  • Reduce the sample size to prevent peak distortion.
• Column flow varies from run to run
  • Ensure the supply pressure at the gas source exceeds the required head pressure by at least 15 psig at the maximum operating temperature.
    Replace the gas cylinder before the pressure drops below 300 psig.
    If pressure control remains unstable, replace the regulator or flow controller.

When installing a new column (same composition as before) but results differ:

Possible Causes and Remedies

• Incorrect column ordered (phase or dimension mismatch)
  • Verify the column tag or manufacturer specifications. Ensure that length, internal diameter, and film thickness match the previous column. Contact the manufacturer if discrepancies are found.
• Column described the same but not identical to previous one
  • Use a certified column from a trusted manufacturer to ensure reproducibility.
• Incorrect linear velocity
  • Adjust the carrier gas linear velocity to the optimal setting for your method.
• Different carrier gas than before
  • Switch to the same carrier gas used in previous analyses for consistent results.